r 
 
 
 REESE LIBRARY 
 
 OF TIIK 
 
 
 
 UNIVERSITY OF' CALIFORNIA. 
 
ELEMENTARY BIOLOGY 
 
 LIBRtffc . 
 
 OF THE 
 
PRINTED BY 
 
 SPOTT1SWOODE AND CO., NEW-STREET SQUARE 
 LONDON 
 
A TEXTBOOK 
 
 OF 
 
 ELEMENTARY BIOLOGY 
 
 BY 
 
 R. J. HARVEY GIBSON, M.A, 
 
 \ i 
 
 FELLOW OF THE ROYAL SOCIETY OF EDINBURGH ; LECTURER ON BOTANY 
 IN VICTORIA UNIVERSITY, UNIVERSITY COLLEGE, LIVERPOOL 
 
 ILLUSTRATED WITH 192 ENGRAVINGS 
 
 LONDON 
 LONGMANS, GREEN, AND CO. 
 
 AND NEW YORK : 15 EAST 16* STREET 
 1889 
 
 All rights reserved 
 
t-f o u 
 
 BiOLOGY 
 
 LIBRARY 
 
 G 
 
PREFACE. 
 
 BY way of preface to this volume it may not, perhaps, be 
 out of place to state briefly the motives which have 
 prompted me to attempt what has proved to be a task of 
 no small difficulty. A not inconsiderable experience as a 
 university teacher of Biology has convinced me that in order 
 to properly appreciate and benefit by a study of that science 
 a student must first undergo a preliminary training in the 
 facts and conclusions of Physics and Chemistry, and in ad- 
 dition must devote not a little time and labour to studying 
 the application of the more general laws of these sciences 
 to the special phenomena of plant and animal life. It is 
 however by no means an easy matter for a beginner in the 
 subject to select from the vast domain of the physical 
 and chemical sciences those generalisations which have an 
 immediate bearing on the problems of Biology. I have 
 therefore endeavoured to summarise briefly in a preliminary 
 chapter the principal conclusions of the inorganic sciences, 
 devoting special attention to those laws on which the higher 
 science of Biology is founded. It is scarcely necessary to 
 
vi Elementary Biology. 
 
 add that that summary is intended to be suggestive rather 
 than exhaustive. 
 
 I have further endeavoured in the succeeding chapters 
 to keep prominently in the foreground the dependence of 
 Biology on Physics and Chemistry, and the relationship of 
 morphological and physiological details to general principles. 
 I have tried to keep constantly before me the evils of the 
 * cram ' system in education, and this must be my apology, 
 if an apology be deemed necessary, for the introduction of 
 so many speculations and explanations of causal relationship. 
 I am fully aware of the fact that many theories referred to 
 in the text are still matters of discussion, yet I am of opinion 
 that working hypotheses not only serve to weave apparently 
 isolated facts together, but give a certain vividness and in- 
 terest to what would otherwise prove too often a bare and 
 lifeless catalogue of data. 
 
 It may seem at first sight that I have given undue pro- 
 minence to the botanical aspect of Biology. I have done 
 so intentionally, and for two reasons. First, because there 
 is no want of sound text-books both practical and theoretical 
 on Animal Biology by most competent authors, and no 
 advantage, it seemed to me, was to be gained by a repe- 
 tition of what had already been so often and so ably done ; 
 and secondly, because Plant Morphology and Physiology, 
 from their relative simplicity and clearness as compared 
 with Animal Morphology and Physiology, are more suit- 
 able for elementary study. ! 
 
 1 A word of explanation seems necessary apropos of the terminology 
 I have adopted in describing the reproductive organs both in the plant 
 and animal kingdoms. In my experience nothing is more confusing to 
 the student than the diversity of the systems of nomenclature at present 
 in use in text-books. The time has now come for the introduction of 
 a uniform terminology in both these subsciences, and reform may not 
 
Preface. vii 
 
 I desire to offer my most grateful thanks to Professor 
 Herdman, D.Sc., for constant advice and help freely given 
 on all occasions, for his kindness in reading the zoological 
 chapters, and for many suggestions with reference to the 
 subjects dealt with therein. My thanks are also due to 
 Professor Lodge, D.Sc., F.R.S.,for similar services rendered 
 in the chemical and physical sections. To my friend Mr. 
 G. G. Chisholm, M.A., I am indebted for much help and 
 criticism of a more general nature during the passage of the 
 book through the press. I desire also to express my best 
 thanks to gentlemen and firms who have been so kind as to 
 allow me to make use of figures from various publications 
 written by or belonging to them, more especially to Professor 
 Marshall, F.R.S., and Mr. Hurst, of Owens College, the 
 Clarendon Press, Messrs. Cassell and Co., Messrs. A. and 
 C. Black, and the publishers of Van Tieghem's Traite de 
 Botanique and Claus's Traite de Zoologie, and of Luerssen's 
 Grundzuge der Botanik. I have been careful t.o indicate in 
 all cases the source from which I have borrowed figures ; 
 those not so distinguished are original. 
 
 My indebtedness to the many works on Botany and 
 Zoology already published in our own or other languages 
 will be manifest to all. I have not directly expressed that in- 
 debtedness by footnote references, simply because a student, 
 it seems to me, wants fust of all a certain general acquaint- 
 ance with his subject before entering on the task of consulting 
 special memoirs, and the larger text books on the two sub- 
 sciences of Botany and Zoology such as the works of Van 
 
 unnaturally commence among the terms applied to the reproductive 
 products where the greatest chaos prevails. I have already given 
 reasons for the adoption of terms similar to those employed in the text 
 in a paper on the subject (Proc. Liverpool Biol. Soc. vol. ii. p. 112). 
 
 a 
 
viii Elementary Biology. 
 
 Tieghem, Goebel, De Bary, and Sachs in the former, and of 
 Huxley, Glaus, Balfour, and Gegenbaur in the latter will 
 supply him with references to such in plenty. 
 
 Lastly, I am fully conscious of the risk I run in ven- 
 turing to add yet another text-book on Natural Science to 
 the long list already before the public; nevertheless the 
 absence of any text-book in the English language dealing 
 with the relationship of Botany to Zoology, and of both to 
 the fundamental sciences of Physics and Chemistry, en- 
 courages me to hope that this little book may fill a gap 
 left unoccupied by the classic treatises I have above re- 
 ferred to. 
 
 R. J. HARVEY GIBSON. 
 
 UNIVERSITY COLLEGE, LIVERPOOL : 
 October 31, 1 888. 
 
Of THE 
 
 CONTENTS. 
 
 INTRODUCTION 
 
 CHAPTER 
 
 I. MATTER AND ENERGY 
 
 SECTION i. Matter 7 
 
 ,, ii. Energy 12 
 
 ,, iii. Classification of chemical compounds . 21 
 
 ,, iv. Laws of chemical change . . . 22 
 
 II. PROTOPLASM 
 
 SECTION i. Chemical composition of protoplasm . 25 
 ,, ii. Morphology and physiology of proto- 
 
 plasm ...... 29 
 
 ,, iii. Phenomena concomitant with the mani- 
 
 festations of energy by living proto- 
 plasm . . . . 35 
 
 ,, iv. Physiological significance of chlorophyll 38 
 
 ,, v. Conditions of the environment necessary 
 
 for the maintenance of life in animal 
 and vegetal protoplasm . . .41 
 ,, vi. The balance of nature . . . . 48 
 
 III. INDIVIDUAL AND TRIBAL LIFE DISTRIBUTION AND 
 
 CLASSIFICATION 50 
 
 SECTION i. Individual life . . . . . 50 
 
 Tribal life ...... 52 
 
 Distribution . . . . , . 54 
 
 Classification 
 
 54 
 
x Elementary Biology. 
 
 CHAPTER PAGE 
 
 IV. THE MORPHOLOGY AND PHYSIOLOGY OF THE SIMPLEST 
 
 LIVING ORGANISMS PROTISTA . . . . 55 
 
 SECTION i. Protista Protamaba . , . -55 
 ,, ii. Protista Protomyxa . . S7 
 
 ,, iii. The relation of unicellular to multicellu- 
 
 lar organisms , . . .62 
 
 V, UNICELLULAR PLANTS PROTOPHYTA Protococcus . 69 
 VI. UNICELLULAR ANIMALS PROTOZOA Amoeba . . 73 
 
 VII. METAPHYTA NON-VASCULARIA 76 
 
 SECTION i. Fresh-water &%&- Spirogyrd . . 76 
 
 ,, ii. Salt-water Algae Fucus . . . 83 
 
 ,, iii. Fungi Penicillium . . . . 91 
 
 ,, iv. Musci Polytrichum , . . . 101 
 
 VIII. METAPHYTA VASCULARIA . . . ; . .113 
 
 SECTION i. Filices Pteris 113 
 
 ,, ii. Ligulatae Selaginella , . -131 
 
 ,, iii. Monocotylcdones Lilium . . . 143 
 
 ,, iv. Dicotyledones Ranunculus . .178 
 
 General physiology of plants . . . 189 
 
 Carnivorous plants . . . .213 
 
 IX. METAZOA INVERTEBRATA 219 
 
 SECTION i. Hydrozoa Obelia . . . .219 
 ,, ii. Vermes Lurnbrictts . . . . 233 
 
 X. METAZOA VERTEBRATA 251 
 
 SECTION i. Cephalochorda Atnphioxiis . . . 251 
 ii. Amphibia Rana .... 263 
 
 ,, iii. General physiology of animals . . 335 
 
 XI. HISTORY OF BIOLOGY . 345 
 
ELEMENTARY BIOLOGY. 
 
 INTRODUCTION. 
 
 Page 84, line 24, for spherical read oblong 
 
 100 ,, 4f>for what is known as read an imperfect 
 174 7 from bottom, <w* like the lily 
 
 appear late in the history of science, and, of necessity, their 
 advent is preceded by a period during which the efforts of 
 scientific workers are, consciously or unconsciously, directed 
 chiefly to the accumulation of stores of information on 
 matters of fact. Moreover, not only have those observations 
 already made to be revised, corrected, and extended, but 
 they require to be directly verified and amplified by ex- 
 periment before the generalisations or inductions formed 
 from their study can be said to be perfectly legitimate and 
 trustworthy. Observations previously isolated become aggre- 
 gated round these generalisations as centres, each generali- 
 sation being known as a natural law. 
 
 The relationships discovered to exist between individual 
 phenomena were soon found to be capable of wide exten- 
 sion ; for the various natural laws which expressed these 
 
 
 
x Elementary Biology. 
 
 CHAPTER PAGE 
 
 IV. THE MORPHOLOGY AND PHYSIOLOGY OF THE SIMPLEST 
 
 LIVING ORGANISMS PROTISTA . . . . 55 
 
 SECTION i. Protista Protamceba . . . -55 
 ,, ii. Protista Protomyxa . . 57 
 
 ,, iii. The relation of unicellular to multicellu- 
 
 lar organisms . . . .62 
 
 V. UNICELLULAR PLANTS PROTOPHYTA Protococcus . 69 
 VI. UNICELLULAR ANIMALS PROTOZOA Amaba . . 73 
 
 NON-VASCIILARIA_ . , . . . 76 
 
 ,, iv. Dicotyledones Ranuncuhis . .178 
 
 ,, v. General physiology of plants . . . 189 
 
 ,, vi. Carnivorous plants . . . .213 
 
 IX. METAZOA INVERTEBRATA ...... 219 
 
 SECTION i. Hydrozoa- Obelia . . . .219 
 ,, ii. Vermes Lumbricus . . . . 233 
 
 X. METAZOA VERTEBRATA 251 
 
 SECTION i. Cephalochorda Amphioxits . . . 251 
 ,, ii. Amphibia Rana .... 263 
 
 ,, iii. General physiology of animals . . 335 
 
 XI. HISTORY OF BIOLOGY . 345 
 
ELEMENTARY BIOLOGY. 
 
 INTRODUCTION. 
 
 THE study of natural phenomena has occupied the atten- 
 tion of mankind from the earliest times of which we have 
 any record. That the observations so made should have 
 been crude, incomplete, and often erroneous, and that the 
 relationship of the several observations to each other should 
 have been, in most cases, entirely overlooked, are results 
 which are completely in accordance with what we know of 
 the development of scientific thought. For generalisations 
 appear late in the history of science, and, of necessity, their 
 advent is preceded by a period during which the efforts of 
 scientific workers are, consciously or unconsciously, directed 
 chiefly to the accumulation of stores of information on 
 matters of fact. Moreover, not only have those observations 
 already made to be revised, corrected, and extended, but 
 they require to be directly verified and amplified by ex- 
 periment before the generalisations or inductions formed 
 from their study can be said to be perfectly legitimate and 
 trustworthy. Observations previously isolated become aggre- 
 gated round these generalisations as centres, each generali- 
 sation being known as a natural law. 
 
 The relationships discovered to exist between individual 
 phenomena were soon found to be capable of wide exten- 
 sion ; for the various natural laws which expressed these 
 
2 Elementary Biology. 
 
 relationships were themselves found to have a genetic con- 
 nection. Natural laws were found to be capable of being 
 systematised into bodies of truth, or departments of science, 
 more commonly, but with less accuracy, termed sciences. 
 
 One step alone remained in this process of unification 
 of knowledge, namely, the determination of the inter- rela- 
 tionships of the various departments of science. Hence 
 arose the various attempts which have been made within 
 comparatively recent years to formulate a classification of 
 the sciences. It can scarcely be said that a uniform agree- 
 ment has been come to as yet upon all points with reference 
 to that classification, but the general order of relationship 
 and development of the several sciences may be said to be 
 now beyond controversy. 
 
 A careful distinction must be drawn at the very outset 
 between what constitutes the subject-matter of the sciences 
 themselves, and the methods or instruments which are em- 
 ployed in their study. These methods are two in number, 
 and entirely different in their nature qualitative, em- 
 bracing the methods of inductive and deductive Logic, 
 and quantitative, or mathematical, embracing that section 
 of mathematics usually known as Pure Mathematics. 
 
 All scientific knowledge rests fundamentally on the belief 
 in the ' order of nature ; ' that is to say, on the belief that 
 under the same conditions the same cause will produce 
 the same effect. The basis for this belief is the universal 
 experience of mankind, obtained by observation of and 
 experiment on natural phenomena, and formulated there- 
 from by inductive reasoning, the induction going by the 
 name of the law of causality. 
 
 In attempting to classify or arrange the sciences in their 
 natural order, regard must be had to the several conditions 
 of a natural classification. 
 
 Firstly, the sciences must be arranged in the order of 
 their complexity ; that is to say, sciences which involve 
 simple and definite generalisations must precede sciences 
 
Introduction. 3 
 
 built out of generalisations involved and complex in their 
 nature. Manifestly, therefore, if we accept this as a principle 
 of classification, the sciences will arrange themselves in order 
 of their difficulty. Further, the sciences dealing with gene- 
 ralities must precede those dealing with specialities, since the 
 latter are dependent on the former for proof. The science, 
 for instance, dealing with the subject of chemical decompo- 
 sition in general must precede the science which discusses 
 the phenomena of life, amongst which the laws of chemical 
 decomposition play so important a part. 
 
 If the sciences be classified according to such principles, 
 they will be found, generally speaking, to arrange themselves 
 in the order of their own historic development. Naturally, 
 therefore, the classification will indicate the order in which 
 they ought to be studied. The absurdity of endeavouring 
 to grapple with the principles of Biology without a prelim- 
 inary acquaintance with the facts and laws of Physics and 
 Chemistry, for instance, becomes at once apparent ; nor will 
 the absurdity be diminished, and the misconceptions which 
 result be less pronounced and mischievous, when attempts 
 are made not only to become acquainted with, but even to 
 teach the principles of the higher sciences of Sociology and 
 Ethics without even an elementary knowledge of the facts 
 and conclusions of Biology. 
 
 The most general of the sciences is manifestly Astro- 
 nomy, since it treats of the universe in the widest possible 
 acceptation of the term. Astronomy may be defined as the 
 science which discusses 'the distribution, motions and 
 characteristics of the heavenly bodies.' Astronomy requires 
 for its proper study an accurate and extensive knowledge of 
 qualitative and quantitative methods, but its conclusions are 
 independent of the special conclusions of the other sciences. 
 
 The first limitation to the scope of scientific enquiry is, 
 naturally, accessibility to the subject matter of science. An 
 extensive acquaintance with matter so far removed as the 
 fixed stars, or even the planets of our own system, is amani- 
 
 B 2 
 
4 Elementary Biology. 
 
 fest impossibility. Consequently, when we define the next 
 three sciences of Physics, Chemistry, and Biology, as ' the 
 sciences of matter and energy,' we mean it to be understood 
 that they are limited to a consideration of the phenomena of 
 matter and energy as manifested, for the most part, on our 
 own earth. We say, * for the most part,' for within recent 
 years the method of spectrum analysis introduced by Bunsen 
 and Kirchhoff have enabled physicists and chemists to make 
 considerable progress towards the attainment of a knowledge 
 of the composition of the heavenly bodies ; whilst our belief 
 in the universal applicability of the laws of Physics has 
 become so firm that, although not submitted to the test of 
 actual experiment or even actual observation, we have no 
 hesitation in affirming that they hold sway in the farthest 
 removed of these bodies known to us. 
 
 Physics may be defined as * the systematic exposition 
 of the phenomena and properties of matter and energy, in 
 so far as these phenomena and properties can be stated in 
 terms of definite measurement and explained by reference 
 to mechanical principles and laws ' (Daniell). Physics is 
 usually divided into two sections, the one dealing with the 
 phenomena and properties of masses of matter, the other 
 concerned with matter in its molecular aspect, and termed 
 molar and molecular physics respectively. The department 
 of molecular physics leads up to, and in some measure 
 overlaps, the science of Chemistry. 
 
 Chemistry may be defined as the science which em- 
 bracss the study (1) of the properties of chemical ele- 
 ments, and of the compounds formed by the union of two 
 or more of them ; and (2) of the laws which regulate the 
 mutual action of elements and compounds on each other. 
 
 We shall have occasion to discuss more fully in Chapters 
 I. and II. the general scope of Physics and Chemistry, more 
 especially with reference to the last of the three sciences of 
 matter and energy, namely, Biology. 
 
 Biology, being 'the science which deals with the 
 
Introduction. 5 
 
 matter and energy of living things ' (using the term ' living 
 thing ' in its ordinary acceptation), manifestly rests on Physics 
 and Chemistry, since it involves the application of the laws 
 and principles of these sciences to the special case of living 
 matter, whether plant or animal. Contrary, perhaps, to our 
 first anticipation, we find that the phenomena of Biology, 
 complex and involved as they admittedly are, are generally 
 speaking capable of expression in physical terms, while the 
 changes that take place in the animal or plant organism 
 are found to agree in all important points with those which 
 form the subject of chemical and physical investigations. 
 Even mental or psychological phenomena, although the 
 nature of their connection with the chemical and other 
 changes taking place contemporaneously in the brain, has 
 not yet been determined, are undeniably accompanied by 
 some such change, and cannot be said to be independent 
 of the physical and chemical basis on which the science of 
 Biology, as a whole, is built. The dependence of Biology 
 on Chemistry becomes still more evident when we learn 
 that no constituent of living matter is incapable of being 
 classified among chemical substances ; that, in other words, 
 the substances which enter into the composition of living 
 things are among the commonest constituents of the mine- 
 rals of the earth's crust. 
 
 It is usual to classify such departments of knowledge as 
 Geology, Mineralogy, Botany, Zoology, Anthropology, and 
 Sociology as distinct sciences. They cannot, however^ 
 claim a higher rank than that of sub-sciences, while some of 
 them are merely special departments of the four great 
 sciences already mentioned. 
 
 This discussion regarding the position of Biology in a 
 classification of the sciences leads us, therefore, to the con- 
 clusion that Biology has for its subject the matter and 
 energy of living things, and is, therefore, dependent on the 
 more general sciences of Physics and Chemistry, and itself 
 forms the starting-point for a series of special sub-sciences, 
 
 *B 3 
 
6 Elementary Biology. 
 
 such as Botany, Zoology, Anthropology, and Sociology, all 
 of which are dependent on it for their fundamental basis. 
 We have also seen that a study of the more general sciences 
 of Physics and Chemistry ought to precede the study of 
 Biology, j list as the discussion of the principles of Biology 
 forms a necessary preliminary to the proper understanding 
 of the phenomena and laws of Psychology, Sociology, and 
 other related sub-sciences. 
 
 The first chapters of this text-book are, therefore, devoted 
 to giving a short account of the principles of Physics and 
 Chemistry, looked at mainly in the light of their application 
 to Biology ; whilst the chemical and physical characters of 
 protoplasm ' the physical basis of life ' led up to by a 
 brief discussion of chemical compounds in general, forms 
 the natural introduction to the subject-matter of Biology 
 itself. 
 
CHAPTER I. 
 
 MATTER AND ENERGY. 
 
 SECTION I. MATTER. 
 
 WHAT is matter ? What is energy ? must of necessity be 
 the first questions asked in a survey of the sciences that deal 
 with these two subjects. A categorical answer to either 
 query is, however, an impossibility. We know matter and 
 energy only as revealed to us through their properties ; nor 
 is there any likelihood of our ever attaining to a knowledge 
 of their essential nature. In order to define the terms 
 'matter,' 'energy,' therefore, it is necessary to enumerate 
 their properties in some detail. 
 
 Fundamental properties of matter. Matter obviously 
 exists in measurable quantities, each quantity occupying a 
 certain volume or bulk. Matter, as occupying a certain 
 volume of space, must possess appreciable length, breadth, 
 and thickness ; that is to say, each volume of matter must 
 possess a definite form. Moreover, in virtue of the universal 
 applicability of the law of gravitation viz. that every par- 
 ticle of matter in the universe attracts every other particle 
 with a force directly proportional to the mass of the attract- 
 ing particle, and inversely proportional to the square of the 
 distance between them every particle of matter must pos- 
 sess weight, that is, a tendency to rush to the centre of the 
 earth, as the nearest great attracting body. 
 
 Matter exists in a variety of states. We are familiar 
 more especially with the solid, liquid, and gaseous states 
 
8 Elementary Biology. 
 
 and others intermediate between any two of them. 1 It 
 is a familiarly known fact, moreover, that the same kind 
 of matter may exist in different states, under different 
 atmospheric and other conditions i.e. we may have trans- 
 formation of matter from one state into another.. Water is 
 probably the most familiar instance of this. Fluid under 
 ordinary temperatures, it becomes solid when the tempera- 
 ture is sufficiently reduced, or gaseous when sufficiently 
 augmented. Matter in its manifold state possesses the 
 same fundamental properties. 
 
 Matter is divisible, but not infinitely so (save in imagina- 
 tion). Practically, and even theoretically, there are limits 
 to the divisibility of matter. The practical limits to the divi- 
 sibility of matter are soon reached ; but, theoretically, matter 
 is believed to consist of extremely minute and perfectly 
 invisible particles, or molecules. Matter is, therefore, not 
 absolutely continuous ; the molecules of which it is com- 
 posed are an appreciable distance apart, i.e. matter is porous, 
 and therefore penetrable or permeable. For it is conceiv- 
 able that smaller molecules of another substance might 
 be insinuated between the larger molecules. Molecules 
 themselves are, however, supposed to be impenetrable. 
 
 The number of molecules in a given volume of a sub- 
 stance varies according as the substance is in the solid, 
 liquid, or gaseous state. Observation and experiment teach 
 us that the molecules are relatively close to one another in 
 the solid and liquid states, and that they are further apart 
 in the gaseous. 
 
 Defined physically, a molecule is the smallest particle 
 of a substance that can possess the properties of the whole; 
 
 1 No reference is made in the text to the recent interesting specula- 
 tion of Crookes and others on the possible existence of a fourth state 
 which matter is believed to be capable of subsisting in, when the tension 
 of its gaseous state is reduced to one-millionth of an atmosphere, as being 
 out of place in an elementary sketch like the present. See ' Radiant 
 Matter,' by W. Crookes, F.R.S., British Association Report^ 1879. 
 
Matter and Energy. 9 
 
 or, from a chemical point of view, the smallest quantity 
 that is able to exist in a free state, or take part in or result 
 from a chemical change. Molecules of the same kind, i.e. 
 of the same substance in whatever state it may occur, agree 
 in possessing the same weight, size, and properties, differing 
 in these respects from molecules of all other substances. 
 
 Constitution of n.atter. Molecules by the employment 
 of certain agencies, chemical or other, are themselves capable 
 of division. Their constituent parts are termed atoms. 
 An atom may therefore be looked upon as the ultimate 
 chemical particle of matter, as distinguished from the mole- 
 cule, the ultimate physical particle. The atom, as its name 
 indicates, is itself indivisible by any means at present in 
 our power. Nearly every molecule is composed of two 
 or more atoms. In a very few cases the molecules con- 
 sist of one atom only, e.g. the metals mercury, cadmium, 
 and zinc, and in these cases the ultimate physical and 
 ultimate chemical particles may be said to be identical. 
 If all the atoms going to form a molecule be identical, then 
 the molecule, or ultimate physical particle, is an elementary 
 molecule, and a substance composed of such molecules is 
 termed a chemical element. If, however, the atoms going 
 to make up the molecule differ in size, weight, and other 
 properties, then the molecule is compound in its nature, and 
 the substance formed of such molecules is known as a 
 chemical compound. 
 
 Molecules vary in size according to the number of atoms 
 entering into their composition that number ranging from 
 one (in a very few cases) to several hundreds. In the case 
 of elementary substances the terms monatomic, diatomic, 
 triatomic, c., are used according to the number of atoms 
 going to make up a molecule of the element in question. 
 
 At the present time about seventy different kinds of 
 atoms, or elementary forms of matter, are known. 1 It is 
 
 1 Since the above sentence was written Kruss and Nilson announce 
 the discovery of twenty new elements in rare Scandinavian minerals. 
 
io Elementary Biology. 
 
 possible, however, that that number may be increased as 
 new methods of investigation are discovered. The method 
 of spectrum analysis, for instance, already alluded to above, 
 has added many to the list. Since not a few of the elements 
 have been discovered by the decomposition of substances 
 which were believed, in a less advanced condition of the 
 science, to be themselves elements, it is by no means im- 
 probable that continued research in the same direction may 
 prove productive of further results of a similar nature. 
 Indeed, the suggestion has been thrown out by some bolder 
 speculators that all the elements are simply different modi- 
 fications or conditions of one primary elemental form of 
 matter. 
 
 We have hitherto purposely omitted to mention one 
 property of matter,, namely inertia, which is defined by 
 Newton as * the tendency of every body to persevere in a 
 state of rest, or of uniform motion in a straight line> 
 unless in so far as it is acted upon by an impressed force/ 
 We are thus introduced to two new ideas * motion ' and 
 * force. 5 The conception of ' motion ' is one so familiar to 
 us, that it does not require further definition. Like 'matter' 
 and ' energy,' the essential nature of force is unknown. It 
 may be defined, however, from a study of its effects, as that 
 which produces, arrests, accelerates, retards, or changes the 
 direction of motion. 
 
 We have already seen that the same kind of matter 
 may exist in the different states known as solid, liquid, and 
 gaseous, and in intermediate states. We have also seen that 
 these states of matter are transformable, the one into the 
 other. The transformation is possible only in consequence 
 of the application of a certain amount of force. 
 
 When left alone, the solid is bounded by definite free sur- 
 faces, often, as in crystals, natural planes. Liquids have one 
 free surface, namely, that exposed to the external atmosphere, 
 and other surfaces which are in contact with the containing 
 
Matter and Energy. 1 1 
 
 vessel, in number and nature corresponding to the sides of 
 the vessel. The one free surface is always horizontal, and is 
 constantly changing owing to the escape of molecules from 
 the surface into the air. A gas has no free surface. The 
 molecules, freed from mutual influence, are constantly tend- 
 ing to become farther and farther separated from each other. 
 
 It has been calculated that in a cubic inch of air the 
 number of molecules can be expressed only by a row of 
 figures twenty-one in number. This calculation can give 
 only a very faint conception of the number of molecules in 
 a given mass, or of their extreme minuteness. Each molecule 
 is believed to be in perpetual and extremely rapid motion in 
 straight lines in all directions, and to hit against its neigh- 
 bours on every side at the rate of something like 18,000 
 million times per second. 
 
 In addition to this motion of translation, molecules are 
 believed to possess intrinsic motions of vibration and rota- 
 tion, dependent for their intensity upon the temperature of 
 the substance. 
 
 As already stated, an application of heat to a solid has 
 the effect of transforming it into the liquid, and through 
 that state into the gas, or, in some cases, into a gas directly, 
 the molecules of which can by further application of the 
 same force be made to recede farther and farther from each 
 other. Change of state is therefore accompanied by change 
 in the rate of motion of the constituent molecules, a high 
 temperature being associated with a rapid motion, and a 
 low temperature with a relatively sluggish motion. The 
 converse is likewise true, abstraction of heat being sufficient 
 to change the gaseous state first into the liquid and lastly 
 into the solid. Water may be again cited as the most 
 familiar example of this change. 
 
 The transformation of one state of matter into another is 
 possible in virtue of a manifestation of energy. 
 
12 Elementary Biology. 
 
 SECTION II. ENERGY. 
 
 Work is the production of motion in mass or molecule 
 against resistance. Work is done when a mass of matter 
 is lifted against the resistance of gravity ; work is done 
 when a chemical compound is decomposed, for the resistance 
 of the peculiar form of attraction known as chemical affinity 
 is overcome and the molecules are separated from each 
 other. 
 
 Energy is the power to do work. It must be carefully 
 distinguished from force, the existence of which it, however, 
 implies, and from motion which, as being produced by force, 
 indicates the presence of energy. Thus, force is exerted 
 when a weight is lifted in virtue cf the arm of the labourer 
 possessing a store of energy, a certain proportion of which 
 energy is used in the act of lifting the weight. Force is 
 similarly exerted when a compound is decomposed in 
 virtue of the agent in the decomposition possessing a store 
 of energy, some of which is expended in the performing of 
 the work of decomposition. 
 
 When a mass of matter rests on the earth's surface, it 
 exerts a certain force due to the action of gravity ; but it 
 possesses no energy, it possesses no motion of its own. 
 When the mass is raised, say to the top of a high tower, 
 the force of gravity has been, in accordance with the law 
 of gravity, reduced, by an extremely small but calculable 
 amount, because its distance from the centre of the attract- 
 ing body has been increased. It has no motion of its own, 
 but it has gained a certain amount of energy energy cf 
 position, or potential energy. It is capable of doing some 
 work ; it is capable, for instance, of driving a pile into the 
 ground if allowed to drop upon it. When the mass reaches 
 the ground again, it has lost its energy. Its energy in the 
 act of falling has become transformed into active or kinetic 
 energy. Kinetic energy is therefore energy of motion. 
 
Matter and Energy. 13 
 
 Further, by the agency of, say, a steam engine, the weight 
 might be again raised to the top of the tower, i.e. the kinetic 
 energy of the engine may be retransformed into the potential 
 energy of the raised weight. 
 
 Again, the weight at the tower's top is prevented from 
 falling by the resistance of the tower. It requires an exer- 
 tion of force to overcome this resistance. That counteract- 
 ing force implies a source of energy which must manifestly 
 be a form of kinetic energy. Imagine the weight referred 
 to as suspended from a hook the energy employed to 
 liberate the weight from its support must be a kinetic 
 energy. The liberating energy need not be by any means 
 commensurate to the effects produced by the liberation. 
 The old proverb, 'a small spark may raise a great con- 
 flagration,' accurately expresses this fact. Indeed, we may 
 take this proverb as an illustration. A barrel of gunpowder 
 consists of a mixture of a large number of molecules of 
 different substances which under certain conditions have a 
 very strong tendency to unite and form new combinations. 
 So long as the condition of their union be. absent, the 
 molecules retain energy of position, and the barrel of gun- 
 powder is a store of potential energy. The condition of 
 their union is the production of a high temperature in their 
 immediate vicinity. That condition being satisfied by the 
 application of a spark to the mass, the affinities of the mole- 
 cules become at once satisfied, and the store of potential 
 energy becomes transformed into kinetic energy, accom- 
 panied by the usual attendant phenomena of an explosion, 
 viz. sound, light, and heat. 
 
 It is necessary that we should trace this conception of 
 energy a little farther. 
 
 Kinetic energy being energy of motion, the various 
 modes of motion may be conversely termed kinetic energies. 
 Sound is produced by the vibration of molecules, and is 
 communicated to the organ of hearing by the sympathetic 
 vibration of the mplec r ul<es-e^ rtie atmosphere between the 
 
14 Elementary Biology. 
 
 ear and the source of the sound. Sound is therefore a 
 mode of motion. Light, according to the undulatory theory 
 first propounded_ by-Huyghens, is to be considered as due 
 to the vibrations of an imponderable medium termed ether, 
 which pervades all space between the particles of matter, and 
 which is by some eminent physicists regarded as in all proba- 
 bility forming the fundamental basis of matter itself. Light 
 may therefore be considered as a mode of motion, though 
 not precisely in the same sense as sound is considered to be 
 so. Lastly, heat is undoubtedly due to the vibration ot 
 molecules of matter. Of course, on the assumption that the 
 . space between all molecules is filled by an ethereal medium, 
 :the vibrations of molecules mean also the simultaneous 
 vibration of their ethereal surroundings. Now, we have seen 
 that the potential energy of the gunpowder became trans- 
 formed into kinetic energy, manifesting sufficient force to 
 cause a considerable alteration in the distance relationships 
 of surrounding objects. But the heat produced during the 
 explosion, which we have just seen to be a form of kinetic 
 energy, must have resulted from the transformation of a 
 portion of the original potential energy of the gunpowder ; 
 similarly for the light and sound. We conclude, there- 
 fore, that all the energy of the gunpowder is not available 
 for the production of ordinary motion of translation amongst 
 masses. Part is transformed into forms of kinetic energy, 
 probably not desired, but nevertheless invariably accompany- 
 ing the chief manifestation. An extended course of obser- 
 vation and experiment teaches us that every transformation 
 of potential energy into kinetic, or of kinetic into potential, 
 is attended by one or more, what might be termed, by- 
 manifestations of energy, the chief of these, and that form 
 of kinetic energy into which all others ultimately become 
 transformed, being heat. 1 
 
 Heat, of course, as kinetic energy, can itself be trans- 
 
 1 The rough determination of the source, destination, and propor- 
 tional amount of the various forms in which energy manifests itself in 
 
Matter and Energy. ' 15 
 
 formed, though only partially, into potential energy. Accord- 
 ing to a law first enunciated by Carnot, it is only when heat 
 passes from a warmer to a colder body, and even then only 
 vpartially, that it can be converted directly or indirectly into 
 mechanical work. That passage, however, cannot be inter- 
 rupted. There is a constant tendency in nature towards 
 the establishment of an equilibrium of temperature ; so that 
 every transformation of energy is accompanied by the dissi- 
 pation of a certain proportion of it into space in the unavail- 
 able form of uniformly diffused heat. It is to be noted that 
 this does not involve a loss of energy ; it merely involves a 
 degradation of energy from an available form into a form 
 unavailable to man. There is never any absolute loss of 
 energy. Like matter, it is indestructible. The relation 
 between the amount of potential and of kinetic energy in the 
 universe is constantly changing the amount of the kinetic 
 energy steadily increasing at the expense of the potential 
 energy. But the sum of the two forms of energy is a 
 constant quantity. This goes by the name of the law of 
 the conservation of energy, and it is one of the most 
 important generalisations to be found in the whole range of 
 science. As stated by Clerk Maxwell, the law may be ex- 
 pressed as follows : ' The total energy of any material system 
 is a quantity which can neither be increased nor diminished 
 by any action between the parts of the system, though it 
 may be transformed into any of the forms of which energy : 
 is susceptible.' 
 
 Although it is true that the sum of the energies of the 
 universe is a constant quantity, yet it must be borne in 
 mind that energy, as has been already stated indirectly, is 
 partly available, partly unavailable, and that energy in its 
 available form is continually becoming less in amount. 
 
 An attempt has been made to measure energy in terms 
 
 ordinary physical or chemical interactions, will form an instructive exer- 
 cise for the junior student, and enable him to become familiar with the 
 transformation of energy in all its aspects. 
 
16 Elementary Biology. 
 
 of the work done by its expenditure. The standard .of 
 measurement was first determined by Joule, by his discovery 
 of the mechanical equivalent of heat. That discovery may 
 be thus expressed : the amount of heat-energy necessary 
 to elevate the temperature of one pound of water through 
 i Fahr., if converted into ordinary motion, is capable of 
 lifting one pound of water to a height of 772 feet from the 
 earth's surface ; and, conversely, a pound of water falling 
 from a height of 772 feet has its temperature raised in 
 consequence i Fahr. If a foot-ponnd be the unit of 
 mechanical work, i.e. the amount of energy required to raise 
 one pound to the height of one foot from the earth's sur- 
 face, and a ponnd-degree be the unit of heat- work, i.e. the 
 amount of energy required to raise one pound of water i 
 Fahr., then a definite quantitative relationship is established 
 between mechanical energy, or energy of visible motion, 
 and heat-energy, viz. 772 foot-pounds are equivalent to i 
 pound-degree. 
 
 All the available terrestrial energy is derived from certain 
 stores of potential energy, and also from various forms of 
 kinetic energy, which are constantly being manifested on 
 the earth's surface. These different sources may be classi- 
 fied as being either directly or indirectly available, the latter 
 class being that from which the direct sources are them- 
 selves derived. 
 
 A. DIRECTLY AVAILABLE SOURCE OF ENERGY 
 (POTENTIAL). 
 
 Fuel Under this term are embraced all substances 
 used in the working of machines, whether these machines 
 be animal, vegetal, physical, or chemical. One instance 
 may suffice by way of illustration. 
 
 Given the elementary substances, carbon (C), hydrogen 
 (H), and oxygen (O), it is possible, by the expenditure of a 
 certain amount of kinetic energy in a manner which we 
 
Matter and Energy. 17 
 
 shall have to consider later on, to build up a complex 
 substance known as grape-sugar. Grape-sugar is a store 
 of potential energy, for by the expenditure of kinetic 
 energy in its formation, the atoms of carbon, hydrogen, 
 and oxygen have obtained potential energy, because their 
 chemical affinities are only partially satisfied (p. 20). It is 
 found that in a molecule of grape-sugar there are six atoms 
 of carbon, twelve of hydrogen, and six of oxygen, which 
 chemists tell us are probably related to one another in the 
 following manner : 
 
 H H H H 
 
 till 
 
 H-C-C-C-C-C-C-H 
 
 I ! I I I I 
 
 o o o o o 
 
 1 I I I I I 
 
 H H H H H H 
 
 the atoms of carbon being linked to each other, and to 
 the hydrogen atoms. But the affinity of the carbon and 
 hydrogen for oxygen is far greater than that of carbon for 
 carbon, or carbon for hydrogen ; therefore, when grape- 
 sugar is burnt or decomposed in the living body, every 
 atom of carbon and of hydrogen immediately quits its 
 former connection and links itself firmly to oxygen, produc- 
 ing in consequence a series of molecules of much simpler 
 composition, known as carbonic acid and water, the excess 
 of oxygen required being obtained from the atmosphere. 
 The potential energy of the grape-sugar becomes transformed 
 mainly into heat 
 
 B. DIRECTLY AVAILABLE SOURCES OF ENERGY 
 (KINETIC). 
 
 From a purely biological point of view these sources are 
 not of very great importance. 
 
 i. Tides. Tides are sources of potential energy four 
 times in the twenty-four hours, becoming kinetic at ebb and 
 
 c 
 
1 8 Elementary Biology. 
 
 flow. Tidal motion is a source of energy little used, 
 although in many cases there seems to be no feasible objec- 
 tion to its employment. 
 
 2. Ordinary water-power and falling bodies in gene- 
 ral. This source of energy has been employed from very 
 ancient times. The possession of potential energy by water 
 depends on its being at an elevation, and its transformation 
 into kinetic energy on its being allowed to fall or flow to a 
 lower level. An undulating or mountainous country will be, 
 therefore, a country with the greatest available water-power, 
 not only on account of the many differences of level, but 
 also on account of the greater rainfall in a mountainous 
 country, and consequent greater absolute supply of water. 
 The energy available by the fall of water from a height is of 
 the same nature as that available by the fall of solids, for 
 example, various forms of hammer, clock-weights, c. 
 
 3. "Wind currents, Universally employed as a motive 
 power in navigation, mills, c., but a source of energy not 
 always to be depended upon, and often adverse to the 
 furtherance of the object for which it is wanted. 
 
 4. Water currents. Ocean currents are, under certain 
 circumstances, useful as an assistant motive power ; whilst 
 river currents are not infrequently employed for the carriage 
 of material to the sea, e.g. the carriage of wood on the St. 
 Lawrence. 
 
 5. Hotsprings, volcanoes, and earthquakes. These 
 sources, although prodigious in amount, in addition to being 
 only locally developed, and in regions where their energy 
 is, as a rule, not wanted, are so utterly unreliable, that they 
 may be left out of account entirely, at least until a method 
 of transforming their energy, by storage or otherwise, into 
 reliable forms be discovered. 
 
Matter and Energy. 19 
 
 C. INDIRECTLY AVAILABLE SOURCES OF ENERGY 
 (KINETIC). 
 
 1. Gravity. The law of gravity has been already stated 
 at page 7, and it is in virtue of that attractive force that 
 we are able to make use of such directly available sources 
 of energy as some of those already mentioned, e.g. tides, 
 ordinary water-power, falling bodies, &c. 
 
 2. Solar radiation. This is the first of the two great 
 sources of energy looked at from a biological point of view. 
 Its relation to plant and animal life will be discussed in 
 detail afterwards. At present we need only direct attention 
 to the irregular heating of the atmosphere by the sun's rays 
 in relation to the formation of winds and of ocean currents. 
 Its influence on the evaporation of water, &c. may also be 
 specially noted. 
 
 3. The rotation of the earth on its axis causes changes 
 in the direction of the atmospheric and oceanic currents, 
 and thus, in conjunction with solar radiation, may be said 
 to be the cause of them. 
 
 4. The earth's internal heat In accordance with the 
 nebular hypothesis, now almost universally accepted in some 
 form by physicists, the earth's internal heat is the residuum 
 of the heat it possessed in its originally molten condition. It 
 manifests itself chiefly in the forms referred to under the 
 fifth heading of the class of kinetic energies entitled ' directly 
 available.' It is a source of energy which must manifestly 
 become gradually exhausted as the age of the earth in- 
 creases, 
 
 5. Chemical affinity. This is to be considered as the 
 second of the two great sources of the energy of living 
 things, and on that account merits fuller treatment 
 
 We have already defined a chemical element as a sub- 
 stance homogeneous in itself, i.e. whose atoms are all of one 
 
 c 2 
 
2O Elementary Biology. 
 
 kind. It has been already stated, that there are about seventy 
 such elements at present known (footnote, p. 9). A com- 
 pound has also been defined as composed of two or more 
 elements. A distinction must be carefully drawn between 
 a compound and a mixture. In a compound each element, 
 or constituent, loses its own peculiar characters, whilst in a 
 mixture each constituent retains its own characters. Thus, 
 for example, hydrogen is a colourless, tasteless, odourless 
 gas, extremely light and inflammable, and possessed also of 
 certain other chemical peculiarities that do not require to 
 be specialised here. Oxygen is also a colourless, odourless, 
 tasteless gas, sixteen times as heavy as hydrogen, not inflam- 
 mable, but a supporter of combustion. If certain definite 
 quantities of these two gases be mixed in a vessel, it is found 
 that the hydrogen in the vessel still retains all the properties 
 of hydrogen, and the oxygen all the properties of oxygen. 
 If, however, a light be applied to the mixture, an explosion 
 immediately takes place, both gases disappear as such, and 
 in their place is left a compound gas, water vapour, which on 
 analysis is found to contain exactly the same amount of 
 hydrogen and oxygen which went to form the mixture, but 
 which differs entirely in properties from either of its consti- 
 tuents. The force which tends to bring about this union is 
 known as the force of chemical affinity, and is one of the 
 most important sources of energy known to us. It may be 
 defined in the following terms : the force which tends to 
 make certain chemical elements or compounds unite with 
 certain other elements or compounds in definite proportions, 
 so as to lead to the formation of a new substance, or sub- 
 stances, with different properties from those of the indivi- 
 dual constituents. 
 
 The original position of separation of the constituents of 
 a compound, previous to combination, is energy of position, 
 or potential energy. The supply of an equal amount of 
 energy to that given off at decomposition is necessary to 
 effect recombination. 
 
Matter and Energy. 21 
 
 Chemical combination, as a form of kinetic energy, is of 
 course always attended by the evolution of heat, or other 
 manifestations of energy resolvable ultimately into heat. 
 
 SECTION III. CLASSIFICATION OF CHEMICAL 
 COMPOUNDS. 
 
 The degree of chemical affinity between elements or com- 
 pounds varies with the elements or compounds under con- 
 sideration. For example, the affinity between carbon and 
 oxygen is much greater than that between carbon and 
 hydrogen ; hence the potential energy in the separation of 
 carbon and oxygen atoms is greater in amount than that in 
 the separation of carbon and hydrogen atoms. There is a 
 marked corresponding difference in the firmness of the re- 
 sulting compounds. Indeed, it is possible to arrange the 
 various, almost innumerable, chemical compounds in a long 
 chain or series, beginning at the one end with those that 
 require the expenditure of an enormous amount of energy to 
 bring about their decomposition, and ending with those that 
 can scarcely be got to hold together at all, that indeed require 
 a constant expenditure of energy to make them do so. The 
 former may be known as stable and the latter as unstable 
 compounds. The classification is a useful one, but is one 
 in which, as in most natural classifications, no hard-and fast 
 line of demarcation can be drawn between the two classes. 
 
 Generally speaking, the more complex a compound is, 
 that is to say, the greater the number of atoms entering into 
 its composition, the more unstable it is, and the fewer the 
 number of atoms in a compound the more stable it is ; 
 soda, water, carbonic acid, iron -rust, and such like, are 
 examples of simple compounds, containing from two to five 
 atoms each ; whilst the majority of the unstable compounds 
 contain twenty, fifty, a hundred, or more atoms. Haemo- 
 globin, the red colouring matter of blood, the most com- 
 -^ 
 
22 Elementary Biology. 
 
 plex compound known, is said to contain no fewer than 
 1,897 atoms (C GO oH 060 N m FeS 3 O 179 ). 
 
 The more complex unstable compounds are chiefly 
 formed as products of the vital processes, and are found 
 associated for the most part with living organisms. They 
 have, therefore, been termed organic. Similarly, owing to 
 the fact that the simple stable compounds are found, as a 
 rule, in the outside world, and in great measure independent 
 of living things, they have been termed inorganic. 
 
 The general tendency of elements is to unite to form, 
 and of complex compounds to decompose into, simple 
 stable compounds. 
 
 SECTION IV. LAWS OF CHEMICAL CHANGE. 
 
 The chemical changes taking place in an organism are 
 of three types, synthesis, isomeric change, and decompo- 
 sition. 
 
 1. By synthesis is understood the building up of com- 
 pounds out of elements, or out of simpler compounds, in 
 obedience to their several chemical affinities. 
 
 2. Isomeric change frequently occurs in the plant and 
 animal organism, and consists in the rearrangement of the 
 atoms of a compound. Hence we have to distinguish 
 between the composition and the constitution of a com- 
 pound, i.e. between simply the number and nature of the 
 atoms, and the relation of the atoms to one another. 
 
 3. Decomposition. A compound is decomposed when 
 (a) the complex molecule breaks up into two or more simpler 
 molecules, the sum of the atoms in all of which is iden- 
 tical with the sum of those in the original molecule. Thus 
 grape-sugar, when subjected to a certain treatment, decom- 
 poses or dissociates into two molecules of alcohol and two 
 molecules of carbonic acid. 
 
 (/>) By dehydration, or the separation of a certain 
 number of atoms of hydrogen and oxygen in the form of 
 
Matter and Energy. 23 
 
 molecules of water, some complex substances are reduced to 
 simpler forms. 
 
 (c) Similarly, the addition of one or more molecules of 
 water hydration may cause a complex molecule to re- 
 solve itself into several molecules, each having a simpler 
 composition than that of the original molecule. 
 
 (d) Undoubtedly the commonest mode of decomposition 
 is oxidation, that is to say, the constant tendency of the con- 
 stituent atoms of complex molecules to give up their weaker 
 affinities, and to unite with free or loosely combined oxygen. 
 All the changes taking place in decaying vegetal or animal 
 matter are dependent, more or less, on the oxidation and 
 removal of the several constituents of the decaying substance 
 in the form of carbonic acid, water, and other simple com- 
 pounds. The principle at the bottom of disinfection is the 
 supply of a large quantity of free or loosely combined oxygen 
 which is available for the oxidation and rendering harm- 
 less of putrescent and injurious matter. 
 
 (e) Deoxidation, or the withdrawal of oxygen, is of much 
 rarer occurrence, but does take place in certain cases. 
 
 Instances of these actions in plenty will present them- 
 selves to us as we proceed. They are omitted here, as they 
 would otherwise be the means of introducing a larger number 
 of new terms than it would be possible to pause to explain 
 at this stage. 
 
 By way of recapitulation, we may classify chemical com- 
 pounds in the following manner : 
 
 Simple Complex 
 
 Stable Unstable 
 
 Inorganic Organic 
 
 Found in the environment Found in the organism 
 
 It must be borne in mind, however, that these different 
 groups flow into one another, and are by no means perfectly 
 distinct. A few illustrations will make this sufficiently 
 clear. 
 
24 Elementary Biology. 
 
 Water is a simple compound ; it is stable, and it is found 
 in by far the greatest abundance in the inorganic environ- 
 ment, but it constitutes ninety-three per cent, of the turnip, 
 for example, among vegetals, and forms about seventy per 
 cent, of the weight of the human body. Salts of potash, 
 lime, and soda, common salt, and other substances found 
 in the environment in prodigious quantities, are nevertheless 
 present in the organism in no insignificant amount. So also 
 many oils and carbohydrates (or compounds of carbon, oxy- 
 gen, and hydrogen), although probably the result of vital 
 processes in some bygone period in the world's history, are 
 nevertheless found existing abundantly, undecomposed, in 
 the environment. 
 
 Many comparatively simple compounds are extremely 
 unstable, such for instance as the compound known as iodide 
 of nitrogen, which, although it consists of only four atoms, 
 explodes at once on being touched. Many other examples 
 will be noted in the sequel. 
 
CHAPTER II. 
 
 PROTOPLASM. 
 
 SECTION I. CHEMICAL COMPOSITION OF PROTOPLASM. 
 
 Protoplasm, the ' physical basis of life,' the granular trans- 
 parent viscous substance present in all living things, and 
 without which no life is possible, may be defined chemically as 
 an immensely complex compound, or mixture of compounds, 
 yielding on analysis l simpler chemical derivatives, which 
 may be classified under the folio wing heads : 
 
 A. Proteids and albuminoids, containing carbon, hydro- 
 gen, oxygen, nitrogen, sulphur, phosphorus, and a variable 
 amount of ash. 
 
 B. Amyloids or carbohydrates, containing the elements, 
 carbon, hydrogen, and oxygen. 
 
 C. Fats, with a similar composition. 
 
 D. Water in large quantity. 
 
 E. Inorganic salts. 
 
 F. Many bodies which are regarded as stages in the form- 
 ation or decomposition of protoplasm. 
 
 All these substances are of the highest importance in a 
 study of biology, and we shall therefore devote some time 
 to their discussion. 
 
 1 The process of analysis consists in (i) desiccation, i.e. drying 
 without combuslion, removing all volatile matters, and leaving only 
 non-volatile substances ; (2) combustion or calcination, which by 
 oxidation, dissociation, &c., removes all combustible bodies and leaves 
 a mineral, incombustible ash. 
 
26 Elementary Biology. 
 
 A. Proteids and albuminoids. Generally speaking, 
 these are substances of unknown constitution, but with a 
 percentage composition, varying in round numbers 
 
 c H N os 
 
 from 51 7 15 21 -5 
 
 to 55 7-5 16-5 23 5 2 
 
 They are all incapable of being crystallised. They are 
 soluble in water, or at least swell up in it. When treated 
 with nitric acid (HNO 3 ), they take on a yellow colour. They 
 are decomposed by the acid, and the yellow deposit, or 
 precipitate, formed dissolves into an orange-red solution 
 on the addition of a solution of ammonia (NH 3 H.,O). 
 Polarised light is turned to the right when passed through 
 a solution of a proteid. 
 
 Proteids may be divided into two chief classes 
 (a) albumins and globulins, which are distinguished from 
 (/;) albuminates, by the fact that when subjected, in solu- 
 tion, to a temperature of over 73 C. they are coagulated 
 or rendered solid, whilst albuminates are not so affected. 
 Albumins, moreover, are soluble in cold water, whilst 
 globulins are not, unless some neutral salt be added, e.g. 
 common salt (NaCl). It is from these two groups of sub- 
 stances that most of the proteid matter of the animal and 
 vegetal worlds is derived. As examples of animal albumins 
 may be mentioned egg-albumin, or white of egg, and serum- 
 albumin, or the albumin found in the fluid portion of blood, 
 whilst the main constituent of vegetal protoplasm is vegetal 
 albumin. 
 
 Of globulins, myosin, the chief constituent of muscle, 
 and glutin, which enters largely into the composition of 
 seeds, may be taken as examples. 
 
 Casein, the chief constituent of milk in the animal world, 
 and the principal source of nourishment in beans, peas, &c., 
 may be given as illustrating the class of nlbuminates. The 
 albumins become changed into albuminates if treated with 
 
Protoplasm. 27 
 
 an acid or an alkali, and receive the names of acid-albumi- 
 nates and alkali-albuminates according to the nature of the 
 agent employed to bring about the alteration. 
 
 Albuminoids very closely resemble proteids in general 
 appearance and in chemical composition, save that some 
 albuminoids contain no sulphur. They differ, however, in 
 one point ; they cannot be made use of by animals as. food, 
 on account of their sparing solubility in water and in the 
 comparatively weak acids and alkalies of the organism. As 
 examples may be mentioned mucin, the substance which 
 gives the viscidity to saliva, gelatin, a familiar commercial 
 product, and keratin, the chief constituent of horn, hoof, 
 nail, hair, and such like. 
 
 B. Amyloids or carbohydrates. These substances, 
 when compared with the group of proteids, are compara- 
 tively simple in their chemical composition. They consist 
 of the three elements carbon, hydrogen, and oxygen, 
 the hydrogen and oxygen being present in the same pro- 
 portion as that in which they occur in water, i.e. two atoms 
 of hydrogen to one of oxygen. The large amount of carbon 
 and hydrogen in their composition gives them their name of 
 carbohydrates, whilst a synonym of starch, viz. amylum, 
 which is one of the chief carbohydrates, accounts for the 
 term amyloid as applied to them. They are sweet to the 
 taste, e.g. sugar, or are capable of being converted into 
 sugar by treatment with a weak acid, or certain other sub- 
 stances found in the plant and animal organism. In addi- 
 tion to ordinary sugar and starch, glycogen or animal 
 starch, milk sugar or lactose, muscle sugar or inosite, may 
 be cited as examples of carbohydrates found in the animal 
 world. 
 
 C. Fats and fatty acids. These substances are closely 
 allied in chemical composition to carbohydrates, from which 
 they are partly derived. They are also largely formed by a 
 transformation of proteids. They contain carbon, hydrogen, 
 and oxygen, usually in the form of more complex molecules 
 
28 Elementary Biology. 
 
 than those of the carbohydrate group. Their general cha- 
 racters are well known. When acted on by an alkali or super- 
 heated steam, 1 they take up water (hydration) and decompose 
 into glycerin and their corresponding fatty acids (dissocia- 
 tion), which unite with the alkalies to form soaps. The chief 
 neutral fats are palmitin, the chief Constituent of palm oil 
 (' railway grease'), olein, the principal component of olive 
 oil, and stearin, of which beef and mutton fat mainly con- 
 sists. 
 
 The fatty acids comprise a series of bodies which occur 
 chiefly in combination with glycerin to form the neutral fats 
 above mentioned, and with alkalies to form soaps. A 
 number of them will call for special mention afterwards ; at 
 present it will be enough to refer to the fatty acids of the 
 neutral fats, i.e. palmitic, stearic, and oleic acids. 
 
 D. Water (H.,O) is the chief mineral constituent of both 
 plants and animals. It often forms a very large proportion by 
 weight of the organism, constituting over 80 per cent, by 
 weight of the kidney, and over 90 per cent, of many succulent 
 plants, e.g. turnip. The well-known 'jelly-fish' of our seas 
 are composed almost entirely of water, a mere film of solid 
 matter being left after desiccation. Some parts of the 
 organism contain very little water, as for example the enamel 
 of the teeth, where it is present in the proportion of only 0*2 
 per cent. 
 
 E. Salts. The chief inorganic salts are the chlorides, 
 phosphates, carbonates, and sulphates of soda, lime, potash, 
 and magnesia. Salts of iron, manganese, and other metals, 
 also occur. These will call for special mention subse- 
 quently. 
 
 F. Transition substances formed in the integration and 
 disintegration of protoplasm. The nature and signifi- 
 cance of these bodies cannot with advantage be treated of 
 before a discussion of the chemical changes taking place 
 in the organism during growth and decay. One or two 
 
 1 I.e. steam treated under pressure to a temperature above 212 F. 
 
Protoplasm. 29 
 
 examples of these transition-substances which are more 
 commonly known may be given, e.g. urea, a nitrogenous 
 derivative excreted by the kidney, bile-acids, ammonia, and 
 various ammoniacal salts, tannin, wax, and many vegetal 
 acids and alkaloids. 1 
 
 SECTION II. MORPHOLOGY AND PHYSIOLOGY OF 
 PROTOPLASM. 
 
 Protoplasm (-n-p^ro<s 'first,' TrAarr/xa 'formed substance ')> 
 when examined in large quantity by the naked eye, appears 
 as a colourless, more or less transparent jelly. In nature it 
 however occurs much more frequently subdivided into ex- 
 tremely minute pieces, resembling each other in general 
 structure and appearance. To study these separate par- 
 ticles, or cells as they are termed, high magnifying powers 
 of the microscope are required. When so examined proto- 
 plasm is found to consist of a clear, glassy, or finely granular 
 soft substance, in which there is frequently to be distinguished 
 an outer layer or ectoplasm, always more transparent than 
 the inner, more granular endoplasm. In consistence proto- 
 plasm varies very greatly. 
 
 In intimate structure it consists of a homogeneous 
 portion or matrix in which are imbedded granules (fig. i). 
 Minute droplets of water also occur (more abundantly in 
 plant than in animal protoplasm), which go by the name of 
 
 1 It is to be distinctly understood that a memory-knowledge of the 
 characters of the substances entering into the composition of protoplasm 
 is of little value, unless coupled with a practical knowledge gained by 
 actual observation and experiment in the laboratory. This synopsis of 
 the characters of proteids and other organic compounds is intended for 
 reference. It is suggested that one or two of the most striking pecu- 
 liarities of the substances, once seen, should be kept in mind, and that 
 the other characters should be learnt gradually and unconsciously as 
 familiarity with the substances themselves is obtained. That know- 
 ledge consists in being able to repeat a list of peculiarities by heart is 
 one of the most mischievous delusions of modern education. 
 
Elementary Biology. 
 
 vacuoles. There may be one or more vacuoles in each cell; 
 indeed, in very many vegetal cells there is more of vacuole 
 than protoplasm (fig. 4 c). Vacuoles are probably reservoirs 
 for substances used in the manufacture of protoplasm or for 
 products of its disintegration. After treatment with certain 
 reagents, or in some cases without such treatment, the proto- 
 plasmic matrix is found to be composed of a sponge-like 
 arrangement of threads or fibrillae (fig. 2) interlacing with 
 one another and forming a supporting framework, while the 
 interstices are filled with a more fluid homogeneous sub- 
 
 Fi. i. DIAGRAM OF AN 
 
 ANIMAL CELL MUCH MAG- 
 NIFIED. (Schafer.) 
 
 FIG. 2. DIAGRAM OF AN ANI- 
 MAL CELL WITH TWO NUCLEI 
 AND SHOWING INTRACELLU- 
 LAR AND INTRANUCLEAR 
 
 NETWORKS. (Klein.) 
 
 , protoplasm, with vacuoles 
 and granules ; n, nucleus, 
 with intranuclear network 
 and nucleolus ('). 
 
 stance. Most recent investigators into the minute structure 
 of protoplasm agree in thinking that the homogeneous matrix 
 above alluded to corresponds to the interfibrillar matter, 
 whilst the knots on the network, as well as the fibrillae them- 
 selves when seen end-on, furnish the granular appearance, 
 True granules are, however, also found in the interfibrillar 
 matter itself. 
 
 In the great majority of cells, generally near the centre, 
 is to be found an oval or rounded, rarely irregular, body 
 termed the nucleus (fig. i). It is usually inclosed in a definite 
 nuclear wall or envelope. Like the cell itself the nucleus 
 consists of a fibrillar network and an interfibrillar substance. 
 
Protoplasm. 
 
 3 1 
 
 In consequence the nucleus presents the apper ranee, undei 
 a low magnifying power, of a homogeneous matrix with 
 scattered granules (fig. 2). 
 
 Usually one or more granules of larger size are to be 
 distinguished in the nucleus to which the name of nucleoli 
 has been given, some of which are probably knots on the 
 intranuclear network. 
 
 A mass of protoplasm possessed of these morphological 
 characters may be ' naked ' or without a cell-wall, and in 
 that condition has a more or less irregular outline (fig. 3), 
 
 FIG. 3. Amxba. polypodia. (Max Schult/e.) 
 
 , nucleus ; Pv, contractile vacuole. 
 
 or it may be inclosed in a definite envelope or cell-wall, 
 composed of substances formed from the protoplasm. Most 
 cells when first formed are ' naked.' They may remain so 
 during their whole existence, or they may become en- 
 veloped by a cell-wall of variable thickness and form. The 
 cell-wall is much more extensively developed in the plant 
 than in the animal organism (fig. 4). 
 
 Physiology of protoplasm. Protoplasm and the various 
 structures differentiated from it may be considered, how- 
 ever, from another point of view. 
 
32 Elementary Biology. 
 
 It is found that all cells undergo certain changes during 
 their existence changes concomitant with the phenomena 
 known popularly as growth, reproduction, decay, and death 
 
 FIG. 4. CELLS FROM THE ROOT OF Frltillaria imperialis ( x 550). (Sachs.) 
 
 c' v *.'' A 
 
 h 
 
 A, very young cells from near apex ; B, from 2 mm. above the apex ; c, from about 
 8 mm. above the apex, /i, cell-wall ; p, protoplasm ; k, nucleus ; k k, nucleoli ; 
 s, vacuoles and cell-sap cavity. 
 
 changes in which the protoplasm and its modifications 
 play different parts, perform different duties or functions. 
 
 The performance of any function, i.e. the doing of any 
 work, as already fully explained (Chapter I., section ii), 
 
Protoplasm. 33 
 
 involves the expenditure of a certain amount of energy. 
 Protoplasm, in virtue of its being composed of a number of 
 very complex and unstable compounds, constitutes a large 
 store of potential energy. In consequence of the trans- 
 formation of part of this rxrtential energy into kinetic energy 
 the various functions of protoplasm come to be performed ; 
 in other words, the- "various functions of protoplasm are 
 manifestations of kinetic energy. In the expenditure of 
 energy the complex compounds become broken down into 
 simpler compounds and often into elements ; hence the 
 presence of so many transitional substances and simple 
 compounds in the chemical composition of proto^asm. 
 We have now to consider what these functions are, and for 
 that purpose we will consider a naked cell, such as that re- 
 presented at fig. 3. 
 
 We note first that it possesses contractility that is to 
 say, that it is capable of motion as a whole or in certain 
 parts. Protoplasmic motion may be definite or indefinite. 
 Indefinite motion consists in the protrusion and withdrawal 
 of finger -like prolongations of protoplasm from any region. 
 The term pseudopodium is applied to such a projection, and 
 the motion itself is usually known as amoeboid motion, from 
 its being specially developed in an extremely simple animal 
 organism known as Amoeba, the discussion of whose cha- 
 racters will occupy us later on. The mass of the protoplasm 
 may follow the pseudopodium, and locomotion of the cell 
 be effected. Definite motion, or motion always in one and 
 the same direction, occurs only in cells which are more 
 differentiated than those we are at present discussing (e.g. 
 muscle). Reference to definite motion is, therefore, post- 
 poned. 
 
 Again, protoplasm is irritable that is to say, if a stimulus 
 be applied to the mass, such as a shock of electricity, an 
 application of gentle heat, or certain chemical substances 
 the protoplasm responds, shows irritability or excitement, 
 local or general. For example, a shock of electricity 
 
34 Elementary Biology. 
 
 immediately produces a general contraction and drawing 
 in of all pseudopodia, amounting, if the shock be a strong 
 one, to actual paralysis and death. 
 
 Motion, however, seems in many cases to be produced 
 independently of external stimulus, and to arise as a conse- 
 quence of some internal chemical changes taking place, not, 
 as in the previous case, in response to stimuli, but automa- 
 tically. Protoplasm is said, therefore, to possess automatism. 
 
 Other stimuli than those mentioned produce local irri- 
 tability and motion of a peculiar kind. Contact with solid 
 particles produces a movement of the neighbouring proto- 
 plasm, and results in the engulfing of the particles into the 
 viscous mass by the protrusion of pseudopodia which sur- 
 round and inclose them. The food-particles thus inclosed 
 undergo certain changes in the interior of the cell during a 
 species of circulation which they experience in its mass 
 changes which result in the absorption, alteration, and 
 preparation of the particles into compounds from which 
 new protoplasm is integrated changes which are no doubt 
 brought about by the chemical action of certain substances 
 present in, and formed from, trie protoplasm. In conse- 
 quence of this power of absorbing or ingesting food particles 
 the protoplasm is said to possess the power of assimilation. 
 These primary changes are followed by others of a more 
 complicated nature, in virtue of which new protoplasm is in- 
 tegrated or built up. It will be convenient to designate all 
 chemical changes taking place in the organism as metabolic 
 changes ; changes concerned in the building up of proto- 
 plasm will therefore constitute constructive metabolism or 
 anabolism ; whilst the changes which take place in the 
 decomposition of protoplasm may be termed collectively 
 destructive metabolism or katabolism. The products of 
 anabolism and katabolism will therefore be naturally termed 
 anastates and katastates respectively. 
 
 The destructive changes to which protoplasm, when 
 formed, becomes at once liable, result in the production of 
 
Protoplasm. 3 5 
 
 two sets of substances (#), secretions, and (p\ excretions, 
 which may be known therefore by the general name of 
 katastates. Both series are chemical compounds, much 
 simpler as a rule in their composition than protoplasm itself, 
 but differing from each other in one important point, viz. 
 that secretions are themselves necessary for the anabolism of 
 food matters, whilst excretions are bodies which are useless 
 and injurious to the organism, and must ultimately be got 
 rid of. Excretions include not only the products of the 
 katabolism of the protoplasm, but also of the secretions 
 after they have performed their functions. They likewise 
 include those portions of the food-material taken into the 
 organism which are useless from a nutritive point of view. 
 
 It has been already pointed out that decomposition con- 
 sists chiefly in the oxidation of the constituent atoms of the 
 complex molecules. Further, on reference to the average 
 composition of proteids (p. 26), it will be seen that carbon 
 is by far the most important and abundant constituent. 
 Manifestly, therefore, oxidation in* the cell must mean chiefly 
 oxidation of carbon and formation of carbonic acid (CO 2 ), 
 the most abundant and most stable oxide of carbon known. 
 Katabolism must, therefore, be accompanied by a plentiful 
 production of carbonic acid, the process of excretion of 
 which is known by the special term of respiration. 
 
 In addition to these changes, there are others which result 
 in the separation of part of the protoplasm to form a young 
 cell or cells. Vegetative multiplication and reproduction 
 are the terms applied to such changes ; the discussion of 
 both is, however, postponed for the present. 
 
 SECTION III. PHENOMENA. CONCOMITANT WITH MANI- 
 FESTATIONS OF ENERGY BY LIVING PROTOPLASM. 
 
 Having considered the various manifestations of energy 
 in living protoplasm viz. contractility, irritability and au- 
 tomatism, reception and assimilation of food, metabolism, 
 
36 Elementary Biology. 
 
 secretion, excretion, and respiration we have now to glance 
 at the general result of these changes on the protoplasmic 
 mass as a whole. 
 
 Commencing at any given moment in the life of a cell, 
 we may find one of three things happening. *In the first 
 place, the chemical substances assimilated and undergoing 
 constructive metabolism may be just equal in amount to the 
 sum of the compounds used up in the concomitant destruc- 
 tive metabolism. In the second place, the amount assimi- 
 lated and absorbed may be greater than the amount 
 expended. Or thirdly, destructive metabolism may be in 
 excess of constructive metabolism. In the first case the 
 protoplasmic mass will be at a standstill it will neither 
 increase nor decrease in size ; in the second case it will 
 grow ; in the third case it will diminish in size and decay. 
 
 Growth is accompanied usually by differentiation of parts 
 or development. Development may be either morpho- 
 logical or physiological or both, and consists in the gradual 
 adaptation of special parts to the performance of special 
 functions. 
 
 Decay is generally ended more or less abruptly by death 
 or the cessation of the various manifestations of energy; no 
 doubt in consequence of a failure on the part of one or 
 other of the functions already mentioned, and the consequent 
 impossibility of maintaining a sufficient store of potential 
 energy on which to draw for the performance of the various 
 functions of the cell. As is to be expected, death is almost 
 always followed by a general breaking down of the complex 
 molecules through the unrestrained play ^f the laws of de- 
 composition. 
 
 We have seen in the simple case which we have used as 
 an illustration that the various functions were all performed 
 by one cell. In other words, while we had physiological 
 differentiation we had complete, or almost complete, 1 
 
 1 It is probable that the vacuoles may perform excretory functions 
 (P- 73)- 
 
Protoplasm. 37 
 
 morphological non -differentiation. Every part of the cell 
 is capable of taking in food-particles, the excreta being also 
 ejected at any point. Emission of carbonic acid or respira- 
 tion takes place over the surface generally, and so on. 
 
 By far the majority of plants and animals are, however, 
 made of vast multitudes or collections of cells, some of 
 which are specially differentiated to perform one function, 
 whilst others are differentiated to perform another. All the 
 cells have the general characters we have already described 
 as belonging to protoplasmic units, but while their other 
 functions are in abeyance, some one function it may be the 
 power of contracting or of exhibiting responses to stimuli 
 is very highly developed. Collections of such similar 
 cells are said to be specialised for the performance of 
 one function. Naturally, also, their form and structure 
 becomes correspondingly modified. Such cell aggregates are 
 termed tissues. Thus we have muscular tissue, whose 
 special function is that of contraction ; nerve tissue, whose 
 special function is that of showing irritability in response to 
 stimuli or automatically ; connective tissue, where the cells 
 are modified to act as padding or as connecting links 
 between other tissues, and so on. Moreover, it is con- 
 ceivable that different parts of the plant or animal may con- 
 tain more than one kind of tissue. Such parts are termed 
 organs ; hence the term organic as applied to chemical com- 
 pounds found in the cells and tissues composing such organs, 
 organised as applied to the plant or animal possessing such 
 differentiation of parts, and organism used as a synonym for 
 living thing. 1 
 
 A reference was made (at p. 35) to the power which cells 
 possessed of separating a part of themselves for the purpose of 
 reproduction and multiplication. When the organism con- 
 sists of a single cell, separation of, so far as we know, any 
 
 1 It must be borne in mind, however, that these terms are applied 
 to many plants and animals which cannot be said to possess tissues, 
 much less organs. 
 
38 Elementary Biology. 
 
 part of the cell is all that is required to form the basis of 
 a new individual. In the higher organisms, where cells are 
 specialised and differentiated, there are certain cells whose 
 sole function in the organism it is to produce other cells 
 capable, under certain conditions, of undergoing develop- 
 ment and growth, and of forming ultimately a new organism. 
 In the lower organisms reproduction takes place just 
 previous to death, and consists, so to speak, in the saving 
 of the last remains of the store of potential energy of the 
 parent organism. 
 
 SECTION IV. PHYSIOLOGICAL SIGNIFICANCE OF CHLORO- 
 PHYLL. 
 
 Hitherto we have spoken of protoplasm in general, 
 but it will have been noted that throughout reference was 
 made to the existence of two varieties of that substance 
 animal protoplasm and vegetal protoplasm. So far as the 
 morphology of protoplasm itself is concerned, the details of 
 difference between the two forms will be more easily under- 
 stood after examples of the animal and vegetal worlds have 
 been examined. At present it behoves us to notice more 
 especially the peculiarities of a substance which is present 
 in by far the majority of plants, and which is characteristic 
 in great measure of vegetal protoplasm. That substance is 
 chlorophyll, to the presence of which in certain cells the 
 familiar green colour of most plants is due. Chlorophyll 
 (save in a few rare cases) is not found uniformly diffused 
 through the protoplasm, but in the form of rounded or (more 
 rarely) of stellate or ribbon-shaped masses, lying imbedded 
 in the protoplasm. These are known technically as chloro- 
 phyll bodies. Chlorophyll bodies consist of a green pigment, 
 chlorophyll itself, united in certain definite proportions with 
 definite masses of protoplasm. The chlorophyll itself is 
 small in amount and can be extracted from the protoplasm 
 by alcohol, ether, and several other allied substances. The 
 
Protoplasm. 
 
 39 
 
 FIG. 5. -CHLOROPHYLL GRANULES IN CEILS 
 OF LEAF OF Funaria hygrometrica (X5;o). 
 (Sachs.) 
 
 protoplasmic residue, or vehicle, remaining after the re- 
 moval of the chlorophyll, is apparently not diminished in 
 amount, but has a vacuolated or frothy appearance. 
 
 Chlorophyll bodies 
 grow and multiply like 
 cells, though, of course, 
 they are not to be con- 
 sidered as cells morpho- 
 logically. The growth 
 and multiplication are 
 due entirely to the ac- 
 tivity of the protoplas- 
 mic vehicle. The chlo- 
 rophyll bodies, of which 
 there are usually a large 
 number in each cell, are 
 formed from the general 
 protoplasm by a process 
 of differentiation. In 
 the chlorophyll bodies 
 themselves secondary 
 products of metabolism 
 are often found. The 
 most common of these 
 is starch, which is pre- 
 sent at first in small 
 quantity, in the form of 
 minute granules, but 
 afterwards increases in 
 amount until the whole 
 chlorophyll body be- 
 comes transformed intO A) S ran ^les of chlorophyll, with contained starch 
 
 grains imbedded in the protoplasm of the 
 
 a maSS Of Starch. Drop- cells; B. separated chlorophyll granules con- 
 
 , ,. ., , taining starch ; a. b. young granules ; b' ', //', 
 
 iCtS Of Oil and Other chlorophyll granules du iding ; c, d, e, old 
 
 L , i chlorophyll granules ; / granule swollen 
 
 Substances are also not up by action ,of water ; >, starch granules 
 
 remaining after chlorophyll has been 
 in destroyed by action of water. 
 
40 Elementary Biology. 
 
 the chlorophyll body. The significance of these changes in 
 constitution will be discussed later on. 
 
 The percentage chemical composition of chlorophyll has 
 been lately ascertained, and is as follows : 
 
 Carbon 73-34 
 
 Hydrogen . . . . . .972 
 
 Isitrogen . . . . . .5-68 
 
 Oxygen 9-54 
 
 Phosphorus . . . . . I -38 
 Magnesium ..... 0-34 
 
 lOO'OO 
 
 It has already been noted that in consequence of meta- 
 bolic changes constantly taking place in organisms, large 
 quantities of carbonic acid were continually being produced. 
 Notwithstanding this fact, the quantity of carbonic acid in 
 the atmosphere is tolerably nearly a constant quantity. The 
 question immediately arises, What becomes of it? In brief 
 terms it may be said that it forms the main constituent of 
 the food of plants, although they themselves, in common with 
 animals, excrete considerable quantities of it. Yet, gene- 
 rally speaking, the quantity they absorb is not only largely in 
 excess of their own production, but balances the production 
 of the same gas by the animal, on which it acts as a poison. 
 Plants are able to make use of this gas as food in virtue ot 
 their possessing chlorophyll, which in presence of sunlight 
 is capable of bringing about the assimilation of carbonic 
 acid by vegetal protoplasm. It is evident, therefore, that 
 chlorophyll is a substance of the very highest importance, 
 not only in the vegetal economy but indirectly in the animal 
 economy also. 
 
 Further, it is manifest that the absorption by the vegetal 
 of large quantities of carbonic acid must mean the reception 
 into the vegetal organism of an enormous amount of oxygen 
 far more than is necessary for the carrying on of the 
 metabolic changes taking place in vegetal cells. As a 
 matter of fact the oxygen is in great part given back into the 
 
Protoplasm. 41 
 
 atmosphere, in this manner keeping up that balance of gases 
 in the atmosphere which is necessary for the maintenance 
 of animal life. 
 
 Before the full bearing of these different phenomena 
 can be seen it will be necessary to refer briefly to certain 
 physical facts of the highest importance in the discussion 
 of the relations of animal and vegetal protoplasm firstly, 
 to their inorganic surroundings, and, secondly, to each 
 other. 
 
 SECTION V. CONDITIONS OF THE ENVIRONMENT NECES- 
 SARY FOR THE MAINTENANCE OF LlFE IN ANIMAL AND 
 
 VEGETAL PROTOPLASM. 
 
 A. Composition of the atmosphere. An * empty' room, 
 10 feet everyway, contains 1,000 cubic feet of air, which, if 
 dry and pure, is a mixture of nearly 
 
 790*2 cubic feet of nitrogen. 
 209-4 ,, oxygen. 
 
 0-4 ,, carbonic acid. 
 
 To such air the terms ' normal,' or ' fresh air ' are applied. 
 In addition to these, traces of other gases, such as ammonia 
 (NH : <), and ammoniacal salts, carbonic oxide (CO) and 
 certain other compounds of carbon. The atmosphere, 
 moreover, always contains a certain proportion of water- 
 vapour, which varies as is well known from day to day and 
 from hour to hour. Normally it amounts to from 5 to 
 1 5 per cent. 
 
 If such a room be occupied by an animal, say an adult 
 human individual, it will be found that after about two hours 
 or so (provided there be no addition of fresh air), the atmo- 
 sphere has become unbearable and highly injurious to life, 
 from the presence in it of certain obnoxious gases. The 
 alteration in the composition of the atmosphere is due to 
 the addition to it of a large quantity of carbonic acid gas 
 (and of other gases in less amount) produced in conse- 
 
42 Elementary Biology. 
 
 quence of the metabolic changes taking place in the organ- 
 ism. Thus air expired from the lungs contains only 16 per cent, 
 instead of 2 1 per cent, of oxygen, and 4^ per cent, of carbonic 
 acid instead of yj^- per cent. The amount of nitrogen remains 
 unaltered. Expired air also contains traces of poisonous 
 and foetid organic matter, and a large amount of water- 
 vapour. Such air is termed ' foul ' air. If the atmosphere 
 of such a room is to remain perfectly wholesome, about 20 
 cubic feet of fresh air per minute should be supplied for each 
 individual in it. 
 
 Small as is the proportion of carbonic acid found in 
 fresh air (only 4 volumes in 10,000), yet that proportion is 
 amply sufficient for the supply of the carbon required by 
 the vegetal economy ; whilst if the proportion be increased 
 beyond that amount, danger would ensue to animal life. 
 The atmospheric conditions, therefore, necessary for the 
 proper maintenance of animal and vegetal life are manifestly 
 that the air should be pure and dry, with a proportion of 
 carbonic acid present not over '04 per cent. 
 
 We have hitherto considered the atmospheric conditions 
 necessary for the maintenance of life in terrestrial plants and 
 animals; we have now to glance at the conditions necessary 
 for the maintenance of aquatic life. 
 
 No life of any kind could be possible under water were 
 it not that all gases are soluble in water. The solubility 
 of different gases, however, depends on first, the tempe- 
 rature and pressure of the gas, and, secondly, the degree 
 of inherent tendency of the gas to dissolve. The last, which 
 is called the coefficient of absorption of the gas, varies very 
 greatly. For example, at a temperature of o C., and under 
 a barometric pressure of 760 mm. one volume of water 
 will absorb, of 
 
 Nitrogen ..... '02035 v l s - 
 
 Oxygen -04114 ,, 
 
 Carbonic acid .... 1 7967 ,, 
 
 Ammonia ... - 1148-8 ,, 
 
Protoplasm. 43 
 
 When a mixture of gases, such as air, is exposed to the 
 action of a solvent such as water, the proportion of each gas 
 absorbed will depend first, on the coefficient of absorption 
 of the gas, and, secondly, on the proportion of it present 
 in the mixture, the temperature and pressure remaining 
 constant. If the temperature vary, however, the amount 
 of each gas absorbed will vary also, increasing as the tem- 
 perature decreases, and decreasing as it increases. If the 
 pressure vary, the amount of gas absorbed will be directly 
 proportional to the pressure. 
 
 It will be seen that, owing to the very low coefficient of 
 absorption of the three chief gases of the atmosphere, the 
 total amount of the gases dissolved in water is very small 
 indeed. Nevertheless, the oxygen present is sufficient for 
 the maintenance of animal life, as the carbonic acid is 
 sufficient to supply the carbon required by submerged 
 plants. Ammonia, which we have cited as an instance of a 
 gas with a very high coefficient of absorption, is present in 
 such small quantity in the atmosphere that, notwithstanding 
 its extreme solubility, the proportion present in water is 
 quite trifling. Moreover, it is quickly licked out of the air by 
 rain and carried down to the earth, where it at once enters 
 into combination with other substances in the soil, to form 
 ammoniacal salts, or becomes oxidised into nitric acid, this 
 latter substance at once forming nitrates by combination 
 with such bases as potash, lime, and soda. This 'nitrifica- 
 tion ' takes place also in the case of ammonia produced by 
 decaying humus. 
 
 B. Temperature. The extremes of temperature between 
 which life is possible vary greatly according as it is vegetal 
 or animal protoplasm that is under consideration. Gene- 
 rally speaking, life cannot be maintained in either kind of 
 protoplasm under a continued exposure to a temperature 
 of above 50 C., or below o C. In the case of certain 
 extremely simple and minute organisms of doubtful affinities, 
 and of the reproductive cells of certain groups of plants, 
 
44 Elementary Biology. 
 
 these limits may be considerably extended, 100 C, or 
 even 120 C-, being insufficient to cause death. 
 
 Cold as a general rule retards, whilst gentle heat accele- 
 rates, amoeboid and other protoplasmic movements. A 
 species of coagulation, however, takes place if the tempera- 
 ture exceed 45 C. 
 
 C. Pressure. The ordinary pressure of the atmosphere 
 at the sea-level is about 1473 Ibs. on the square inch, 
 which indicates the weight of a column of air of the same 
 sectional area. Manifestly the atmospheric pressure will 
 vary according to the height above the sea-level at which 
 the observation is made. Atmospheric pressure also varies 
 with the latitude and longitude, the temperature, the season 
 of the year, and the hour of the day. 
 
 The total atmospheric pressure on the surface of the 
 body of an average-sized human adult is about 14 tons, a 
 pressure which is, however, equalised by the. outward pres- 
 sure of the air permeating the tissues. As a general rule, 
 higher organisms are constructed so as to subsist under an 
 atmospheric pressure not varying widely on either side of 
 the average, viz. 30 in. or 760 mm. of the mercurial baro- 
 meter. Many of the lower forms of life, however, can 
 tolerate without injury a much higher pressure than that. 
 With reference to aquatic, and more especially marine, 
 animals the limits of pressure are much wider. Many fish, 
 for example, live at a depth of from 300 to 2,000 fathoms, 
 that is to say, they are capable of accommodating themselves 
 to a pressure varying from one-half to two tons on the square 
 inch. Fish living at greater depths must, of course, be subject 
 to still greater pressures, amounting in some cases to as much 
 as four-and-a-half tons on the square inch, always of course 
 balanced by an equally great outward pressure of the water 
 with which their tissues are permeated. These forms, when 
 brought to the surface, are found to be greatly injured owing 
 to the effect on their bodies of the removal of much of the 
 external pressure at the surface. 
 
Pretoplasm. 45 
 
 D. Light. (a) In relation to vegetal life, it has already 
 been explained that the life of plants containing chlorophyll 
 is entirely dependent on the action of light, inasmuch as 
 the assimilation of carbonic acid is impossible without it. 
 The higher metabolic processes can, however, go on equally 
 well in darkness. It follows that while these processes are 
 active in all parts of the plant, the assimilation of carbon 
 takes place only in the parts which contain chlorophyll. 
 Plants not containing chlorophyll, such as the parasitic 
 fungi, are dependent on chlorophyll-bearing plants for the 
 products of assimilation on which they subsist. 
 
 Sunlight, as is well known, is a mixture of a number of 
 coloured rays. Sunlight may be readily resolved into its 
 constituent coloured rays by passing it through a prism. 
 In accordance with certain laws of optics, the light comes 
 out of the prism at a different angle to its surface from that 
 at which it enters. Moreover, each of the constituent 
 coloured rays issues at an angle peculiar to itseJf. The 
 rays are said, therefore, to be differently refrangible. The 
 result is, consequently, that the broken-up ray of white 
 light forms a band of colour known as the solar spectrum. 
 The colours are red, orange, yellow, green, blue, indigo, 
 and violet, and are arranged in that order. These colours 
 merge gradually into one another; hence the term 'con- 
 tinuous ' as applied to such a spectrum. The red, orange, 
 yellow, and green are the rays of low refrangibility, while 
 the refrangibility gradually increases through blue and indigo 
 to the violet-end of the spectrum. There is abundant 
 evidence to show that there are other rays, non -luminous, 
 and therefore invisible, beyond the red rays on the one 
 hand, and beyond the violet on the other. 
 
 It is found that these different rays have not all the 
 same effect on vegetal life. For instance, the rays of low 
 refrangibility are those concerned in the chemical changes 
 in the plant, whilst mechanical changes are furthered by the 
 rays at and beyond the violet end of the spectrum. It must 
 
46 Elementary Biology. 
 
 be borne in mind in this relation that many chemical changes 
 are furthered by the rays of high refrangibility. For example, 
 the decomposition of silver salts in photography takes place 
 under the influence of the violet rays of the spectrum. 
 Probably special rays a 1 through the spectrum are capable 
 of bringing about or influencing in some way or another 
 special chemical changes, while only some (those beyond 
 the red end of the spectrum) are capable of producing the 
 sensation of heat, and others (the red and yellow) the 
 sensation of light, in any appreciable degree. 
 
 In reference to the decomposition of carbonic acid by 
 chlorophyll-bearing cells, it is found that the less refrangible 
 rays are those through whose influence carbonic acid is 
 decomposed and chlorophyll is produced, while the more 
 refrangible rays have not that effect. 
 
 The influence of light on plants varies with its intensity. 
 This subject is, however, still far from being thoroughly 
 investigated, so that little may be said on this subject beyond 
 the general statement that diffused daylight seems more 
 suitable for the furtherance of plant life than direct sunlight, 
 while assimilation ceases after sunset. 
 
 Manifestly this subject is closely connected with the depth 
 to which the light penetrates into the plant tissues, since 
 the light loses in intensity according to the thickness of the 
 tissue through which it has passed. The rays of least re- 
 frangibility, that is to say those chiefly concerned in chemical 
 changes in plants, are found to penetrate most deeply. The 
 rays at the violet end of the spectrum are in great part 
 absorbed by the chlorophyll and the colouring matters in the 
 superficial cells. In regard to aquatic, and especially marine, 
 plmts, the depth to which light penetrates through water has 
 to be considered. Experiments have shown that darkness 
 prevails at all depths beneath 100 fathoms, and conse- 
 quently that no vegetal life can exist beneath that depth, if, 
 indeed, it ever exists so far. 
 
 (b) In regard to animal life, the presence or absence of 
 
Protoplasm. 47 
 
 light seems to have little or no particular effect. Some 
 animals live throughout their whole existence in darkness, 
 and very many others in light so dim that they may be said 
 to be practically independent of its influence altogether. 
 The special luminosity known as phosphorescence, which 
 many of these animals are themselves capable of producing, 
 may in some part act as a substitute for sunlight. 
 
 E. Food-supply. Since animals have no . power of 
 building up complex organic compounds out of simple 
 inorganic materials, but are dependent directly or indirectly 
 on the vegetal kingdom for their food, manifestly it is of the 
 first importance for the maintenance of animal life that 
 vegetals should have the means of nourishment in abund- 
 ance and in an accessible form. The food of animals is 
 extremely variable. Some animals are entirely herbivorous, 
 others entirely carnivorous, while others still are omnivorous. 
 It must be at once evident, in order that any group of 
 animals may remain in existence, that a sufficient supply of 
 that particular food on which they subsist should be available. 
 In some cases that supply is extremely localised and small 
 in amount. The larvce of certain flies, for example, are de- 
 pendent for the maintenance of their life on the larvae of a 
 particular species of bee. 
 
 The food materials required by plants are most easily 
 determined by analysing the plant itself (p. 187). An ex- 
 amination of the products of desiccation, of calcination, and 
 of the ash shows that the following elements are required 
 for the maintenance of plant life : 
 
 I. and chiefly, carbon, hydrogen, oxygen, and nitrogen. 
 
 II. and less important, sulphur, potassium, calcium, 
 iron, magnesium, phosphorus, chlorine, sodium, and silicon. 
 
 The source of the carbon has already been sufficiently 
 explained ; nitrogen is obtained by the plant from com- 
 pounds of ammonia and nitric acid in the soil, and not as 
 might have been expected from the enormous supply in the at- 
 mosphere; oxygen is obtained by the decomposition of water, 
 
48 Elementary Biology. 
 
 carbonic acid, and oxy-salts; the source of the hydrogen 
 is water, which is decomposed in the chlorophyll -bearing 
 cells in presence of sunlight ; sulphur is absorbed in the 
 form of soluble salts ot sulphuric acid ; the other elements 
 also are absorbed in the form of various salts from the soil 
 or other medium in which the plant lives. The various 
 sources of these different substances and the changes which 
 they undergo in the phnt organism will be dealt with more 
 fully afterwards (p. 188). Reference is made to the subject 
 at this point chiefly to show what are the main conditions 
 of life in regard to food-supply. 
 
 In section iv it was pointed out that in consequence of 
 the relationship of the plant world to carbonic acid and cf 
 the animal world to oxygen, a balance of gases was main- 
 tained in the atmosphere. Having now reviewed generally the 
 condition of environment necessary for the maintenance 
 of life, we shall be able to understand more readily the 
 importance of the subject of gaseous balance in the air. 
 
 SECTION VI. THE BALANCE OF NATURE. 
 
 In explaining what is meant by the balance of nature it 
 will be necessary to repeat certain statements already made, 
 with the view of explaining the relationship in- which they 
 stand to each other. 
 
 We have seen that animals and plants or parts of plants not 
 possessed of chlorophyll require organic compounds for their 
 sustenance, as well as salts and water, the organic compounds 
 being obtained directly or indirectly from the vegetal world. 
 They remove oxygen from the atmosphere, and return 
 carbonic acid with the evolution of energy ; i.e. the solar 
 energy originally stored in the organic compound manifests 
 itself in the various phenomena of life, ultimately becoming 
 dissipated as heat into space, the products of disintegration 
 being at the same time returned to the inorganic world. 
 
 On the other hand we have seen that plants possessed of 
 
Protoplasm. 
 
 49 
 
 chlorophyll are independent of organic compounds for their 
 nutrition, since they seize the solar energy and use it to 
 decompose the carbonic acid of the atmosphere. Oxygen is 
 returned to the atmosphere, carbon is retained, and, uniting 
 with the elements of water, is finally precipitated in the form 
 of some comparatively simple organic compound. This 
 being digested as required, and water, mineral matters, and 
 nitrogen being obtained from the soil by the roots, the re- 
 integration of new protoplasm is effected. 
 
 As explained in section iv., the same destructive meta- 
 bolism exemplified by animals is also exhibited by plants, 
 but masked in great measure by constructive metabolism 
 where chlorophyll is present. 
 
 There is thus in the economy ot nature a double 
 balance : 
 
 (i) Between animal and vegetal life, or more strictly 
 between colourless and green protoplasm ; and (2) between 
 fresh and foul air, or more strictly between oxygen and 
 carbonic acM. 
 
 Thi APct is so important in Biology that it warrants 
 repetition yet once mj^^n the diagrammatic form appended 
 which wieadil understood from the above 
 
 NH, 
 
 anima s 
 
 and plants not 
 
 possessing chlorophyll, 
 
 which disintegrate 
 complex chemical com- 
 pounds, liberating 
 energy in the 
 process. 
 
 Plants 
 possessing chlorophyll 
 
 integrate complex 
 
 chemical compounds, 
 
 storing up solar energy 
 
 in the process. 
 
 They 
 
 ->NH 3 
 
 \ 
 
 <^. c Energy originally obtained from the sun 
 radiated by the animal (chiefly) into 
 space as heat, and thereby becoming 
 ultimately unavailable. 
 
50 Elementary Biology. 
 
 CHAPTER III. 
 
 INDIVIDUAL AND TRIBAL LIFE DISTRIBUTION AND 
 CLASSIFICATION. 
 
 SECTION I. INDIVIDUAL LIFE. 
 
 THE transference of kinetic solar energy into potential 
 energy stored in complex chemical compounds involves the 
 necessity for the existence of collecting or preparing organs 
 in the organism by and in which the food materials may 
 be brought to it and elaborated for its use. These organs 
 are technically called organs of nutrition, alimentation, 
 digestion, or assimilation. Further, the fo^d materials 
 when prepared must be distributed to everfPpMt of the 
 organism, hence the necessity for distributing organs or 
 organs of circulation. The cells* ultimate constituents 
 of the tissues to which the food materials are 6%tributed 
 then exercise a selective action on them, and new proto- 
 plasm is integrated, thus forming a store of potential energy 
 in each individual cell. Liberation of this store of potential 
 energy as kinetic energy is afterwards effected, manifesting 
 itself in the various phenomena of life. The compounds 
 are decomposed and the energy let loose by the action on 
 the cells of a liberating energy from without, or in virtue of 
 the property of automatism possessed by the cell itself. It 
 still remains to be determined, however, how far automatism 
 is dependent on, and influenced by, external changes not 
 necessarily immediately precedent to the exhibition of au- 
 tomatism. 
 
 The ultimate results are seen in the various phenomena 
 
Individual Life, 51 
 
 of life, such as secretion, motion, nervation, &c., for the 
 manifestation of which, special organs and tissues are set 
 apart, as explained in section iii. Organs are consequently 
 developed to fulfil these functions. The muscular or con- 
 tractile system subserves the function of motion ; some 
 parts are specially adapted to protect the more delicate 
 organs from external injury, whilst others are specially modi- 
 fied to act as a supporting framework to the same. More- 
 over, the nervous system with its accompanying sense organs, 
 is that which is especially differentiated to receive and 
 retain impressions from the external world, to regulate the 
 due performance of the functions of the other systems, and 
 generally, to instigate in the cells and tissues chemical changes 
 which result in the several phenomena we have already 
 considered. Lastly, it is necessary that certain organs 
 should be set apart for the removal of the bye-products of 
 the decomposition of complex compounds ; these may be 
 termed purificatory organs. Since it is manifest that the 
 bye-products must consist either of solid, fluid or gaseous con- 
 stituents, the purificatory organs will differ according as it is 
 their function to remove solid, fluid or gaseous bye-products. 
 The solid or fluid refuse matters are got rid of mainly by 
 the excretory or renal organs, whilst the gaseous bye-pro- 
 ducts are removed chiefly by the respiratory system. 
 
 We have thus seen the individual to consist of a co- 
 ordinated series of organs, or systems, each performing its 
 own function, but all subservient to one end, viz. the proper 
 maintenance of individual life. 
 
 We may arrange these organs and their functions in two 
 parallel columns, thus : 
 
 Morphology (structure). Physiology (function). 
 
 A. Nutritive or alimentary system. Nutrition. 
 
 B. Circulatory ,, Circulation. 
 
 C. Purificatory ,, Purification. 
 
 i. Renal (fluid &c. excreta) i. Excretion, 
 
 ii. Respiratory (gaseous &c. excreta) ii. Respiration. 
 
 E 2 
 
52 Elementary Biology. 
 
 Morphology (structure). cont. Physiology (function). cent. 
 
 D Contractile system Contraction motion. 
 
 E. Supporting ,, Support. 
 
 F. Protecting ,, Protection. 
 
 G. Nervous ,, Nervation. 
 
 Sense organs. Sensation. 
 
 SECTION II. TRIBAL LIFE. 
 
 The various systems we have already discussed are 
 concerned entirely with the maintenance of individual life. 
 One system present in the individual, however, we have 
 yet to examine, viz. the reproductive system, which is con- 
 cerned entirely with the maintenance of tribal life, i.e. the 
 maintenance on the earth's surface of organisms of the 
 same kind as that in which the special reproductive elements 
 under consideration are developed. 
 
 Reproduction may be either sexual or asexual. Asexual 
 reproduction consists in the separation of a part (usually a 
 single cell) of the individual, which part is capable (without 
 union with any other part) of becoming an adult organism 
 like its parent 
 
 By sexual reproduction is meant the production by the 
 same or different individuals of two kinds of cells, one of 
 which is known as the male cell (sperm or spermatozoon), 
 the other as the female (ovum), the union of which results 
 in a product (embryo) capable of forming, after it has passed 
 through several developmental changes, an adult organism 
 like one or other of its parents. It is worthy of note at 
 this point, that the male reproductive cell is almost always 
 in the animal, and very generally in the vegetal king- 
 dom, an extremely minute and very active body, whilst 
 the female cell in all cases (with a few exceptions in the 
 very lowest forms) is comparatively large and immobile. 
 After union with the male cell the female cell begins to 
 undergo changes which are probably due to the instigat- 
 ing influence of the male cell. In other words the female 
 
Tribal Life. 53 
 
 cell is a store ot potential energy, while the male is a store 
 of kinetic energy, which acts as a liberating energy on the 
 female cell, permitting of the transference of its potential 
 into kinetic energy. 1 In certain rare cases the female cell 
 has the power of developing without previous union with 
 the male cell. This is known as parthenogenesis. 
 
 These two methods of reproduction are usually conjoined, 
 especially in the vegetal kingdom, save in very low forms of 
 life, where, so far as we know, asexual reproduction alone 
 exists. Sexual reproduction prevails more generally among 
 the highest forms of life, especially in the animal world. 
 
 After fertilisation (as sexual union is termed) of the 
 female cell by the male, the female cell may be considered 
 as and styled an embryo. The embryo passes through a 
 variety of stages, sometimes very extraordinary in their 
 nature, before arriving at the adult condition. The history 
 of these successive changes is termed its ontogeny, or 
 individual life-history. It is, moreover, apparent that every 
 organism must have a genealogy or tribal history in addition 
 to its individual history. To the genealogical history the 
 term phytogeny is applied. 
 
 As a result of the brilliant researches of biologists during 
 the past century, amongst whom Lamarck, Geoffrey St. 
 Hilaire, Haeckel, Darwin, and Wallace hold the first places, 
 a definite relation has been established between these two 
 histories. In an elementary text-book like the present, it 
 would be entirely out of place to go into this subject with 
 requisite detail ; it will be sufficient to state briefly that 
 there is abundant evidence to show that the ontogeny of 
 any organism is an epitome of its own phylogeny, and that 
 the various stages in its development indicate landmarks 
 guiding us in the tracing of its genealogical history each 
 
 1 Probably the male cell at the same time brings to the female cell 
 certain chemical compounds necessary for the further development of 
 the ovum. 
 
54 Elementary Biology. 
 
 stage exhibiting, in a greater or less degree, a likeness to 
 extinct or now existing organisms of a lower grade of organ- 
 isation. 
 
 SECTION III. DISTRIBUTION. 
 
 It may have been noted that in the preceding sections 
 we have answered three out of the four questions which 
 may be asked concerning every living thing. Morphology 
 answers the question, What is it? Physiology answers the 
 question, How does it live ? Phylogeny answers the ques- 
 tion, Whence came it? What was its origin ? One query 
 yet remains, Where is the organism found ? An answer to 
 that query is given by the section which treats of the 
 distribution of organisms. 
 
 Any given group of animals may have a distribution in 
 space, or a geographical distribution, and also a distri- 
 bution in time, or a geological distribution. In the former 
 case the habitat and distribution over the earth's surface of 
 the members of the tribe is taken account of, in the latter, 
 note is taken of the occurrence of remains of the same forms 
 as fossils in the strata of the earth's crust. 
 
 SECTION IV. CLASSIFICATION. 
 
 It has been already explained that organisms widely differ 
 in the structure of their different organs. Hence we have 
 varying degrees of likeness and unlikeness among organisms, 
 and a starting-point is thus afforded us for classifying them. 
 Classification consists essentially in the grouping together 
 of like and the separation of unlike forms, not merely in 
 their adult condition, but after taking into account their 
 entire life-histories. Classifications may be either natural 
 or unnatural. By an unnatural classification is meant 
 one based on superficial or apparent resemblance, whilst a 
 natural classification is one based, not only on accurate 
 morphological investigation, but also on the story of relation- 
 ship with other organisms told by ontogeny. 
 
55 
 
 CHAPTER IV. 
 
 THE MORPHOLOGY AND PHYSIOLOGY OF THE SIMPLEST 
 LIVING ORGANISMS PROTISTA. 
 
 SECTION I. PROTISTA PROTAMCEBA. 
 
 IN the preceding chapters an account has been given of the 
 main principles on which Biology is based, and of the chief 
 aspects or ways of looking at the subject. We have in the 
 chapters that follow to discuss the subject-matter of Biology, 
 dealing with it in the manner already sketched out. We 
 have seen that progress from lowly organised plants and 
 animals to those of a higher grade in either kingdom, is 
 .marked by what we have termed 'differentiation of parts,' 
 that is by the specialisation or setting aside of special organs, 
 tissues, and cells in the organism for special duties the per- 
 formance of which is necessary for the maintenance of 
 individual and tribal life. It is natural that we should find 
 that the lower we go in the scale of being the less differen- 
 tiation we should meet with, until we ultimately arrive at 
 forms that exhibit no structural or morphological differen- 
 tiation at all, although they do show, as has been pointed 
 out (at page 37), physiological differentiation, or division 
 of labour. In order to understand rightly the principle of 
 differentiation in the higher forms it is necessary that we 
 should study the structure and life-history of one of the 
 simplest types known to us. Such a form is that known as 
 Protamceba primitiva (fig. 6). The utmost that can be said 
 of it is that it is a microscopic speck of almost unmodified 
 protoplasm, presenting at most a differentiation into a denser, 
 
56 Elementary Biology. 
 
 more hyaline or glassy, outer layer or ectosarc, and a more 
 fluid, and granular, inner portion or endosarc. 1 In form 
 Protamteba is irregular, and varies from time to time, the ecto- 
 sarc becoming produced into pseudopodia in consequence of 
 changes taking place automatically in the organism, or in re- 
 sponse to stimuli from without (page 34). The pseudopodia 
 may vary in shape and size, being blunt or filamentous, and 
 are capable of retraction. Frequently the pseudopodia, 
 especially if filamentous, fuse with one another, forming 
 a network, or a mass, at a little distance from the parent 
 body (fig. 7). 
 
 FIG. 6. P rot amoeba primitiva. (Haeckel.) 
 
 A, before division ; B, in process of dividing ; Ca and Cb, two 
 new individuals resulting from division. 
 
 The protrusion of a pseudopodium from the hyaline ectosarc 
 is followed by the streaming into the pseudopodium of the 
 granular endosarc. Locomotion is effected by the gradual 
 movement of the entire body in the wake of a pseudopodium. 
 Food-particles are taken into the protoplasm at any point, 
 and the excreta, or indigestible and useless parts of the food, 
 are extruded at any point. At a certain period in the life- 
 history of Protamceba the protoplasm of the body becomes 
 divided into two or more parts (fig. 6). The organism either 
 separates into halves, either half receding from the other, 
 or small portions of the mass are nipped off. These halves 
 or smaller portions are capable of at once starting life on their 
 
 1 These terms, ecto- and endo-sarc, must be carefully distinguished 
 from the terms ecto- and endo-derm, mentioned at p. 22^.1 
 
Protamoeba Protomyxa. 
 
 57 
 
 own account, engulfing food particles, exhibiting amoeboid 
 motion, and dividing just as in the case of the parent. 
 
 A simpler state of things than this could scarcely be 
 imagined. Here we have morphological differentiation at 
 
 FIG. 7. Protogenes porrecta. (Max Schultze.) 
 
 tepii^s^ 
 
 ItlSiS -"- K = =TO ,,-:~ 
 
 / i\ A\\ 
 
 / I .K\ \\ "^ 
 
 I iU\ \ ' 
 
 its lowest ; whilst it is to be noted that all the various 
 functions exhibited by higher forms are manifested by this 
 exceedingly simple organism. 
 
 SECTION II. PROTISTA PROTOMYXA. 
 
 Closely allied to Protamceba we find another simple 
 organism, with an even more instructive life- history. 
 
 Crawling over the shells of some dead molluscs in the 
 Canary Isles, Hgeckel found an organism, the study of whose 
 structure and life-history has thrown a flood of light on the 
 relationships of forms such as we are discussing to each 
 
58 Elementary Biology. 
 
 other and to the higher plants and animals. That form 
 Hceckel termed Protomyxa aurantiaca (fig. 8). Protomyxa 
 
 FIG. %.*Protcmyxa aurantiaca. (Haeckel.) 
 
 d c 
 
 For explanation see text. 
 
 exhibits four very different stages in its life-history, each of 
 which has a distinct significance. 
 
Protomyxa. 59 
 
 At one period of its existence Protomyxa exists as a 
 minute Protamceba-Xfot creature (fig. 8*) possessing the gene- 
 ral characters already described as possessed by that form. 
 Numbers of these Protafnaba-\&. bodies unite to form a large 
 aggregation of protoplasm, to which the name of plasmo- 
 dium is given (fig. 8/). The plasmodium behaves exactly 
 like a large Prdiam&ba, throwing out and retracting pseudo- 
 podia, engulfing food particles, &c. The individuals going 
 to form a plasmodium cannot, however, be distinguished 
 in its mass as individuals ; their identity becomes lost. 
 After a time, however, the plasmodium comes to rest and 
 the pseudopodia are withdrawn. The mass collects into a 
 round ball which soon becomes covered by a capsular invest- 
 ment or cyst ; hence this stage is spoken of as the encysted 
 stage (fig. 8#). Subsequently the protoplasm within the 
 cyst becomes segmented or divided into distinct masses 
 forming a mulberry-like bunch within the cyst (fig. 8/). 
 Under certain conditions the cyst bursts and the separate 
 protoplasmic masses escape (fig. 8<r). Each unit is an 
 actively moving pear-shaped mass of protoplasm, possessed 
 of a. fine flagellum or whip at the pointed end by the con- 
 stant motion of which the little organism is propelled 
 through the fluid medium. This motion is, however, transi- 
 tory, for the flagellate mass speedily becomes more and 
 more sluggish in its movements, and ultimately comes to 
 rest, drawing in its flagellum and developing pseudopodia 
 like those of the form we started with. Such are the stages 
 in the life-history of this important type, and they are of 
 sufficient interest to warrant more detailed treatment before 
 we proceed to the considerati6n of higher forms. 
 
 In the first or amoeboid stage we have to note especially 
 the apparently exceeding simplicity of structure accompanied 
 by complexity of function. In other words, we have in this 
 stage complete physiological differentiation without corre- 
 sponding morphological differentiation. 
 
 Passing to the second or plasmodial stage, we have 
 
60 Elementary Biology. 
 
 exemplified the union of a number of amoeboid units after 
 they have to a certain extent exhausted their store of energy 
 by active motion. Evidence produced of late years tends 
 to show that plasmodium formation, or the union of sepa- 
 rate amoeboid units, in other forms as well as in this, is an 
 act intimately connected with reproduction. As explained 
 above (p. 52), sexual reproduction consists in the union of 
 a male with a female cell, followed by the development of 
 the product of union into a new organism or organisms. 
 Here we have union, not of two dissimilar cells, but of many 
 more or less similar cells. Nevertheless there is evidence 
 to show that the union, if it be not truly sexual in its nature, is, 
 so far as we know at present, at least accompanied by several 
 of the phenomena which characterise sexual union of cells. 
 In other words, in the conjugation of the separate amoeboid 
 stages of this simple form we have probably the first appear- 
 ance of sexual union in living things. 
 
 We have seen that the plasmodial stage is sooner or 
 later followed by an encysted or resting- stage, where the 
 protoplasm contracts into a spherical mass, and becomes 
 clothed by a capsule. In the period during which the 
 protoplasm remains encysted, probably certain constructive 
 molecular rearrangements are taking place in the proto- 
 plasm, which result in the revivifying or rejuvenescence of 
 the mass, the energy of the various amoeboid units originally 
 entering into the formation of the plasmodium being, so to 
 speak, concentrated. What meaning must we attach to the 
 cyst? During active amoeboid life carbonic acid and 
 water were being abundantly excreted, as already explained. 
 During the encysted stage, of course, the organism is 
 still living, only the vital phenomena are not so actively 
 manifested. There must, therefore, be a certain quantity 
 of carbonic acid and water produced even then. What be- 
 comes of the excreta ? During the active amoeboid stage, 
 owing to the constant motion and change of position 
 of the different parts of the organism, the excreta escape 
 
Protomyxa. 6 1 
 
 easily into the surrounding medium ; but during the 
 resting stage, manifestly, the rapid escape of effete matter 
 would not be possible. Hence the excreta would become 
 more or less aggregated round the organism itself. The 
 organism becomes, in other words, encrusted or enclosed by 
 its own excreta. The fact that the cyst is at first wanting, 
 ,then thin, and gradually becomes thicker, is thus explained. 
 Further, when we examine into the chemical nature of the 
 cyst we find it to be composed of a substance exceedingly 
 common in the plant world, namely cellulose. Now, one of 
 the most apparent physiological distinctions between animals 
 and plants is, that while the power of motion is characteristic 
 of the former it is not so of the latter. Hence we are not 
 astonished to find that cellulose, which we have seen to be 
 in some way associated with rest, is a substance of rare 
 occurrence in the animal world ; and that in those animals 
 in which it, or some substance chemically allied, does occur 
 the power of movement has been nearly or entirely lost. 
 
 Chemically, cellulose (which must be the subject of more 
 careful consideration subsequently) is composed of carbon, 
 hydrogen, and oxygen (the two latter in the proportion in 
 which they occur in water). We have here the elements of 
 carbonic acid and water, while the oxygen which the car- 
 bon has lost is probably used up in various concomitant 
 chemical changes taking place in the encysted organism. 
 
 The first result of this temporary cessation of activity, 
 represented by the encysted stage, is the formation of a 
 store of energy (potential), which first manifests itself 
 kinetically, under special circumstances, in the subdivision 
 or segmentation of the protoplasm into masses, which by- 
 and-by escape upon the rupture of the cyst (probably by 
 internal pressure). The organisms which escape are actively 
 motile masses of protoplasm, each provided with a flagellum 
 by means of which motion is effected. Hence this may be 
 known as the flagellate stage. As in the amoeboid condi- 
 tion, it is possible, by the application of a gentle heat, to 
 
62 Elementary Biology. 
 
 accelerate this motion, while excess of heat paralyses and 
 kills the organism. After a time the flagellum is gradually 
 withdrawn, the motion becoming slower and slower until 
 the amoeboid stage is again reached, and it becomes again 
 necessary for the organism to take in a stock of food to 
 make good the waste and enable it to remain in existence. 
 
 The importance of these four stages in the study of the 
 interrelationships of the lower organisms, whether plant or 
 animal, and indeed in a study of the physiology of the 
 higher forms as well, cannot well be exaggerated. It will 
 be necessary, however, to devote a special section to the 
 discussion of the subject. 
 
 SECTION III. THE RELATION OF UNICELLULAR TO 
 
 MULTICELLULAR ORGANISMS. 
 
 On reviewing the multiplicity of forms which we meet 
 with among the lowest plants and the lowest animals, 
 together with those organisms which are doubtfully classified 
 under either category, we soon discover that each and all 
 live for a longer or shorter period of their existence in one 
 or other of the four stages exemplified by the life-history of 
 Protomyxa, while most, if not all, show some indication 
 of a tendency to pass through a corresponding series of 
 stages in their own life-histories. One, for example, lives 
 during the greater part of its existence in the form of an 
 encysted cell ; but at another time, possibly only for an 
 exceedingly short period, the protoplasm escapes from its 
 cyst and becomes a motile flagellate organism. In other 
 cases the plasmodium stage is the one emphasised. In 
 many cases one or more stages of the general type of life- 
 history (that of Protomyxa) are omitted, or slurred over 
 very rapidly. Manifestly, if inquiry be made into the reason 
 for this emphasising of some one stage, the explanation will 
 be found in that adaptation of the organism to its environ- 
 ment which has already been discussed at some length in 
 
Protozoa and Protophyta. 63 
 
 Chapter II. section v. For instance, flagellate and amoe- 
 boid motion are possible only in a liquid (water). If that 
 were absent the protoplasm would dry up and shrivel, or the 
 surface would become covered by a skin ; in other words, 
 encystation would take place. Those forms, therefore, 
 which exhibit the complete life-history of Protomyxa are 
 necessarily such as live in liquid media. It will be easily seen 
 from this that the varying length in the duration of the seve- 
 ral stages can be accounted for by the habits of the form in 
 question, whether living all its life in a fluid, or only a part 
 of its existence in such a medium. 
 
 In the cases hitherto discussed we have dealt with 
 organisms composed of one cell only. To such organisms 
 the term unicellular has been applied. Now, manifestly, 
 the term * unicellular organism ' might be applied to or- 
 ganisms which were referable either to the plant or to the 
 animal world. We have already seen (p. 38) that the green 
 colouring matter chlorophyll is essentially characteristic of 
 the plant as distinguished from the animal, and we have 
 just learnt that a cellular investment is an additional charac 
 teristic of a vegetal cell. We are able, therefore, roughly, to 
 divide unicellular organisms into vegetal unicellular organ- 
 isms and animal unicellular organisms. To the former class 
 we give the name of Protophyta, or ' primitive plants ' ; while 
 to the latter we give the name of Protozoa, or ' primitive 
 animals.' There are, however, many organisms which, as 
 has been already indicated, cannot be said to belong 
 directly to either group, e.g. Protomyxa itself. These are 
 grouped along with the Protophyta and Protozoa in one 
 general collection, to which the term Protista, or ' primitive 
 living things,' is applied. A protist is therefore a unicellular 
 organism, whether truly vegetal or animal or doubtful in its 
 affinities ; in other words, an extremely simple living thing. 
 A protozoon is a unicellular organism, generally without 
 chlorophyll, and without a cellulose investment ; in other 
 words, an extremely simple animal. A protophyte is a 
 
 V 
 
 ^, n 
 
64 Elementary Biology. 
 
 unicellular organism usually possessing chlorophyll and a 
 cellulose cell-wall ; that is to say, an extremely simple 
 plant. 
 
 It is important to notice that this classification into uni- 
 cellular plants and unicellular animals is a very loose one, 
 and that no doubt many of the organisms at present classed 
 as doubtfully vegetal or animal in their relationships, are in 
 reality transition stages between the two types, or forms 
 which represent the generalised type from which both ani- 
 mal and vegetal have been derived. It is further worthy 
 of note that the presence or absence of chlorophyll is a 
 more important distinction than the presence or absence of 
 a cellulose capsule, since the capsule of cellulose, or of a 
 substance chemically related to cellulose, is developed, at 
 least temporarily, in not a few Protozoa. We have already 
 seen (p. 38) that chlorophyll is far more important from 
 a physiological than from a morphological point of view ; 
 hence we may say that the distinction between unicellular 
 animals and unicellular plants is mainly a physiological one. 
 
 Unicellular organisms make up, however, only a small 
 proportion of the organisms on the earth's surface. By far 
 the majority of organisms are composed of a vast number 
 of cells which are specially modified for the performance 
 of special functions that is to say, differentiated in the 
 mariner already fully explained. Such organisms are said 
 to be nmlticellular, and among these organisms there are 
 several well-marked differences which enable us at a glance 
 to say whether any given organism is a plant or an animal. 
 Such differences are the presence or absence of chlorophyll, 
 the presence or absence of cellulose, the possession or not 
 of the power of movement, the method of feeding, and 
 character of the foodstuffs, &c. We divide these multi- 
 cellular organisms into multicellular plants, or Metaphyta, 
 and multicellular animals, or Metazoa. Even here, how- 
 ever, the lowest group of Metaphyta includes forms which, 
 from their possessing the power of motion, have been often 
 
Metazoa and Metaphyta. 
 
 FIG. 9. Volvox Globator. (Stein.) 
 
 classed with the Metazoa. Their metaphytal affinities are, 
 however, now deemed indubitable. 
 
 It is necessary at this point that we should clearly under- 
 stand the relationship of the Metaphyta and Metazoa to the 
 Protista as a whole. 
 
 A metaphyte is essentially a 
 vast collection of encysted cells, 
 or cells invested with cellulose 
 capsules (fig. 4). In Protomyxa 
 the encysted protoplasm be- 
 comes segmented into a number 
 of parts. Now if each of these 
 parts were to develop round it a 
 cellulose capsule, and to remain 
 attached to the neighbouring 
 segments, we should have formed 
 an elementary metaphyte. Such elementary Metaphyta are 
 found in great profusion ; and no doubt we may look upon 
 this as the probable way in which the group Metaphyta has 
 
 FIG. io. LIFE-HISTORY OF Physarum album. (Cienkowski ) 
 
 originated, namely, by the multiplication and cohesion of 
 encysted cells. We have only a few instances of Metaphyta 
 
 F 
 
66 
 
 Elementary Biology. 
 
 formed by the cohesion of flagellate cells (fig. 9), or of 
 amoeboid cells (fig. 15). 
 
 On the other hand, Metazoa may be looked upon as re- 
 sulting from the multiplication and cohesion of cells in one 
 or other or both of these latter conditions. In the multi- 
 plication of the amoeboid stage movement is usually very 
 much diminished, and is as a rule in one direction, i.e. de- 
 
 Fin, it.- CILIATED CELLS FROM THE 
 FROG'S MOUTH. (Sharpey.) 
 
 FIG. 12. PELLS FROM THE LINING 
 
 OF THE LRETEK. (Kollikei.) 
 
 finite. In most cases the amoeboid condition is represented 
 simply by the absence of the cell-wall (fig. 12). Flagellate 
 Metazoa are of common occurrence among the lower groups 
 
 FIG. 13. Magospha-ra 
 p'.anu'.j.. (Herdman.) 
 
 FlG. 14. MULTINUCLEATED CELL FROM 
 THE MARROW HlGHLY MAGMFIEU. 
 
 (Schafcr.) 
 
 of that sub-kingdom (fig. 13); whilst masses of flagellate, or, 
 if there be a number of small flagella on each cell, ciliated 
 cells occur frequently scattered here and there in the bodies 
 of the higher Metazoa (fig. n). The plasmodial condition 
 
Metazoa and Metaphyta. 67 
 
 may be looked upon as in reality a multicellular stage in the 
 life-history of a unicellular organism ; but that condition is 
 retained as the permanent adult form only in one or 
 two aberrant groups of plants (fig. 15). In the sponge 
 group and allied forms amongst animals the plasmodial 
 stage is permanently exemplified ; it also appears in ab- 
 
 Fio. 15. PLASMODIUM OF Didymium leucopus. (Sachs.) 
 
 normal (diseased) cells and, in certain cases also, normally 
 in the higher Metazoa (fig. 14). In disease the ciliated cells 
 are those which most commonly, losing their cilia, become 
 plasmodial. Such collections of diseased cells are plasmodia 
 in which the nuclei of the constituent cells are still visible, 
 
 F 2 
 
68 
 
 Elementary Biology. 
 
 thereby at once indicating the multicellular origin of the 
 mass. It must be clearly remembered that no hard and fast 
 lines can be drawn with reference to such phenomena ; 
 
 FIG. 16. WHITE FIBRO-CARTILAGE FROM AN INTERVRRTEBRAL DISK (HUMAN). 
 HIGHLY MAGNIFIED. (Schafer). 
 
 The concentric lines around the cells indicate the limits of deposit of successive cap- 
 sules or cell-walls. 
 
 animal cells, for instance, especially those entering into the 
 formation of the supporting system, have very distinct cell- 
 walls, though not of cellulose (fig. 16). The absence of the 
 power of motion in these cells is to be noted. 
 
 The author is indebted for many of the generalisations discussed 
 in this chapter to the works of his friend Mr. Patrick Geddes, Lecturer 
 on Botany in University College, Dundee. 
 
6 9 
 
 CHAPTER V. 
 
 UNICELLULAR PLANTS PROTOPHYTA. 
 PROTOCOCCUS. 
 
 As an example of the simplest form of plant known, we may 
 select Protococcus pluvialiS) one of the organisms which gives 
 the green colour to damp wood and stones, and which is 
 found also in large quantities wherever rain-water collects. 
 
 The plant is of extremely minute size, varying from 
 ToiioT^h to 47j th of an inch in diameter. In shape it is 
 ovoid or elliptical, and presents, when examined under a 
 high magnifying power, a differentiation into protoplasm 
 and cell- wall. The protoplasm has diffused through its mass 
 or aggregated in the form of rounded masses the green 
 colouring matter chlorophyll. In addition there may be 
 seen a nucleus and granules. Some forms have, in addition 
 to the chlorophyll, red colouring matter present, usually in 
 the form of a spot at one end. The cell-wall is of an appre- 
 ciable thickness, is colourless, and composed of cellulose. 
 
 Occasionally Protococcus becomes motile, and migrates 
 from place to place in the medium (water) by means of the 
 vibration of two flagella or cilia which are extruded from 
 one end of the cell. Careful examination under a high 
 magnifying power shows that the protoplasm has contracted 
 generally from the cell-wall, but is continuous with the cilia. 
 Not infrequently the cell-wall becomes entirely lost, and 
 Protococcus moves about as a naked mass of protoplasm 
 provided with the two cilia already mentioned. The nucleus 
 and red spot are distinctly visible at this stage, usually close 
 
7O Elementary Biology. 
 
 to the origin of the cilia. After a variable period of exist- 
 ence in this free-swimming condition the cilia are absorbed, 
 the cell comes to rest, and the reformation of a cellulose coat 
 takes place. 
 
 With regard to the physiology of Protococcus, being a 
 plant possessed of chlorophyll, it decomposes water and 
 carbonic acid in the presence of sunlight, and integrates new 
 protoplasm, setting free oxygen into the air in the process. 
 In this instance, of course, the carbonic acid is obtained 
 from the rain-water. The nitrogenous elements in its com- 
 
 FIG. 17. LIFE-HISTORY OF Protococcus pluvialis. 
 
 
 a-e, stages in vegetative division \f,g t motile stage. 
 
 position Protococcus obtains from the air or from the sur- 
 rounding objects upon which it may happen to be growing. 
 All the phenomena spoken of as characteristic of plants 
 possessing chlorophyll are exhibited by Protococcus and 
 therefore need not be repeated here. 
 
 When the plant is provided with the necessary substances 
 for its nutrition and life, multiplication takes place. No 
 sexual union or other phenomenon connected with sexual 
 reproduction is, so far as we at present know, manifested by 
 Protococcus. Multiplication is entirely a vegetative process. 
 The protoplasm, when multiplication takes place, becomes 
 divided into two parts, and a cellulose partition is formed 
 between them (fig. 17). Each half may again divide, either 
 simultaneously or successively, and the new cells become 
 
Unicellular Plants Protophyta. 71 
 
 gradually detached to form new individuals. This process is 
 known as cell-division or fission, and is the simplest known 
 mode of multiplication. These new cells are naturally at 
 first small, and have thinner walls than the older cells ; but 
 increase in age is accompanied by increase in thickness of 
 the cell wall, and increase in size as a whole. 
 
 Protococcus is very widely diffused, because of its power 
 of migration by the possession of a motile stage. Such 
 migrations, of course, take place only in a liquid medium. 
 In addition, however, Protocoaus has the power of retaining 
 its vitality even though dried. The extreme minuteness 
 and lightness of the dried plant manifestly permit of its 
 being carried about by winds, and its area of distribution is 
 thus materially widened. 
 
 There are many different species and varieties of the 
 genus Protococcus, all more or less like each other, and all 
 possessing the general features above enumerated. 
 
 Comparing the life-history of Protococcus with that of 
 Protomyxa, we notice that we have represented the encysted 
 stage in the adult plant, whilst the incipient multicellular 
 condition resulting from rapid division indicates a transi- 
 tion to the Metaphyta, which, as already stated, are merely 
 aggregates of encysted protoplasmic units. Further, the 
 ciliated stage is represented in the motile Protococcus, whilst 
 the amoeboid condition is hinted at in the quiescent naked 
 phase immediately succeeding the withdrawal of the cilia. 
 The plasmodial stage is apparently wanting ; its absence 
 may, perhaps, be explained by the presence of chlorophyll 
 in the protoplasmic unit enabling it to replenish its store of 
 energy directly, without needing to unite with other units, 
 and to undergo molecular rearrangement. 1 
 
 Protococcus is a true plant for three reasons : first, it is 
 invested by a cellulose cr.psule ; secondly, it is possessed of 
 
 1 It is to be noted that in the Myxomycetes, where plasmodium-for- 
 mation is the rule, chlorophyll is wanting. Chlorophyll is also wanting 
 in animals, where the formation of plasmodia is of frequent occurrence. 
 
72 Elementary Biology. 
 
 chlorophyll ; and thirdly, it is therefore able to integrate 
 new protoplasm from inorganic material. 
 
 The varieties of form and life-history exemplified amongst 
 the Protophyta are very numerous. Although it is not 
 within the scope of this book to enter into a description of 
 these allied forms, yet mention must be made of the Diatom- 
 acece, of the Saccharomycetes and Schizomycetes, as examples 
 of this important group. The unicellular Diatomacece are 
 particularly remarkable for their variety of form, and for the 
 great beauty of their silicious skeletons. It is possible that 
 some of the types included under the Protophyta may re- 
 present stages in the life-history of the Metaphyta, and 
 hence ought by rights to be considered in that relation. 
 The Saccharomycetes, represented by the common Yeast - 
 plant (Toruld), are by many botanists looked upon as 
 degraded Metaphyta. For further details with regard to 
 these forms, however, reference must be made to such text- 
 books of Botany as those of Gcebel, Strasburger, De Bary, 
 and others. 
 
73 
 
 CHAPTER VI. 
 
 UNICELLULAR ANIMALS PROTOZOA. 
 
 AAfCEBA. 
 
 IN order to emphasise the distinction between a unicellular 
 animal and a unicellular plant, we devote this chapter to a 
 sketch of the organisation and life-history of one of the 
 simplest of the animal forms, viz. Amoeba. 
 
 Much of what has been said of protoplasm in general 
 is applicable here. Indeed, as will be seen on reference to 
 Chapter II., Amoeba was there instanced as an example of 
 protoplasm in one of its simplest and most easily accessible 
 forms. It may be well, however, to repeat briefly the main 
 points there discussed in detail. 1 
 
 Amoeba (fig. 18), as there stated, has generally the ap- 
 pearance of a minute particle of transparent or finely granular 
 jelly, having a nucleus, contractile vacuole, and granules, 
 and showing an elementary differentiation into ecto- and 
 endo-sarc. During its life it is constantly undergoing slow 
 changes in form, due to the protrusion and withdrawal of 
 masses of its own substance (pseudopodia). Its method of 
 ingesting and circulating food-material has already been de- 
 scribed (p. 34). Careful observation shows that the con- 
 tractile vacuole undergoes periodic changes of form which 
 partake of the nature of alternate swellings and contractions, 
 known technically as diastole and systole. It is probable 
 that the contractile vacuole is some sort of renal organ 
 
 1 It is advisable that the student re read Chapter II. in this relation. 
 
74 
 
 Elementary Biology. 
 
 whereby the water, and possibly also certain nitrogenous 
 excreta, are separated from the protoplasm and ejected on 
 the surface. The general physiology of Amoeba^ the effect 
 of the application of heat, &c., has been already described 
 fully above (pp. 33-35). 
 
 At a certain period at present, so far as we know, unde- 
 terminable, but probably in some way related to the condi- 
 tion of the environment and the state of nutrition of the 
 
 FIG. 18. Amoeba polypodia (Max Schultze.) 
 
 , nucleus ; '/Y, contractile vacuole, 
 
 animal itself the Amoeba assumes a more or less globular 
 form and becomes enclosed in a cellulose cyst. No seg- 
 mentation, however, takes place as in Protomyxa. After 
 a period of a few hours the cyst bursts and the Amoeba 
 escapes and becomes actively motile. This process is 
 known as rejuvenescence. 
 
 Amoebae are inhabitants of damp earth and stagnant 
 water, and are decidedly local in their distribution, as might 
 be expected from the absence of special possibilities for 
 diffusion. They vary much in size, but a common size is 
 from T -^o7) tn to Tou tn f an mcn - Vegetative multiplication 
 
Unicellular Animals Protozoa. 75 
 
 by fission is common here also, as in the case of Protococcns, 
 although in Amoeba no cellulose cell-wall is formed. As in 
 Protococcus so in Amoeba, division of the nucleus precedes 
 division of the mass as a whole. 
 
 Anmba is an animal because it possesses no cellulose 
 cell-wall ; because chlorophyll is absent ; and because the 
 organism is for that reason incapable of using the carbonic 
 acid of the atmosphere and the water and simple salts of 
 the soil in the manufacture of new protoplasm, but requires 
 to be supplied with organised matter or protoplasm already 
 partially or entirely integrated. 
 
 As regards its physiology, it is to be noted (p. 35) that 
 carbonic acid is constantly being produced, both in the dark 
 and in sunlight, and thus it presents superficially a great 
 difference to the plant where carbonic acid is exhaled in 
 quantity in the dark only, whilst in daylight its exhalation 
 is masked by the comparatively large amount of oxygen 
 given forth contemporaneously. 
 
76 Elementary Biology. 
 
 CHAPTER VII. 
 
 METAPHYTA NON-VASCULARIA. 
 
 SECTION I. FRESH-WATER Ki&fc 
 
 IN the two preceding chapters we have discussed the struc- 
 ture and life-history of a representative of each of the two 
 groups known as Protophyta and Protozoa, or unicellular 
 plants and unicellular animals. We have seen, moreover, 
 that at one stage in the life-history of the example taken to 
 illustrate the Protophyta it assumed a temporary resem- 
 blance to a very elementary metaphyte. Protococcus was 
 found to divide into three or four pieces, which for a short 
 space of time remained attached to each other, forming a 
 very simple multicellular organism. It is to be noted, how- 
 ever, that the cells are really physiologically independent of 
 each other, being rather members of a colony than units in 
 one whole. The organism at this stage does not exhibit 
 that division of labour which has been already explained (at 
 page 37) as being characteristic of multicellular organisms, 
 It is quite true that many multicellular plants and animals 
 do show only a very slight approach to morphological 
 differentiation ; and we cannot be surprised that it is so, 
 since manifestly in the simplest beginnings of a metaphyte, 
 morphological sameness must precede differentiation. Every 
 plant and every animal has its cells at first perfectly similar. 
 It is only after a period, varying in duration with the nature 
 of the plani or the animal in question, that the cells begin 
 to be differentiated and become specially modified for the 
 performance of their several duties. 
 
Metaphyta Spirogyra. 
 
 77 
 
 FlG. 19. Spirogyra longata. 
 
 We commence our study of the Metaphyta, or multi- 
 cellular plants, by examining a plant which exhibits very 
 little morphological differentiation. As an example, we take 
 the common pond weed known 
 as Spirogyra longata. In struc- 
 ture Spirogyra has the form of 
 a green thread composed of a 
 variable number of cells, placed 
 end to end. The cells are all 
 exactly alike, save for certain 
 slight variations in size. Such a 
 collection of more or less simi- 
 lar cells is termed a thallus, a 
 flattened or thread-like cellular 
 expansion. In some cases the 
 thallus may be large, in other 
 cases small. For example, the 
 familiar seaweeds on the coast 
 are instances of large flattened 
 thalli, while Spirogyra itself is an 
 instance of a delicate, narrow, 
 and thread-like thallus. 
 
 We shall see in the present 
 chapter that thalli differ very 
 greatly in form and size. Each 
 cell is composed of a cellulose 
 coat, or cell-wall, with contents. 
 The contents are granular proto- 
 plasm, a nucleus, a green spiral 
 band, and a variable quantity of 
 a watery substance known as cell 
 sap. 
 
 Further details can be made out only by employing high 
 magnifying powers of the microscope. When that is done 
 it is seen that the protoplasm in the cells, at least at some 
 
 Incipient conjugation, a, pro- 
 cesses approaching each other ; 
 6, processes united. 
 
78 Elementary Biology. 
 
 distance from the free ends of the thread, exists as a layer 
 lying immediately within the cell-wall. From this layer 
 there stretch to the nucleus threads of protoplasm, the 
 nucleus itself being surrounded by a layer of the same 
 substance. The nucleus may, however, be found lying 
 close to the cell-wall, but always separated from it by the 
 layer of protoplasm above referred to. The cell-sap, a 
 watery fluid containing certain organic substances which are 
 probably products of the metabolism of protoplasm, varies 
 in amount according to the age of the cell. Spirogyra, 
 being an unattached floating plant, has neither apex nor 
 base, though the terminal cells have rounded ends, and 
 less cell-sap than other cells. The nucleus contains a 
 nucleolus, and that again is said by some writers to contain 
 a distinctly differentiated endonucleolus. The structure of 
 che spiral band varies with the species of Spirogyra under 
 consideration. In some forms it is single, in others double, 
 but in all cases it possesses more or less irregular margins, 
 , is of a green colour, and contains starch granules, aggregated 
 at definite intervals or scattered irregularly through it. The 
 band itself consists of a protoplasmic basis and chlorophyll, 
 which is in this case diffused in the protoplasmic band, and 
 not in the form of granules as was the case in Protococcus. 
 
 Increase in length is effected by the division of a pre- 
 viously existing cell into two parts. Division takes place 
 only during the night. The process of division is a some- 
 what complicated one, and is accompanied by a series of 
 changes in the nucleus which have been the subject of much 
 recent study and debate. Without entering into detail, it 
 may be sufficient to indicate the chief features in the pro- 
 cess as follows. 
 
 The division of a cell is preceded by division of the 
 nucleus. The process of division of the nucleus is known 
 as karyokinesis. The structure of the nucleus has already 
 been described (p. 30), and it is necessary for the com- 
 
Metaphyta Spirogyra. 
 
 79 
 
 prehension of the phenomena of karyokinesis to bear that 
 structure in mind. The first stage consists in the partial 
 or complete disappearance of the fibrillae, or network, and 
 the assumption of a more markedly granular character by 
 the protoplasm ; the granules then unite into the form of 
 curved rods or threads and arrange themselves parallel to 
 the long axis of the nucleus. This is known as the spindle 
 stage in division (fig. 20, iv.). At the same time the proto- 
 plasm of the cell generally aggregates round either pole of 
 
 FIG. 20, KARYOKINESIS AND CELL DIVISION. (Strasburger.) 
 
 the spindle and exhibits a radial arrangement of its granules. 
 The next stage consists in the appearance of a plate or 
 thickening round the equator of the spindle, known as the 
 nuclear, or equatorial disc. The equatorial disc next splits 
 into two plates, either half travelling to either pole to form 
 the two new nuclei (fig 20, vi.). 
 
 The protoplasm of the cell itself has meantime been 
 undergoing the preliminary alterations necessary to the 
 formation of a new cell- wall between the two nuclei. These 
 changes consist in the differentiation of a special plate of 
 protoplasm across the middle of the cell. In this there 
 appears a deposit of cellulose, which gradually increases in 
 
8o 
 
 Elementary Biology. 
 
 amount, until it ultimately cuts the old cell into two (fig. 
 20, ix.). The nuclei then take up a central position, and 
 the two new cells gradually assume the characters of the 
 parent cell from which they arose. 
 
 Spirogyra does not differ essentially in its physiology from 
 the general type (Chap. VIII. sect. v.). The phenomena of 
 constructive and destructive metabolism are the same as 
 those exhibited by all green plants, save that its gaseous 
 food is obtained from carbonic acid dissolved in water. 
 
 FIG. 2i. CONJUGATION IN Spirogyra, longata. (Sachs.) 
 
 A, a, passage of the male protoplasm into the female cell ; f>, B, c, embryos 
 in different stages of development within the mother cell-wall. 
 
 A species of sexual union and reproduction prevails in 
 Spirogyra, whereby new individuals are formed. Conjuga- 
 tion, as it is sometimes called, takes place between two cells 
 of different filaments lying close to each other (fig. 19). It 
 is to be noted that the conjugating cells are, to the eye, 
 precisely similar, although doubtless there are molecular 
 differences of importance ; the basis for that belief will be 
 given presently. The first stage in the union consists in 
 either cell sending out late v al protrusions or buds which 
 grow towards each other (fig. 19), and increase in length and 
 
Metaphyta Spirogyra. . 81 
 
 size until they touch. Fusion of the walls at the point of 
 contact takes place, and a direct communication is estab- 
 lished therefore between the two cells. Meantime the 
 protoplasmic contents of both cells contract and withdraw 
 from the cell-wall, forming a dense rounded mass in the 
 interior of either cell. This contraction is accompanied and 
 rendered possible by the expulsion of a quantity of water 
 from the cell. The contraction may take place in both 
 cells at the same time, or more usually in one cell before 
 the other. This would seem to point to some molecular 
 difference between the cells, probably of nutritive condition 
 (p. 90). The next step consists in the passage of one or 
 other of the protoplasmic masses into the other cell in which 
 latter no doubt preparatory metabolic changes are taking 
 place (fig. 21). This fact would again point to some mole- 
 cular difference between the cells. Union of the proto- 
 plasmic masses follows, and results in the formation of an 
 ovoid mass which is smaller in bulk than the sum of the 
 two masses before union, doubtless owing again to the 
 expulsion of water such a body is called in most text-books 
 a ' zygospore,' or 'resting spore.' The term 'spore,' as 
 applied to such a body, is most undoubtedly inappropriate, 
 if not positively wrong, because that name is applied to 
 cells which multiply the plant asexually (p. 95), and which 
 do not result from conjugation of two reproductive cells of 
 different sexes. l Although the conjugating cells of Spiro- 
 gyra are apparently alike, yet there can be no doubt that 
 this is a case of primitive sexual union, and therefore if we 
 decide to call the egg or female reproductive cell of a plant 
 before fertilisation an ovum, this 'zygospore' is simply a 
 fertilised ovum or embryo. It is true we cannot say before- 
 hand which of the conjugating cells is the ovum, and which 
 
 1 The term ' spore ' is used by Vines in his Physiology of Plants to 
 designate both the asexually produced spore and the embryo, or product 
 of union of the ovum and sperm. 
 
 *G 
 
82 
 
 Elementary Biology. 
 
 corresponds to the male element, until movement of the 
 protoplasm of one of the cells takes place. Then, on the 
 general analogy that the male element is usually the more 
 active of the two (p. 52), we are able to guess that the more 
 passive cell is that which corresponds to the ovum. 1 
 
 After undergoing a short period of rest, during which, no 
 doubt, various important rearrangements of the protoplasmic 
 materials take place, the body resulting from the union of 
 the two cells ('zygospore'), which has meantime obtained 
 a thick cell-wall, bursts first its own special capsule, and 
 
 FIG. 22. GERMINATION OF Spirogyra jugalis. (Pringsheim.) 
 
 I, 'zygospore' ; e,f, layers of cell-wall. II, germination of embryo, g: 
 III, young Spirogyra, with three cells already formed; c, cell-wall 
 of parent (female) cell ; /, ', new cell-walls ; c, conjugating processes. 
 
 afterwards the mother cell-wall, and pushes out a long 
 filament, which develops gradually into a new Spirogyra. 
 
 Such is the life-history of Spirogyra^ and it is to be 
 noted by way of recapitulation that we have here an 
 exceedingly simple thallus, producing two cells of different 
 
 1 It is desirable that the student should as far as possible familiarise 
 himself with the terms in use in other text-books, after he has, by means 
 of a uniform terminology such as that adopted here, mastered the exact 
 signification of the various parts of plants, more especially of their re- 
 productive organs. 
 
Metaphyta Fucus. 8 3 
 
 sex, which unite together, the product of union developing 
 into a new thallus. 
 
 Taking a general view of the allies of Spirogyra, we find 
 that they embrace a most varied collection of forms. The 
 modification lies principally in the form of the thallus. For 
 example, some take on the form of a hollow sphere, the 
 component cells being provided with flagella, as in Pan- 
 donna. Others again, such as the Coleochcetece, by succes- 
 sive division, assume the form of rounded or irregular plates 
 built out of branched multicellular threads. 
 
 The familiar though aberrant Chara is more thread-like 
 in form, but the delicate stem and branches into which the 
 thallus is divided are more highly differentiated than those 
 of the filamentous forms of which Spirogyra is the type. 
 
 All the forms mentioned are inhabitants of fresh water, 
 and differ only in the structure of the thallus, as above stated, 
 and in the nature of their reproductive cells and the parts of 
 the thallus which bear them. All shades of differentiation 
 are exemplified, from the simple form we have just ex- 
 amined to the fully developed sexual apparatus we are 
 about to describe in the next vegetal type that falls to be 
 discussed. 
 
 SECTION II. SALT-WATER ALG&FUCC7S. 
 
 In this and succeeding vegetal types where the repro- 
 ductive organs are distinctly differentiated, it will be most 
 convenient to discuss the plant first from a vegetative point 
 of view, and then to treat of its reproductive apparatus. 
 
 Vegetative organs. To illustrate the group of salt- 
 water Algae, or seaweeds, and as giving a good instance of a 
 typical plant of the lower or less advanced type, of structure, 
 no better example could be found than one of the common 
 seaweeds Fucus platy carpus. The plant consists of a much- 
 branched and flattened thallus attached by means of cylin- 
 drical and branched roots to some fixed object. The 
 
 G 2 
 
8 4 
 
 Elementary Biology. 
 
 FIG. 23. PART OF THE TH LLUS 
 OF Fitcus platycarpns. (Thuret.) 
 
 dimensions of the thallus vary extremely ; but it usually 
 attains a length of from one to five or six feet in the group 
 to which F. platycarpus belongs. 
 
 The branching is dichotomous i.e. the growing apex 
 divides into two nearly equal por- 
 tions. All the branches lie in 
 the same plane, and all more or 
 less resemble each other. Each 
 branch consists of a cylindrical 
 core or mid-rib with a lamella 
 on either side, both lamellae 
 lying also in the plane of branch- 
 ing. Every here and there along 
 the lamellae air-sacs or bladders 
 are found, which are, morpho- 
 logically, simply spaces in the 
 cellular tissue. 
 
 If a section of the branch be 
 examined microscopically it will 
 be found to consist of cells which 
 differ in shape and size according 
 to the part of the branch under 
 examination. On the surface the 
 cells are small, spherical, and have 
 no intercellular spaces between 
 them ; the cells of the centre are 
 elongated and branched, and 
 form a loose spongy mass, with 
 necessarily large intercellular 
 spaces. The cell-walls are very 
 thick and mucilaginous in their nature, and swell up readily 
 under the action of fresh-water. Hence the slimy cha- 
 racter of the seaweeds when handled, and especially if ex- 
 amined alter steeping in fresh-water, or preservation in 
 alcohol or spirit. 
 
 Although the colour of Fucus is a dull brown it is not 
 
 A, dichotomous branching ; f, f, 
 fertile branches. 
 
MetapJiyta F 11 ens. 8 5 
 
 to he supposed that there is no chlorophyll present. Its 
 presence is masked by an admixture of a brown colouring- 
 matter. It performs the same function as the unmixed 
 chloropKyll of Spirogyra. The carbonic acid is, of course, 
 as in the case of Spirogyra, obtained from the water (in this 
 case salt-water), and the various physiological processes are 
 conducted as in that type. 
 
 Reproductive organs. The reproductive apparatus of 
 Fucus is of a very complete and highly differentiated nature. 
 At the proper season, certain (usually many) of the branches 
 exhibit at their terminations swellings which are covered 
 over with minute mounds or pimples. These are fertile 
 branches, and the pimples represent the mouths of small 
 depressions or sacs ('conceptacles') sunk in the tissue of the 
 thallus. Each sac contains (in F. platycarpus] both male 
 and female cells. If a vertical section of a sac be examined 
 (fig. 24) it will be found to consist of a bounding wall of cells 
 not unlike, and continuous with, the cells forming the super- 
 ficial layer of the thallus, from which spring numerous hairs 
 of variable shape some long and branching, others thick, 
 short, and in some cases almost spherical. These are respec- 
 tively the spermaria, or sperm-producing organs, and the 
 ovaria, or ovum-producing organs. The cavity of the sac 
 is filled with sea-water, mingled with the mucus secreted 
 from the slimy tissue of the thallus. 
 
 A more careful examination of the hairs in the sac dis- 
 covers that they spring from the lining-wall of the sac, and 
 that, since the sac itself is formed in all probability by an 
 indentation or imagination of the superficial cells, 1 the 
 reproductive hairs are really equivalent morphologically to 
 superficial or epidermal hairs. 
 
 Taking the male elements first, we find that the sperms 
 (fig. 25) are minute unicellular bodies without any visible 
 
 1 According to Bower, Q.J.M.S. 1876, the formation of the con- 
 ceptacle, at least in some forms, commences with the decay of a super- 
 ficial cell. 
 
86 
 
 Elementary Biology. 
 
 nucleus, but each provided with a red spot of colouring- 
 matter and two fine flagella, by the constant vibration of 
 which they are able to move about with extreme rapidity. 
 The sperms are developed in the interior of club-shaped 
 
 FIG. 24. SECTION OF THE CONCEPT ACLE OF Fucus platycarpus. (Thuret.) 
 f, 
 
 a, b, sterile hairs ; c, ovaria ; e, spermaria ; d thallus. 
 
 branches of the male hairs, each branch being a spermarium 
 (antheridium). Each spermarium consists at first of a cell- 
 wall and granular protoplasm, which latter gradually be- 
 comes segmented and modified into the sperms already 
 described, which escape on the rupture of the wall of the 
 
MetapJiyta Fucus. 87 
 
 spermarium. Scattered amongst the male hairs are other 
 hairs, which are unbranched, but destitute of spermaria. 
 These are sterile hairs, which have been unable to develop 
 into true sexual hairs owing to the pressure for space, etc. 
 Some of these sterile hairs are very long, and extend out 
 through the mouth of the sac to the exterior. 
 
 The ovaria are comparatively large spherical bodies rest- 
 ing on a basal cell which arises from the lining-wall of the 
 
 FIG. 25. Fncus vesiculosJis. (Thuret.) 
 
 A, branched hairs bearing spermaria. a,. J3, sperms. //, ovarium (outer 
 wall, a ; inner wall, i) containing eight ova. ///, ovum surrounded 
 by sperms. J V, V, development of embryo. 
 
 sac. Each ovarium consists originally of a cell-wall which is 
 composed of an inner and an outer layer, filed with dense 
 dark protoplasm. The ovarium in F. platycarpus divides 
 into eight ova, although in other forms a smaller number of 
 ova are occasionally formed from the contents of each 
 mature ovarium. The ovarium on bursting ejects its ova 
 
 *G 4 
 
88 Elementary Biology. 
 
 into the cavity of the sac, and from thence to the exterior, 
 where they are fertilised by sperms which have meantime 
 escaped from the spermaria. Although many sperms 
 attack each ovum, and by rheir active motion cause the 
 passive ovum to move also, yet probably only one sperm 
 fuses with the ovum (fig. 25). Immediately after the act 
 of fertilisation the ovum becomes an embryo. As the first 
 consequence of union with the male cell, the previously 
 naked ovum takes on a cell-wall and begins to germinate, 
 elongating and dividing, at first transversely, and then ver- 
 tically, or in the direction of the long axis of the embryo. 
 Very soon after fertilisation the embryo comes to rest, 
 attaching itself to some fixed body, whilst the free end 
 develops by repeated cell- division into an organism like the 
 parent. 
 
 To recapitulate, we have in Fucus a thallus giving rise 
 to two undoubtedly sexual cells which after union produce 
 a thallus like that from which they arose. There is thus 
 exemplified in this form a certain amount of morphological 
 differentiation. We find that a distinction can be drawn 
 between the purely vegetative portion of thallus and th^ 
 purely reproductive portion. That part which has for its 
 duty the formation of cells which have to do with the main- 
 tenance of tribal life is altered in outward form and micro- 
 scopic or histological structure from the purely vegetative 
 portions of the thallus ; yet both the purely vegetative and 
 the purely reproductive branches are modifications of the 
 same type. It can scarcely be said that the thallus of 
 Fucus, from a vegetative point of view, is morphologically 
 differentiated ; since, although we find that there is an 
 approximation to a stem, from which the branches spring, 
 and to a root, yet these are not at the same time physio- 
 logically differentiated, that is to say, they do not perform 
 the functions of circulation and absorption respectively, 
 which are the chief functions of the similar parts known by 
 these terms in the higher plants. Probably every cell in the 
 
MetaphytaFucus. 89 
 
 thallus performs for itself the important functions requisite 
 for the maintenance of life. 
 
 Amongst the cells forming the thallus, however, we have 
 a certain amount of modification of form. As already seen, 
 the cells of the centre, or medulla, are different from those 
 nearer the surface, or cortex ; whilst those associated with 
 the reproductive sacs are still more variable in shape. 
 Collections of more or less similar cells are called tissues. 
 Thus we may speak of medullary tissue and cortical 
 tissue, or we may describe in general term the whole thallus 
 as being made up of a cellular tissue. Organs, therefore, 
 are made up of tissues, which are themselves collections of 
 more or less similar cells. 
 
 It is important that we should pause at this point and 
 endeavour to ascertain what is the rationale of the pheno- 
 mena of true sexual reproduction which we have, in Fucus, 
 met with for the first time. 
 
 In the first place, what is the connection, if any, between 
 asexual and sexual reproduction ? It was pointed out 
 (p. 52) that asexual reproduction meant the separation of any 
 cell from the parent, which, without union with any other 
 cell, was capable of developing into an organism like the 
 parent. A modification of this process is exhibited among 
 the higher plants, and more rarely among the higher animals ; 
 namely, the separation of many cells in the form of a ' bud,' 
 * shoot,' * stolon, 7 etc., which is similarly capable of deve- 
 loping into an adult organism. 
 
 The process of protoplasmic anabolism (p. 36) was seen 
 to be one usually accompanied by growth in the early stages 
 of a plant's or animal's life-history. The various pheno- 
 mena of life are, of course, always possible only if the pro- 
 ducts of katabolism be got rid of. Now since the mass of 
 any growing cell increases as the cube of the dimensions, 
 while the surface only increases as the square (H. Spencer), 
 katabolism must sooner or later overtake and outstrip 
 anabolism, with one of two results to the cell, namely, 
 
9 Elementary Biology. 
 
 either it must be poisoned by its own excreta, or it must 
 divide into two or more parts, so as to restore the proper 
 proportion of surface and mass. Hence at the limit of 
 growth, in Protococcus for example, division takes place. 
 This division amongst the lowest types represents asexual 
 multiplication. 
 
 It is conceivable, however, that the division might result 
 in the formation of two cells not equally anabolic or kata- 
 bolic. For instance, one half might contain an excess of 
 protoplasm which had a tendency to break down, or exhibit 
 energy in movement ; the other half might be more ana- 
 bolic, passive, and sluggish. We should have thus formed 
 an incipient sexual differentiation ; the more katabolic cell 
 would represent the male cell, or sperm, whilst the more 
 anabolic would represent the female cell, or ovum. Atten- 
 tion was directed in the case of Spirogyra to the difference 
 in physiological activity observed between the two conju- 
 gating cells, and it was there pointed out that it was pro- 
 bable that the more kinetic cell represented the male and 
 the more sluggish the female element in true sexual conju- 
 gation. 
 
 The kinetic or katabolic cell may be looked upon as the 
 liberating energy which sets off, so to speak, the potential 
 energy, just as the spark sets off the gunpowder; only in this 
 case the conditions are more complicated. The kinetic 
 energy must be energy resulting from the decomposition of 
 special chemical compounds, and the products of kata- 
 bolism must themselves be such as are able to stimulate the 
 ovum to undergo those changes which convert it into an 
 embryo. This subject will be referred to again when we 
 come to consider the fertilisation of the higher plants and 
 animals. 
 
 Amongst salt-water Algse, as amongst fresh- water forms, 
 we meet with great variety in the form and structure of the 
 thallus. All agreeing in the general characters already de- 
 scribed in the type discussed above, they yet present almost 
 
MetapJiyta Penicillium. g i 
 
 every conceivable modification of such a primitive type. 
 The threadlike thallus is represented by Ectocarpus, which 
 is otherwise specially interesting as exhibiting an incipient 
 sexual differentiation among its reproductive cells. Lami- 
 naria, again, has a flat and much-branched thallus. Amongst 
 the red seaweeds, or JFloridece, there is great variety of form, 
 and one large group the Corallines derive their name 
 from the fact that they have an incrustation of carbonate of 
 lime which covers the thallus and causes them to mimic in 
 some respects the corals among animals. 
 
 The reproductive cells, the method of fertilisation, and 
 the subsequent changes in the ovum are also somewhat 
 different from the corresponding phenomena in the type 
 selected ; but nevertheless the essential points are precisely 
 similar, and in such a text- book as the present it is not 
 possible to do more than point out the general relationship 
 amongst individual differences. 
 
 SECTION III. FUNGI PENiciLLiUM. 
 
 We have now to direct our attention to a group of plants 
 very different in external appearance and in minute structure 
 from those we have been hitherto discussing, viz. the Fungi. 
 Under this extensive class are included such forms as the 
 common grey moulds, the mushrooms, and many others 
 more or less popularly familiar. In microscopic structure 
 and external configuration they present very great variety, 
 but there are certain well-defined characters which they all 
 possess, and which separate them from the Algae on the one 
 hand, and the higher plants on the other. It will be of 
 advantage to briefly emphasise these characteristics and in 
 some measure to account for them. 
 
 In the first place, it is to be noted that the cells of Fungi 
 contain no chlorophyll, and in the second place that the 
 Fungi live parasitically on dead or living organisms. Can 
 we trace any relationship between these two phenomena? 
 
92 Elementary Biology. 
 
 It will be remembered that the presence of chlorophyll 
 (Chap. II. sect, iv.) in the green plant was an essential 
 condition of the decomposition of carbonic acid, and that by 
 the assistance of chlorophyll in the presence of sunlight the 
 green plant was able to build up complex organic com- 
 pounds out of simple inorganic compounds and elements. 
 Now a fungus lives entirely upon the already organised 
 compounds found in the living or dead organism on which 
 it is a parasite. The necessity for the complex anabolic 
 processes above referred to is therefore in great measure 
 avoided, and consequently chlorophyll is not required. 
 
 FIG. 26. MYCELIUM OF Penicillium glaucnin. 
 
 Hence we see that absence of chlorophyll in the Fungi is 
 a result of parasitic habits. 
 
 Further, we notice that the form of the fungus as a whole 
 is by no means definite ; that there is to the naked eye a want 
 of that individuality which we see developed in such a form 
 as Fucus. Indeed it will be found that many of the moulds 
 form colonial masses, varying in extent with that of the 
 nutritive surfaces on which they live. 
 
 It will be best at this point to describe a typical fungus, 
 availing ourselves, as far as we can, of the knowledge of 
 vegetal life-history we have derived from a study of the 
 immediately preceding type. 
 
 If any moist organic substance, such as a piece of thin 
 
MetapJiyta Penicillium. 
 
 93 
 
 bread, be exposed to air and light for some days there will 
 usually be found over its surface at the end of that time a 
 dense white felt-work, which forms a tolerably firm cover- 
 ing to the underlying organic material. This is the so-called 
 * mycelium ' of Penicillium glaucum. We will assume that 
 the cultivation has been obtained pure, that is, free from 
 admixture with other moulds a result not always obtained. 
 Vegetative organs. The felt-like mass (fig. 26), when 
 examined under the microscope, is discovered to consist of 
 
 FIG. 27. SPORE BEARING FILAMENTS OF Penicillium glaucum. 
 
 fl /i 
 
 1-6. Stages in the formation of the branched ends of the filaments. 
 
 closely interwoven threads (hyphae) which branch and twist 
 in all directions. Each thread is composed of a variable 
 number of elongated cells placed end to end. Each cell con- 
 sists of a cellulose cell- wall, containing colourless granular 
 protoplasm. The threads branch and form a loose cellular 
 tissue, which may be termed a thallus, as being comparable 
 in all respects to the thallus of the Fucus. In Penicillium 
 the thallus is entirely composed of elongated cells loosely 
 
94 
 
 Elementary Biology. 
 
 connected, and showing large intercellular spaces ; in Fucus 
 only the medullary portion of its mass is so constituted. 
 
 The thallus does not long retain its white appearance, it 
 soon begins to show greenish patches at intervals over its 
 
 FIG. 28. Eurotium repens. (De Bary.) 
 Jt 
 
 A, portion of thallus with erect spore-bearing filament (c); young 
 ovarium (as). B, young ovarium (as) and spermarium (/j. C, same 
 beginning to be surrounded by sterile filaments, two shown in the 
 front. D, ' fructification.' 
 
 surface. We must now endeavour to throw some light on 
 this phenomenon. Microscopic examination of a portion of 
 the thallus which shows this alteration in colour demonstrates 
 
Metaphyta Penicilltum. 
 
 95 
 
 Peniciilium glaucum. 
 
 at once a series of threads or elongated cells springing verti- 
 cally from the thallus into the air and bearing at their free 
 ends many strings or chains of minute rounded greenish 
 bodies, commonly known as spores. More careful exami- 
 nation shows that these spore-bearing filaments are simply 
 modified cells or threads of the thallus (fig. 27). Apparently 
 any cell, may give rise to a filament, which, when it reaches a 
 certain size, begins to branch at its free termination. The 
 successive stages in that division will be best understood by 
 a study of the figure (fig. 
 
 27). The terminal Cells, FlG * ^.-GERMINATION OF SPORES OF 
 
 after the branching has 
 sufficiently progressed, 
 bud off parts of them- 
 selves, the youngest 
 buds being those near- 
 est to the parent cell. 
 This budding is a purely 
 vegetative process, and, 
 though the buds at first 
 remain attached, they 
 are very soon set free 
 and blown about in great 
 numbers. Each bud or 
 spore consists of a very 
 
 faintly yellowish-green cell-wall and colourless protoplasm. 
 A nucleus is said to be present, but it is not easily seen 
 without the assistance of reagents. 
 
 If sown in or on a suitable medium, the spore begins to 
 germinate. At one or more places on the spore wall a 
 bud appears, which develops into a much elongated thread 
 not unlike one of the threads of the parent thallus (fig. 29). 
 The filament afterwards divides, and, alone, or in company 
 with filaments formed by other spores, forms a felt-like 
 thallus, like that which originally produced the spores. 
 
g6 Elementary Biology. 
 
 So far, it would seem that we have to deal with a plant 
 very different in life, history from such a form as Fucus. In 
 the above account we have nothing comparable to the 
 spermaria and ovaria and their contents ; the two plants 
 have only a thallus in common. More careful observation, 
 however, discovers that we have by no means exhausted the 
 life-history of Penidllium. 
 
 If the mycelium be placed under such conditions that 
 oxygen is partially excluded, whilst the cultivation goes on 
 in darkness, spore-bearing filaments after a time are not 
 formed so plentifully, and the budding off of spores comes 
 consequently to an end. In place of these, minute yellowish 
 bodies make their appearance the so-called ' fructifica- 
 tions.' It will conduce to clearness if we trace the de- 
 velopment of these ab initio. 
 
 The first stage in the formation of a * fructification ' is the 
 spiral coiling of the terminal cell of one of the threads of the 
 thallus (fig. 28, A) ; the spiral coil so formed is a close one, 
 and the cell becomes by transverse partitions divided into 
 as many cells as there are turns in the spiral. At the same 
 time a small branch from the same terminal cell grows up in 
 a spiral manner round the other spiral and closely intertwines 
 itself with it. Ultimately fusion of the two spirals takes 
 place and the protoplasmic contents mingle with each other. 
 After fusion of the two spirals, a series of filaments spring 
 out from the parent filaments from which the spirals arose, 
 which enclose and protect them. In some forms of mould 
 very elaborate capsules are so formed. In Penidllium the 
 surrounding envelope closely embraces the spirals and is 
 of a spongy texture, owing to the continuous growth and 
 division of the enveloping filaments. The chief or primary 
 spiral, however, after fusion with the secondary spiral 
 develops branches (0.rcz)which push their way in amongst the 
 enveloping filaments. At this stage the mass (already re- 
 ferred to above as the ' fructification ' ) becomes detached 
 from its thallus and may undergo further development either 
 
Metaphyta Penicillium. 
 
 97 
 
 FIG. 30. Eitrotium re&cns, 
 Bary.) 
 
 at an early date or after a long period of rest. If the condi- 
 tions be favourable, a few weeks are required for the deve- 
 lopment of the asci (fig. 30), in the interior of each cf 
 which by a process of segregation, or free-cell formation, a 
 number of cells are formed. As 
 a general rule, eight cells appear 
 in each ascus, each of which 
 cells is in all essential points 
 extremely like the spore formed 
 from the thallus by ordinary 
 vegetative division. They must 
 not, however, be confounded 
 with them, and the commonly 
 accepted term of 'ascospore' 
 given to these cells is misleading 
 in that respect. Each of these 
 spore-like bodies may, if suitably 
 nourished, develop into a thallus 
 exactly like that formed by the 
 true spore, and in a precisely 
 similar manner. 
 
 It will be necessary for us now 
 to endeavour to obtain some clear 
 conception of the relationship of 
 these different series of pheno- 
 mena to each other, and to trace 
 the homologies between the 
 various stages of the life-history 
 
 Of FllCUS and those Of the life- E, F, two stageslri the d-.velopment 
 
 history of the mould we have just J^. 
 
 been considering. It will con- G ' ascus: H > <asc ' s P re -' 
 
 duce to clearness if we repeat briefly what has been already 
 
 said on these points. 
 
 First, what corresponds in Penicillium to the thallus of 
 Fucusl Obviously what we have named the thallus viz. 
 the felt-like tissue. The thallus of Fucus produced male and 
 
 H 
 
98 Elementary Biology. 
 
 female cells, which we termed respectively sperms and ova. 
 These we found were produced in spermaria and ovaria. 
 Have we anything comparable to this in Fenidllium ? Cer- 
 tainly, the primary spiral filament in the so-called 'fructifi- 
 cation ' is undoubtedly an ovarium, while the secondary 
 spiral is as undoubtedly a spermarium. 1 At this point, 
 however, a certain amount of divergence is observable. The 
 whole contents of the spermarium fuse with or fertilise the 
 contents of the ovarium. Again we notice that the spermarial 
 or male filament is the active or kinetic filament coiling up 
 to meet the terminal portion of the ovarium. 
 
 The phenomena taking place subsequently to fertilisa- 
 tion are also considerably modified. In Fucus the embryo 
 at once developed into the adult plant, in Penicillium that 
 development is postponed ; in fact, the stimulus of the male 
 fertilising matter seems needed for bringing about the forma- 
 tion of ova (fertilised, however) from the protoplasm of the 
 ovarium and its branches ( * asci ' ). The so-called * asco- 
 spores ' are, therefore, the delayed products of sexual union, 
 and for that reason ought to be called embryos, just as the 
 products of sexual union in Fucus were called embryos. 
 
 How shall we exp'ain, then, the occurrence of an asexual 
 method of multiplication ? 
 
 It has been seen -that Pcnicillium is dependent for its 
 life on the obtaining of complex organic food matter, and 
 is unable to decompose carbonic acid and build up organic 
 compounds out of inorganic constituents. Manifestly, there- 
 fore, the food supply of Penicillium is considerably more 
 limited than that of green plants in consequence of its para- 
 sitic habits. Moreover, it has been seen that its sexual 
 method of reproduction is one of considerable complexity, 
 
 1 Brefeld, who first made out the stages in the formation of asco- 
 spores in Penicilliuw, now believes that the process is vegetative and 
 not sexual. De Bary's researches establish beyond doubt that Brefeld's 
 first conclusion is the correct one. 
 
Metaphyta Penicillium. 99 
 
 and requiring a long time for its proper development. The 
 chances of the ultimate formation of embryos in this manner 
 are obviously much fewer than if the plant were not a para- 
 site, and if its embryos were rapidly formed as in Fucus. 
 There might be a possibility under these circumstances of 
 the plant being entirely destroyed off the face of the earth 
 were it not that the very efficient asexual mode of multipli- 
 cation comes, so to speak, to the rescue, so that, by means 
 of the simple vegetative budding above described, a plentiful 
 supply of new plants may be formed with a large margin for 
 waste. The spore stage is not represented in FUCUS, where 
 the, in that case, very efficient sexual method is amply 
 sufficient to replenish loss of individuals and spread the 
 seaweed wherever the salt-water flows. 
 
 By way of summary, then, we may say that while the 
 thallus of Fucus produces male and female cells, the latter 
 of which, after fertilisation by the former, develop into 
 thalli similar to that from which we started, the thallus of 
 Penicillium produces male and female cells, similarly capa- 
 ble after fusion of reproducing a thallus, but is able also to 
 separate off certain asexual cells, conveniently termed 
 spores, which may develop into thalii capable of forming 
 either another and yet another generation of spores, or true 
 sexual organs and a series of embryos. 
 
 If we agree to term the thallus of Fucus the sexual gener- 
 ation, as being the generation which produces sexual cells, 
 then we would write its life-history thus : 
 
 ST--<? + $ ST <?+ ? 
 
 and so on, where S T stands for the sexual thallus and $ 
 and $ for the male and female sexual cells respectively. 
 If we agree, in like manner, to call the spore-bearing thallus 
 an asexual generation, and to denominate it by the letters 
 AST and the spore by the symbol o, we might write the 
 history of Penicillium thus : 
 
 H 2 
 
TOO Elementary Biology. 
 
 ST - <? + $ - AST - o - AST 
 ST -- + 
 
 Amongst Fungi, then, for the first time we meet with 
 what is known as altercation of generations between an 
 asexual and a sexual condition a phenomenon which we 
 shall find lies at the base of all knowledge of the life-histories 
 of the higher plants. 
 
 It will be seen that the asexual method of multiplication 
 is much the more efficient of the two, inasmuch as it is in- 
 finitely simpler, waste of reproductive cells is not of so much 
 moment, their formation takes up a very short space of 
 time, and they are, at least for many generations, quite as 
 fertile as the true products of sexual union. It is con- 
 ceivable that under these circumstances the sexual method 
 of multiplication might in the Fungi fall into disuse and 
 ultimately disappear altogether. Hence arises the phe- 
 nomenon of apogamy in Fungi, i.e. the absence of sexual 
 reproductive powers. We have all stages in the degenera- 
 tion exemplified by different forms. We have instances, for 
 example, of male and female organs being formed which 
 never unite, of dwarf male organs which perform apparently 
 no function, and of forms like the common mushroom 
 (Agaricus}, where, so far as we know, no sexual organs are 
 formed at all. Apogamy is especially interesting when viewed 
 in relation to the life-history and habits of these plants. 
 
 A physiological classification of Fungi into two groups 
 may be formulated, based on their habits. Some live on 
 dead or decaying organic matter ; such are termed sapro- 
 phytes ; others live parasitically on living plants and 
 animals usually causing a more or less serious disease in 
 the host ; such forms may be termed parasites. As a 
 general rule, it may be laid down that the more pronounced 
 
 1 The colonial habit of the moulds accounts for the co-existence of 
 the two generations on the same plant. A thallus may bear in one 
 place sexual reproductive organs, in another spore-producing filaments. 
 
Metaphyta Polytrichum. ici 
 
 the degree of parasitism in any fungus, the more degenerate 
 are the reproductive organs. The impulse given to the 
 plant causing it to produce reproductive cells probably 
 comes from the protoplasm of the host instead of from the 
 protoplasm of a male sexual cell (Marshall Ward). When 
 surrounding conditions are such that extraneous impulse 
 cannot be obtained, then true sexual reproduction super- 
 venes and saves the parasite from extinction. 
 
 SECTION IV. Musci POL YTR1CHUM. 
 
 We began our study of Penicillium by emphasising the 
 salient features in its structure and life-history as popularly 
 known. Similarly, in the case of 'the moss, we will begin our 
 discussion of the type by noting certain general points which 
 at once strike the eye (fig. 31). 
 
 First we note that the thallus, which in Fucus was a 
 flattened branched brown expansion, and in Penicillium a 
 colourless loose filamentous mass without definite form, is 
 here distinctly more in accordance with what we are com- 
 monly disposed to associate the term plant. We note the 
 existence of a distinct stem, from which leaves arise, and 
 the continuation of the stem below ground as a root. We 
 note, moreover, that the thallus has chlorophyll present in 
 its cells. Further, at certain seasons of the year we find 
 small cylindrical urns or boxes surmounted by pointed caps 
 which spring from the free termination of the thallus and are 
 separated from it by a long stalk. So far for the popular 
 knowledge of the life-history of the moss. AVe must now 
 deal more in detail with these various parts, and endeavour 
 to trace a relationship between the stages in the life-history of 
 the moss and those of the two immediately preceding types. 
 
 Treating first of the thallus, we may for convenience of 
 description separate it into stem, root, and leaves. 
 
 Stem. The stem of the moss presents us with an example 
 
102 
 
 Elementary Biology. 
 
 of differentiation, for we find in it still further developed the 
 different systems of tissues foreshadowed in the stem of 
 Fucus (fig. 32). Superficially, the cells have their walls 
 
 FIG.3X.-/W,***,* thickened > and of a dark er colour (yellow 
 commune, TERMINA- or brown). The cells forming the cortical 
 
 TION OF THE FEMALE . . . . . , 
 
 PLANT. (Maoutand tissue have also thickened cell-walls, but 
 becoming thin-walled as they approach 
 the centre. Such cells of tolerably uniform 
 diameter and imdifferentiated character 
 receive the name of parenchymaj or 
 parenchymatous tissue. The central 
 cells of the stem differ in character in 
 different species of moss. In the type 
 chosen for consideration the cells are 
 elongated and thick walled, forming a 
 tolerably firm axis or support, to which 
 the name of sclerenchyma or sclerenchy- 
 matous tissue is given. In higher plants 
 there is found running through the stem, 
 branches, roots, and leaves, a very perfect 
 system of strands of sclerenchyma accom- 
 panied by vessels and variously modified 
 cells, of which we shall have to speak 
 afterwards more in detail. That system 
 has been called the fibre-vascular system. 
 One is at first inclined to look upon this 
 axial strand in the moss stem as a rudi- 
 mentary fibro-vascular strand. It is 
 scarcely correct, however, to adopt that 
 view, seeing that the moss plant and the 
 ordinary flowering plant are not compar- 
 able organisms in fact, as we shall after- 
 wards see, do not belong to the same 
 generation. Even the terms stem, leaf, root, are used to 
 indicate structures, at the most, only analogous to the corre- 
 sponding parts in higher plants. Only a few species of moss 
 
Metaphyta Polytrichu m. 103 
 
 show this amount of differentiation. Whilst a few possess a 
 strand of elongated thick-walled cells in the axis of the stem, 
 many have only thin-walled medullary parenchyma. 
 
 The stem of the moss may branch, whilst not in- 
 frequently a shoot or stolon, which may run along under- 
 ground, or just on the surface, may be given off. The stolon 
 usually takes root at some little distance from the parent 
 plant, and forming an upright stem begins life indepen- 
 dently. 
 
 The roots of the moss are more correctly termed rhizoids, 
 as being the organs by which the thallus fixes itself to, and 
 absorbs nourishment from, the ground. The rhizoids spring 
 in a tuft from the base of the stem, differing, therefore, 
 markedly from the continuation of the stem below ground, 
 to which the term root is applied 
 
 11 ric. 32. Polytrichiim comtm*:ie, 
 
 in most Of the higher plants. \\ hen TRANSVERSE SECTION OF THE 
 
 STEM. 
 
 examined microscopically the rhi- 
 zoids are found to be merely cellu- 
 lar outgrowths from the epidermis, 
 and do not contain any of the sub- 
 jacent tissues. They correspond to 
 the epidermal hairs which are found 
 so abundantly springing from the 
 various organs, more especially of 
 flowering plants. The rhizoids, 
 unlike the stem, branch very freely, 
 and usually form a very dense mat- 
 ting below ground, or just on the surface. The cells forming 
 the rhizoids are elongated, and contain granular protoplasm, 
 oil globules, &c. enclosed in an orange-coloured or brown 
 cell-wall, the outer surface of which becomes gradually 
 clothed with particles of the soil. 
 
 The leaves are also merely cellular outgrowths of the 
 stem, and are composed of almost undifferentiated paren- 
 chyma, the cells of which, however, contain chlorophyll in 
 addition to the ordinary constituents of such cells. In form 
 
IO4 Elementary Biology. 
 
 the leaf is broad at its base and pointed at its free end. It is 
 sessile, i.e. has no stalk, and is attached directly to the stem. 
 In those species which possess an axial bundle of elongated 
 cells, the leaf shows a central nerve, or midrib, of similar 
 cells, continuous with those of the axial strand. The margin 
 of the leaf commonly bears minute spines. The leaves are 
 arranged in a definite order round the stem, approaching 
 to a more cr less perfect spiral. 
 
 It has been already remarked that the thallus had the 
 power of throwing out from itself a stolon or shoot, which, 
 after creeping along the ground for some distance, could 
 take root, and develop into an independent plant. This 
 method of multiplication is a form of vegetative repro- 
 duction, and it is a method especially common amongst 
 mosses, and is by no means confined to the production of 
 stolons. The first stage in the formation of a new plant, 
 by vegetative multiplication from the root, is the production 
 of a small branched green intermediate thallus known as a 
 protonema, from which springs the erect stem of the thallus 
 proper. 
 
 Reproductive organs. The essential organs of repro- 
 duction are borne at the free termination of the stem 
 surrounded by an involucre of leaves. The male organs 
 may be borne on the same stem as the female organs, or on 
 different plants. If on the same thallus, the spermaria and 
 ovaria may be intermingled or may be arranged so that the 
 ovaria are central whilst the spermaria surround them. The 
 involucres differ slightly in the character of their leaves 
 according to the sex. In Polytrichum the sexual organs are 
 borne on different stems on distinct plants. 
 
 The spermarium is an elongated sac, the wall of which 
 is composed of many cells arranged in a single layer deep. 
 The cells contain chlorophyll bodies imbedded in the gran- 
 ular protoplasm. The sac is filled with cellular tissue, the 
 mother cells of the sperms. The contents of each cell 
 become gradually transformed into a sperm, round which, 
 
Metaphyta Polytrichum. 
 
 105 
 
 when mature, the cell-wall remains as an envelope. Each 
 sperm consists of an elongated spirally twisted body, which 
 terminates in two delicate flagella, by means of which it is 
 
 FIG. 33. Polytrichum 
 
 COJIlMline, TERMINA- 
 TION OF A MALE 
 
 PLANT. (Maout and 
 Decaisne.) 
 
 FIG. 34. Polytrichum commune. PARAI HVRES 
 
 AND SPERM ARIA, ONE OF WHICH EMITS SPERMS. 
 
 (Maout and Decaisne.) 
 
 
 FIG. 35.P0?ytrjchitm cammune. SPERMS. 
 (Maout and Decaisne.) 
 
 able to move in a fluid medium. When they first escape, 
 the sperms with their enclosing cysts lie in a mucilaginous 
 matrix, which, however, ere long becomes dissolved and 
 disappears. In addition to the spermaria there are nume- 
 rous sterile hairs, or paraphyses, which are slightly different 
 
io6 
 
 Elementary Biology. 
 
 FIG. 37. Funaria J:ygrometvica. 
 
 (Sachs.) 
 VI 
 
 in form, being more club-shaped and with a narrower stalk. 
 They carry chlorophyll grains and resemble in other respects 
 ordinary epidermal hairs, or the paraphyses of Fucus, with 
 which they are homologous. 
 
 The female sexual organ or 
 ovarium consists of a swollen basal 
 portion, commonly called the 
 venter, and a long terminal tube B 
 or neck (fig. 37). Both venter 
 and neck are made of cells, two 
 layers thick in the venter, and one 
 layer thick in the neck. The 
 venter when mature contains one 
 large central cell, the ovum, whilst 
 the canal in the neck is filled by 
 
 FlG. 36. DIAGRAMMATIC REPRESENTATION OF 
 THEHOMOLOGIESOF THE FEMALE REPRODUC- 
 TIVE ORGANS OF Fucus AND Polytriclimn. 
 
 A, apex of stem of a female thallus 
 ( x 100) ; a, female reproductive 
 organs ; b, leaves, u, female 
 reproductive organ ; b, venter 
 with ovum ; //, neck with seven 
 canal cells. 
 
 the mucilaginous remains of half-a-dozen long narrow cells, 
 whose duty it is before fertilisation to swell and force open 
 
Metaphyta PolytricJin m. 
 
 107 
 
 the neck to permit of the entrance of the sperms. The 
 ovum, by transverse division, adds one more to the row of 
 canal cells. 
 
 Notwithstanding the great apparent dissimilarity of the 
 various organs to those of Fucus, it is possible to see in the 
 venter the very much reduced cavity of a female ' concep- 
 tacle. 1 The comparison of the two cases will best be under- 
 stood by reference to fig. 36, where a diagrammatic repre- 
 
 FIG. 38. Polytrichum commune. (Maout and Decaisne.) 
 
 i, Theca with calyptra. 2, Calyptra removed to show operculum. 3, Operculum 
 removed. 4, Transverse section of theca. 
 
 sentation of the two ' conceptacles ' is given showing them, 
 as it were, originating from the same thallus. It will be seen 
 that the conceptacle of the moss is simply an upraised and 
 free conceptacle of Fucus. In Fucus the contents of the 
 ovarium broke up into eight cells, all of which became ova. 
 Here seven of these cells perform a very subsidiary function, 
 namely, that of forcing the mouth of the * conceptacle ' open 
 for the entrance of the sperm. Naturally the close-fitting 
 and protecting venter and neck do away with the necessity 
 of a special protecting cell- wall or capsule for the ovum and 
 the seven modified ova, or canal cells. 
 
 It is subsequent to fertilisation that the difficulty of 
 
io8 
 
 Elementary Biology. 
 
 FIG. 40. - -TRANSVERSE SECTION 
 THROUGH THE SPORE-SAC OF 
 Funaria hygrometrica. (Sachs.) 
 
 explaining the homologies of the various parts begins. On 
 
 fertilisation the ovum becomes an embryo. The canal 
 
 cells are now completely disorganised and disappear. The 
 
 embryo rapidly divides and forms a mass of cells which soon 
 
 differentiates into a stalk, or 
 
 seta, and a capsule, or theca. 
 
 As the development of this 
 
 organism takes place in the 
 
 venter, naturally the venter 
 
 very soon becomes too large 
 
 to contain it, and its walls give 
 
 FIG. 35. Funaria. hygrometrica. 
 (Sachs.) 
 
 A, female plant, with root hairs, and young 
 theca enclosed in the calyptra, c. B, 
 asexual generation nearly mature ; s, 
 seta ; J\ iheca. C. longitudinal section of 
 theca (magnified) ; tec, columella ; d, 
 operculum ; /, peristome ; s. archespo- 
 riam ; h, air-spaces. 
 
 A, sit, archesporium. B, s i, 
 mother spore-cells ; a, outer, /', 
 inner side of the spcre-sac. 
 
 way at the base, whilst the embryo bears on its head, or 
 theca, the upper portion of the venter and its neck as a cap, 
 or calyptra. The calyptra remains attached to the embryo 
 
Metapliyta Polytrichiim. 1 09 
 
 for a time, and then drops off. It will now be necessary to 
 study this product of sexual union, an organism, it is to be 
 noted, quite different from the thallus which produced the 
 sexual cells and which now bears the embryonic new gene- 
 ration as a parasite on itself. 
 
 As already stated, the new generation consists of a stalk, 
 or seta, surmounted by a capsule, or theca. The seta is fixed 
 firmly in the basal portion of the venter, and consists of a 
 core of slightly elongated cells covered by a thickened 
 epidermis. The seta swells slightly below the theca forming 
 the apophysis. The seta is continued up through the theca 
 as the columella. Surrounding the columella is an annular 
 spare traversed by thread-like ceils, which stretch from the 
 columella to the outer wall of the theca. The wall of the 
 theca is several layers of cells deep, and has its epidermal 
 layer strongly thickened. Closely surrounding the colum- 
 ella, but separated from the annular air-space, by two or 
 more layers of cells containing chlorophyll, forming the 
 spore-sac or sporangium, lies a layer of cells which are 
 highly protoplasmic and are capable of active division 
 This forms what is known as the archesporium (fig. 40). 
 These cells do not become spores directly, but give rise to 
 what are known as mother spore-cells lying in the interior 
 of the now much enlarged sporangium. The walls of the 
 mother spore-cells deliquesce, or become watery, forming 
 a fluid medium in which float the rapidly developing spores, 
 formed by subdivision into four of each of the mother 
 spore-cells. The upper part of the columella is much 
 enlarged, and forms a disc covering the sporangium and 
 its contained spores. The margin of the disc-like head of 
 the columella becomes continuous with the outer wall of 
 the theca, which is itself in this region greatly modified. 
 The terminal pointed portion of the theca separates as a 
 lid or operculum. At the point of junction between the 
 operculum and the theca, there is developed a series of 
 multicellular hairs, the>e^^aJla;O^which are thickened and 
 
 * 
 
110 
 
 Elementary Biology. 
 
 FIG. 4\.Polytrichum piliforme. 
 (Lantzius- Beninga.) 
 
 deep brown in colour. This 
 fringe of hairs forms the 
 peristomium. The hairs 
 are broad at their bases 
 and taper inwards, where 
 they are attached to a thin 
 plate or epiphragm, which 
 forms a temporary lid to 
 the theca after the removal 
 of the operculum. 
 
 When the spores are 
 ripe the operculum is 
 forced off by the expansion 
 of the peristomium, and 
 the spores are shaken out 
 by the swaying of the long 
 seta in the wind. Each 
 spore is a cell covered by 
 an outer brown cell-wall or 
 exosporium and an inner 
 colourless cell- wall or en- 
 dosporium, and contains 
 protoplasm, oil globules, 
 and chlorophyll. The time 
 required for the develop- 
 ment of the asexual para- 
 sitic generation varies in 
 different species, but an 
 average period is three or 
 four months. The spore 
 germinates rapidly when 
 sown on moist soil, and 
 forms a protonema. The 
 
 A, longitudinal section of theca. B, transverse general character of the 
 section : ?*/, wall of theca ; c//, operculum ; , , , 
 
 c,c, columella ; p, peristomium; c p, epi- prOtOnema has already 
 
 E^tf*3i^*S32i'4beai described (p. 104). 
 
 peristomium. 
 
MetapJiyta Polytridtum. 
 
 i ii 
 
 From this intermediate thallus the true thallus springs as a 
 lateral outgrowth. The mode of development and the 
 varieties of form observable among 
 
 i ji i FIG. 42. Fontinalis 
 
 protonemata cannot be dealt with in antipyretic*. - PERIS- 
 the present volume. The thallus ori- 
 ginates from the subdivision of a single 
 cell of the protonema, which may be 
 termed an apical cell since it, or a 
 descendant of it, retains its apical posi- 
 tion, and by its subdivision increases 
 the length of the shoot. 
 
 Having gone over briefly the struc- 
 ture and life-history of a typical moss, it may be well by 
 way of summary to follow the plan adopted at the end of 
 
 FIG. ^.Funaria kygrometrica. (Sachs.) 
 
 A. germnatng spores XSSQ); s, exosporium ; w, root hair ; v, yacuole. B, part of 
 protonema three weeks after germination ( x QO) ; h. main branch ; ^, side 
 branch ; /i, formation of a bud from which the stem of the moss arises. 
 
 the preceding sections, and give a short comparison of the 
 stages in the life-history of the moss with those of the lower 
 types already discussed 
 
112 Elementary Biology. 
 
 We have to note in the first place the comparative 
 importance of the thallus or sexual generation, which in 
 Polytrichum assumes the external appearance of, and simu- 
 lates in internal structure, an ordinary flowering plant We 
 note further the existence of an asexual generation which 
 is parasitic on the sexual generation, although the succes- 
 sion of asexual generations in Penicillium is here represented 
 by the power on the part of the sexual thalli to separate off 
 portions of themselves in the form of protonemata, which 
 are capable of producing again a sexual thallus by ordinary 
 vegetative division. We note, moreover, that the spore- 
 producing generation is a highly organised structure, and 
 that it carries distinctively a sporangium, some of the con- 
 tained cells of which form an archesporium, i.e. a group or 
 layer of cells capable indirectly of producing spores or 
 asexual cells, which, without union with any other cell, are 
 able to form protonemata and new sexual plants. The seta 
 and theca of the moss correspond, therefore, to the stalk 
 and spore-bearing head of the fungus, together with that 
 part of the mycelium which bears the sporangium. We 
 have in the moss, in short, a plant which has made the 
 most possible of its sexual stage, while the asexual plant, by 
 being parasitic on the sexual, has arrested that development, 
 and laid the foundation of a new type of plant altogether, 
 viz. the asexual spore-bearing generation foreshadowed in 
 Penicillium. 
 
Metaphyta Pteris. 1 1 3 
 
 CHAPTER VIII. 
 
 METAPHYTA VASCULAR I A. 
 
 SECTION I. FILICES PTERIS, 
 
 IN discussing the morphology and life-history of the fern 
 we are upon more familiar ground ; the various parts of the 
 adult plant, and its habit of producing much simpler inter- 
 mediate plants capable again of giving rise to the plant 
 from which it was itself developed, are all matters of common 
 knowledge. It may be well, however, to summarise that 
 knowledge in technical terminology. 
 
 Starting from the fern, commonly so called, we are able 
 to distinguish an underground stem or rhizome from which 
 roots are given off. We can, moreover, differentiate shoots 
 springing from this rhizome and fronds or leaves, usually 
 of large size, which appear above ground, and constitute the 
 visible part of the plant. On the under surface of these 
 fronds at certain seasons, and covered over and protected 
 by an inturning of the leaf, or a scale-like projection of it, 
 are to be found collections of brown granular-like bodies, 
 known as sporangia, from which can be shaken a fine dust, 
 composed of what are popularly and scientifically known as 
 spores. These spores, if sown in a suitable soil, germinate 
 and form small flattened green plants, anchored to the ground 
 by minute rootlets, and not exhibiting any differentiation 
 into the stem, root, leaves, &c. which characterised the true 
 fern. To this organism the term ' prothallus ' has been 
 applied. It will be known throughout this chapter as the 
 tnallus. Upon this thallus male and female reproductive 
 
114 Elementary Biology. 
 
 organs are developed, and from the female organ after it 
 has been fertilised the new fern springs. 
 
 Following the course we adopted in the description of 
 the moss, we ought to begin our study of the life-history of 
 the fern by an account of the thallus ; it will be more con- 
 venient, however, to commence with the product of sexual 
 reproduction, i.e. the fern so called. As already stated, we 
 distinguish in the adult fern, rhizome, roots, shoots, and 
 fronds. It will be necessary to describe these successively 
 in detail. 
 
 Rhizome. The rhizome of Pteris aquilina consists of 
 an elongated, brown, scaly body, irregularly thickened at 
 intervals where the fronds are given off. To these thickened 
 portions the term node is given, whilst the space between 
 any two nodes is naturally termed an internode. The 
 rhizome itself is covered by a scaly integument, dark 
 brown in colour save along either side, where there is a 
 lighter strip termed the lateral line. The rhizome shows 
 as a whole a difference in age from one end to the other. 
 One extremity is pointed the growing point whilst the 
 other end is thicker, darker, and apparently withering away. 
 The nodes also give rise to shoots which are successively 
 older from the pointed to the withered end of the rhizome. 
 It is worthy of note that the growing point is not bud-like 
 whilst the shoots are so. It is the second youngest shoot 
 that shows fronds above ground, the older shoots having 
 done so in past seasons, and the youngest shoot being 
 that which will take the place of the present bunch of 
 fronds in the following year. Over the outside of the 
 rhizome the leaf-scales, ramenta, or paleae have already 
 been alluded to. They must not be mistaken for the true 
 leaves whose origin they surround. 
 
 The minute structure of the rhizome is of considerable 
 importance, as in it we come face to face for the first time 
 with that great differentiation of tissues so characteristic of 
 the higher plants. If a transverse section be microscopically 
 
MetapJiyta Pteris. 1 1 5 
 
 FIG. 44. Asplenium adiantum nigrwn. RHIZOME AND PINNAE. (Maout and 
 Decaisne.) 
 
 \ 
 
 examined it will be found that externally the rhizome is 
 covered by a single layer of tabular cells, the epidermis, 
 the cell-walls of which are very thick and of a deep brown 
 
Elementary Biology, 
 
 colour (fig. 46). From this layer may be given off uni- or 
 multicellular hairs, technically called trichonies. These 
 may be simple and hair-like, but frequently take on the 
 appearance of sessile leaves (ramenta). 
 
 Enclosed by the epidermal layer on all sides lies the 
 fundamental tissue, which composes the main mass of the 
 stem, and which is modified differently in different parts of 
 the section. Immediately beneath the epidermis lies the 
 
 subepidermis, consisting of 
 
 FIG. 45. TRANSVERSE SECTION OF THE _ p _ .1 l nvpr c n f t v,j r l, W o11 Pr | 
 
 RHIZOME OF p tens aguiiina. several ia) ers or micK-^ ailed 
 
 cells, or sclerenchyma, gra- 
 dually shading off into the 
 general thin-walled paren- 
 chyma of the fundamental 
 tissue. The epidermal and 
 subepidermal cells are filled 
 only with water or granular 
 debris ; the parenchyma of 
 the fundamental tissue, on 
 the other hand, contains nu- 
 cleated granular protoplasm 
 
 a, fundamental parenchyma; l>, epider- and Starch grains closely ap- 
 mal hair; c, cortex ; d, fibro-vascular ^i.-^j f ~ t Vi~ ^ii ... O ii TJ~,.~ 
 strand ; e, sclerenchymatous strands. pll< a 10 me CCll-wail. 1616 
 
 and there in the fundamental 
 
 tissue are to be seen groups of sclerenchymatous cells, 
 with thickened cell-walls. Such sclerenchyma not infre- 
 quently takes on a horse-shoe or other characteristic shape. 
 Most important of all, however, are the fibro-vascular 
 strands, which are plunged irregularly in the fundamental 
 tissue, and are variable in size. In transverse section one 
 of these strands shows externally, and next the fundamental 
 tissue a layer of cells known as the endodermis (fig. 47). 
 These cells are formed from the fundamental tissue and are 
 dead in the sense of containing no protoplasm ; their cell- 
 walls have been modified and form a definite boundary 
 for the fibro-vascular strand. Immediately within the en- 
 
FIG. 46. TRANSVERSE SECTION RHIZOME OK Pter'is aqitilina. (Thome.) 
 It 
 
 o, epidermis ; R', thick-walled, R" thin-walled cortical cell : s, endodermis 
 s t, sclerenchyma ; x, fundamtntal parenchyma. 
 
Ii8 Elementary Biology. 
 
 dodermis lies another layer of cells, also derived from the 
 fundamental tissue, but differing from the endodermis in 
 that the cells contain starch-granules in considerable abun- 
 dance. Forming a zone of irregular thickness within this 
 
 FIG. 47. PART OF A TRANSVERSE SECTION OF FIBRO- VASCULAR STRAND 
 OK Pteris aquilina. (Sachs.) 
 
 ~~ ^ u ^f- - 
 
 P, starch-bearing parenchyma ; S, spiral vessel surrounded by proser.- 
 chyma ; gg, scalariform vessels ; s p^ sieve tubes ; b, bast ; s^, endo- 
 dermis outside the phloem sheath. 
 
 inner sheath (sometimes known as the phloem- or bast- 
 sheath) lies the phloem, or bast, one of the two essen- 
 tial constituents of the fibro-vascular strand. The phloem 
 consists chiefly of vessels, along with a small amount of 
 starch-bearing parenchyma. The cells are known as bast 
 parenchyma ; they are closely packed, elongated, and 
 
Metaphyta Pteris. 119 
 
 much crushed, the walls being considerably thickened. The 
 vessels are of peculiar form and deserve special notice. In 
 section they appear as large apertures lying internal to 
 the bast fibres, and often surrounded by the starch-bearing 
 parenchyma above referred to. Each vessel is an elongated 
 tube whose walls are riddled with minute apertures. These 
 apertures are not uniformly distributed, but are collected in 
 areas, the so-called sieve plates (see fig. 68). From their 
 possession of these curious discs the vessels of the phloeir 
 are known as sieve tubes. Enclosed by the phloem lies a 
 mass of parenchymatous tissue surrounding the vessels, and 
 collectively known as the xylem or wood. The paren- 
 chyma does not differ from specimens of that tissue already 
 described. The vessels are complicated in structure, and 
 are known as tracheae. The wall of each has laid down 
 upon it internally a layer of secondary deposit, and that not 
 uniformly, but in the shape of transverse bars. These bars 
 are joined together at their ends ; hence a series of slit-like 
 spaces are left where the primary cell-wall can be seen. 
 The appearance of such a vessel reminds one forcibly of a 
 ladder, the thickenings corresponding to rungs : hence the 
 name given to these vessels, viz. scalariform (fig. 48). 
 Associated with the tracheae are found long, narrow vessels 
 in which a spiral thickening has been laid down : these are 
 characteristic of the xylem and are called spiral vessels. 
 They are central in position (fig. 47). The structure of these 
 several elements and their relationship to each other may be 
 best made out by comparing a transverse and a longi- 
 tudinal section of a strand. 
 
 A fibro -vascular strand, then, consists of two kinds of 
 vessels surrounded by certain fibres and cells which strengthen, 
 protect, or act as padding to the vessels, viz. the vessels of 
 the xylem and the vessels of the phloem the tracheae 
 and sieve tubes respectively. These two important elements 
 are differently arranged with reference to each other in 
 different plants. In Pteris the arrangement is said to be 
 
I2O Elementary Biology' 
 
 concentric, seeing that the xylem is internal, and that the 
 phloem encloses it. 
 
 It will be necessary at this point to investigate the struc- 
 ture of the growing point of the rhizome in order to gain 
 some idea as to the mode of origin of these various tissues. 
 
 FIG. 48. Pteris aqnilina.- S - AI ARIFORM VESSEL. (Thome.) 
 
 The growing point of the stem consists of one apical 
 cell, full of protoplasm, the parent of innumerable cells 
 surrounding it, and together with them forming the growing 
 point of the rhizome These growing cells go collectively 
 by the name of meristem. The growing apex is carefully 
 protected by a series of loosely arranged ramenta, but 
 possesses no distinct cap, such as we shall find true roots 
 have. No differentiation of the cells into fibres, vessels, 
 sclerenchyma, &c. is observable near the apex. At some 
 distance from the growing point the cells will be seen to 
 become elongated, thickened, and otherwise metamorphosed 
 to form those modified cells described as composing the 
 fibro-vascular strand. 
 
 It is possible, by prolonged maceration in water, or very 
 dilute potash, to obtain a skeleton of the fibro-vascular system 
 of the fern rhizome. The skeleton forms a very perfect net- 
 work, or netted cylinder, from which smaller strands pass 
 off into the leaves. 
 
 Roots. Passing now to the roots we find that they are 
 given off from the rhizome behind the growing point, and 
 consist primarily of a group of actively dividing cells. The 
 root is covered terminally by a root-cap, which originates by 
 the subdivision of a cell segmented off from the apical 
 cell, whilst other cells, likewise formed from the apical cell, 
 become epidermis, fundamental tissue, and fibro-vascular 
 
Metaphyta Pteris. 
 
 121 
 
 strands (fig. 49). The parent roots give off lateral rootlets, 
 which spring from cells of the fundamental tissue of the 
 parent root near its apex, and before it has become differen- 
 tiated into the elements of the fibro- vascular bundles. These 
 again give origin to root-hairs like those of the moss. A 
 transverse section of a true root exhibits an appearance not 
 unlike that described for the stem, i.e. externally an epider- 
 mis, which as the root grows older may be replaced by sub- 
 
 FIG. \g. Pteris hastata.-h.VTS.-x. OF ROOT. (Naegeli and Leitgeb.) 
 
 v, apical cell ; c, o, e, tissues of the root ; k, /, m> n, tissue of the root cap. 
 
 epidermal tissue, fundamental tissue traversed by fibre-vas- 
 cular bundles, and with portions of it metamorphosed into 
 sclerenchyma. 
 
 Shoots, The leaves appear on the rhizome as small 
 buds, gradually elongating until they reach the surface of the 
 ground. They are covered by ramenta which protect the 
 young fronds from injury. Each leaf when in the bud has 
 its several parts rolled up in crozier form (fig. 44). The 
 arrangement of leaves in the bud is termed vernation or 
 praefoliation. The leaves of all plants are not arranged in 
 the same manner. Those of the ferns are said to have 
 circinate vernation. 
 
122 
 
 Elementary Biology. 
 
 FIG. 50. PINNA AND 
 PrNNULEs or A FROND 
 OF Pleris aguitina. 
 
 Frond. The leaf or frond is usually a much branched 
 structure of considerable size. It consists of a stout rachis, 
 leaf-stalk, or petiole, and in Pteris of 
 considerable length. From the petiole 
 are given off veins, and from these 
 again veinlets. These strands support 
 the flattened green lamina, which is 
 thus subdivided into pinnae and pin- 
 nules, corresponding to the veins and 
 veinlets respectively. The entire la- 
 mina is covered on both sides by an 
 epidermis of flat green cells, bound- 
 ing and enclosing loose parenchyma 
 plentifully supplied with chlorophyll, 
 starch, &c. The epidermal cells are 
 prolonged into epidermal hairs, but 
 have no intercellular spaces, with the 
 exception of the stomata. A stoma is 
 a minute aperture in the epidermis 
 bounded by two chlorophyll-bearing 
 guard cells, capable of altering the size 
 of the stoma by their contraction or 
 expansion under different hygroscopic 
 conditions of the atmosphere, and according as the frond 
 is or is not exposed to sunlight. The stomata are as a rule 
 more abundant on the under surface of the leaf (fig. 71). 
 Since the histological structure of a fern leaf is fundamentally 
 the same as that of the angiosperm leaf a detailed descrip- 
 tion of that organ is postponed (page 152). 
 
 One important structure developed on the leaf remains 
 to be described, viz. the asexual reproductive organ or 
 sporangium. At certain seasons of the year the edges of 
 the laminae will be found to be curled in towards the under 
 surface, and will be found to enclose a number of small dark 
 brown stalked bodies. These are the sporangia. In many 
 ferns, where the sporangia are produced on the under sur- 
 
Metaphyta Pteris. 
 
 123 
 
 face of the leaf more towards its centre, there are developed 
 scale-like outgrowths from the epidermis of the leaf, which 
 have for their function the protection of the sporangia before 
 they are mature. The epidermal covering is known as an 
 indusmm, and a group of sporangia enclosed by an indusium 
 is termed a sorus. In Pteris^ however, the edge of the leaf 
 forms a false indusium, whilst the term sorus must be applied 
 to the entire infolded margin and its contents. All the 
 leaves of the fern do not carry sporangia a point of some 
 
 FIG. 51. Pteris aqiiUina.- SORUS AND INDUSIUM. (Maout and Decaisne ) 
 
 Pinnule with incurved edge. 
 
 Sporangia exposed by rup- 
 ture of indusium. 
 
 importance, as we shall see subsequently. Those leaves 
 which do carry sori, i.e. the fertile leaves, are termed sporo- 
 phylla. 
 
 It will be necessary now to examine a sorus in greater 
 detail. 
 
 If a section of the margin of a sporophyllum be made 
 the sporangia will be found to be stalked capsules springing 
 from the under surface of the leaf (fig. 51). Each sporan- 
 gium is multicellular, and continuous with the epidermis of 
 the leaf, the point of origin of a sporangium being termed 
 the placenta. 
 
 The stalk of the sporangium, or funicle, is itself multi- 
 cellular, but does not contain any fibro-vascular or funda- 
 mental tissue. In its mature state it usually consists of two or 
 more rows of cells, although very young sporangia have only 
 
124 
 
 Elementary Biology. 
 
 a single linear series. The capsule consists of a single layer 
 of thin squames enclosing a space, filled in the mature sporan- 
 gium by a large number of spores. The cells of the wall of 
 
 FIG. v.AspidiicmJilix-nias. .(Sachs.) 
 
 A, section of leaf with sorus ; s, sporangia ; /. indusium. B, young sporangium ; 
 r, annulus. c, mature sporangium containing spores ; d, glandular hair. 
 
 the sporangium are modified in one region, so as to form what 
 is known as the annulus. The row of cells composing it have 
 their walls greatly thickened and are of a dark brown colour. 
 Rupture of the thin parietal cells is brought about by drying 
 
Metaphyta Pteris. 12$ 
 
 of the annular cells. The contents of the sporangium are 
 thus ejected, If one of the spores be examined under a 
 high power of the microscope it will be found to consist of 
 a minute sac filled with granular protoplasm, with a nucleus- 
 The wall is differentiated into a thin inner 
 
 i j i i , 
 
 layer or endosporium, and a thick outer layer 
 or exosporium. 
 
 Among the fertile sporangia are occa- 
 sionally to be seen barren sporangia, multi- 
 cellular club-shaped hairs which have been 
 termed paraphyses ; in other words, sporangia 
 which have not come to maturity, which have been crowded 
 out in fact, and have not had room to develop, owing to the 
 more vigorous growth of their neighbours. 
 
 As already stated, the sporangium and its contents 
 are formed from the epidermal layer of the sporophyll. 
 Its mode of origin is of importance and demands a brief 
 description in passing. The sporangium first appears as a 
 bud from an epidermal cell on that region of the leaf to 
 which the name placenta has been given. The bud becomes 
 segmented off from the epidermal cell by a transverse par- 
 tition, just as is the case with multicellular or unicellular 
 epidermal hairs. Indeed, a sporangium in the primary 
 stages of its development is morphologically a hair or 
 trichome. The unicellular trichome segments transversely 
 into a proximal cell, which gives origin to the funicle, and a 
 distal cell which gives origin to the capsule. The proximal 
 cell segments repeatedly both transversely and longitud- 
 inally to form the funicle ; the distal cell also repeatedly 
 segments, but the details of segmentation are in that case of 
 more importance. In the first place four cells are cut off 
 tangentially, leaving the central cell to form what is known 
 as the archesporium. These tangentially formed cells be- 
 come the sporangial wall by repeated subdivision perpen- 
 dicular to the surface of the sporangium. The archesporium 
 next segments off again tangentially a layer of cells which 
 
 - 
 
126 Elementary Biology. 
 
 may by subsequent division become a double layer, to which 
 the name of tapetum is given. The remainder of the 
 archesporium gives rise by subdivision to what are known 
 as the mother spore-cells, usually sixteen in number. The 
 tapetal cells do not remain long in existence as distinct cells, 
 but deliquesce and form a sort of watery jelly in which the 
 sixteen mother spore-cells float. Thereafter each mother 
 spore-cell divides into four spores, each of which in the 
 process of growth assumes the characters described above 
 as possessed by the adult spore. 
 
 The various changes which take place in the develop- 
 ment of a sporangium must be carefully borne in mind when 
 the mode of origin of the sporangia of the higher plants 
 comes to be considered. The main features in their develop- 
 ment will be found to be the same, and homologies at present 
 obscured by an old-fashioned terminology will, in the light 
 of the life-history of the fern, become at once evident. 
 
 We have now considered at sufficient length the morpho- 
 logy of the vegetative and asexual reproductive organs of 
 Pteris ; we must now follow the spore through the changes 
 which it undergoes when sown on moist, warm soil. 
 
 The first appearance of the future plant is the protrusion 
 of a minute protoplasmic bud through the exosporium. 
 This bud by growth and division becomes an elongated 
 thread, which by future division becomes a flat expansion, 
 the cells of which contain chlorophyll. Growth takes place 
 chiefly at the distal end of the thallus, and in such a manner 
 that the thallus becomes heart-shaped. Though at first 
 composed of only one layer of cells, two or more layers soon 
 make their appearance. The thallus is especially thick at 
 some distance behind the growing point, and rises in a 
 mound-like elevation on the under surface. From this 
 mound root-hairs, or rhizoids, are given off which penetrate 
 the soil and perform the same function that true root-hairs 
 perform in the spore-producing plant. From the same 
 mound are developed the female sexual organs, whilst the 
 
MetapJiyta Pteris. 1 27 
 
 male sexual organs originate in the immediate vicinity or 
 generally over the thallus. The true sexual organs of re- 
 production are constructed on precisely the same pattern as 
 those of the moss. 
 
 FIG. 54. Pteris aquilina,. DEVELOPING THALLUS. (Maout and Decaisne.) 
 
 The male organs, or spermaria (antheridia), consist of 
 extremely short multicellular hairs, composed of one central 
 cell and a covering of chlorophyll-bearing cells, originating 
 after -the manner of the sporangia of the sporophyllum. 
 The contents of the central cell become transformed into 
 sperms of peculiar form. Each sperm originates from, and, 
 
128 
 
 Elementary Biology. 
 
 until shed, is enclosed by, the cell-wall of the cell which 
 arises by division of the central cell. The sperm is a 
 spirally coiled body bearing a tuft of cilia at one end, and 
 having attached to the other a delicate sac containing cell- 
 sap and granules. The sperms according to recent re- 
 searches are produced almost entirely from the nuclei of 
 the daughter cells into which the central cell of the sperm- 
 arium divides. This discovery is quite in harmony with the 
 
 FIG. 56. SPERMARIUM OF Adi ant nut 
 capillus-veneris{x 550). (Sachs.) 
 
 IG. 55. THALI.US OF FERN (DIA- 
 GRAMMATIC). (Prantl.) 
 
 h, root-hairs ; an, spermaria ; ar, ovaria. 
 
 /, tissue of thallus ; a, wall of sperma- 
 rium ; s, sperm with attached sac, b. 
 
 conclusions of many workers in the development of the 
 sperms of animals. 
 
 Theovauum (archegonium) is not unlike the spermarium 
 in general appearance. Like the spermarium also, it is 
 formed from a bud of an epidermal cell, and is morphologi- 
 cally a trichome. As in the moss, it consists of a venter 
 and a neck, but differs from the ovarium of that type in 
 having the venter sunk in and continuous with the general 
 
Metaphyta Pteris. 
 
 129 
 
 tissue of the thallus. It thus approaches more closely to 
 the ovarium of a unisexual Fucus. The neck consists of four 
 vertical rows of cells each from four to six in number. The 
 enclosed canal contains two or more canal cells, while the 
 venter contains the large naked ovum. As in the moss, the 
 canal cells, before maturation of the ovum, become mucila- 
 ginous, and act in the first place as a wedge to force open 
 the canal mouth, and secondly, when expelled, as a trap to 
 
 FIG. <-$.Ad ; antuin capillus- 
 venerls. THALLUS AND 
 
 YOUNG FERN. (Sachs.) 
 
 FIG. 57. Pteris scrrulata. 
 OVARIUM. (Sachs.) 
 
 b, first leaf ; w'. w", roots ; 
 h, root-hairs of thallus,/,/. 
 
 catch the sperms and conduct them to the mouth of the 
 ovarium. One or more sperms touch and fuse with the 
 ovum, thus fertilising it. The fertilised ovum, or embryo, 
 soon begins to segment into a number of cells, which early 
 in their history show indications of the respective parts of 
 the mature plant to which they are to give rise. That por- 
 tion of the embryo next the bottom of the venter becomes 
 the apex of the young stem, and a peculiar organ known as 
 the foot ; the part of the embryo pointing towards the neck 
 becomes the root and first leaf. The foot acts as an organ 
 for the transference of the nourishment from the nurse-like 
 
 K 
 
130 Elementary Biology. 
 
 thallus to the rapidly growing embryo, and corresponds to 
 that which was so largely developed in the embryo of the 
 moss, namely, the seta. The primary root appears first and 
 penetrates the ground, taking on its proper nutritive func- 
 tion ; subsequently the first leaf and the apex of the stem 
 emerge, and, bending round, pass to the upper surface of 
 the thallus. The thallus, or sexual generation, having now 
 fulfilled its function, withers away, whilst the young fern 
 develops gradually by division and differentiation of cells 
 into the organism already fully described above. 
 
 It may prove by no means unprofitable if we pause at 
 this point and briefly review the phenomena of the life- 
 history of the fern, and draw attention to some of the more 
 important features viewed comparatively. 
 
 In the first place, we have to observe the growing 
 importance of the asexual generation and the waning of 
 the sexual thallus. In the moss, it is true, we have this 
 condition reversed, the thallus being the important organism, 
 whilst the asexual plant was a mere parasite on it. In the 
 fern the asexual plant is by far the more important of the 
 two. 
 
 Again, we have in the fern a perfect example of alterna- 
 tion of generations, unlike that of the fungus in being a 
 regular alternation, and differing from that of the moss in 
 that the two generations are for the greater part of their 
 lives totally independent of each other. 
 
 Once more we have to observe the gradual simplifica- 
 tion and degeneration of the sexual organs. The ovarium 
 is having its canal cells reduced in number, and the venter 
 is becoming merely a hollow in the thallus, with which, 
 indeed, its tissue is continuous. The spermarium is also 
 simpler in structure, and gives rise to fewer sperms. 
 
 It is also worth observing that the embryo fern is for 
 some time contained in the thallus which thus acts as a 
 nurse, and from which the embryo obtains nourishment by 
 means of a special organ, the so-called 'foot.' The embryo, 
 
MetapJiyta Selaginella. 1 3 1 
 
 moreover, has its parts arranged so that the primary root 
 points towards the mouth of the canal, and consequently is 
 able to reach the exterior with the minimum amount of 
 difficulty, whilst one primary leaf is developed by which the 
 embryo first obtains nourishment from the atmosphere on 
 its own account. These various peculiarities will be referred 
 to later on when we come to examine the much more 
 complicated embryology of the ' flowering plant.' 
 
 There is a large variety of ferns, most of which are 
 familiar objects to many whose tastes do not lie exactly in 
 the way of morphological and etiological research. Osmund.i, 
 Polypodium, Asplenium, Scolopendrium, Aspidium, Adiantum, 
 are all familiar names to fern collectors. There are, how- 
 ever, verv many extremely interesting allied forms, amongst 
 which botanists must look for the explanation of the mode 
 of origin of those anomalies in structure and development 
 which yet await elucidation. Such forms as Marsilea, Sal- 
 7'ifiia,Afara//ia,Sind Ophioglossum, especially claim attention, 
 but the extent of the present volume forbids more than the 
 mention of their names. 
 
 SECTION II. LIGULATVE SELAGINELLA. 
 
 In the preceding section we saw that the fern illustrated 
 very well the principle of alternation of generations in 
 plant life-history, i.e. the intervention of an asexual plant 
 between two sexual thalli. In the moss we found that this 
 alternation of generations was present, but masked by the 
 parasitism of the asexual on the sexual plant. In the type 
 we have now to consider we shall find that the converse 
 holds good, namely, that the sexual thallus is parasitic, for a 
 considerable time at least, on the asexual plant. Selagi- 
 nella, the plant commonly known as a Lycopodium or club- 
 moss in hot-houses, i.e. the asexual plant, produces spores, 
 which in their turn produce sexual thalli, only the sexual 
 thalli never leave their parent asexual plant, and, indeed, 
 
 K 2 
 
132 Elementary Biology. 
 
 develop inside the wall of the spore itself ; another and still 
 more striking example of one of the generalisations empha- 
 sised at the end of the preceding section, viz. that in the pro- 
 gress from lower to higher forms of plant life the importance 
 of the sexual thallus gradually diminishes as that of the 
 asexual plant gradually increases. Botanists are accustomed 
 to give names to these two generations. That bearing the 
 ovum is known as the oophyte, that bearing the spore as the 
 sporophyte. The oophyte and the sporophyte therefore 
 correspond to our thallus and asexual plant respectively. 
 The term gamophyte will be employed throughout in pre- 
 ference to oophyte, as taking into account both the male and 
 the female sexual organs. In the life-history of the moss, 
 therefore, the oophyte bears the sporophyte parasitic on it, 
 while in the fern, the oophyte (thallus) and sporophyte (fern 
 proper) are distinct plants. In Sf.lagirulla, the sporophyte 
 carries the oophyte for some time upon itself, and a con- 
 siderable part of its development is gone through in that 
 condition. 
 
 Sporophyte. Let us now examine the sporophyte in 
 detail, and, taking the vegetative organs first, as in the last 
 type, we will consider it under the headings of stem, root, 
 and leaf, dealing subsequently with the asexual reproductive 
 organs, or sporangia. 
 
 Stem. 'There are several striking differences between 
 the stem of Selaginella and that of Pteris. In-the first place, 
 it is almost entirely aerial and not underground. It is 
 cylindrical, long, and thin, and branches repeatedly in what is 
 known as the monopodial system, i.e. where the branch is a 
 lateral outgrowth from the principal axis, and not the result 
 of dichotomous division of the same (p. 84). The arrange- 
 ment of the branches, however, resembles that of Fucus in 
 that they are all on the same plane. The stem is very 
 slender, and consequently a considerable part of it rests on 
 the ground ; the primary part, indeed, may simulate the fern 
 rhizome in being underground. Moreover, its relationship 
 
Metaphyta Selaginella. 133 
 
 to the rhizome is further hinted at by the habit it has of 
 giving origin to rhizophores, which spring from the origin of 
 
 FIG. 59. Selaginella, FERTILE BRANCH. (Maout and Decaisne.) 
 
 secondary branches and descend to the earth. The stem is 
 green, and no doubt takes part in the function of nutrition 
 
 * K 3 
 
134 Elementary Biology. 
 
 (p. 197), in that respect assisting the leaves, which are small 
 and inconspicuous. 
 
 The microscopic structure of the stem also shows a like- 
 ness to that of the fern. Externally the stem is covered by 
 epidermis, the cells composing which are elongated and 
 contain chlorophyll grains. Beneath the epidermis are 
 several layers of closely packed cortical cells whose walls 
 are much thickened. Inside this stratum and gradually 
 merging into it lies the general fundamental parenchyma, 
 the cells of which are larger and thinner walled. Plunged 
 in the fundamental tissue are the fibro-vascular strands. 
 These differ from the strands in -the rhizome of the fern in 
 that they are surrounded by a large annular air-space, in 
 the centre of which the fibro-vascular strand is slung by 
 means of anchoring strands composed of small parenchy- 
 matous cells. The fibro-vascular strand itself is very similar 
 in structure to that of the fern, viz. a broad band of scalari- 
 form vessels with a few spiral vessels at either end of the 
 band to represent the xylem, and an enclosing layer of 
 phloem consisting of small parenchymatous cells, the whole 
 enclosed by one or more layers of phloem sheath. 
 
 Leaf. The leaves in Selaginella are of two kinds, small 
 and large. These leaves are arranged one series on one 
 side of the stem, the other bcrics on the other. The ventral 
 leaves, those lying next the ground, are the larger. The 
 leaves of both types are sessile and very simple in structure. 
 They are heart-shape, the broad base being next the stem. 
 In microscopic structure they are covered by an epidermis, 
 the cells of which contain chlorophyll granules, with stomata 
 on the under surface only. The cells composing the 
 epidermis are very similar to the general fundamental 
 tissue of the leaf, which is composed of loosely arranged 
 parenchyma with large and irregular intercellular spaces. 
 Each leaf has one fibro-vascular bundle in the form of a 
 midrib, consisting of the same elements as those constitut- 
 ing the fibro-vascular strands of the stem, only not so plentiful 
 
Metaphyla Selaginella. 135 
 
 in amount. The fibro-vascular strand of the leaf is an offset 
 from that of the stem. 
 
 Root. The roots do not require detailed mention, being 
 fundamentally the same in structure as the stem, though 
 the arrangement of the tissues of the fibro-vascular strand 
 is somewhat different. Numerous root-hairs are given off 
 from the rootlets as they enter the soil. Structures known 
 as rhizophores are developed, as already mentioned, from 
 certain parts of the stem, near the bases of branches. 
 These find their way to the ground and there give origin to 
 true roots. The rhizophores have no root-caps, but are not 
 therefore necessarily branches ; for we have in botanical 
 morphology numerous instances of true roots which have, 
 while still aerial, no root-cap, afterwards obtaining one when 
 they touch the soil. 
 
 It was stated in the last section FIG. 60. Selaginella. APEX 
 
 . OF THE STEM. 
 
 that the apex of the stem in the lower 
 
 plants consisted usually of a single 
 
 cell, known as the apical cell, whilst 
 
 the terminal growing points of the 
 
 higher plants usually consisted of a 
 
 group of cells known as primary 
 
 meristem. In Selaginella and its 
 
 allies we have numerous transitional 
 
 stages between the single apical cell and the multicellular 
 
 state. We will take an instance where the unicellular state 
 
 is retained. Even in the very terminal portion of the stem, 
 
 and not far from the apex itself, we find the young leaves 
 
 mapped out in primary parenchyma. The meristem is 
 
 formed by successive segmentations from either side of the 
 
 apical cell, each segment producing by subdivision a group 
 
 of cells forming the rudiment of a leaf. 
 
 Asexual reproductive organs. The sporangia are not 
 developed on all leaves of the sporophyte, but on certain 
 leaves towards the ends of certain branches. The fertile 
 branches have their terminal leaves modified (a) in form, 
 
136 
 
 Elementary Biology. 
 
 FIG. 61. Selaginella incEqualifolia. 
 (Sachs.) ' 
 
 and (b) in arrangement. The leaves are all the same size, 
 and are arranged so as to form a four-sided spike, or cone, 
 
 not unlike a fir cone pulled 
 out. As we shall find later 
 on, this simile is a strictly 
 correct one, for the cone 
 of Selaginella is morpho- 
 logically comparable to the 
 cone of a fir, since both 
 bear sporangia or asexual 
 organs of multiplication, 
 although terms which are 
 associated with the true 
 sexual organs have become 
 by long usage attached to 
 the sporangia of the fir 
 cone. 
 
 The sporophyll of Sela- 
 ginella is hollow and spoon- 
 like, broad at its base, and 
 rapidly narrowing to a 
 sharp point. It bears in 
 its axil the sporangium, 
 while between the leaf and 
 the sporangium, and aris- 
 ing from the base of the 
 leaf, is the so-called ligule, 
 a structure which, since it 
 encloses the sporangium 
 and is epidermal in origin, 
 
 A, fertile branch with cone. B. section of -r^t-,^!/-^ \ 11 , in/ln 
 
 cone (enlarged), showing sporangia, con- 1S morphologically an indU- 
 
 J^^5ri^* e1 *^'^ sium - Each sporangium Is 
 
 composed of a short stout 
 
 funicle and a bivalved capsule, composed of one or more 
 layers of chlorophyll-bearing cells. The sporangium appears 
 in Selaginella to originate from the stem, but that fact need 
 
Metaphy ta Selaginella. 137 
 
 not prevent us from comparing its origin with that of the fern 
 sporangium, since the leaf in Selaginella is extremely small, 
 whilst the sporangium is relatively large. Moreover in many 
 allies of the type we are considering the sporangia are borne 
 by the leaf itself. As already stated, the sporangium is, 
 when ripe, a bivalved capsule, although when young the wall 
 of the capsule is complete. On examining the sporangia in 
 more detail, we find that their contents differ from each 
 other and also from the contents of the sporangia of the 
 fern. 
 
 Before discussing the sporangia of Selaginella, it may 
 conduce to clearness if we glance briefly at the sporangia of 
 an allied type, Lycopodium. The sporangia in that type are 
 developed in the axils of the sporophylla, and spring from 
 their bases. The spores are small and rounded in form and 
 all of .the same size. They are comparable to the spores of 
 the fern, and, by one unaccustomed to the microscopic study 
 of plants, might be readily mistaken for them. Each spore 
 is composed of a mass of protoplasm surrounded by firstly 
 an endosporium, and secondly by an exosporium, which (as 
 is usually the case) is raised into spines and prominences. 
 This spore when sown is capable of producing a thallus 
 bearing spermaria and ovaria. An examination of Selaginella 
 at once exhibits to us a great advance in differentiation, for 
 although the sporangia appear all perfectly alike, yet some 
 contain what appear to be numerous small spores, like those 
 of Lycopodium^ whilst others contain a few very large spores, 
 totally different in appearance. These have been known as' 
 microspores and macrospores respectively. These terms are 
 by no means satisfactory, as we shall presently see. A ripe 
 microspore, when artificially clarified and examined under a 
 high power, shows externally a thick exosporium and thinner 
 clear endosporium, but contains, instead of granular pro- 
 toplasm, a considerable number of cells, one of which is 
 segmented off by a distinct cellulose cell-wall. The rest of 
 the cellular mass consists of small naked cells, which are 
 
138 
 
 Elementary Biology. 
 
 apparently in process of transformation into sperms. Simi- 
 larly when the so-called ripe macrospore is examined it is 
 found to consist of a very considerable collection of cells 
 enclosed in an endo- and exosporium, which latter has three 
 very well marked ridges on one surface. The cellular tissue 
 sho -vs more or less distinct demarcation into two areas ; the 
 
 FIG. 62. STRUCTURE OF THE SPORES IN Selaginclla. (Pfeffer.) 
 
 3 
 
 i, 2, 3, ovospore of .$". martens IL A, D, spermospore of S. caulescens. 
 
 upper of which, that is to say the part nearest the point of 
 union of the three ridges, is termed the thallus, whilst the 
 lower part has been termed the endosperm. In the thallus 
 are found three or four funnel-shaped depressions, leading 
 into minute cavities, which contain naked protoplasmic cells. 
 These cells are on further examination found to be com- 
 
Metaphyta Selaginella. 1 39 
 
 parable to the cells found in the cavity of the ovarium of the 
 fern thallus. There are in each cavity one large basal cell? 
 or ovum, and one or more cells filling up the neck, i.e. canal 
 cells. We have no option, then, but to consider this so- 
 called macrospore as a thallus which is developing its ovaria 
 within the spore-wall. The entire second or sexual gene- 
 ration has remained hidden in and protected by the spore- 
 wall. What shall we say then of the microspore viewed in 
 the light of this discovery ? Simply that here, too, the thallus 
 has never left the spore-wall, but has first of all divided 
 into a vegetative cell, probably of no service save to guide 
 us towards an explanation of this anomaly, and a number 
 of reproductive cells which at once proceed to form sperms 
 without forming spermaria at all. 
 
 It appears, therefore, that we have in the spores of Seia- 
 ginella a great advance on the condition of affairs in the 
 fern. In the first place we find two kinds of thalli instead 
 of one, and these ihalli are unisexual instead of herm- 
 aphrodite. How is this to be explained ? Probably in terms 
 of a law which we shall find of supreme importance in higher 
 plants, namely, the law of cross-fertilisation. It is an ad- 
 vantage for a plant to be fertilised by male elements derived 
 from another plant than itself. In the fern, because the 
 thallus was a comparatively large organism and because it 
 obtained its nourishment from the environment, probably 
 the protoplasm of the thallus was sufficiently differentiated in 
 the region of the ovaria from the protoplasm in the region 
 of the spermaria to allow of the invigorating effect of inter- 
 crossing being thus obtained. Possibly future experiments 
 may show that true cross-fertilisation is as helpful t6 the 
 Algae and the Non-vascularia as it is in the case of the 
 flowering plants ; indeed there are numerous observations on 
 record pointing to this conclusion. In Selaginella, since the 
 thalli are not independent living plants, we have male and 
 female elements produced in different thalli, which are pro- 
 duced from different spores, instead of being developed, as 
 
140 Elementary Biology. 
 
 they might well be, on the same thallus inside the same 
 spore-wall. 
 
 Again, how must we account for the disappearance of 
 the thallus which ought to have been formed by the micro- 
 spore, and the retaining of the thallus formed by the 
 macrospore ? Obviously when the cells which will produce 
 sperms have been formed there can be no further use for a 
 thallus bearing them only ; whilst not only has the thallus 
 producing ovaria to give origin to sexual cells, but it has also 
 to nourish for a time the embryo that results from the ferti- 
 lisation of the ovum. The thallus, in short, has to act as 
 nurse to the fertilised ovum it has itself given birth to. 
 Hence its persistence for a variable period dependent on the 
 time taken by the embryo to develop. Moreover, we saw 
 that the tissue in the interior of the macrospore is divisible 
 into what has been termed thallus and endosperm. This 
 latter would apparently correspond to the purely vegetative 
 portion of the thallus of a fern, whilst the thallus proper is 
 that portion which gives rise to the ovaria. It is possible, 
 however, that the endosperm is an altogether new formation 
 which has arisen during the gradual assumption of para- 
 sitism by the sexual generation, and for the special purpose 
 of affording nourishment to the embryo during its develop- 
 ment whilst inside the spore-wall. 1 
 
 1 Although speculations without detailed proofs are in many cases 
 looked on with suspicion (and rightly so), yet they do so much towards 
 relieving detailed description from the charge of dulness that no further 
 apology is given for their introduction here. In the present instance, 
 when one considers the fact that the unisexual thallus, whether formed 
 from a microspore or a macrospore, must have, at one time in phylo- 
 genetic history, been a hermaphrodite structure, and that the spermaria 
 are developed on a different part of the thallus from the ovaria ; that, 
 moreover, the presence of root-hairs on the exposed ovaria-bearing 
 portion of the enclosed thallus of Seiiginella clearly points to that being 
 the morphologically ventral surface of the thallus, one cannot but sus- 
 pect that the endosperm really corresponds to the spermaria-bearing 
 portion of the hermaphrodite thallus. The division of the nucleus of 
 
Metapliyta Selaginella. 141 
 
 We have yet to describe briefly the mode of development 
 of the sporangia and their contents, and to follow the 
 development of the embryo into the young Selaginella. A 
 word on the relation of Selaginella to the pine group of 
 plants will complete the section. 
 
 It is needless to describe in detail the development of 
 the sporangium, since it is, broadly speaking, similar to that 
 of the fern. A tapetal layer is formed, which subsequently 
 becomes absorbed. The archesporium also forms mother 
 spore-cells, capable of developing microspores or macro- 
 spores. It would be preferable to distinguish these spores 
 by terms indicative rather of their sex than of their size. The 
 microspore is a spore capable of forming a male thallus, 
 hence it might be termed a spermospore, whilst the macro- 
 spore might be termed an ovospore, an unavoidable hybrid, 
 since the word oospore has already been given to the fer- 
 tilised ovum or embryo. 1 
 
 The development of the spores up to this stage is alike 
 for both sexes, but here their development diverges. The 
 mother cells of the spermospores are numerous, and each 
 subdivides into four daughter cells which become the 
 spermospores in question ; the mother cells of the ovo- 
 spores are also numerous, but only one of these subdivides 
 into four daughter cells which become the four ovospores of 
 the mature sporangium, the other mother cells remaining 
 undeveloped. 
 
 After fertilisation of the ovum by a sperm the ovum 
 rapidly segments, forming a pro-embryo, which differentiates 
 
 the embryo-sac of the angiosperm (p. 167), and the subsequent union 
 of a portion of one daughter nucleus with a portion of the other, might 
 on this view be looked upon as a species of cross- fertilisation (resulting 
 in the more vigorous formation of endosperm) of a female nucleus by a 
 male nucleus, rather than a union of two female nuclei as suggested 
 by Marshall Ward. 
 
 1 The term macrospore is even etymologically incorrect, since 
 naKpbs means ' long,' not 'large.' Megaspore would be more correct 
 if we must use these tei 
 
142 Elementary Biology. 
 
 into a portion known as the suspensor and the embryo 
 proper. The suspensor forms the upper portion of the pro- 
 embryo, that is, the part next the canal cells. The suspensor 
 by increase in length pushes the embryo proper deep into 
 the endosperm or vegetative part of the thallus. There the 
 mapping out of the embryo into cotyledon (or seed-leaf), 
 stem, and root takes place before its independent existence 
 outside the spore-wall is established. During the embryonic 
 period, as already explained, the thallus acts as nourishment 
 to the young plant. 
 
 There are many points of resemblance between the 
 structure and life-history of Selaginella and the Gymno- 
 spennce (pine group), which point to their being transition 
 forms between the Selaginellida and the Angiosperma. 
 There is no difficulty in tracing the relationship between 
 Selaginella and such a type as Pinus, notwithstanding the 
 tremendous difference in size ; for in past ages of the earth's 
 history closely allied forms have flourished which were even 
 more gigantic than the largest pines, the spores of which 
 form no small part of the coal of ordinary use. We have 
 already drawn attention to the likeness between the cones 
 of the two forms, and the structure of the sporangia also 
 shows no great morphological differences. The anatomy of 
 the stem, however, is much closer in its nature to that of the 
 flowering plant than to that of Selaginella ; but beyond this 
 general statement further exposition of their agreements and 
 differences must be omitted here. 
 
 Lastly, and by way of summary, we must emphasise the 
 fact that we have in Selaginella true alternation of generations 
 just as in the fern, only here the sexual generation is para- 
 sitic on, or at least commensal with, the asexual. If we were 
 to attempt to represent this relationship diagram matically we 
 might do so thus : 
 
 """;st"(J i X ST J 
 
 AST< + ; : AST< + 
 
 \8 T 2 H X S T ? 
 
 -- ! 
 
MetapJiyta Liliu-iii. 143 
 
 where AST stands for asexual generation, and S T for sexual 
 thallus. 
 
 SECTION III. MONOCOTYLEDONES LILIUM. 
 
 We now reach that point in our examination of the mor- 
 phology of plants when it is necessary to consider what are 
 popularly known as the flowering plants. For that purpose 
 we shall devote our attention to two forms which illus- 
 trate well the characters of that group. A thorough grasp 
 of the preceding section, where we examined Selaginella in 
 detail, will help us considerably towards understanding the 
 at first sight very different organisation of the types now 
 before us. 
 
 In the first place we meet with what appears to be a 
 perfectly new development in the morphology of plants, 
 namely, the flower. That structure, however, turns out on 
 closer examination to be simply a cone the constituent leaves 
 of which have become variously modified, some coloured, 
 some remaining green, and others, i.e. those bearing the 
 sporangia, which are themselves considerably modified, very 
 much altered in shape and microscopic structure. We find 
 in the second place that certain of the sporangia those 
 which produce ovospores do not shed their contents, but, 
 covered over by the leaf on which they were produced, form 
 with the sporophyll itself what is commonly known as the 
 fruit. Indeed, we might briefly state the case thus : while 
 the fern and Selaginella shed their spores, a flowering plant 
 sheds its sporangia and their contents, and very often the 
 sporophylla also. 
 
 Again, we find that there are many points of difference, 
 or, rather, advance in the structure of the fibro-vascular 
 strands, and in their mode of arrangement both in the stem 
 and in the root. The leaves also are found to possess, what 
 we may term, higher organisation, in virtue of there being 
 greater differentiation of parts and more division of labour. 
 The Angiospermcz, as the plants we are now considering 
 
144 Elementary Biology. 
 
 are termed, are divided into two large groups, the Monocotyle- 
 dones and the Dicotyledones. As an example of the first 
 group we may take the lily or hyacinth ; as an example of 
 the second, the buttercup. The former we shall discuss in 
 this, the latter in the following, section. 
 
 First of all, it may be well to point out several obvious 
 points of distinction between these two plants, which are 
 easily observed and appreciated on the basis of the know- 
 ledge of plant structure we have already gained. 
 
 A section of a stem of Lilium shows what we know now 
 as fibro- vascular strands scattered apparently irregularly 
 through the fundamental tissue, whilst a similar section 
 through the stem of Ranunculus (buttercup) shows the 
 strands arranged concentrically round the centre, which is 
 either hollow or occupied by a mass of fundamental tissue 
 known as pith. Again, the strands in Lilium are crowded 
 towards the surface, whilst in Ranunculus there is a toler- 
 ably thick layer of cortex between the outer edge of the 
 strands and the epidermis. Further, when we examine the 
 ordinary foliage leaves the veins in Lilium are found to run 
 parallel to one another, whilst those in Ranunculus start at 
 various angles from a central midrib, and the veinlets branch 
 and anastomose, or form a network over the entire lamina. 
 Lastly, when we look at the cone or flower, we discover that 
 although the leaves composing it are very different in 
 colour, shape, and microscopic structure from the leaves 
 which go to form a fir cone or a Selaginella cone, yet they 
 are, broadly speaking, the same in general character in Lilium 
 and Ranunculus, but differ in their arrangement on the 
 cone-axis in these two types. In the lily they are obviously 
 arranged in circles of three each, whilst the outer leaves at 
 least in the buttercup are arranged in circles of five. These 
 flowers are therefore said to belong to the trimerous and 
 pentamerous types respectively. There are other points of 
 distinction between the two forms we have selected, but 
 these will be referred to later on. 
 
Metaphyta L ilium. 1 4 ; 
 
 Still continuing our general survey, we may observe that 
 in the flower or cone of these angiosperms we have far fewer 
 sporophylla than in the cone 
 
 . , . 77 . . . .. FIG. 63. Nymphffa alba. TRANSITION 
 
 Of Selagintita, but this dlS- FROM PETALS TO STAMENS. (Maout 
 
 tinction is bridged over by /s !^ d D 
 the flowers of such exam- 
 ples as the water-lily, a plant 
 which, despite its name, is 
 a far nearer relation to the 
 buttercup than to the true 
 lily. Again we have to note 
 that some of the sporo- 
 phylla are barren, namely, 
 those which are coloured 
 and lower down m the cone. 
 Were space a matter of no object, numerous intermediate 
 conditions might be referred to which would help to con- 
 vince us, if that were necessary, that in the lily and buttercup 
 we have to deal with plants in no respect distinct in nature 
 and life-history from those we have already considered in 
 the preceding sections. 
 
 Stem. The stem of Lilium y like that of the fern, is 
 underground, but from its peculiar shape it receives the 
 name of bulb. It is difficult at first sight to see exactly 
 where the stem proper is. If a section be made of the bulb 
 (e.g. of an onion or hyacinth) it will be seen to consist of two 
 parts a disc shaped lower portion, from the under-surface 
 of which the roots spring, and an upper portion which is 
 discovered to be composed of the thickened bases of the 
 closely-packed leaves. The typical stem we have already 
 seen consists of nodes, from which the leaves spring, and of 
 internodes between the nodes. In the bulb the internodes 
 are absent, and the nodes are closely crushed together, so 
 that the stem is exceedingly short and reduced to a mere 
 disc- shaped mass, from which the leaves spring. The only 
 part of the stem remaining of tolerable length is the terminal 
 
 L 
 
146 
 
 Elementary Biology. 
 
 FIG. 64. Attiiim cepa. BULB. 
 (Edmonds.) 
 
 shoot, which is elongated into a long axis on which the 
 flower is borne. Between the bases of the leaves we may 
 find buds or branch shoots about to give forth another floral 
 axis in the next season. The likeness to the underground 
 rhizome of the fern is thus again brought up. 
 
 The floral axis presents a 
 more suitable subject for ex- 
 amining the structure of the 
 monocotyledonous stem than 
 the modified underground bulb. 
 If a section be made across it 
 it will show externally a colour- 
 less epidermis, enclosing a large 
 soft fundamental parenchyma 
 in which run the fibro- vascular 
 strands (fig. 65). The mode of 
 origin of these we shall notice 
 later on ; meantime it will be 
 sufficient to draw attention to 
 the fact that the smallest strands 
 are placed close to the epider- 
 mis, while they increase regu- 
 larly in size as the centre is 
 approached. There cannot be 
 said to be any distinct pith, that is, fundamental tissue left 
 unmodified in the centre of the stem, for there is no clear 
 line of demarcation between the fundamental tissue of the 
 centre and that between the several strands. The course of 
 the fibre-vascular strands in the stem may be best studied in 
 a longitudinal section, such as that represented at fig. 65 B. 
 There it will be seen (in an allied type) that the strands 
 pursue a wavy course, arising close to the outside of the 
 stem, curving inwards, and, after recurving outwards again 
 and crossing the strands higher up, pass out entirely into 
 the base of a leaf. These fibro- vascular strands when fur- 
 ther examined are found to contain elements similar in 
 general structure to those described in the stem of the fern, 
 
 a, a, bu ds ; b, stem proper 
 which roots originate. 
 
 from 
 
Metaphyta L ilium. 
 
 but differing in certain points of detail The tissue of the 
 strand is confluent with the fundamental parenchyma, but 
 the cells bounding the strand are much smaller and thicker 
 walled: these cells are elongated and form a prosenchy- 
 matous sheath to the strand. Towards the outer side of the 
 strand, i.e. that next the surface of the stem, immediately 
 
 FIG. 65. MOXOCOTYLEDONOUS STEM, (Maout and Decaisne.) 
 
 A. Diagrammatic transverse section. 
 
 B. Diagrammatic longitudinal section. 
 
 inside the prosenchymatous sheath, 
 is found a mass of tissue which cor- 
 responds to the phloem or bast of 
 the fern strand, and goes by that 
 name. It consists of sieve-tubes, 
 long, thick- walled prosenchymaand 
 thin-walled parenchyma. Next the 
 inner side of the strand lies the 
 xylem, or wood, which also consists 
 
 of three elements, tracheides, or vessels with dotted walls, 
 one or more spiral and annular vessels, a quantity of pro- 
 senchyma, and a little parenchyma. We have already 
 briefly glanced at the general structure of the cells compos- 
 ing these several types of tissue. As we shall, however, 
 require to use these terms constantly in the remainder of 
 this and in the succeeding sections it may be well to give 
 at this point a few more details regarding them. 
 
 L 2 
 
148 
 
 Elementary Biology, 
 
 Parenchyma. The parenchyma of the xylem and 
 phloem is mainly important as presenting us with the re- 
 
 FIG. 66. TRANSVERSE SECTION OF FIBRO-VASCULAR STRAND OF Zeamait. 
 (x 5 oo). (Sachs.) 
 
 \ 
 
 \ 
 
 /, fundamental parenchyma of a, outer and /, inner part of the stem, en- 
 closing prosenchyma ; g; g, pitted ducts ; ,y, spiral vessel ; r, annular 
 vessel ; /, air cavity ; v, v t phloem. 
 
 mains of the primitive cambium or growing tissue from 
 which the entire strand was originally formed. 
 
Metaphyta L ilium. 
 
 149 
 
 Prosenchyma. The prosenchyma represents parenchy- 
 matous cells which have been elongated in a fusiform 
 manner (usually) in one direction (fig. 67). Their walls 
 may become thickened either by apposition, i.e. the deposi- 
 tion of new particles of cellulose on the inner surface of the 
 cell-wall, or by intussusception, i.e. the intercalation of 
 similar particles between those of the primary cell-wall. 
 Possibly both methods of cell-wall thickening may take 
 place simultaneously. Chemical 
 changes usually take place in the 
 wall at the same time. Secondary 
 thickening may take place regu- 
 larly ; in that case we have a uni- 
 formly thickened prosenchymatous 
 cell of the type represented at fig. 
 66. In other cases we may have 
 parts of the primary cell-wall left 
 uncovered, when a pit-like depres- 
 sion results (fig. 67). In other 
 cases still we may find that the 
 secondary thickening has been laid 
 down (in this case by apposition) 
 in the form of a spiral band or 
 annular rings upon the primary 
 cell-wall. These varieties of cell- 
 wall thickenings are commonest 
 amongst the next type of tissue. 
 
 Cell-fusions, vessels. Vessels 
 result from the fusion of two or more elongated cells 
 arranged end to end. The adjacent walls have become 
 completely broken down, or the walls may become perfo- 
 rated by a series of minute apertures through which the 
 contents of the one cell may communicate with the con- 
 tents of the other : good examples of this type of vessel 
 are found in the sieve-tubes of the phloem (fig. 68). 
 It is probable, according to recent researches, that this 
 
150 
 
 Elementary Biology. 
 
 intercellular communication occurs in a great number of 
 tissues besides the sieve-tubes. Among the vessels of the 
 wood, as already stated, a great variety of types exist, known 
 
 as sftiral. annular. 
 
 FIG. 68. SIEVE-TUBES FROM PHLOEM OF Cucurbita 
 
 (Sachs.) reticulated, scalari- 
 
 forrn, and other ves- 
 sels. 
 
 It is important to 
 notice, in regard to 
 the fibro - vascular 
 strand of a monoco- 
 tyledon, that there is 
 no growing tissue 
 present it has all 
 become permanent 
 tissue. The strand 
 is incapable, there- 
 fore, of growing larger 
 when once formed ; 
 the strand is there- 
 fore said to be closed. 
 We shall find this an 
 important point of 
 distinction between 
 the monocotyledo- 
 nous and the dico- 
 tyledonous type of 
 stem, in which latter 
 growing tissue or 
 cambium is present, 
 
 q, transverse section of a sieve ; sL sieve on side 
 wall ; x, pits on face and /, on side view ; ps, 
 protoplasm contracted from the cell-wall ; z, in- 
 termediate prosenchyma. 
 
 and where the fibro- 
 vascular strands are 
 therefore known as 
 
 open strands, i.e. capable of further growth. 
 
 Hoot. Passing next to the root, we find that the struc- 
 
 ture differs considerably from that of the stem, not in the 
 
Metaphyta L ilium. 151 
 
 character of the tissues present, but in their arrangement. 
 It is easy to make out the fibro-vascular strands grouped 
 together and forming a perfect cylinder in the centre, and 
 surrounded by the vascular sheath or layer of fundamental 
 tissue next to the cylinder. Outside the fibro-vascular 
 cylinder is a very thick cortex (fig. 69). The epidermis 
 soon disappears, and its place is taken by the outer layers 
 
 FIG. 69. TRANSVERSE SECTION- ROOT OF Acorus calamus. (Sachs.) 
 
 PP 
 
 s, fibro-vascular sheath, surrounded by cortical parenchyma and external to 
 the pericambium ; pp, xylem ; ph, phloem ; g, g, p, p, vessels of xylem. 
 
 of the cortex. Within the sheath, we again find points of 
 difference from the stem in the arrangement of the various 
 parts of the strands themselves. Immediately internal to 
 the vascular sheath is a layer of cells known as the peri- 
 cambium, or phloem-sheath, which encloses the strands. 
 The bast and wood of the strand, however, are not arranged 
 
152 Elementary Biology. 
 
 so that the bast is outside and the wood inside, but so that 
 the two portions lie alternating with each other. Such an 
 arrangement is known as a radial strand. 
 
 The roots, as a whole, originate from the base of the 
 flattened stem in the form of long branched or unbranched 
 filaments, each terminated by a root-cap. The cap may be 
 very well studied in the aquatic roots of the duckweed, an 
 aberrant monocotyledon which exhibits a root-cap not dif- 
 fering essentially in this respect from that of the monocoty- 
 ledon. The roots give origin to rootlets and root-hairs by 
 which the absorption of food from the soil is carried on. 
 
 Leaf. The leaves of the lily are peculiar in being 
 sessile ; their bases are generally the widest parts, and, in 
 the case of the outer leaves of the bulb, enclose the entire 
 
 FIG. 70. ROOT-CAP OF Lemna minor. 
 
 6. 
 
 d 
 
 a, root-cap ; c, epidermis of the root ; b, growing cells. 
 
 stem. The fibro- vascular strands, as already stated, are 
 arranged in a parallel manner, the veins running without 
 branching from end to end of the leaf or merging into a 
 vein which runs round the margin of the leaf. The micro- 
 scopic structure of the leaf must now occupy our attention 
 for a moment. Superficially the leaf is covered by an 
 epidermis composed of flat tabular cells. The upper epi- 
 dermis has numerous stomata; these are, however, much 
 more numerous on the under epidermis (especially of dico- 
 tyledonous leaves). The guard-cells are, as a rule, smaller 
 than the general epidermal cells, and are concavo-convex 
 in outline, the space between being the stoma. The guard- 
 cells contain chlorophyll, which is absent from the other 
 epidermal cells. The stoma leads into a large intercellular 
 
Metaphyta L ilium. 
 
 153 
 
 space in the fundamental tissue of the leaf. This funda- 
 mental tissue is composed of chlorophyll -bearing cells, 
 which are, next the epidermis, arranged in one or more 
 regular layers and termed palisade parenchyma, whilst to- 
 wards the middle of 
 
 FlG> 7I '~ A ' EPIDERMIS OF Leucojum Ternum. 
 B. TRANSVERSE SECTION OF EPIDERMIS OF 
 
 tV>A 
 
 form What is known ********* umbellatus. (Behrens.) 
 
 as the mesophyll, 
 
 or spongy paren- 
 
 chyma. Through 
 
 the mesophyll run 
 
 the fibro-vascular 
 
 strands, which are 
 
 similar in structure 
 
 to those of the stem. 
 
 The xylem lies next 
 
 the upper surface of 
 
 the leaf, since that 
 
 is morphologically 
 
 the side next the 
 
 centre of stem. 
 
 From either epider- 
 
 mis (though not in 
 
 that of the type we 
 
 are considering) 
 
 arise epidermal 
 
 hairs, which are 
 
 prolongations of ep- 
 
 idermal cells (fig. 
 
 72). Such hairs are 
 
 termed unicellular 
 
 when their cavities 
 
 are continuous with 
 
 those of the cells of which they are prolongations. Fre- 
 
 quently, however, these hairs become multicellular by the 
 
 subdivision of their cavities into chambers. A number of 
 
 s, guard-cells containing chlorophyll grai 
 1 cells ; k, nuclei ; h, 
 
 cuticle ; e, epidermal 
 /, general parenchyma. 
 
 ins, a ; c, 
 air-space ; 
 
FIG. 72. FORMS OF HAIRS. (Thome".) 
 
 A, unicellular hair of Pelargonium ; B, multicellular hair of Geranium : c, 
 glandular hair of nettle ; D, prickle of hop ; E, glandular hair of Lamium. 
 
Metaphyta L ilium. 
 
 155 
 
 hairs may become matted together, giving origin to a prickle, 
 as in the hop, whilst in the nettle the leaf is covered with 
 multicellular hairs, with the contents of the terminal cells 
 modified into a substance capable of producing considerable 
 irritation should its end be broken off in the skin. These 
 may be termed glandular hairs. 
 
 Before passing to the consideration of the sporophylla, 
 we must glance at the nature and mode of origin of certain 
 substances which we have had occasion frequently to men- 
 tion in reference to many points in our past survey, and 
 which may conveniently be discussed at this point. They 
 are substances found in the cells of plants, such as starch, 
 inulin, aleurone, crystalloids, crystals, and such-like ; chloro- 
 phyll we have already discussed (p. 38). 
 
 FIG. 73. STARCH GRANULES OF Solatium tuberosum. (Thom6.) 
 
 Starch. Starch is one of the most important of the 
 derivatives of plant protoplasm. It is a compound of 
 extreme abundance, some parts of plants, such as the potato 
 tuber, consisting almost entirely of starch. Starch occurs 
 in the form of granules, which possess a characteristic form 
 for different plants. We may take those of the potato as 
 a type. Each granule is ovoid in shape, and presents a 
 usually eccentrically placed point known as the hilum 
 round which the starch is arranged in a number of layers, 
 alternating with thin films of water. The hilum is the 
 
156 
 
 Elementary Biology. 
 
 most watery part of the granule. Starch has the same, com- 
 position as cellulose (C 6 H IO O 5 ), but a different constitution. 
 The granule is believed to consist of a skeleton of cellulose, 
 
 FIG. 73 A. -STARCH GRAINS UNDER THE POLARISCOPE. (Vines and Dippel.) 
 
 ha. (Thome.) 
 
 the interstices of which contain the starch proper, or granu- 
 lose. The physiological importance of starch, and of the 
 other differentiation products of protoplasm about to be 
 mentioned, will be referred to in the 
 - next chapter, where a general sketch 
 w m j^ gj ven o f t ^ e more important 
 phenomena in the physiology of plants. 
 Inulin, a chemical ally of starch, is 
 found abundantly in the roots of many 
 plants (e.g. the daisy tribe). It however 
 differs from starch in that it exists in 
 solution in the cell-sap, from which it 
 may be precipitated by the action of 
 alcohol, in the form of crystalline 
 spheres, usually adhering to the cell- 
 wall (fig. 74). 
 
 Aleurone is to be considered as a 
 proteid reserve material (fig. 75). It is usually found in the 
 form of grains which lie embedded in an oily matrix. It 
 
MetapJiyta L iliu m. 
 
 157 
 
 also occurs in a semi-crystalline form with which are asso- 
 ciated minute rounded masses of a double salt of magne- 
 sium and calcium termed globoids. All these bodies have 
 a nutritive value, to which reference will afterwards be 
 made. 
 
 True crystals are also of common occurrence, especially 
 in the cell-sap. They are most commonly composed of 
 oxalate of lime, appearing in the form of short prisms or 
 
 FIG. 75. ALEURONE. (Sachs.) 
 
 A, seed of Pisnm sativum ; a, granular form of aleurone ; ?, intercellular 
 bpace; st, starch grains ; B, seed of Ricinns com mum's, crystalline form. 
 
 long and slender needles known as raphides. Calcic carbo- 
 nate also occurs, but more rarely, and only, at least to any 
 extent, in a few groups of plants. Club-shaped masses of 
 calcic carbonate, pendent in certain cells of the leaf (e.g. of 
 Ficus elastica\ have been called cystoliths (% 77). 
 
 The reproductive organs. We have already likened 
 the flower of the lily to a fir cone, and emphasised the mor- 
 phological identity of these two structures. We notice 
 
158 
 
 Elementary Biology. 
 
 however, that the flower is much depressed, the diameter 
 (short axis) of the cone being as great if not greater than 
 the length (long axis). The examination of the flower of the 
 
 FIG. 76.-CRVSTAU, ( A) Sac%s;n, Thome.) ^ter-lily, Or of MyO- 
 
 sums, close allies of the 
 buttercup, exhibits to us 
 a condition of things 
 where even the external 
 likeness between the 
 flower and the fir cone 
 is unmistakable. The 
 sporophylla are much 
 reduced in number in 
 Lilium ; and further ex- 
 amination proves to us 
 that that number is defi- 
 nite and constant for 
 the whole group to 
 which the lily belongs. 
 We find that there are 
 nine fertile sporophylla 
 in all, six of which are male and three 
 female. Surrounding the fertile sporo- 
 phylla are six barren leaves, which differ 
 from the ordinary foliage leaves in being 
 coloured yellow, blue, red, or some other 
 hue. These leaves form together what 
 is known as the perianth. They are 
 arranged in two whorls, the outer being 
 crystals of calcic oxa- termed the calyx, the inner the corolla. 
 !&TJ5SBd5 <rte leaves composing the calyx and 
 bom AM retusa. coro lla go by the name of sepals and 
 petals respectively. The sepaline and petaline leaves differ 
 in several points from the ordinary foliage leaves. In the 
 first place they are coloured, but not green. An exception 
 to this occurs in the sepaline whorl, where the outer surface 
 
Metaphyta L iliu m. 
 
 159 
 
 of the sepals is frequently green. Amongst the dicotyledons 
 the calyx is generally green, although endless variations 
 
 FIG. 77. CYSTOLITH FROM THE LEAF OF Ficns 
 elcutica. (Thom6.) 
 
 FIG. 78. VERTICAL SECTION OF FIG. 79. FLOWER AND OVARY (IN SECTION) 
 FLOWER OF Myosurus. (Maout OF Lilinm album. (Allen.) 
 
 and Decaisne.) 
 
 occur. The colouring matter is found to be in the form of 
 crystals or granules, or simply diffused in the epidermal 
 
160 Elementary Biology. 
 
 cells. In the second place the shape of the perianth leaves 
 differs markedly from that of the ordinary leaf type. We 
 cannot go into the discussion of the variation in the mor- 
 phology of the perianth leaves, but must content ourselves 
 with saying that their form is as various as their hue. In 
 the lily they do not present shapes differing much from the 
 tapering form assumed by the foliage leaves. 
 
 The arrangement of leaves on the stem has been con- 
 sidered of some im- 
 
 FIG. 80. FLORAL DIAGRAM OF Liltum. 
 
 (Alien.) portance by botan- 
 
 ists, and its value 
 cannot be exaggerated. 
 There can be no doubt 
 that primitively the 
 mode of arrangement 
 of the perianth leaves 
 and sporophylla on the 
 floral axis must have 
 shown some likeness to 
 the mode of arrange- 
 ment of leaves on the 
 ordinary branch, just 
 
 ,, I, a, 3, sepals ; /, I, 3 , 3,. petals ; ,, ., 2 , 3, ^ ^ arrangement Q f 
 
 outer stamens ; 
 
 2 ' 3 > car P els - the sporophylla in Sela- 
 
 ginella resembles that of the foliage leaves in the same type. 
 Owing, however, to the great shortening and condensation 
 which the flower axis has undergone (compare the bulb), 
 the likeness has become in most cases completely oblite- 
 rated. We are compelled, therefore, to discuss the arrange- 
 ment of the floral leaves (anthotaxis) separately from that 
 of the foliage leaves (phyllotaxis). The methods of ar- 
 rangement in both cases may be very well studied in the 
 foliage or floral bud. 
 
 The study of the relationships between flowers is greatly 
 aided by the use of what are known as floral diagrams. In 
 such a diagram the various perianth leaves and sporophylla 
 
Metaphyta L ilium. 
 
 161 
 
 are represented in ground plan. The floral diagram of a lily 
 is represented at fig. 80. There the sepals are shown as 
 forming an outer whorl of three parts, the petals as forming 
 an inner whorl alternating with the parts of the calyx. 
 Within these two whorls we find the nine sporophylla in 
 three whorls, all alternating with each other and with the 
 members of the perianth whorl. Notwithstanding the 
 extreme dissimilarity of structure between the sporophylla 
 and the perianth leaves we have no difficulty in deciding 
 that both belong to the phyllon type, not only from this 
 
 FIG. 81. STAMEV. (Semi- 
 diagrammatic.) 
 
 FIG. 82. POLLEN GRAINS OF 
 Cnpressns sempennrens. 
 (Thome\) 
 
 II 
 
 The anther has been sectionised to 
 show the loculi. 
 
 t, a, entin ; />, intin ; II, forma- 
 tion of the pollen tube, c. 
 
 arrangement, but also from the study of such flowers as those 
 of the water-lily, where we find a gradual transition from 
 the sepals to the petals, and from the petals to the sporo- 
 phylla (fig. 63). 
 
 The sporophylla, as we have already seen, are of two 
 kinds, male and female. The male sporophylla form the 
 two outer whorls of three each, and are known as stamens; 
 the three central female sporophylla are known as carpels. 
 
 M 
 
1 62 Elementary Biology. 
 
 Each stamen consists of a petiole, or filament, and a 
 club-shaped head, the anther (fig. 81). The anther itself 
 consists of a centrally placed continuation of the filament 
 known as the connective, which is, however, nothing more 
 nor less than the midrib of the sporophyll, and four elong- 
 ated bodies, two on either side of the midrib, all of them 
 pointing towards the centre 6f the flower (introrse). These 
 bodies are the sporangia which are sessile in the great 
 majority of angiosperms. The sporangia are therefore, in 
 the lily, developed not in the axilla, but on the edge of the 
 sporophyll, towards its upper surface. The spermospores 
 fill the interior, or loculus, of the sporangium, and are 
 known when ripe as pollen grains. 
 
 Passing now to the central sporophylla, or carpels, we 
 find that, in addition to still further modifications in form 
 the three carpels have become united to form one mass 
 known as the ovary, which is in this case described as 
 compound (fig. 79). If a section be made across the 
 ovary the three separate chambers which are then exposed 
 demonstrate its compound nature. In each cavity we ob- 
 serve the sporangia, here known as ovules. The spo- 
 rangia are stalked, are numerous, and are arranged in six 
 rows, two rows in each chamber. 
 
 The ovary is continued upwards as the style, and ends 
 in a bulb-like furrowed head, the stigma (fig. 79). The 
 style, on a transverse section being made, is found to be 
 hollow, and the stigma, if microscopically examined, is seen 
 to be covered by a number of short hairs. 
 
 On what part of the sporophyll are the sporangia borne ? 
 A careful study of the development of the carpel shows us 
 that the sporangia are really developed, as in the case of 
 the male sporophylla, from the edge of the carpellary leaf. 
 The edges are, however, during development turned inwards 
 upon themselves, and ultimately unite so as to form a cavity, 
 with, consequently, the upper surface of the carpellary leaf in 
 the interior (fig. 83). The sporangia are thus completely en- 
 
Metaphyta L ilium. 
 
 163 
 
 closed in their sporophyll ; and since sporangia are deve- 
 loped on both edges, the double row is thus accounted for. 
 When the three carpels are fused together to form one ovary, 
 manifestly the ovules (sporangia) will appear to originate 
 from a central axis, in the formation of which the floral axis, 
 or central part of the cone, may participate. The style, 
 then, is simply the upper part of the sporophylla, the edges 
 of which have not united in the centre, and the trilobed 
 stigma is composed of the terminal points of the same, 
 swollen up and covered with epidermal hairs for a purpose 
 we shall discover presently. 
 
 FIG. 83. -PLACEXTATION. (Prantl.) 
 
 A, one carpel, one loculus ; B, three carpels, one loculus ; c, four carpels, 
 four loculi by upgrowth of thalamus ; D, five carpels, five loculi ; /, 
 placenta. 
 
 Turning now to the sporangium itself we meet with 
 still further differentiation from the type we described in 
 the fern. The sporangium is stalked, but the capsule is 
 doubled down on the funicle, and is adherent to it for a 
 considerable distance. The portion of the funicle which is 
 attached to the sporangium is termed the raphe. A spo- 
 rangium turned upside down thus is said to be anatropal 
 (fig. 84). The sporangium is pear-shaped, its pointed end 
 being directed downwards towards the placenta or point of 
 origin from the sporophyll. The wall of the sporangium is 
 incomplete at this point, and the aperture left is termed 
 the micropyle. The sporangia are not always anatropal. In 
 many plants the sporangium is perfectly straight, with the 
 micropyle pointing upwards (orthotropal) ; in other cases a 
 half-way condition is maintained between the anatropal and 
 
 M 2 
 
164 
 
 Elementary Biology. 
 
 the orthotropal, known as campylotropal. The sporangium 
 has usually two cellular coverings, the outer coat being 
 known as the ' primine,' the inner as the * secundine.' It 
 is preferable to discard these terms altogether and talk of 
 them simply as the outer and inner coats of the ovule. 
 The central tissue of the ovule is known as the ' nucellus, 
 sunk in the interior of which is found one ovospore, which 
 goes by the name of the ' embryo-sac.' If we study the deve- 
 lopment of the sporangia (male and female), we shall be able 
 to distinguish clearly the homologies of the several parts. 1 
 
 FIG. 84. DIFFERENT POSITIONS OF OVULES. (Prantl.) 
 
 A, orthotropal ; B, anatropal ; c, campylotropal ; a z, ii, coats of the ovule ; 
 A, nucellus; w, micropyle ; e i, embryo-sac ; f, funicle ; r, raphe. 
 
 The development of the stamen. The stamen appears 
 first as a small prominence on the floral axis (or thalamus, 
 as it is sometimes called). Very early a division of the 
 prominence takes place, indicating the primary divisions into 
 two pairs of spermo sporangia. The tissue between the 
 
 1 It is very unfortunate that the terminology of the reproductive 
 organs of the Angiospermce is so different from that of the lower plants. 
 The terms ovule, anther, ovary, and such like, have, however, become 
 by long usage so firmly associated with these structures that it is almost 
 impossible now to get rid of them. It is necessary, however, that the 
 student should in all cases know not only the terms in ordinary use, 
 but also the terms which are morphologically correct. In order to 
 familiarise the reader with either terminology, use will be made of both 
 series of names. 
 
Metaphyta L iliu m 165 
 
 two lobes becomes the future connective ; the filament is 
 an after- formation. The lobes are covered by epidermis 
 and the layer immediately beneath it divides into two layers, 
 the inner of which becomes the archespormm, whilst the 
 outer layer subdivides into three layers, the inner two of 
 
 FIG. 85. -DEVELOPMENT OF THE STAMEN. 
 
 A, transverse section of young stamen ; B, c, stages in the development of 
 the anther ; u, pollen grain ; E, pollen tube developing from the repro- 
 ductive cell ; a, epidermis ; 3, bundle sheath ; c, fibro-vascular bundle ; 
 J, layer which will become the tapetal layers, d' t d" ; f, vegetative 
 cells ; /J reproductive cell ; g, intin. 
 
 which become disorganised and form tapetal tissue ; the 
 outer becomes, with the epidermis, the wall of the sporan- 
 gium. The cells composing this inner layer have their walls 
 irregularly thickened, and form what is known as the endo- 
 thecium, while the term exothecium is sometimes given to 
 the epidermis. The archesporium forms mother spore-cells, 
 
1 66 
 
 Elementary Biology. 
 
 each of which subdivides into fcur daughter spore -cells, 
 which in turn become, after the development of exo- and 
 endo-sporium, the spermospores. As in the case of the 
 spores of the fern, the exosporium (or extin) is ridged or 
 elevated into spines, or otherwise variously sculptured. 
 There are numerous apertures in the exosporium through 
 which the endosporiurn (or intin) may be seen. Careful 
 examination of the spermospore when fully developed reveals 
 a structure highly suggestive of that of the spermospore of 
 Selagintlla, namely, the division of its contents into two por- 
 tions, a vegetative cell or cells and a reproductive cell. 
 The reproductive cell, however, does not undergo further 
 division, and forms no sperms. Both cells are nucleated, 
 and the much smaller vegetative cell is separated off from 
 the other by a cellulose cell-wall. The ultimate fate of 
 these cells we shall see later on, after we have studied the 
 ovosporangium and its contents. 
 
 The development of the carpel. The carpel, like the 
 stamen, originates as a prominence of the thalamus, and 
 after developing into a leaf-like structure forms on its in- 
 turned edges minute projections the future cvosporangia 
 
 FIG. 86. DEVELOPMENT OF THE OVULE. (Luerssen.) 
 
 For explanation see text. 
 
 (fig. 86, A, B). The projection becomes multicellular, and 
 is covered over by the epidermis of the placenta. The pro- 
 jection gradually increases in size until it becomes differen- 
 tiated into a head and stalk. The head forms the imcellu* 
 
Metaphyta L iliu m. 1 67 
 
 (c, n\ the stalk becomes the funicle. The cells immedi- 
 ately beneath the nucellus multiply tangentially and form 
 the rudiments of the coverings of the ovule, the inner coat 
 first, and the outer from beneath the origin of, and subse- 
 quently to, the inner coat (D, /"/', at). The ovule, though 
 anatropai when ripe, is orthotropal at first, gradually becom- 
 ing anatropal as it increases in size. These coverings to the 
 sporangium are obviously new developments not present at 
 all in the sporangia of the lower plants. The nucellus alone 
 is the morphological spoiangium comparable to that of the 
 fern. The cell in the terminal portion of the nucellus 
 immediately beneath the epidermis divides into two an 
 upper and a lower. The former becomes the tapetum, 
 which in the ovosporangium is composed of one, or a very 
 few, cells, whilst the latter becomes the archesporium (B, m). 
 The archesporium whilst sunk in the nucellus next divides 
 into four cells, of which one only becomes an ovosoore 
 (macrospore, embryo-sac). The other three cells, which are 
 thus to be considered as barren ovospores, remain in a semi- 
 disorganised condition on the summit of the fertile ovospore. 
 This latter increases in size and very soon shows signs of 
 division. The nucleus divides into two, each daughter 
 nucleus retreating to opposite ends of the ovospore. Each 
 of the two new nuclei again subdivides into four, and round 
 three of either set protoplasm collects, thus forming three 
 cells at either end of the ovospore. The two remaining 
 nuclei reapproach the centre of the ovospore and there fuse, 
 forming a new secondary nucleus for the ovospore. We 
 have thus the contents of the ovospore subdivided into three 
 small cells at the end nearest to the tapetum, and three small 
 cells at the opposite end, with a large secondary nucleus 
 between them (fig. 88). The cells of the upper of these two 
 sets are arranged so that two cells are above and one be- 
 neath, the odd one having for its nucleus the sister of that 
 nucleus which helped to form the new secondary nucleus of 
 the ovospore. This lower cell is the ovum proper, whilst 
 
1 68 Elementary Biology. 
 
 the two upper cells form the sole remnant left of the canal 
 walls of the primitive ovarium. The three cells formed at 
 the opposite pole may similarly be looked on as the rudi- 
 ment of the thallus, probably that portion of it which would 
 give rise, if completely developed, to spermaria, just as the 
 vegetative cell of the spermospore, no doubt, contains 
 chemical compounds necessary to the development of an 
 ovarium. 
 
 Fertilisation. - In the life-history of vascular plants we 
 have hitherto found that the male elements have been 
 motile cells termed sperms, which in virtue of their move- 
 ment have been able to migrate from the thallus bearing 
 them to the female organs carried in other thalli. The 
 pollen gram of a lily is itself, however, a non-motile struc- 
 ture, and no sperms are formed. How, then, is the male 
 protoplasm to be carried to the ovum, and what in this case 
 takes the place of the sperm in fertilisation ? 
 
 Notwithstanding the fact that the majority of flowers are 
 hermaphrodite, i.e. bear both male and female elements, 
 it has been experimentally proved that, if an ovum be fer- 
 tilised by the pollen of the same flower, the result, if 
 there be any, is a weak and diminutive embryo, often in- 
 capable of developing into a perfect plant ; whilst, on the 
 other hand, if the ovum be fertilised by pollen taken from 
 another flower, a healthy and vigorous embryo is produced. 
 Thus cross-fertilisation tends to the maintenance of healthy 
 tribal life, whilst self-fertilisation tends to produce a dwarf 
 and weakly progeny, and ultimately extinction of the race 
 unless cross-fertilisation intervenes and saves it. This law 
 has a number of exceptions, but holds good in the great 
 majority of cases. Granted, then, that foreign pollen (of 
 course from a plant of the same kind) must be employed in 
 fertilisation, how is the pollen to be conveyed from the 
 male sporophyll of one flower to the female sporophyll of 
 another, since the pollen grains, or spermospores, are them- 
 selves non-motile, and do not produce any motile sperms ? 
 
Metaphyta L ilium. 1 69 
 
 We have noticed already that the perianth leaves of the 
 lily are brightly coloured. We may now notice further 
 that they have a distinct odour, or scent. Lastly, an ex- 
 amination of the bases of the petals reveals the presence of 
 small disc-shaped glands, to which the name of nectaries are 
 applied, and which secrete a copious supply of nectar, a 
 sweet, sticky fluid, not unlike honey, and containing a large 
 amount of sugar in its composition. Everyone is familiar 
 with the fact that insects of various kinds, especially bees, 
 butterflies, and moths, visit flowers for the sake of this 
 nectar, which they use as food. Further, it has been 
 proved that they are attracted to the flower both by the 
 scent (produced by the evaporation of volatile oils) and by 
 the colour of the perianth. The insect in its movements 
 in the interior of the flower, whilst sucking the nectar, rubs 
 itself over with pollen, which when ripe tumbles out of the 
 open sporangia. The pollen grains are viscid and sticky 
 through having lain amongst the gelatinous tapetal cells. 
 The insect's back and legs are covered with hairs, to which 
 the viscid grains adhere. In this way the pollen grains are 
 conveyed to another flower, where they fall, or are rubbed 
 off by the insect, on the stigma. It is needless to say the 
 action of the insect in this matter is a quite unconscious one. 
 The stigma, it is to be remembered, is covered with hairs, 
 also viscid and sticky, and to these some of the pollen 
 grains adhere. They are nourished there by the secretion 
 in which they are caught, and in some way or another are 
 stimulated to further development. 
 
 The development of a pollen grain consists in the ex- 
 trusion of part of its contents through the exosporium. The 
 protuberance gradually assumes the form of a long thread 
 or tube, the pollen tube, covered by the endosporium. 
 The pollen tube lengthens and bores its way through the 
 tissue of the style, and enters the micropyle of the ovule. 
 There the termination of the pollen tube swells, and its wall 
 becomes apparently more porous and suitable for the pass- 
 
Elementary Biology 
 
 age through it of the male protoplasm. The pollen tube 
 fixes itself to the two upper cells of the rudimentary ovarium, 
 and seems to become continuous with their substance. The 
 nucleus of the male reproductive cell, which has meanwhile 
 migrated down the pollen tube, becomes dissolved in the 
 general protoplasm, and apparently in that form penetrates 
 the endosporium and enters into the two upper cells, which 
 
 FIG. 87. FERTILISATION. (Vines.) 
 
 o- 
 
 A, B, and C, succes-ive stages in ihe fusion of the male and female pro- 
 nuclei : pt, pollen tube ; ;/, male pronucleus ;.//, female pronucleus ; 
 o, ovum. 
 
 are frequently spoken of as the synergidse. These in turn 
 fuse with the ovum, which thus has transferred to it some of 
 the male fertilising matter. The nucleus (of the reproductive 
 cell of the pollen grain) reaggregates and appears in the 
 ovum, with the nucleus of which it fuses, completing the act 
 of fertilisation. The pollen tube now withers away and the 
 synergidae disappear. 
 
 Results of fertilisation. The first result of fertilisation 
 is the assumption by the fertilised ovum or embryo of a 
 
'Fie. 88. -VEHTICAL SECTION OF THE CARPEL. (I.uerssen.) 
 
 a i, outer ovular integument; zz, inner ovular integument; o, wall of 
 ovary : g, style : s, stigma ; f, funicle ; />, pollen tube ; , nuce'.lus ; b, 
 antipodal cells ; k, ovum ; e, embryo-sac ; m, micropyle. 
 
Elementary Biology. 
 
 cell wall (compare the period of rest after sexual union in 
 Spirogyra and other types). Meanwhile rapid subdivision 
 of the secondary nucleus of the ovospore (embryo-sac) takes 
 place, accompanied by the aggregation of protoplasm round 
 the new nuclei to form cells. The result is a mass of 
 cellular tissue, the endosperm, whose function it is to pro- 
 vide nourishment for the embryo about to be developed. 
 In relation to the development of the embryo and the 
 formation of a store of nourishment, it may be well to note 
 
 FIG. 89. -Viola tricolor. (Sachs.) 
 
 A, longitudinal section of anatropous ovule after fertilisation. //, pla- 
 centa ; /, swelling on the raphe ; a, outer integument ; /, inner inte- 
 gument ; /, pollen-tube entering micropyle ; e, embryo-sac, with the 
 fertilised ovum at the micropyle end, and numerous endosperm cells at 
 the other. B, apex of embryo-sac, e, with young embryo, eb, of three 
 cells, and one cell forming the suspei.sor. C, same, further advanced. 
 
 the existence in the funicle of a fibro-vascular strand, a 
 feature absent from the funiculi of the lower types, as being 
 unnecessary owing to the simplicity and short life of the 
 sporangium (Selaginella^ or to the fact that the embryo is 
 not developed in the sporangium at all (Pteris). The ovo- 
 spore now increases greatly in size and becomes completely 
 filled with the endosperm, which appears first round the 
 
Metaphyta Lilium. 1 7 3 
 
 spore- wall and later in the centre. The cellular tissue 
 (nucellus) surrounding the ovospore meantime disappears, 
 becoming absorbed by the endosperm, 1 which takes its place 
 morphologically and physiologically. 
 
 The development of the embryo. The fertilised ovum 
 soon segments into a short string of cells, one extremity 
 being fixed to the end of the ovospore next the micropyle, 
 the other end of the string lying free in the endosperm. 
 This row of cells forms the FlG> ^_ VMa tricolor , (Sac hs.) 
 pro-embryo. The proximal 
 cells of the row, i.e. those 
 next the micropyle, become 
 the suspensor, and the 
 single distal cell becomes 
 
 the embryO proper. Posterior end of embryo-sac : e, wall ; S, 
 
 _,, j i f cavity of the embryo-sac ; A", A", yoijng 
 
 1 lie development OI endosperm cells ; pr, protoplasm of the 
 
 this distal cell into the 
 
 embryo is of some importance, and it is at this stage that 
 we meet with the first great difference between monoco- 
 tyledons and dicotyledons. The account here given is true 
 for both types up to a certain stage, when the points of 
 difference will be emphasised. 
 
 The distal cell first undergoes subdivision, so that eight 
 daughter cells are formed, which form a blastoderm (to use a 
 convenient zoological term). These eight cells next become 
 tangentially divided into eight concavo-convex cells exter- 
 nally and eight small tetrahedral cells internally (fig. 91). 
 The external concavo-convex cells become dermatogen, or 
 the layer destined to give rise to the epidermis ; the inter- 
 nal tetrahedral cells form the embryonic periblem, or 
 fundamental tissue, and plerome, or fibro-vascular tissue. 
 In the dicotyledons the embryo becomes flattened and 
 heart-shaped, and shows a differentiation into two lobes, the 
 cotyledons, or primary leaves, between the bases of which 
 
 1 In some few plants it remains, and is then known as perisperm. 
 
174 Elementary Biology. 
 
 lies an actively growing mass of cells, the future stem, and 
 a body known as the hypocotyledonary axis, the tip- of 
 which is covered by the terminal cell of the suspensor. 
 This suspensorial cell divides transversely into two cells, the 
 cell next the embryo (the hypophysis) giving origin by 
 subdivision to a number of layers, the distal forming the 
 terminal portion of the root, while the proximal cells form 
 the embryonic root-cap, or calyptrogen. 
 
 The embryology of the monocotyledon does not differ 
 from that just described, save that only one cotyledon is 
 formed, consequently the young stem seems to spring from 
 the side of the cotyledon rather than from the terminal part 
 of the embryo. The manner of arrangement of the cells of 
 the embryo is also somewhat different 
 
 While these various changes have been taking place in 
 the interior of the ovospore, concomitant changes have been 
 occurring in the surrounding tissues of the ovule and ovary. 
 After fertilisation the ovule becomes the seed, and the 
 ovary, with its contained seed, the fruit. The outer covering 
 of the ovule (now of the seed), is termed the testa, the inner 
 the tegmen. Similarly, the ovarial wall, when it forms the 
 wall of the fruit, receives the name of the pericarp, not in- 
 frequently differentiated into three layers, the epi- meso- and 
 endo-caip. In some fruits the floral axis swells up round 
 the ovary, and, becoming succulent, forms what is known as 
 a false fruit, or pseudocarp. This does net, however, take 
 place in the type which we are considering. 
 
 We have already seen how the various tissues of the 
 adult plant are differentiated from meristem. The plant so 
 formed may, like the lily, live only one year ; it is then 
 termed an annual, and its individual life, as a rule, ceases 
 after it has matured its fruit. Plants which thus flower only 
 once are termed monocarpic ; others, again, flower after 
 they are two years old and then die (biennial monocarpic) ; 
 others only after they have lived for a number of years 
 (perennial monocarpic). The majority (especially of dico- 
 
Metaphyta Lilium. 
 
 175 
 
 tyledons) flower again and again year after year. These 
 may therefore be termed perennial polycarpic plants. 
 
 When ripe the fruit falls off, and by withering or de- 
 
 FIG. 91. -DEVELOPMENT OF EMBRYO OF DICOTYLEDON. (Sachs.) 
 
 / 
 
 The numbers indicate planes of division ; v, suspensor ; k, hypophysis ; 
 h 1 , cell formed from the hypophysis ; w, tissues of the embryonic root ; 
 s, stem ; c, c, cotyledons. 
 
 hiscing allows the seeds to escape, or the fruit may dehisce 
 while still attached to the stem. The seeds germinate when 
 sown on a suitable soil under the proper conditions of heat 
 
FIG. 92. DEVELOPMENT OF THE EMBRYO. (Maoqt and Decaisne.) 
 c h a f 
 
 t-\ 
 
 i, i 3, Development of a dicoty- 
 ledonous embryo. 4, 5, 6, 7, De- 
 velopment of a monocotyledon- 
 ous embryo, a, raphe ; c, cotyle- 
 don ; e, inner coat of seed ; /, 
 outer coat ; f, funicle ; g; plu- 
 mule ; A, hilum ; t, hypocoty- 
 ledonary axis ; r, radicle ; in, 
 micropyle ; k, groove occupied 
 by the plumule ; s, secondary 
 roots, i. Shows half a pea still 
 attached to the placenta. 2. 
 The embryo removed and split 
 open. 3. The embryo after a 
 few days' growth. 4. Shows a 
 section of the oat with small co- 
 tyledon and large mass of endo- 
 sperm. . 5. The embryo re- 
 moved from endosperm. 6. First 
 stage in its germination. 7. Ger- 
 minating embryo of maize. 
 
Metaphyta L ilium. 
 
 177 
 
 FIG. 93. ORIGIN OF SECONDARY 
 ROOTS. (Prantl.) 
 
 and moisture. The root is the first part of the embryo to 
 make its way through the micropyle and elongate. The 
 plumule or young stem remains hidden for some time in the 
 seed. When the primary root has taken fast hold of the 
 soil the secondary roots begin to make their appearance to 
 take its place functionally, and the primary root soon ceases 
 to elongate. The plumule next emerges and increases in 
 size. The stems of plants of the lily type increase in thick- 
 ness very rapidly, but do not add secondary wood in future 
 years if perennial. There are 
 a number of arborescent lilies 
 such as the Yucca, Dracaena, 
 and such like which, how- 
 ever, form interesting tran- 
 sition links to the dicotyle- 
 donous type which we shall 
 discuss in the following sec- 
 tion. One point of some im- 
 portance remains to be noticed 
 with regard to the mode of 
 origin of lateral branches of 
 the stem and of the root axis. 
 Lateral roots always originate 
 from what was described at p. 
 150 as the pericambium layer, 
 
 , , secondary roots in various stages 
 of growth ; k, root-cap ; r, cortex ; 
 f, fibro-vascular core. 
 
 especially that portion of it which lies opposite to the xylem 
 of the fibro-vascular strands, whilst branches of the stem 
 axis originate from superficial layers of cells of the cortex 
 into which the fibro-vascular strands afterwards become 
 developed. The root method of development is termed 
 endogenous, whilst the shoot method is termed exogenous. 
 One result of the endogenous mode of origin is that the 
 roots have to push their way through the cortex of the root ; 
 hence, where each root appears on the surface, it is sur- 
 rounded by a collar of tUsue which it has pushed aside, the 
 coleorhiza. /^ 
 
 N 
 
178 Elementary Biology. 
 
 We have now completed our examination of the lily, 
 and pass on in the next section to a survey of the points of 
 difference and agreement between it and the buttercup. 
 As already stated, the points of difference are chiefly in 
 external configuration and arrangement of parts, in the 
 arrangement of the tissues of the nbro-vascular strand, and 
 in their mode of secondary growth; and, lastly, in the 
 structure of the embryo. We will devote special attention 
 to these points. It will have been noticed that very little has 
 been said hitherto of the physiology of plants ; that subject 
 is postponed until our consideration of the morphology is 
 completed, since the function of the several structures and 
 the phenomena of nutrition, &c. will be thus more easily 
 understood. The physiology of the several types, moreover, 
 agrees in most essential points, hence by thus postponing 
 our treatment of that subject until we have surveyed all the 
 types we shall escape constant repetition. 
 
 SECTION I V. DICOTYLEDONES RANUNCUL US. 
 
 In selecting a type of the dicotyledonous flowering 
 plants it is advisable to choose one where the various parts 
 are as nearly as possible in a primitive or simple condition, 
 where no parts of the flower are wanting (a complete flower) 
 and where none have been specially modified for purposes 
 connected with cross-fertilisation (a regular flower). The 
 buttercup is a particularly favourable subject for investiga- 
 tion, for it is a complete and regular flower of the penta- 
 merous type. 
 
 Stem. The most important point of difference between 
 the stem of the buttercup and that of the lily is in the 
 arrangement of the fibro-vascular strands. As already seen, 
 they are in the lily arranged irregularly in the fundamental 
 tissue ; in the buttercup they are arranged concentrically. 
 They therefore subdivide the fundamental tissue into a cen- 
 trally placed medulla or pith, and a peripherally placed 
 
Metaphyta Ranunculus. \ 79 
 
 cortex, whilst joining these two tissues and running between 
 the fibro- vascular strands lie plates of fundamental tissue to 
 which the name of medullary rays has been given. The 
 tissues of the strand are, moreover, also arranged concen- 
 trically. It is easily possible to differentiate three layers in 
 the strand : internally and next the pith, the xylem, con- 
 sisting of prosenchyma, parenchyma, and vessels. These 
 latter are partly distributed through the wood (dotted ducts) 
 and partly collected at the tips of the wood next the pith 
 (spiral vessels), where the latter form what is known as the 
 medullary sheath. Next we meet with a layer of actively 
 growing tissue, the cambium, composed of parenchyma rich 
 in protoplasm ; and externally the bast, or phloem, which is 
 composed on its inner side of parenchyma, prosenchyma, 
 and sieve-tubes, on its outer aspect of thick-walled prosen- 
 chyma. 
 
 As the stem grows older the strands increase in size, and 
 approach one another so that the medullary rays are di- 
 minished. Moreover, a layer of cambium appears, uniting 
 the cambium regions of the various strands together. The 
 cambium of the strands and the cambium between the 
 strands are known as fascicular and interfascicular cam- 
 bium respectively. The interfascicular cambium has the 
 power of forming xylem on its inner and phloem on its outer 
 side. Thus are formed new or secondary strands, which 
 become wedged in between the primary strands, squeezing 
 and crushing the medullary rays, of which nothing is ulti- 
 mately left save a few thin plates or isolated patches of cells. 
 The cambium of both kinds has, however, another duty to 
 perform, namely, to add new wood and (to a less extent) 
 new bast to the wood and bast already formed. Towards 
 the end of the year when the vitality of the plant is ebbing, 
 the cambium cells divide less rapidly and the cells are much 
 smaller. Again, on the return of spring, when vitality is in 
 full flow, the cells become much larger and divide rapidly. 
 This cessation and renewal of growth form what are popu- 
 
 N 2 
 
i8o 
 
 Elementary Biology. 
 
 larly known as annual rings. These rings are not so distinct 
 in the bast, for there is not so much of it formed, nor is it 
 formed so regularly. This formation of new layers of wood 
 and bast may go on for an indefinite number of years, hence 
 
 FIG. 94. DEVELOPMENT OF THE DICOTYLEDONOUS STEM. (Sachs.) 
 
 K, cortex : M, pith ; zV, interfascicular cambium ; fc, fascicular cimbium ; 
 /, primary phloem ; .r, primary xylem ; l>, hard bast ; f/i, ifh. fascicular 
 and interfascicular wood. 
 
 the term open, or capable of continuous growth, as applied 
 to strands of the dicotyledonous stem. Manifestly as the 
 stem thus increases in size, some arrangement must be made 
 whereby the cortex may increase concomitantly. Such an 
 
MetapJiyta Ranunculus. 
 
 iSi 
 
 arrangement is found in the cork cambium, a layer of grow- 
 ing cells lying outside the bast. This layer is capable of 
 growth in two directions outwards and inwards. Outwards 
 
 FIG. Q5. DICOTYLEDONOUS FIBRO-VASCULAR STRAND. (Sachs.) 
 
 V 
 
 l>, hard bast ; c, fascicular cambium ; c b, interfascicular cambium ;y, sieve-tubes 
 in soft bast ; m, fundamental tissue ; g, dotted ducts ; t, pitted vessels. 
 
 it forms a layer of periderm, or cork, the cells of which are 
 dead and filled with air, and have their walls rendered almost 
 impermeable to water ; and inwards a layer of phelloderm, 
 
1 82 
 
 Elementary Biology. 
 
 or green cortex. The cork cambium itself is often known as 
 phellogen. These successive layers of cork form the bark ; 
 even in young twigs little patches of cork form below the 
 epidermis (usually beneath a stoma) and bulge it outwards, 
 forming what are known as lenticels. 
 
 In the cortex, and frequently amongst the other tissues 
 as well, are found vessels of an entirely different character 
 
 FIG. 9^. ORIGIN OF CORK. (Sachs.) 
 
 f, epidermis ; A', cork or periderm ; /*/*, phellogen, or cork cambium ; 
 pd, phellodenn. 
 
 from those already described. Two forms will be here de- 
 scribed, the laticiferous vessels and the resin canals. 
 Laticiferous vessels usually run in an irregular manner 
 through the tissue in which they are found. They branch 
 and anastomose. They contain a watery fluid known as 
 latex, seen very well exuding from the cut stem of a dande- 
 lion. The latex is not always white, however. It may be 
 red, blue, or yellow. Some varieties of latex are of great 
 commercial value, such as, for instance, india-rubber, gutta- 
 
MetapJiyta Ranunculus. 
 
 183 
 
 percha, and opium. Whilst laticiferous vessels are formed 
 of straight or irregular cells, placed end to end and having 
 
 FIG. 97. LATICIFEROUS VESSELS. (Sachs.) 
 
 A, slightly, B, highly magnified. 
 
 their common walls absorbed, resin canals are formed by a 
 
184 Elementary Biology. 
 
 number of cells enclosing a cavity, which is thus an inter- 
 cellular space. They also contain substances of economic 
 value, as turpentine, resin, &c. 
 
 Numerous other varieties of vessels, ducts, and glands 
 might be mentioned, such as oil glands, nectar glands, 
 cavities containing volatile oils, and such like. The pecu- 
 liar glands of insectivorous plants will be described in 
 sect. vi. 
 
 FIG. 98. TRANSVERSE SECTION OF A RESIN CANAL. (Vines.) 
 
 Root. The chief point of importance in the dicotyledon- 
 ous root in which it differs from that of the monocotyledon 
 is its power of increasing in thickness by growth of second- 
 ary wood. It will be remembered that while the primary 
 root of the monocotyledon soon ceases to grow, and has its 
 place taken by adventitious roots, the primary root of the 
 dicotyledon goes on growing, and increases in thickness by 
 
Metaphyta Ranunculus. 185 
 
 the addition of new wood and new bast to that already 
 formed. The secondary layers of wood and bast arise from 
 a fascicular and interfascicular cambium, which lies internal 
 to the pencambium, and, owing to the position of the two 
 parts of the primary strands, winds in a sinuous mariner out 
 and in between the primary masses of bast and the primary 
 masses of wood. The cambium first makes its appearance 
 opposite the bast portions of the strands. 
 
 Leaf. The arrangement of the fibro- vascular strands in 
 the dicotyledonous leaf differs from that of the monoco- 
 tyledon in being netted or reticulated, the strands starting 
 from one or more chief ribs, and gradually becoming smaller 
 towards the leaf edge, branching and anastomosing as 
 they go. 
 
 The shape of the leaf as a whole is most varied. We 
 may distinguish, however, between simple and compound 
 leaves, and reduce each leaf or leaflet (if compound) to one 
 or other of the three types, circular, elliptical, and ovate. 
 The shape of the leaf is always related to the arrangement of 
 the leaves on the stem (phyllotaxis). It must be remem- 
 bered that the main object of phyllotaxis is to enable the 
 leaves to get a maximum of light and air. We will first con- 
 sider one or two of the chief phenomena of phyllotaxis and 
 then it will at once become apparent that the shape of the 
 leaf and the phyllotaxis are closely correlated (Lubbock). 
 
 There are two chief methods of leaf arrangement, 
 namely, where the leaves are arranged in pairs opposite to 
 each other or in circles or whorls of more than two, and they 
 are arranged in an alternate or spiral manner round the 
 stem. These two methods may be termed the verticillate 
 and the spiral respectively. The verticillate is often 
 mimicked by the spiral when the internodes of the latter are 
 very closely approximated, as, for instance, on the floral axjs. 
 There are many varieties of the spiral arrangement, but all 
 are found to obey a definite law, viz. that if the numerator 
 of a vulgar fraction indicate the number of times it is neces- 
 
186 
 
 Elementary Biology. 
 
 sary to go round the stem before coming to the leaf directly 
 above that from which the start is made (following the bases 
 of successive leaves on the branch in regular order), and if 
 the denominator represent the number of leaves passed, then 
 the numerators of two successive fractions added together 
 give the numerator, and the denominators added together 
 the denominator of the succeeding fraction. Thus the 
 
 FIG. 99. DICOTYLEDONOUS LEAF. (Thom.) 
 
 simplest spiral phyllotaxis must be that where the leaves are 
 alternate, one on one side and the other on the other side 
 of the stem. Manifestly, under such circumstances, we 
 must pass two leaves and go once round the stem to reach 
 the leaf directly above that from which we started. Hence 
 we have here a \ spiral. Again, still circling the stem once, 
 every fourth leaf might be that directly above the starting 
 point. The fraction then would be ^. According to the 
 
MeiapJiyta Ran uncnlus. 
 
 law, the succeeding spirals must be f, f, ^Vim an( * so on - 
 The law, however, does not always hold good amongst the 
 higher fractions, and seldom among the sporophylla and 
 perianth leaves, where various modifying influences come 
 into play. Now if a plant has large broad leaves, manifestly 
 
 Fto. rco. COMPOUND 
 LEAF. (Thome). 
 
 FIG. lot. DIAGRAM OF 
 SHRAU (Prantl). 
 
 if they are to obtain a maximum of light and air, they must 
 be arranged on a wide spiral, and not in a whorl, else they 
 will cover each other. So, also, if the leaves are small and 
 narrow, they might well be arranged in whorls or close 
 spirals, and still be sufficiently exposed to the air and 
 light. 
 
188 
 
 Elementary Biology. 
 
 The flower. It has already been pointed out that the 
 flower of the buttercup conforms to what is known as the 
 
 FIG. ^.-VERTICAL .SECTION OK RANUNCULUS. PentamerOUS type, 
 (Maout and Decai^ne.) that is tO Say, has itS 
 
 parts arranged in 
 whorls of five each. 
 This may be best 
 seen by comparing 
 the floral diagram 
 (fig. 103) with the 
 vertical section of the 
 flower (fig. 102). The 
 sporophylla are more numerous than in the lily, and are 
 spoken of as indefinite. The stamens conform to the type 
 already described for the lily. The carpels are, however, 
 in Ranunculus free as well as numerous. Moreover, each 
 carpel contains only one anatropal sporangium (ovule). 
 After the ovum in its interior has been fertilised the carpel 
 is then a one-celled one-seeded fruit. The carpels in the 
 
 lily formed a three- 
 celled many-seeded 
 fruit. All the sporo- 
 phylla, as well as the 
 perianth leaves, spring 
 from a convex thalamus 
 or floral axis, and the 
 stamens rise free from 
 each other and the peri- 
 anth, and at a lower 
 level than the carpels. 
 The stamens (andrce- 
 cium) are therefore said 
 to be hypogynous, or 
 beneath the carpels 
 (gyno3cium). The ac- 
 count of the structure of the sporophylla and the essential 
 
 FIG. 103. FLORAL DIAGRAM OF RANUN- 
 CULUS. (Allen.) 
 
 a a, carpels ; b b. stamens ; cc, petals ; 
 dd, sepals. 
 
General Physiology of the Plant. 1 89 
 
 elements of reproduction already given for the lily will, with 
 very slight modification, do also for the type now before us. 
 Repetition, therefore, is unnecessary. We have also directed 
 attention to the chief points in the embryology in connection 
 with that of the lily. We proceed now to give a brief ac- 
 count of the general physiology of the plant in application 
 (with exceptions which will be pointed out) to all the types 
 considered in the preceding sections. 
 
 SECTION V. GENERAL PHYSIOLOGY OF PLANTS. 
 
 As was pointed out in sect. i. of Chap. III. every organ in the 
 plant body has a special duty or function to perform, a duty 
 directly subservient to the welfare of the individual as a 
 whole. Just as we may consider the plant body as a whole 
 from a morphological point of view, so also we may discuss 
 the general physiology of the vegetal organism as a whole. 
 These two aspects may be denominated the statical and the 
 dynamical aspects respectively. Previously, however, to 
 entering upon the detailed examination ol the dynamical or 
 physiological aspect of plant life, it will be found of great 
 assistance if we devote a short time to the survey of the 
 special chemical elements which enter into the composi- 
 tion of the plant, and the conditions under which they occur 
 there. 
 
 The easiest method of ascertaining the elements which 
 compose the food of plants is by subjecting the entire 
 plant to destructive analysis (footnote, page 25). The 
 plant organism contains a very large percentage of water 
 (uncombined) which can be dried off by application of a 
 gentle heat (desiccation). Many of the volatile oils and 
 aromatic substances are also got rid of by this means ; we 
 will not, however, consider them at present The amount 
 of the water present varies with the season of the year at 
 which the examination is conducted, and the plant and part 
 of the plant examine<J<^^dtib '^su^s, such as the Algse, 
 
1 90 Elementary Biology. 
 
 contain a much larger percentage of water than terrestrial or 
 aerial plants, though there are numerous exceptions to this 
 rule. In spring, when active growth is commencing, the 
 proportion of water rapidly increases. The younger parts of 
 the plant also contain more water than older parts. 
 
 The dried or desiccated plant may further be subjected 
 to calcination, or red heat, when the greater proportion of 
 the solid residue is removed in the form of simple com- 
 pounds, such as carbonic acid, water of combination, and 
 ammonia. The elements found to be present by an exami- 
 nation of the products of calcination are carbon, hydrogen, 
 oxygen, and nitrogen ; whilst the ash left over contains, as 
 a general rule, sulphur, phosphorus, chlorine, silicon, potas- 
 sium, calcium, magnesium, sodium, and iron, all in the form 
 of simple salts, or even as bases or oxides. Many other 
 substances are found in particular plants, those, that is 
 to say, growing in certain districts where such substances 
 occur in the soil, or where the plant has peculiar facilities 
 for the absorption of such substances. These accidental 
 constituents are iodine and bromine, which are found in 
 many marine plants, lithium, zinc, copper, aluminium, man- 
 ganese, cobalt, nickel, strontium, and barium. 
 
 It will be convenient to classify these elements in the 
 order of their relative importance, thus : 
 
 I. Essential elements present in the plant organism in 
 large amount. (C, H, O, and N.) 
 
 II. Essential elements present in small amount. (S, P, 
 K, Ca, Mg, Fe.) 
 
 III. Non-essential elements present in variable amount, 
 and not in all plants. (I, Br, Zu, Cu, Al, Mn, Co, Ni, Sr, 
 Li, Ba, Si, Na, Cl.) 
 
 I. Essential elements in the plant organism, present 
 in large amount. 
 
 CARBON. 
 
 Origin. The carbon is obtained from the carbonic acid 
 
General PJiysiology of tJie Plant. 191 
 
 of the atmosphere (p. 42). Parasites and saprophytes (p. 
 100) and carnivorous plants (p. 213), however, obtain their 
 supply of carbon from organic compounds present in living 
 or dead animals and plants. 
 
 Importance. Carbon forms fully half the dry weight of 
 the plant, and forms an essential constituent of proteids, 
 carbohydrates, fats, and many transition substances (p. 26). 
 
 HYDROGEN. 
 
 Origin. From the water obtained from the soil and 
 from the air : also from ammonia and its compounds present 
 in the soil. 
 
 Importance. As in the case of the carbon, hydrogen 
 is an essential constituent of proteids, carbohydrates, fats, 
 and manv transition substances. It also, of course, forms 
 an essential constituent of the omnipresent water. 
 
 OXYGEN. 
 
 Origin. Most of the oxygen used in the formation of 
 organic compounds in the plant is obtained from water 
 and salts containing oxygen. The oxygen, on the other hand, 
 required in the oxidising of protoplasm in the liberation of 
 energy, is obtained from the boundless supply of free oxygen 
 in the air. 
 
 Importance. Again an essential constituent of the 
 substances mentioned under hydrogen, as well as forming a 
 necessary condition of plant life in its free form for purposes 
 of oxidation (respiration, p. 35). 
 
 NITROGEN. 
 
 Origin. The nitrogen found in combination in the 
 plant body is entirely obtained from nitrogen in combination 
 in the soil, as nitrates, nitrites, &c. Notwithstanding the 
 enormous quantity of free nitrogen in the air, none of it 
 seems to be used directly by the plant as food. 
 
1 92 Elementary Biology. 
 
 Importance. Like carbon, hydrogen, and oxygen, an 
 essential constituent of the same organic and inorganic 
 compounds already shown to enter into the formation of 
 protoplasm. 
 
 Not only carbon, but also hydrogen, nitrogen, and 
 oxygen are, in the case of the saprophytes, parasites, and 
 carnivorous plants, obtained from the organic compounds 
 contained in the media in or on which they live. 
 
 II. Essential elements in the plant organism, present 
 in small amount, 
 
 SULPHUR. 
 
 Origin. The sulphur is obtained by the decomposition 
 of salts of sulphur present in the soil. Undoubtedly the 
 chief salt from which the sulphur is procured is sulphate of 
 lime. This salt is probably decomposed by some organic 
 acid in the plant, such as oxalic acid, crystals of calcic- 
 oxalate being formed, the nascent sulphur combining with 
 higher compounds. Many other sulphates, such as those of 
 potassium, magnesium, and ammonium, are probably used 
 in this way. 
 
 Importance. Although sulphur is itself volatile, it is not 
 burnt off during calcination, but is found in the form of 
 sulphuric acid united with bases present in the ash to form 
 sulphates. It forms an essential constituent of proteids and 
 many transition substances. 
 
 PHOSPHORUS. 
 
 Origin. Phosphorus, like sulphur, is obtained from its 
 salts present in the soil. These salts are mainly those of 
 lime, potash, and magnesia. 
 
 Importance. Phosphorus, though present in very small 
 amount, nevertheless seems to be in some way essential to 
 the vitality of the organism. In addition to forming a 
 
General Physiology of the Plant, 193 
 
 constituent of the substance nuclein, of which the cell- nucleus 
 is mainly composed, and of various proteids or derivatives 
 of them, it also plays an important part in the metabolism 
 of protoplasm ; for no albuminoids can be formed if it be 
 absent. It also enters into the composition of chlorophyll 
 (P- 40). 
 
 POTASSIUM. 
 
 Origin. Potassium is obtained from a variety of salts 
 in the soil, of which the chief are the sulphate, phosphate, 
 and chloride. 
 
 Importance. Potassium, like phosphorus, is necessary 
 for the metabolic processes in the plant. While phosphorus 
 assists in the formation of proteids, potassium exerts a like 
 influence in the formation of carbohydrates. 
 
 CALCIUM. 
 
 Origin. Salts of lime in the soil, such as the sulphate, 
 phosphate, nitrate, and carbonate. 
 
 Importance. The use of calcium in the plant economy 
 is still a matter of some doubt. Probably one of its uses is 
 to form a medium whereby the excess of organic acids may 
 be got rid of (see under sulphur). 
 
 MAGNESIUM. 
 
 Origin. From the same salts as those of calcium. 
 The chlorides of magnesium and of calcium are, however, 
 unfavourable to the growth of the plant. 
 
 Importance. The value of magnesium to the plant is 
 involved in even greater doubt than is that of calcium. It 
 has been found to be a constituent of the globoid associated 
 with grains of aleurone where these occur (p. 157). 
 
 IRON. 
 
 Origin. Many compounds of iron are employed to give 
 the very small amount of that metal required by the plant. 
 
 o 
 
194 Elementary Biology., 
 
 Importance. Iron is associated in some way with the 
 formation of chlorophyll. Without it no chlorophyll is 
 formed, and yet if more than a minute trace be present it 
 acts as a poison on the plant. It does not, however, form 
 a constituent of chlorophyll. It must, therefore, act simply 
 as an essential agent in its production. 
 
 Ill, Non-essential elements in the plant organism, 
 present in variable amount. 
 
 All these substances are not found in the same plant, 
 nor at the same time. Some, indeed, are invariable consti- 
 tuents though they do not seem essential to life, e.g. sodium. 
 
 Sodium. This metal might be said to be ubiquitous, 
 and yet it cannot be said to be of essential service to the 
 plant. It is obtained from salts in the soil, and is taken up 
 chiefly in the form of sodium chloride. 
 
 Chlorine. Chlorine, as has been mentioned above, is 
 necessarily taken into the plant in the process of the absorp- 
 tion of potassium and sodium, but it does not seem to be 
 necessary to the life of the plant. 
 
 Silicon, however, enters in considerable quantity into 
 the composition of many plants of the grass tribe ; whilst 
 many of the lower plants have large deposits of silica 
 (SiO- 2 ) in the cells of the thallus. The silicious covering of 
 the Diatoms and allied forms are composed almost entirely 
 of this compound (p. 72). 
 
 Iodine and bromine are found in tolerably large quanti- 
 ties in certain of the marine Algae, whence, indeed, a large 
 proportion of the iodine and bromine of commerce is 
 obtained. 
 
 With regard to the other elements mentioned under 
 this head, we need only say that they are found rather as 
 accidental constituents than as essentials in the plant organ- 
 ism. They are probably not peculiar to plant life at all. 
 
 The forms in which these various elements appear in the 
 
General Physiology of the Plant. 
 
 195 
 
 plant will form a subject of study when we consider the 
 dynamical aspect of plant physiology. 
 
 It is worth pointing out here that this analysis of plants 
 yields us most important results when we consider their life- 
 histories from an agricultural point of view. The whole 
 question of manuring is founded on a knowledge of the 
 chemical composition of plants. By the rotation of crops, 
 the farmer is able to grow a plant on a soil impoverished by 
 the preceding crop of many substances which its successor, 
 however, does not require. Meantime, by rainfall and the 
 natural disintegration of the soil, the land is becoming re- 
 plenished with exactly those substances in which it has be- 
 come deficient, enabling it, therefore, to supply food-stuffs 
 for another crop of the former kind in a succeeding year. 
 A comparison of the chemical composition of a few typical 
 crops, as, for example, those in the subjoined table, will 
 make this fact clearer. 
 
 1,000 PARTS OF DRY SOLID MATTER CONTAIN (PrantI) : 
 
 PLANT 
 
 - 
 
 1 
 
 - 
 
 i 
 
 .3 ! 1-8 
 | -c.-S ' -2 
 
 .a 
 
 1 
 
 orine 
 
 
 
 1 
 
 
 a 
 
 \ ^ ' < 
 
 C/3 
 
 
 js 
 
 
 Clover in bloom 
 
 68-3 
 
 21*96 
 
 1*39 
 
 24*06 
 
 7-44 0-72 ' 6-74 
 
 2'06 
 
 1-62 
 
 2 -C6 
 
 Wheat (grain) 
 
 19-7 
 
 6-14 
 
 '44 
 
 0-66 
 
 2*36 o'26 9'26 
 
 1 
 o'c7 o'42 
 
 0*04 
 
 ,, (straw) 
 
 53'7 
 
 7'. 3 0-74 
 
 3-09 i 1-33 i 0-33 2-58 ; 1-32 : 36-25 
 
 0-90 
 
 
 
 
 
 
 
 Potato tubers 
 
 37'7 
 
 22*76 
 
 0-99 
 
 0-97 
 
 I- 77 0*45 . 6'53 
 
 2 '4 5 
 
 o'8o 
 
 i-i7 
 
 Apples 
 
 14-4 
 
 5 14 
 
 3-76 
 
 o'sg 1*26 o'2o , 1*96 
 
 o'88 
 
 O'62 
 
 - 
 
 Peas (seed) 
 
 27'3 
 
 n'4i | 0*26 1 i '36 2*17 o'i6 9'95 
 
 o-95 
 
 0-24 
 
 0-42 
 
 We have seen, then, that the food of plants is solid, liquid, 
 and gaseous, and that it is obtained (in the case of green 
 plants) from the soil and the atmosphere. We have already 
 considered the composition of the atmosphere (p. 41) ; we 
 have now to glance for a moment at the composition of the 
 soil. 
 
 -O 2 
 
196 Ehmcntary Biology. 
 
 Composition of the soil, The composition of the soil 
 is as varied as that of the rocks by the disintegration of 
 which it is in the main produced. The animals and plants 
 living on it, however, add a considerable quantity of matter 
 to the soil by their excrement while living, as well as by the 
 products arising from the decomposition of their dead 
 bodies. The character of the soil is dependent in the first 
 instance on the nature of the subsoil and rock beneath, i.e. 
 on the nature of the geological formation of the district in 
 question. The character of the vegetation, as a rule, alters 
 very considerably according as the soil is clayey, loamy, 
 sandy, gravelly, chalky, or peaty. Not only SD, but the 
 presence or absence of various constituents determines 
 whether the soil will be wet or dry, cold or warm, rich or 
 poor. Particular crops are adapted to some soils rather 
 than to others a fact very familiar to all agriculturists. In- 
 deed, the occurrence and distribution of "many wild plants 
 within their area of habitat depends entirely on the presence 
 or absence of the suitable soil and the requisite amount of 
 moisture. 
 
 General physiology of the plant It has already been 
 pointed out in the early chapters of this book how necessary 
 it is that the great law of the conservation of energy should 
 be continually borne in mind when the physiology of pro- 
 toplasm is being discussed, and it will be found most advis- 
 able if we adopt this law for our guide in discussing the 
 physiology of the plant in general. Protoplasm is a highly 
 complex store of potential energy, in virtue of the possession 
 of which the plant is able to perform certain duties, related 
 to the maintenance, partly of tribal, partly of individual life. 
 Without protoplasm, so far as we know, no protoplasm is 
 formed. Since, then, this store must be built up ere it can 
 be broken down and used, it follows that we must discuss 
 the physiology of the plants first from the anabolic, and 
 secondly from the katabolic point of view. (Compare sec. ii. 
 Chap. L, sees. ii. and vi. Chap. II., and sees. i. andii. Chap. 
 
General Physiology of the Plant. 1 97 
 
 III. in this relation). Under the head of anabolism we will 
 discuss the absorption and assimilation of food-stuffs, both 
 of which processes may be summed up under the one term 
 nutrition. We will also consider the circulation of the 
 various food-stuffs, both in their prepared and unprepared 
 states, through the plant-body, and the storage of the excess. 
 Under the head of katabolism, on the other hand, we will 
 discuss the breaking down of protoplasm, and the formation 
 in consequence of degradation products, or katastates, 
 which may be again subdivided into secretions, or kata- 
 states still useful in the plant economy, and excretions, or 
 katastates which must be got rid of. Lastly we will deal with 
 the results of katabolisn?, or the various ways in which the 
 energy liberated by the decomposition of protoplasm mani- 
 fests itself in the plant These results will be found to group 
 themselves under the following headings : first, growth and 
 movement ; secondly, irritability or sensitivity ; thirdly, 
 heat, light, and electricity ; and lastly, phenomena con- 
 nected with the formation of reproductive cells. 
 
 I. Anabolism. 
 
 A. Nutrition. All the food of the plant must necessa- 
 rily be absorbed in the fluid form, i.e. in the form of a liquid 
 or a gas. Solids absorbed are taken up in solution in water, 
 those which are naturally insoluble in water being made so 
 by some substance (acid) produced by the plant. 
 
 We must first consider the laws which govern the 
 absorption of liquids and gases. All food material must 
 pass through the cell-walls of the leaf and root-hairs, which, 
 we have already seen, are the organs of absorption. Let us 
 examine first the passage of liquids, i.e. water with salts in 
 solution, through the cell-walls of the root-hairs ; and it may 
 be well to illustrate this subject by a simple physical experi- 
 ment, before taking up the special case before us. 
 
 A glass vessel is taken containing distilled water, and in 
 it is suspended a smalle/ glass vessel, which, however, has a 
 
198 Elementary Biology. 
 
 bottom made of parchment or bladder instead of glass. In 
 the smaller vessel is put a solution of common salt (NaCl). 
 If the apparatus be left for some time, and the two liquids 
 be then examined, it will be found that, although none of the 
 salt solution would have passed through the membrane had 
 the smaller vessel been suspended in air instead of in w r ater, 
 under the latter circumstances some of the salt solution has 
 permeated the membrane, and can be detected in the sur- 
 rounding water of the larger vessel by adding a small quantity 
 of a solution of nitrate of silver, which gives with sodium 
 chloride a white precipitate not obtainable from the dis- 
 tilled water before the experiment was begun. Moreover, 
 if the contents of the inner vessel be examined, it will be 
 found to contain a much weaker percentage of salt than 
 at the beginning of the experiment, showing that the solution 
 has been diluted by the addition of waiter. The quantity of 
 the solution is, moreover, greater absolutely, showing that 
 the passage of the salt alone, without water, will not account 
 for the weakening of the solution (fig. 104). Both water 
 and sodium chloride are crystalline substances, the former 
 at a temperature of o C., the latter at all ordinary tempera- 
 tures. Now if for the sodium chloride a solution of gum, 
 or of white of egg, or other non-crystallisable substance, be 
 substituted, it will be found that very little, if any ; of the 
 gum will pass through into the outer solution, though the 
 density of the gum solution will become less in consequence 
 of the passage of water into the inner vessel. Further, if 
 sodium chloride solutions of different density be placed on 
 either side of the membrane, a current is established between 
 the two solutions, which continues until the density of both 
 is identical. Lastly, if the membrane separate two solutions, 
 both uncrystallisable, neither substance will pass over into 
 the other, though the water of the solution may, until the 
 density of the two fluids be the same. 
 
 From these experiments we may formulate a law to the 
 effect that if a vegetal or animal membrane separate two 
 
General PJiysiology of tJie Plant. 
 
 199 
 
 FIG. 104. -DIALYSIS 
 (Edmoiv's.) 
 
 solutions, both of which are crystallisable, a current will be 
 
 set up through the membrane between them. .This passage 
 
 is known as osmosis. The current from the outer to the 
 
 inner vessel in the experiment is known as an endosmotic 
 
 current ; that from the inner to the outer is termed an 
 
 exosmotic current. Almost all crystallisable substances, 
 
 which can wet the membrane, are capable 
 
 of undergoing osmosis ; non-crystallisable 
 
 substances, as a rule, are not. Further, 
 
 many substances pass much more rapidly 
 
 through organic membranes than others, 
 
 so that there may be a very rapid endos- 
 
 mosis accompanied by a very slow exos- 
 
 mosis, or 77?* versa. Moreover, the rate 
 
 is also modified by the chemical nature 
 
 of the substances inside or outside the 
 
 membrane. 
 
 A root-hair is a vessel, a very small 
 one it is true, containing in its cell-sap 
 certain crystallisable and certain non- 
 crystallisable substances. These are se- 
 parated from the soil by two vegetal 
 membranes, the cell-wall and the proto- 
 plasm. The soil contains various crys- 
 tallisable salts in solution in water, or 
 capable of being made so by the action 
 of various acids &c. Here, then, we 
 have all the conditions for osmosis. 
 There will be a considerable endosmosis 
 and little exosmosis, for the substances 
 in the cell being in great part protoplasmic in their nature, 
 and therefore non-crystallisable, will have little tendency to 
 pass outwards into the soil, whilst the crystallisable solutions 
 in the soil will tend to pass inwards through the cell-wall 
 and protoplasm into the cell. It must be remembered, 
 however, that the protoplasm itself acts in many cases 
 
 n, level of water in outer 
 vessel ; /', inner ves- 
 sel containing salt so- 
 lution ; r, level of salt 
 solution after experi- 
 ment. 
 
2OO Elementary Biology. 
 
 differently from the cell-wall, and hinders many substances 
 from passing inwards which the cell- wall would allow to 
 pass through. 
 
 The chemical nature of the cell-contents influences to 
 a very great extent the nature and rapidity of the endosmotic 
 flow. It is believed that the organic acids and their salts, 
 always present in living cells, exert an attractive influence 
 on the water, although probably other substances as well 
 have a like, though not so great an, affinity for water. After 
 exosmosis of the water of the cell-sap, the protoplasm 
 contracts, leaving a space between it and the cell-wall. 
 This phenomenon is known as plasmolysis, and takes place 
 when the cells are treated with a solution of a substance 
 having a strong affinity for water. 
 
 We have now to examine the laws governing the absorp- 
 tion of gases by plants, 
 
 From the composition of the atmosphere given at p. 41 
 it will be seen that it is a mixture of several gases, the 
 principal of which, from a biological point of view, are 
 oxygen and carbonic acid. It must be remembered that 
 these gases are in no way chemically united, but are simply 
 mixed completely and equally, in obedience to the law of 
 diffusion, viz. that gases mix equally and in all proportions. 
 The method by which these gases are absorbed by the plant 
 cell is the same as that by which solids are absorbed. No 
 gas can be assimilated unless it be first of all in solution in 
 water or in cell-sap. The solubility of gases in water is, 
 therefore, a subject of extreme importance in a discussion 
 of the nutrition not only of aquatic, but also of terrestrial 
 plants (p. 42). 
 
 (a,) Absorption of water and dissolved salts. As 
 already seen (p. 196), the soil consists of a mixture of a 
 great many chemical compounds obtained from the dis- 
 integration of rock masses, and also from the ultimate 
 decay and disintegration of animal and vegetal matter. 
 The particles of which the soil is made up are surrounded 
 
General Physiology of the Plant. 
 
 201 
 
 by thin films of water, and it is this water (hygroscopic 
 water) which is believed to be the medium for the trans- 
 ference of the salts from the soil to the cell. The water 
 thus absorbed is replaced by more water drawn by capillary 
 attraction from the more distant soil. It has been found 
 that a dilute solution of a salt is more easily absorbed than a 
 strong solution, and that the power of roots to absorb is 
 greater when the temperature is high than when it is low. 
 
 FIG. 103. PLASMOLYSIS, illustrated on parenchyma from the peduncle 
 of Cephalarta leucantha. (De Vries.) 
 
 A, turgid cell ; B, the same cell in 4 p.c. nitre solution ; c, in 6 p.c. so'u- 
 tion ; P, in 10 p.c. solution, showing complete plasmolysis ; A, cell- 
 wall ; /, protoplasm ; A; nucleus ; c, chlorophyll corpuscles ; s, cell- 
 sap ; e, nitre solution. 
 
 This latter fact is to be explained by the increased evapora- 
 tion of water from the leaves under a high temperature. 
 
 Different plants, as we have already seen, require different 
 salts in varying proportions. What, then, governs the ab- 
 sorption of a salt ? Apparently the need of it by the plant, 
 that is to say, a vital (for want of a more scientific term) and 
 not a physical necessity. The extent to which any plant 
 absorbs a particular salt is termed its specific absorbent 
 capacity for that salt. The salt absorbed is assimilated, 
 
202 Elementary Biology. 
 
 undergoes, that is to say, the preliminary changes of ana- 
 bolism. The supply outside the root is greater than that 
 inside, hence more is absorbed, anabolism constantly re- 
 moving what osmosis is constantly supplying. We are here, 
 of course, considering only such substances as are soluble in 
 water, and which can penetrate through cell- wall and proto- 
 plasm into the cell-sap. What of those substances which 
 are insoluble in water? How, for instance, can calcic 
 carbonate be absorbed by the root when that substance is 
 insoluble in water? It has been already shown that all 
 living cells, in virtue of the katabolism which they are 
 undergoing, are constantly giving off, or respiring, carbonic 
 acid (p. 40). This carbonic acid, in the case of the cells of 
 the roots, does not readily escape, but, becoming dissolved 
 in the water surrounding the root cells, acts on the calcic 
 carbonate, changing it into the bicarbonate, which is soluble 
 in water. In this form calcium is taken into- the plant. No 
 doubt many organic acids are also employed by the plant in 
 the work of rendering insoluble substances soluble. 
 
 The amount, then, of the various compounds in the soil 
 which are found in the individual plant depends on the 
 specific absorbent capacity of the plant, and on the compo- 
 sition of the soil. Several examples of the composition of 
 the ash of plants have been already given at p. 195. 
 
 Leaves, stems, and even flowers are able under certain 
 circumstances to absorb water and salts in solution, a fact 
 familiarly known to all lovers of floral decoration. 
 
 (b.) Ab:orption of gases. Since, as mentioned above 
 (p. 191), free nitrogen is not used by the plant, we need not 
 consider its absorption at all. Indeed, the plant is always 
 bathed in free nitrogen, though no direct assimilation of the 
 gas takes place. The two gases whose absorption we have 
 specially to study are oxygen and carbonic acid. 
 
 Absorption of oxygen. Oxygen is absorbed by the plant 
 for two purposes, to assist in the formation of organic com- 
 pounds and also to oxidise these same compounds in order 
 
General Physiology of the Plant. 203 
 
 to liberate the potential energy stored up in them. Oxygen 
 then takes part both in anabolism and katabolism ; that em- 
 ployed in the former process is obtained from the compounds 
 of oxygen in the soil (water and oxy- salts), that employed in 
 the latter process is obtained from the atmosphere in solu- 
 tion in water. In the one case it performs a nutritive, in 
 the other a respiratory function. 
 
 The absorption of carbonic acid is a purely nutritive 
 process, taking place in sunlight and in the presence of 
 chlorophyll (p. 40). 
 
 What becomes of the various food-stuffs absorbed by the 
 plant ? They become transformed into organic compounds 
 under certain conditions. This transformation takes place 
 in two stages primarily the formation of certain compara- 
 tively simple organic compounds, which may be termed 
 primary products of assimilation, which are afterwards built 
 up into higher organic compounds. 
 
 Assimilation. The process known as assimilation con- 
 sists in the decomposition of the carbonic acid and water 
 absorbed by chlorophyll-bearing cells and the building up 
 of the products of decomposition into some comparatively 
 simple anastate. This process is accompanied by the libe- 
 ration of oxygen. Probably the anastate so formed is 
 formic aldehyde. In that case the primary change might 
 be represented thus : 
 
 CO 2 -f H,O=CH 2 O(formic aldehyde) + O 2 
 
 by the abstraction of one atom of oxygen from water and one 
 from carbonic acid. The carbonic oxide and hydrogen are 
 then said to be in a nascent condition, and would readily 
 unite to form formic aldehyde with the liberation of a mole- 
 cule of oxygen. These changes take place in the chloro- 
 phyll-bearing parts and in sunlight only. By the formation 
 of such anastates the dry weight of the plant is increased 
 considerably. The conditions of assimilation have already 
 been discussed in sections v. and vi. of Chap. II. 
 
204 Elementary Biology 
 
 The construction of nitrogenous compounds out of such 
 primary products of assimilation and the nitrogen salts ab- 
 sorbed by the roots forms the second stage in the process of 
 anabolism. At this point, however, the nature of the che- 
 mical changes involved becomes difficult to follow, and the 
 results of experiment and even of observation are anything 
 but beyond doubt. It is probable, however, that the com- 
 pounds of nitrogen unite with such compounds as formic 
 aldehyde, to form substances known as amides, the chief of 
 which in the plant is asparagin. These amides are then 
 transformed into proteids, and then into protoplasm, or into 
 protoplasm directly, by the action of already formed proto- 
 plasm. These changes take place quite independently of 
 sunlight and chlorophyll, which are necessary only for the 
 primary stages of anabolism. 
 
 Circulation. We have already had frequent occasion 
 to assume the transference of substances from one part of 
 the plant to another. We must now glance at the methods 
 by which the food-stuffs in the plant are distributed through 
 the organism. 
 
 Water is of course the great solvent in the plant, so that 
 the question of the circulation of food-stuffs really resolves 
 itself into a discussion of the movements of water. The 
 manner of entrance of water with salts in solution through 
 the root-hairs has already been considered under the head 
 of osmosis. The same principle holds good in the passage 
 of water with food- stuffs in solution from cell to cell. We 
 have to consider two types of plants, however, in dealing 
 with this question, viz. the vascular and the non-vascular. 
 In the latter the entire circulation is one of osmosis from 
 cell to cell, the cause of the movement being the disturbance 
 of the equilibrium between different cells by the removal, 
 by anabolic changes, of many of the substances undergoing 
 osmosis, the object of the movement being the re-establish- 
 ment of the equilibrium. Naturally this movement is a very 
 inconstant and irregular one. It also takes place in vas- 
 
General Physiology of the Plant. 205 
 
 cular plantb ui those parts where there are no vessels, as, for 
 example, in the cortex of the root, the parenchyma of grow- 
 ing parts, c. The turgid cells near the vessels, however, 
 give up some of their contents, which become squeezed into 
 the latter, more especially when the quantity of liquid in 
 plant is at a maximum, e.g. in spring. 
 
 The extent of surface exposed by a plant by means of 
 its leaves is enormous, and from this evaporation of water 
 is constantly taking place. Since, however, the cells of the 
 leaves and neighbouring parts remain turgid, it follows 
 that the supply of water must be constantly renewed ; that, 
 in fact, the amount of water got rid of by transpiration must 
 be balanced by the amount absorbed ; and that in conse- 
 quence there must be a constant flow, at least during the 
 time when the leaves are out, from root to leaf. What is the 
 course of this regular circulation ? Numerous experiments 
 have proved that the water travels by the young wood of 
 the fibre-vascular strand, and by the substance of the cell- 
 walls, and not through the cavities, of the vessels save 
 under the conditions mentioned above. Trie amount of 
 water transpire! by the leaves varies of course with the 
 nature of the plant and of the atmospheric conditions, 
 according to laws which may easily be deduced. The 
 stomata have an important influence also on the amount tran- 
 spired. The rate of circulation of water in the stem is very 
 considerable, in some cases from 100 cm. to 150 cm. per hour. 
 
 It has already been stated that the primary stages in 
 anabolism take place in the leaf, the result being the forma- 
 tion of, in the first instance, some comparatively simple 
 anastate such as formic aldehyde and then of some proteid. 
 Starch, the great reserve store compound, is also employed 
 as an anastate, though probably it is a product of the kata- 
 bolism of protoplasm in the first instance. In order that 
 the plant may be nourished a circulation of these substances 
 must take place. Since, however, starch and proteids are 
 practically insoluble it follows that they must be digested, or 
 
206 Elementary Biology. 
 
 turned into soluble substances, before they can be circulated. 
 This alteration of substances so as to make them soluble is 
 known as metastasis, and is brought about by means of 
 compounds produced by the plant itself which are known 
 as ferments. There are four chief kinds of ferments 
 known : the first is termed the diastatic ferment, its func- 
 tion being to render starch soluble ; this it does by trans- 
 forming it into sugar. The second, the proteolytic ferment, 
 has for its function the transformation of proteids into 
 peptones (soluble proteids) in the presence of an acid. The 
 other two ferments are less important : one has for its func- 
 tion the decomposing of substances known as glucosides 
 (compounds of sugar with some aromatic principle), the 
 other the converting of cane into grape sugar. All four 
 perform their function in the same way, viz. by hydration 
 (p. 23), the ferment itself not being altered in the process. 
 
 The paths by which these various products of fermenta- 
 tion move may be summarised thus : 
 
 (a) By osmosis, from cell to cell in the parenchyma. 
 
 (b) By intercellular communication of protoplasm in 
 
 sieve tubes (and doubtless many other tissues). 
 
 (<r) By laticiferous vessels. 
 
 The object of this circulation is twofold, viz. to supply 
 plastic material wherewith t3 build up new protoplasm, 
 and to carry material to storehouses in the stem, or root, 
 or other parts of the plants. In these stores the food-stuffs 
 are modified again into insoluble forms, such as starch, oil, 
 aleurone and various amides. They are again transformed 
 into soluble carbohydrates and proteids as they are wanted, 
 and transferred up the stem to the seat of anabolic activity. 
 This storage of plastic material in an insoluble form is the 
 visible expression of the transformation of kinetic into 
 potential energy ; while the reconversion of the insoluble 
 store into soluble plastic material is the first step in the 
 transformation of the potential energy of the stored food 
 into kinetic energy. Both transformations are accom- 
 
General Physiology of the Plant. 207 
 
 panied by a manifestation of heat, a very noticeable 
 instance being the heat developed during the germination 
 of the embryo. 
 
 In order that the various gases used in the metabolism 
 of plants may be supplied as directly as possible to the cells 
 where that metabolism is going on, it is necessary that they 
 should permeate every part of the plant. This they do by 
 
 FIG, 106. INTRRCELLULAR COMMUNICATION OF PROTOPLASM. (Vines.) 
 
 a 1 / 
 
 / 
 
 a, protoplasm of cell ; 3, protoplasmic threads connecting adjacent cells. 
 
 way of the intercellular spaces and in obedience to the 
 ordinary laws of gaseous diffusion. The stomata aid 
 greatly of course in the entrance of the gases into the 
 interior of the plant, although much of the gas passes 
 directly through the cuticle into the subjacent tissues. 
 Where stomata do not occur as in the Algae, the latter is the 
 only method. 
 
 Having briefly glanced at the various stages in the 
 anabolism of protoplasm,^we-roay A t^v consider its katabol- 
 ism, which must take - place before the energy stored up in 
 
208 Elementary Biology. 
 
 the complex compounds of which we have found protoplasm 
 to be composed becomes available. 
 
 KatabDlism. The chief agent in the decomposition of 
 protoplasm is oxygen. The oxidation of carbon compounds 
 and the liberation of carbonic acid we have already termed 
 respiration, so that we may conveniently consider that 
 subject first. 
 
 The inhalation of oxygen and exhalation of carbonic 
 acid must be most carefully distinguished from the absorp- 
 tion of carbonic acid and liberation of oxygen. In the 
 case of the animal, the determination of the ratio between 
 the gas inhaled and the gas exhaled is a comparatively easy 
 process ; in the plant it is complicated by the reabsorption 
 of ths carbonic acid so produced in the anabolism which 
 goes on during sunlight. In the dark, however, in the great 
 majority of green plants, and also in colourless plants both 
 at night and in daylight, it is easy to prove the exhalation 
 of carbonic acid in respiration. In the animal the oxygen 
 which is inhaled is almost immediately employed in the 
 oxidation of organic compounds ; in the plant, on the other 
 hand, the oxygen may be absorbed and stored up as intra- 
 molecular oxygen, and used as an oxidising agent long 
 afterwards. The chief products of respiration are the same 
 in both plant and animal, viz. water-vapour and carbonic 
 acid. It has been found, however, that in some plants car- 
 bonic acid is replaced as a product of respiration by other 
 so-called organic acids, such as oxalic and malic acid. No 
 doubt also much of the carbonic acid and water is trans- 
 formed into cellulose, as suggested at page 60, or excreted 
 directly as such. 
 
 The other products of katabolism are some of them 
 useful (secretions), some of them useless (excretions). It 
 has already been seen that what are considered as waste 
 products in the animal world, e.g. carbonic acid, water, and 
 such like, are far from being so in the plant world. In 
 addition to these substances, however, there is a large 
 
General Physiology of the Plant. 209 
 
 number of others secreted, or excreted by the plant, into 
 the discussion of which, however, we cannot enter here. 
 Many of these bodies, such as the aromatic substances, the 
 colouring matters of flowers, the vegetal alkaloids, perform 
 very important services in metabolism. Nectar (p. 169) 
 and the odorous substances employed in cross-fertilisation 
 in the manner already explained, are instances in point. 
 For a complete discussion of the subject of waste pro- 
 ducts, reference must be made to Vines' Lectures on Plant 
 Physiology^ lect. xii. 
 
 Results of metabolism. We have lastly to glance at 
 the results of metabolism, which are chiefly expressible in 
 terms of the law of the conservation of energy. We may 
 consider these results under two headings : first, those 
 connected with the evolution of heat, light, and electricity ; 
 and secondly, those which are related to growth and move- 
 ment. Connected with the phenomena of movement may 
 be considered those of sensitivity, or irritability. 
 
 Heat. Heat, we have already seen, is the ultimate form 
 into which all kinetic energy becomes transformed (p. 14). 
 The energy evolved by the plant is very considerable, and all 
 forms of it very rapidly undergo degradation into the final 
 form of energy. The surface of a plant, however, exposed 
 to the air is very great, so that the heat evolved is very 
 speedily dissipated. Of couise, where the greatest metabo- 
 lism is going on, as, for instance, in the reproductive organs, 
 in~ the growing shoots, and in germinating seeds, there the 
 greatest amount of heat is evolved. Unless specially pro- 
 tected, however, the temperature of a plant seldom rises 
 above that of the surrounding air, for the reason above 
 stated. It is more frequently beneath the temperature of 
 the air, owing to the cooling effect of evaporation from the 
 exposed parts. 
 
 Light. The manifestation of light by plants, often 
 known as phosphorescence, is a phenomenon exhibited only 
 by the lower plants, such as some of the Fungi and Algae. 
 
 p 
 
2IO Elementary Biology. 
 
 Like heat, light is a by-manifestation of kinetic energy, of 
 course connected with the general metabolism of the plant. 
 The origin and mode of development of the luminosity have 
 not as yet been satisfactorily explained. 
 
 Electricity. The existence of electrical currents has 
 been much more carefully investigated, and numerous valu- 
 able results have been arrived at. The connection between 
 the electrical phenomena displayed and the vital processes 
 awaits, however, further elucidation. It is well known that 
 certain chemical changes are always accompanied by an 
 electrical disturbance, and probably similar chemical changes 
 in the plant metabolism give rise likewise to similar electrical 
 manifestations. The well-known carnivorous plant Dioncea 
 muscipula has been very carefully investigated, with the 
 result that a very distinct electrical current has been ob- 
 served during the folding of the leaf ; a movement which 
 takes place when the leaf is irritated by a. suitable stimulus. 
 The results of many researches on this subject may be 
 summed up in few words. During the life of a plant, 
 normal electrical currents are constantly traversing its various 
 parts, though considerable doubt prevails as to the true 
 nature of these currents, and their connection with the ex- 
 penditure of energy, which, we have seen, is so intimately 
 related with the evolution of heat and light. In addition to 
 these ordinary or normal currents, other special currents are 
 developed in parts of plants, which, like the leaves of 
 Dioncea, are capable of movement upon stimulation. In 
 these cases the electrical disturbance is undoubtedly inti- 
 mately connected with the chemical changes suddenly set 
 up in the organ by the stimulus applied. 
 
 Among the directly visible results of the expenditure of 
 energy are, however, the very marked ones of growth and 
 movement. 
 
 Growth. Without the expenditure of energy no growth 
 can take place, for growth is an increase in the bulk of the 
 plant, usually accompanied by a temporary or permanent 
 
General Physiology of the Plant. 2 1 1 
 
 change of form. Growth is, of course, the result of active 
 constructive metabolism, and as such involves an increase 
 in the dry weight of the plant. We shall glance first at the 
 general conditions cf growth, and then briefly summarise the 
 phenomena of growth. 
 
 Conditions of growth. Growth can take place only in 
 the living plant, the growth of unorganised substances being 
 merely the result of accretion. Growth is, in the first place, 
 due to expenditure of energy taking place under certain 
 conditions of temperature. Again, an adequate food supply 
 in the form of the substances essential to the nutrition of 
 plants is necessary. Water more especially is of importance. 
 In addition, however, to these general conditions, growth is 
 modified by the ability of the protoplasm itself to assimilate 
 and respire, i.e. the anabolic and katabolic powers of each 
 indmdual cell must be taken into account. Now it has 
 been shown by Spencer that the mass of a cell increases as 
 the cube of the dimensions, while the surface increases only 
 as the square. The surface of the cell, however, is the area 
 of nutrition, i.e. the larger the surface, other things being 
 equal, the more nourishment can be absorbed. Again, the 
 same surface is the area of excretion, so that the surface of 
 the cell is at once alimentary and purificatory. In the early 
 life of the cell the anabolism is in excess of katabolism. 
 As, however, that excess gradually makes itself apparent in 
 increase of bulk, Spencer's law comes into operation, and 
 with the increase of size the possibility of the maintenance 
 of the same equilibrium between the anabolic and katabolic 
 process comes to an end. The relative excess of waste 
 products may kill the cell, but much more frequently, as in 
 young cells, restoration of the original conditions takes place 
 by division of the cell into two or more parts. Thus growth 
 of the individual cell brings about growth of the plant as a 
 whole. 
 
 The phenomena of growth are too complex for any 
 detailed treatment here. The most that we have space to 
 
 p 2 
 
212 Elementary Biology. 
 
 notice are some of the general characters of growing parts, 
 and their behaviour in response to certain changes in 
 external conditions. 
 
 Growing parts, e.g. the tip of the shoot, root, &c., are 
 remarkable for possessing an extreme flexibility though 
 they have small power of elastic recoil. If a young shoot 
 or branch be twisted it frequently remains so deformed 
 during its entire life ; a fact that is well known and even 
 embodied in a proverb. Again, all growing parts are 
 extremely succulent. This might be expected, since a 
 great mass of the food supply comes to the growing parts in 
 the form of a watery solution. The abundance of water in- 
 deed makes the cells turgid and tense ; a condition in itself 
 favourable to assimilation and growth. The growing parts 
 are, moreover, very extensible. As the very tip (punc- 
 tum vegetationis) is the region of most active cell division, 
 so the region behind the tip is that of maximum growth. 
 
 The greatest amount of growth takes place during ths 
 night. In the dark, when assimilation is in abeyance, cell 
 division is at a maximum. So in the same way the shaded 
 side of a plant grows more than the illuminated side, with 
 the result that the shaded side gradually bends round to 
 the light. This phenomenon is known as heliotropism. 
 Many parts of plants have, however, a tendency to bend 
 away from the light. Such parts may be termed aphelio- 
 tropic. The term geotropism has been applied to the 
 tendency which roots have to bend towards the earth's 
 centre. Although it is probable that gravitation has not a 
 little influence in producing this movement, yet the fact 
 that the shoot grows upwards, in opposition to the same 
 force, seems to throw some doubt on the assertion that this 
 is the only influence at work. In addition to these move- 
 ments growing parts seem also to be subject to a movement 
 termed nutation, due to unequal growth of different parts. 
 Examples of this movement are universally found in the 
 growing shoot and radicle, while modifications of the same 
 
Carnivorous Plants. 
 
 213 
 
 type are exhibited by the twining of tendrils, climbing stems, 
 twisting of elongated fruits, flower peduncles, &c. 
 
 There are many examples of movement exhibited by 
 leaves (e.g. of sensitive plant), flowers (stamens), and other 
 parts, which are not due to growth, but manifest themselves 
 in response to external stimulus, or take place at certain 
 periods of the day or of the life of the plant, as a result of 
 certain chemical changes going on in the plant itself. An 
 instance will be given in the concluding section of this 
 
 FIG. 107. LEAF OF Mimosa pudica. (Duchatre.) 
 
 A, during the day. B, during the night or after irritation. 
 
 chapter when we discuss the remarkable phenomena pre- 
 sented by carnivorous plants. 
 
 SECTION VI. CARNIVOROUS PLANTS. 
 
 Even an elementary text-book on biology would be 
 incomplete without an account of the remarkable phenomena 
 presented by the so-called carnivorous plants. Although 
 these forms belong to a number of different tribes, they all 
 agree in being able to absorb nitrogenous organic com- 
 pounds from the dead bodies of insects which they are able 
 
214 
 
 Elementary Biology, 
 
 to catch and kill by means of remarkable modification of 
 their leaves. The number of species which possess this 
 power is not great. They belong to widely separated 
 families, and are found scattered all over the globe. They 
 comprise some well-known forms, such as the famous 
 pitcher-plant, Nepenthes, the familiar Drosera or sundew, and 
 Dionaa, Venus's fly-trap. On examination we find that itis 
 
 FIG. 109. LEAF OF Drosera. (Darwin.) 
 
 FIG. 108. Drosera rotundi- 
 folia. (Bentham.) 
 
 possible to make a physiological classification of carnivorous 
 plants according to the way in which they treat the animals 
 they catch. One group is represented by the sundew, which 
 is able, not only to kill, but to digest the insects which it 
 catches ; the other, represented by Sarracenia, is unable to 
 do more than absorb the gases which arise by decomposition 
 of the dead bodies of the insects caught by the leaf. A, 
 
Carnivorous Plants. 
 
 215 
 
 short account of the structure and physiology of these two 
 types will sufficiently explain the mode of operation of the 
 mechanism employed in either case. 
 
 Drosera rotundifolia. The sundew, found in quantities" 
 on our marshy hill-sides, is a small plant consisting of a few 
 small roots, a rosette of peculiarly shaped leaves, and an 
 upright flower-bearing stem. The leaves are the organs by 
 
 FIG. no. -LEAF-TENTACLE OF Drosera. (Darwin.) 
 
 A, glandular head with drop-et of the secretion, B, anatomy of the gland. 
 
 \\hich the nutrition is carried out, and therefore merit fuller 
 description. Each leaf consists of a long petiole with a 
 flat spoon-shaped lamina, which is of a greenish purple or 
 red colour, and beset with tentacles, which are morpho- 
 logically soft thorns, from one to two hundred in number. 
 Each tentacle is tipped by a bulb-like gland, surrounded by 
 a minute drop of a viscid secretion, which gives to the plant 
 its popular name of sundew. Microscopic examination 
 shows the tentacle to be composed of a small fibre-vascular 
 
2l6 
 
 Elementary Biology. 
 
 FIG. in. LEAK OF Drosera WITH 
 
 TENTACLES PARTIALLY INFLEXED. 
 
 (Dr.rwin.) 
 
 strand reduced to its simplest elements, surrounded by 
 parenchyma, and covered by epidermis. Should a fly 
 chance to alight on an uninjured leaf, it is immediately 
 caught by the viscid secretion on the glands of the short 
 -central tentacles. Gradually, apparently in response to a 
 stimulus conveyed from the centre of the leaf to the peri- 
 phery, the much longer marginal tentacles begin slowly to 
 
 curve over their victim. The 
 fly soon becomes drowned in 
 the secretion produced by the 
 large number of tentacles 
 which ere long will have folded 
 over it. The entire movement 
 takes about a quarter of an 
 hour. 
 
 The secretion, however, 
 seems to have the power not 
 only of killing the insect, but 
 afterwards of digesting it that 
 is to say, of rendering the 
 insoluble nitrogenous com- 
 pounds composing its body 
 soluble and easily absorbed. 
 The ferment (proteolytic) in 
 the secretion acts in presence 
 of an acid also found in the secretion, and is therefore, as 
 we shall see afterwards, quite comparable to the ferment 
 found in the stomachs of animals. 
 
 The tentacles may be made to bend inward by a simple 
 touch, but not by wind or rain. Contact with a solid or the 
 absorption of the minutest trace of a nitrogenous fluid is, 
 however, the most efficient stimulus. 
 
 Sarracenia purpurea. The extraordinary leaves of this 
 plant are particularly well adapted to fulfil the function of 
 catching and destroying insects and such like, which may be 
 unfortunate enough to tumble into their widely open trum- 
 
Carnivorous Plants. 
 
 217 
 
 pet-shaped mouths. The secretion, however, produced by 
 the glands in the interior does not seem to have the power 
 of digesting the dead bodies. The genus Sarracenia itself is 
 
 FIG. ii2. -LEAVES OF Sarraccnia. (Geddes.) 
 
 A, attracting and secreting surface. B, glassy epidermis, c, secreting 
 surface. D, detentive surface. 
 
 distributed over the eastern States of North America, and is 
 abundantly cultivated also in hothouses in this country. 
 
 The leaves, like those of Drosera, are radical. Each 
 trumpet-like leaf is surrounded by a large gaudy nectar- 
 secreting blade, serving as a protection from the rain, which 
 
218 Elementary Biology. 
 
 would otherwise soon fill the hollow portion of the leaf and 
 render it useless. The microscopic structure of the trumpet 
 is of some importance. The lid and the mouth of the tube 
 are covered by honey glands, and by small downwardly 
 directed hairs. Some of these glands are also distributed 
 on the outside of the trumpet on the ladder-like ledge which 
 runs up one side. Following upon the gland-bearing area 
 is a zone covered by slippery epidermal cells ; after that a 
 zone on which are numerous glands secreting the fluid with 
 which the trumpet is partly filled. Lastly, there comes a 
 zone where the epidermal cells are pulled out into long, 
 stiff downwardly directed hairs or bristles. An insect, lured 
 up the honey-strewn path on the outside of the pitcher in 
 search of more honey, climbs into the mouth itself, and, 
 gently goaded by the short hairs of that region, reaches the 
 glassy epidermis over which it slides rapidly downwards 
 amongst the secretory glands, only to be stranded on the 
 long bristles, which, though they permit of a downward, effec- 
 tually prevent an upward, movement. Indeed, the struggle 
 of the hapless visitor only serves to stimulate the glands to 
 more active secretion, washing it into the putrid fluid below. 
 
 Virgil's lines 
 
 Facilis descensus Averno ; 
 
 Seel revocare graclum superasque evadere ad auras, 
 Hoc opus, hie labor est 
 
 are not more applicable to the descent into Avernus than 
 they are to trumpet-shaped leaves of Sarracenia. 
 
 The leaf seems to have the power only of absorbing the 
 gases arising from the decomposition of the dead bodies and 
 decaying animal matter lying in the pitcher. 
 
 The description of these two species must suffice to illus- 
 trate this subject. Full details of the marvellous adaptations 
 for the absorption of nitrogenous organic compounds occur- 
 ring in other species and genera will be found in the article 
 on * Insectivorous Plants ' in the Encyclopedia Britannica 
 or in Darwin's Insectivorous Plants. 
 
Metazoa Obelia. 2 \ 9 
 
 CHAPTER IX. 
 
 METAZOA 1NVERTEBRATA. 
 
 SECTION I. HYDROZOA OBELIA. 
 
 IT will be remembered that in Chap. IV. sect. iii. we divided 
 plants into two great groups, Protophyta and Metaphyta. 
 Amongst animals we similarly formed the two divisions of 
 Protozoa and Metazoa. We have now completed our sur- 
 vey of the chief types of the Metaphyta, and are prepared 
 to take up the thread dropped in Chap. VI., and follow out 
 a few of the principal modifications exhibited by the various 
 groups of the Metazoa. 
 
 As examples of the lower Metaphyta we selected Spiro- 
 gyra and Fucus. Both these forms belong to the Algae. 
 Among the lower Metazoa a similar large group exists, the 
 Hydrozoa, some of the members of which are, indeed, 
 frequently mistaken for Algae by the uninitiated. There are 
 numerous points of superficial resemblance between the two 
 groups, far apart as they really are in structure and physio- 
 logy. 
 
 The type most frequently selected to illustrate the group 
 of the Hydrozoa is Hydra, the fresh -water 'polype,' a selection, 
 however, by no means in all respects fortunate, since its 
 method of development is far from typical. We will employ 
 it only in order to illustrate the minute structure of the 
 Hydrozoa, and describe the mode of reproduction and life- 
 history in a more generalised form, such as Obelia genicu- 
 lata. 
 
220 Elementary Biology. 
 
 Obelia is one of the salt-water Hydrozoa, variable in size, 
 but seldom reaching a greater height than six inches. It is 
 
 FIG. 113. PORTION OF A COLONY OF Obelia ^enlcnlata. 
 
 a, c, coenosarc ; l>, perisarc ; d, ?ooid ; c, sexual zooid with rudimentary 
 gonophores. 
 
 fixed to the rock by a network of root-like filaments, which, 
 however, are only superficially comparable to roots. The 
 
Metazoa Obeli a. 221 
 
 upright stems spring from the rooted portion, each stem 
 having smaller branches. There may be few or many up- 
 light stems, and the root- formation or stolon may show a 
 corresponding variability in extent. Each branch is termi- 
 nated by a zooid, and the entire structure is a colony, [he 
 persons of which are thus intimately connected with each 
 other. 
 
 The stolon is a branching and anastomosing network of 
 delicate tubes, the cavities of which are perfectly continuous 
 with each other and with the upright stems which spring from 
 them. The tubes of the stolon are covered externally by a 
 horny, laminated cuticle, known as the perisarc, enclosing 
 a cellular axis or coenosarc. The outer layer of the coeno- 
 sarc shows no very clear division into cells, though nuclei 
 are scattered at intervals through it, pointing to the fact 
 that the layer is of cellular origin. The cavity of the tube 
 is lined by a layer of cells bearing cilia. To the outer or 
 protective layer the name of ectoderm is given ; to the 
 inner or digestive layer, that of endoderm. These two 
 terms must on no account be confounded with the terms 
 ecto- and endo-sarc used in describing the differentiated 
 layers of protoplasm in the Protozoa (p. 56). The endoderm 
 and ectoderm are layers of cells ; the ectosarc and endosarc 
 are layers of protoplasm of one cell. The structure of the 
 upright branches is the same as that of the stolon, save that 
 the central cavity becomes larger and more distinct as the 
 ends of the various branches are reached. 
 
 It will be advantageous at this point to digress and 
 examine another related Hydrozoon known as Hydractinia. 
 A common species (ff. echinatd] is found covering the shells 
 of many hermit-crabs. Like Obelia it consists of a rooted 
 branched stolon and upright stems. But the stems are short, 
 and each is terminated by a single zooid. Four distinct 
 types of zooid can be identified. One known as the ali- 
 mentary zooid consists of an upright stem or body, con- 
 tinuous with the stolon at its fixed extremity, and terminated 
 
222 
 
 Elementary Biology. 
 
 at the other by a swollen head, bearing in the centre a 
 mouth, and fringed with a circle of tentacles. The head 
 is conical, and the tentacles spring from its broadest portion. 
 The conical eminence over the origin of the tentacles is 
 known as the hypostome. The hypostome bears the termi- 
 nal mouth aperture leading into the interior of the zooid. 
 The cavity into which the mouth leads we shall term the 
 
 FIG. 114.-- PORTION OF A COLONY OF Hydractinia. (Glaus.) 
 
 a, alimentary zooid ; d, tentacular zooid ; e, reproductive zooid 
 
 enteron, but defer the explanation of that term until we 
 have completed our study of the general morphology. 
 Another kind of zooid is seen near the alimentary zooid, 
 possessing a body and swollen head ; but the head carries 
 no tentacles, and there is no mouth. This degraded form 
 we shall term a tentacular zooid. Yet another type is to 
 
Metazoa Obelia. 223 
 
 be distinguished, like the tentacular zooid in general appear- 
 ance, but bearing two-thirds of the way up a number of 
 swellings. This is the reproductive zooid. 1 Lastly, there 
 are to be found scattered here and there throughout the 
 colony upright stiff branches, incapable of movement and 
 tapering to a point. The perisarc in this type of zooid is 
 largely developed. This we may term a protective zooil 
 Before proceeding to the comparison of Hydractinia and 
 Obelia we must endeavour to explain this peculiar arrange- 
 ment and modification of parts. 
 
 We have often had occasion to refer to division of labour 
 as the key to the understanding of the evolution of highly 
 developed from lowly developed types, briefly sketched out 
 in Chapters II. and III. We must again employ this expla- 
 nation in the present instance. Examining into the func- 
 tions of the several zooids in the colony, we find that the 
 alimentary zooid digests food not only for itself but for the 
 whole colony ; the tentacular zooid works towards the same 
 end by catching and killing small animals floating in the 
 water ; the reproductive zooid, fed like the others through 
 the stolon by the nourishment prepared by the alimentary 
 zooid, confines its attention to the production of gonophores 
 (p. 227) capable of forming sexual elements ; whilst lastly, 
 on the approach of danger, all three varieties of zooid rapidly 
 contract under shelter of the strong pointed protecting 
 zooids, which stand up firm and erect to shield the others 
 from harm. The tentacular zooid no doubt also serves to 
 warn off enemies by its active movements on the border of 
 the colony. In other words, we have in a Hydractinia 
 colony a number of persons all reducible to the same type 
 but each modified in some peculiar way to perform one or 
 other of the functions necessary to the maintenance of indi- 
 vidual and tribal life. We have morphological differentia- 
 tion accompanied by physiological specialisation or division 
 
 1 Though termed the reproductive zooid, it is to be noted that it 
 does not produce sexual cells (p. 227). 
 
224 Elementary Biology. 
 
 of labour. As in the plant we found leaves nutritive in 
 their function and leaves reproductive (sporophylla), leaves 
 protective (sepals) and leaves attractive (petals), so in the 
 Hydrozoa we have zooids performing nutritive, protective, 
 and reproductive functions ; only in the case of the plant 
 we had differentiations of organs, in the Hydrozoa differen- 
 tiation of persons or zooids. 
 
 If we now return to the consideration of Obelia, we 
 find that the various zooids are becoming more closely, i.e. 
 organically, united, but physiologically less dependent on 
 each other. All the zooids (the protective zooids are want- 
 ing) are, moreover, aggregated or gathered together on one 
 branch ; a manifest gain, for not only is a far smaller 
 amount of stolon required, but the colony being closely 
 packed can maintain itself in healthy life, with less expendi- 
 ture of energy, and is moreover not so liable to injury in 
 its compact form. If space permitted, we might instance 
 Hydrozoa that showed still further aggregation. We must 
 content ourselves by a reference to Hydra only. In this 
 form we have not only the functions performed by one 
 person, but we have degeneration and simplification taking 
 place as well. Hydra does not form colonies, and the in- 
 dividual consists of one zooid similar to the alimentary zooid 
 of the Hydractinia, but with the power of producing sexual 
 organs, which latter are of a far simpler type than those 
 possessed by Obelia or even Hydractinia. 
 
 We must now leave general questions and devote our- 
 selves to the study of the social economy of the colony, and 
 the life-history through which it passes. 
 
 As already mentioned, the alimentary zooid of a Hy- 
 dractinia or an Obelia does not materially differ from a 
 Hydra. It will, therefore, be convenient to describe the 
 histological structure and mode of life of Hydra as an ex- 
 ample of an alimentary zooid. One point of difference we 
 must, however, note, viz. that the perisarc is absent in Hydra. 
 The body and tentacles, eight in number, are eminently 
 
Metazoa Obelia. 
 
 22$ 
 
 FIG. 115. TRANSVERSE SECTION OF 
 THE WALL OF Hydra. 
 
 contractile, and are composed of two layers an outer ecto- 
 derm and inner endoderm. The cavity, or enteron, opens 
 to the exterior by the terminal mouth, and is continuous 
 throughout the body and into the tentacles. The ectoderm 
 is composed of large pyramidal cells, having the pointed 
 ends, which are internal, pulled out into long processes ; 
 these processes from the ectoderm cells form a tolerably 
 complete layer beneath 
 the ectoderm termed the 
 neuro - muscular layer. 
 The ectoderm cells are 
 nucleated and granular, 
 and contain, in addition, 
 pear - shaped bodies, 
 known as nematocysts. 
 The nematocysts are 
 organs of offence and de- 
 fence. Each consists of 
 a small sac in the interior 
 of which is coiled a very 
 long barbed thread, lying 
 bathed in what is no 
 doubt a poisonous fluid. 
 The animal can at will 
 evert the thread with great 
 rapidity, with the effect of 
 stupefying or poisoning 
 the prey it strikes. Nema- 
 tocysts occur in the endoderm also. The tentacles of the 
 alimentary and tentacular zooids of Obelia and other genera 
 are plentifully supplied with nematocysts. Numerous young 
 cells lie in nests round the pointed inner ends of the ecto- 
 derm cells (fig. 114), ready to take the place of the ecto- 
 derm cells when these are shed, as they frequently are when 
 a nematocyst is fired. 
 
 The endoderm differs considerably in character from 
 
 ectoderm ; 6, endoderm ; c, neuromuscular 
 layer ; d, basemem membrane ; <?, cilium on 
 an en odeTn cell ; _/, amoeboid endodermal 
 cell ; , ectoderm cell with young cells at 
 iis base ; Ji, /', nematocyst. 
 
226 
 
 Elementary Biology. 
 
 FIG. 116. LONGITUDINAL SECTION OF THE BODY 
 WALL OF Hydra. KILLED DURING DIGESTION. 
 (T. J. Parker.) 
 
 the ectoderm. The cells are large and irregular, and are at 
 one time furnished with one or more cilia ; at other times 
 they are amoeboid and throw out blunt pseudopodia. They 
 rest on a fine structureless lamina, which again supports 
 the neuro- muscular layer above mentioned. It is a point 
 
 of considerable biologi- 
 cal interest to find, as 
 we do in Hydra viridis, 
 chlorophyll corpus- 
 cles, more especially 
 when we learn that 
 these have the power 
 of decomposing car- 
 bonic acid just as in 
 the plant. 
 
 Food material taken 
 into the mouth disin- 
 tegrates in the enteron, 
 and the food particles 
 are taken into the in- 
 terior of the endoderm 
 cells, which lose their 
 cilia and become am- 
 oeboid for the purpose- 
 In the interior of the 
 endoderm cells the 
 food particles gradually 
 become transformed 
 into soluble food-stuffs 
 
 termed peptones. This process is known as intracellular 
 digestion, and is of common occurrence in many of the 
 lower animals. The other parts of the body are no doubt 
 nourished by osmosis of the soluble peptones from cell to 
 cell. The excreta and useless parts of the food are ejected 
 by the mouth. 
 
 This account of the structure and physiology of Hydra 
 
 ec, ectoderm ; en, endoderm ; mp, neuro-muscu 
 lar fibres ; d,f, food particles. 
 
]\Ietazoa Obelia. 
 
 227 
 
 FIG. 117. TYPICAL MEDUSOID. (Hincks.) 
 
 will serve also for that of the alimentary zooid in Obelia or 
 Hydr actinia. 
 
 Turning now to the reproductive zooids, we are at once 
 brought face to face with the phenomenon of alternation of 
 generations, which we found to be so important a feature in 
 the life-history of plants. It is strange, however, that, whilst 
 it is the higher plants 
 that exhibit this pheno- 
 menon best, it is, on the 
 other hand, most per- 
 fectly developed in the 
 lower animals. The re- 
 productive zooid in 
 Obelia consists of a 
 short branch over which, 
 however, the perisarc is 
 continuous. The modi- 
 fied zooid within the 
 perisarc gives off buds, 
 consisting of protrusions 
 of both endoderm and 
 ectoderm. The bud in- 
 creases in size, and, at 
 the same time, takes on 
 a bell-shaped form with 
 a projecting tongue pen- 
 dant from the concavity. 
 We need not follow all 
 the stages in its development, but rather describe briefly 
 the completed product. The bud when ripe drops off and 
 swims away a free, independent organism, a gonophore or 
 medusoid. These medusoids were long looked on as distinct 
 animals, before their development from the fixed colony 
 was clearly made out. 
 
 A medusoid, as already stated, is bell-shaped, the con- 
 cavo-convex body being known as the disc, or necto-calyx 
 
 Q2 
 
228 Elementary Biology. 
 
 The edge of the bell has an inwardly directed rim, or velum, 
 and carries four or a multiple of four long tentacles (fig. 1 1 7). 
 The tongue, or manubrium, is not unlike the head of an 
 alimentary zooid turned upside, down and denuded of 
 tentacles. The cavity of the manubrium leads into a quad- 
 rangular space, or enteron, which again is prolonged into 
 four radial canals, hollowed out of the tissue of the disc 
 and united by a circular canal which runs round the rim. 
 Prolongations of this canal occupy the axes of the ten- 
 tacles. The cavity of the manubrium, enteric cavity, and 
 
 FIG. 118. MUSCLE-FIBRES AND NERVE CELLS FROM THE NECTO-CALYX 
 OK Aurelia aurita. (Schafer.) 
 
 the entire series of canals are lined by endoderm. In fact, 
 the medusoid is merely a greatly modified zooid (which has 
 arisen as a bud from the reproductive zooid) turned upside 
 down and free swimming. The medusoid, however, is a 
 much more highly organised creature than its parent. In 
 the first place, its tissues are more highly differentiated. In 
 the hydroid form there were no cells specially set apart to 
 contract ; each cell retained that power. In the gonophore 
 there are often distinct muscular fibres scattered through 
 the nectocalyx, by the rhythmic contraction of which the 
 
Metazoa Obelia. 
 
 229 
 
 umbrella is made to pulsate and the gonophore to move. 
 The muscular fibres are cells which have become elongated 
 in one direction, and though still nucleated they show a 
 differentiation of the protoplasm which takes the shape of 
 a very marked transverse striation. In Hydra every cell 
 was sensitive to touch. 1 In the medusoid there are, scattered 
 
 FIG. 119. MESODERMAL TISSUE FROM THE NECTO-CALVX OF 
 Aurelia aurita. (M. Schultze.) 
 
 through the necto-calyx, special branched cells, which have 
 probably for their function the regulation of the muscular 
 movements, and, therefore, termed nerve-cells. These cells 
 and fibres are specially numerous near the edge of the bell. 
 Sensitive cells, since they convey to the animal the impres- 
 
 1 There is some ground for believing that the processes of the ecto- 
 dermal cells of Hydra are nervous as well as muscular in function ; 
 hence the term ' neuromuscular ' applied to them. Jickeli has, more- 
 over, lately described true nerve-cells in Hydta. 
 
230 Elementary Biology. 
 
 sions made upon them by the external environment, might 
 naturally be expected to be ectodermal in origin, and the 
 nerve-cells of the medusoid have been found to originate 
 from superficial cells which have become sunk in the more 
 or less gelatinous tissue that fills the spaces between the 
 ectoderm and endoderm. This gelatinous tissue, through 
 which numerous cells are scattered, is looked upon as a 
 middle layer, much more highly differentiated in the higher 
 animals, and termed there the mesodenn. 
 
 Moreover in the free-swimming gonophore we meet for 
 the first time with sense-organs, special nervous organs for 
 the reception of impressions of the outside world, whether 
 in the form of vibrations of light or of sound. In the 
 neighbourhood of these organs the nervous elements are 
 very fully developed ; indeed, the cells which make up the 
 sense-organ are in direct continuation with the terminations 
 of the nerve-cells. The sense-organs are of four different 
 kinds, ocelli or eyespots, otocysts and tentaculocysts or 
 organs of hearing, and olfactory pits or nasal organs, all of 
 which are found round the margin of the bell, and most of 
 them near the bases of the marginal tentacles. Since, how- 
 ever, these organs are specialised in their character we need 
 not enter into further detail with regard to them. 
 
 Just as the thallus of the fern was an independent gene- 
 ration for the dissemination of the type, producing sexual 
 organs at a distance from the parent asexual plant, so the 
 medusoid is a very effective means for the dispersal of the 
 embryos of the Hydrozoa. After a certain period of free 
 existence the gonophore develops ovaria or spermaria be- 
 tween the ectoderm and endoderm layers in the manubrium 
 or on the course of the radial canals. 1 The sexual products 
 escape by the mouth or by rupture of the wall of the canal, 
 fertilisation taking place in the sea-water. The ovum is a 
 
 1 Weisman has shown that the ova and sperms are really produced 
 in the stem and afterwards migrate into the gonophore. 
 
Metazoa Obeli a. 2 3 1 
 
 naiced, often amoeboid cell ; thespermhas the typical head and 
 flagellum which are so characteristic of the male sexual cell. 
 
 The fertilised ovum or embryo segments into a large 
 number of cells, which arrange themselves into a hollow 
 sphere. This is known as a blastula, and the central cavity 
 as the blastoccele or segmentation-cavity. From the wall- 
 cells, however, many cells are given off by budding until the 
 wall has been made two layers thick. The embryo is now 
 known as a planula, and the cavity left or afterwards 
 formed in it is known as the enteron or primitive alimen- 
 tary canal. The outer layer is that which will give rise 
 to the ectoderm, and is called the epiblast ; the inner 
 layer gives rise to the endoderm, and is known as the 
 hypoblast The epiblast cells at this stage are ciliated. 
 
 The planula is a free swimming organism, using its cilia 
 as organs of locomotion. After a time the cilia disappear 
 and the embryo settles down on one end. It then forms an 
 adherent disc and begins to branch over the rock or sea- 
 weed on which it has fixed itself, elongating and sending 
 out buds, which develop into zooids like those already 
 described in the adult fixed form with which we started. 
 The perisarc meantime forms, and by a branching of the 
 stolon in all directions a new colony is produced. 
 
 There are several points of great importance to be looked 
 at before we pass on to the consideration of a higher type. In 
 the first place we may emphasise the appearance of a differ- 
 entiation of the cells formed by segmentation of the ovum, 
 into two or, in the medusoid, possibly three layers, which 
 give origin in process of development to entirely different 
 series of organs. We may note especially the development 
 of the nervous elements from the ectoderm and the lining 
 of the alimentary canal from endoderm. Again, the occur- 
 rence of marked alternation of generations in both kingdoms 
 is worthy of thought. The early differentiation of tissues in 
 the animal and the appearance of the sensitive system are 
 also points of great importance. With regard to the latter 
 
2 32 Elementary Biology. 
 
 we must here briefly describe how the nervous system of the 
 higher animals has probably arisen from the simple elements 
 we find in the Hydrozoa. 
 
 Briefly stated, in the animal groups immediately above 
 the Hydrozoa the tendency is first of all towards elongation 
 of the body and concentration of the sense-organs at the. 
 anterior or head end. Naturally that would lead to aggre- 
 gation of the nervous elements at the anterior end and along 
 either side as being the points of most frequent contact. 
 The head as containing the sense-organs, and therefore being 
 the regulating centre for the entire body, would contain, 
 naturally enough, the largest number of nerve-cells, which 
 would form one or more clumps or ganglia at either side of 
 the mouth, connected by commissures or connecting strands 
 of nervous tissue for the sake of co-ordination ; and from 
 these ganglia nerve impulses would be sent off along the 
 nerve fibres collected in the form of lateral cords or bands 
 along either side of the body. This is the actual arrange- 
 ment in many of the lower worms, and intermediate stages 
 are not wanting connecting such types with the diffused 
 nerve plexus of the Hydrozoa. 
 
 We have thus seen that the hydrozoon begins its life- 
 history as an amoeboid cell ; that by segmentation and 
 foimation first of a one-layered and then of a two-layered 
 sac or planula it passes on to the adult stage by subsequent 
 differentiation. We may now examine a type of which the 
 same is true, but which goes further, and where the embryo 
 is itself segmented. Further, an aperture is opened at the 
 posterior end of the enteronto permit of the more convenient 
 escape of excreta, and special organs are developed out of 
 mesoblast for the performance of duties which each in- 
 dividual cell of Hydra has to perform for itself. These 
 organs are lodged in a space hollowed out of the mesoblast 
 itself, the ccelom, or body-cavity, or in outgrowths from the 
 primitive alimentary canal. 
 
Metazoa Lumbricus. 233 
 
 SECTION II. VERMES LUMBRICUS. 
 
 One of the largest of the natural groups into which the 
 animal kingdom is divided is the Vermes ; and this group is 
 of special importance, not only on account of its size, but 
 also because it is amongst the forms included within it that 
 we must look for the type on which the majority of the 
 lower animals are built. Moreover there is good reason to 
 believe that even the higher animals are closely related 
 genealogically to the worm group, for they undoubtedly still 
 retain many vermian characteristics, if not in their adult 
 state, yet in the embryonic phases of their life-history. 
 
 To illustrate the group of the Vermes, and at the same 
 time to obtain a basis for the correct understanding of the 
 morphology of the higher forms of animal life, we may take 
 the common earthworm. Although Lumbricus is perhaps not 
 a perfectly satisfactory type of vermian structure, yet that dis- 
 advantage is more than counterbalanced by the readiness 
 with which it can be obtained, and the ease with which the 
 principal features in its anatomy may be made out. 
 
 Lumbricus terrestris is familiarly known to everyone as 
 the animal which burrows in soft, moist earth, appearing on the 
 surface during rainy weather and sinking far down as drought 
 comes on. Its habits and general physiology are also well 
 known after their admirable expression in Darwin's work on 
 The Formation of Vegetable Mould by the Action of Worms. 
 That the earthworm swallows large quantities of mould from 
 which it extracts the small quantity of food material it re- 
 quires (in the form of decaying vegetable matter), and that 
 it has the remarkable power of replacing lost parts of its 
 body, and recovering from very serious injury, and that 
 rapidly too, are facts very familiar to everyone who has 
 handled a spade. The general appearance of the animal 
 also, the long, tapering, many jointed body, without any dis- 
 tinction into head, neck, trunk, and appendages, the marked 
 yellow band present nearer one end at certain seasons of 
 
234 Elementary Biology. 
 
 the year, the serpentine motion, and perhaps even the exist- 
 ence of the rows of bristles on its under surface by which 
 that movement is facilitated are likewise common know- 
 ledge. 
 
 It will be remembered that in Chap. III. sect. i. we dis- 
 cussed the differentiation of organs according to the duties 
 which they severally had' to perform ; and we saw that it 
 was possible to classify these organs according to the share 
 they had in collecting, assimilating, and distributing the food 
 (p. 51). It will be found most convenient if we adopt this 
 classification in the discussion of the morphology of the 
 animals with which we have yet to deal. In the types of 
 plant life which we considered, and in the Hydrozoa, the 
 examination of which we have just concluded, it was not 
 possible nor advisable to treat of the various organs sepa- 
 rately from each other according to that plan ; for what 
 particularly strikes us in these types is the want of that 
 extreme differentiation of organs which we shall afterwards 
 see is so characteristic of the higher animals. The leaves 
 were the organs of respiration as well as of nutrition, in both of 
 which functions the stem and branches, if green, participated. 
 The branches, stem, and roots were the organs of circulation, 
 but they were also organs of support, and frequently played 
 an important part in nutrition, in addition to that already 
 referred to. Indeed it might be said that the highest plant 
 is far below an animal even so low as the worm, because of 
 this want of differentiation, although the power the worm 
 possesses of replacing lost parts also points to a lowness of 
 organisation, expressed in the plant by the power it likewise 
 possesses of reproducing itself by buds, shoots, and cuttings, 
 and of giving origin to new leaves and branches when the 
 former ones are removed. We have just seen that among 
 the lowly organised Hydrozoa the same characters are 
 prominent. 
 
 We may best commence our examination of Lumbricus 
 by a survey of its external characters. The animal varies 
 
Metazoa L umbricus. 
 
 235 
 
 FIG. 120. Linnb 
 terrestris. 
 
 greatly in length, some giant earthworms being known to 
 naturalists who have investigated speci- 
 mens from tropical countries. A com- 
 mon length for the species so abundant 
 in our own country is from three to 
 nine inches. The body is of tolerably 
 uniform diameter throughout, and is 
 pointed at either end. Its length is 
 subdivided into compartments by a 
 series of rings from one to three hundred 
 in number. The compartment between 
 every two rings is known as a segment 
 or somite. Although at first sight the 
 distinction is not very apparent, yet if 
 a careful examination be made it will 
 be found that the earthworm possesses, 
 if not a head, at least an anterior or 
 head end, which may be recognised by 
 its more pointed character and darker 
 colour. The posterior end, though also 
 tapering, ends more abruptly. The 
 dorsal aspect is more darkly coloured 
 than the ventral aspect, which latter is 
 further distinguished by the presence of 
 bristles, or setae. The setae may be 
 readily felt if the forefinger and thumb 
 be gently drawn from the tail to the 
 head of the living worm. On the ventral 
 surface of the anterior end will be found 
 the mouth or anterior opening of the 
 alimentary system. The projection of 
 the body above and in front of the 
 mouth is known as the prostomium. 
 Similarily at the posterior end, but in p, pnstomium; c , ciitei- 
 
 .1 . -ii i lum. (The number of 
 
 this case at its very extremity, will be Semites Lehind the 
 found the posterior opening of the ali- g3 should be 
 
 -XXVIII 
 
 -mvi 
 
236 Elementary Biology. 
 
 mentary system, the anus. Some distance from the anterior 
 end there occurs a swelling on the body, usually of a 
 yellowish colour, thus contrasting with the dark reddish 
 brown of the general body surface. This swollen portion 
 goes by the name of the clitellum. The clitellum usually 
 occupies the position of the twelve somites after the twenty- 
 eighth. The ventral aspects of the ninth to the fifteenth 
 somites are also slightly tumid, marking the openings of the 
 ducts from the reproductive organs and from certain organs 
 accessory in function to the ovaria and spermaria. These 
 are the chief features which strike the eye in an examination 
 of the external surface of the worm. If, however, a hand 
 lens be employed two minute apertures will be found on the 
 ventral surface of each somite, which mark the openings 
 of the purifactory or renal organs, known in the worm as 
 nephridia. 
 
 We may now suppose the worm- to be laid open by a 
 median longitudinal incision along the dorsal surface. If 
 this incision be carefully made, and if the skin be pinned 
 back, the internal anatomy may be seen with very little 
 supplementary dissection. 
 
 The alimentary system. The alimentary system con- 
 sists of a straight tube running from the mouth to the anus, 
 with certain dilatations at intervals. The dilatations are 
 of great importance as indicating morphological differentia, 
 tions in the alimentary system into parts which have differ- 
 ent functions to perform in the preparation of food. We 
 may note first the cavity into which the food enters, the 
 buccal cavity, which opens immediately into a part of the 
 alimentary tube provided with exceedingly thick walls, known 
 as the pharynx. The anterior part of the pharyngeal wall 
 is composed of muscular tissue, and is connected with the 
 wall of the body by a large number of radiating muscle 
 bands. After the pharynx there follows the oesophagus, a 
 narrow slightly sacculated tube, which opens at its posterior 
 end into a large sacculation or crop. Attached to the wall 
 
Metazoa L umbricus. 
 
 237 
 
 of the oesophagus are three (or in some worms two) pairs of 
 glands, known as calciferous glands. These glands contain 
 crystals or granules of carbonate of lime. They appear to 
 communicate with the oesophagus, but their function has not 
 as yet been determined. The crop is, like the pharynx, an 
 extensible muscular organ, and in it the food matter is no 
 doubt stored before it passes into the drum-shaped gizzard, 
 whose powerful muscular walls and thick horny lining crush 
 
 FIG. 121. LONGITUDINAL SECTION THROUGH THE ANTERIOR PORTION [OF 
 Lumbricus terrestris. (Hurst.) 
 
 BC, buccal cavity ; c, crop ; cu, cuticle ; D, thickened cuticle of the gizzard ; 
 E, epithelium of the alimentary canal ; EP, epidermis ; G, gizzard ; IN, in- 
 testine ; M, retractor muscles of the anterior part of the oesophagus ; MC, 
 circular muscles of the body-wall ; MG, muscular wall of the gizzard ; ML, 
 longitudinal muscles of the body-wall ; MV, posterior median vesieula semi- 
 naiis ; N, dorsal portion of the nerve collar cut across ; NC, ventral nerve- 
 chain ; NP, nephndium : OD, oviduct ; OE, oesophagus ; OG, aperture of cal- 
 ciferous gland ,'ov, ovary ; PH, pharynx ; P.M, retractor muscles of pharynx ; 
 PS, prostomium ; SF, mouth of vas deferens ; T, anterior testis ; TY, typhlo- 
 sole ; VD, vas deferens. 
 
 any foodstuffs that require mastication or grinding. The 
 posterior portion (third) of the gizzard has thinner walls, 
 and leads into the long glandular and sacculated intestine. 
 Down the dorsal surface of the intestine, and hanging free in 
 its interior, runs a curious fold, the typhlosole, a structure 
 which is found in a large number of animals. It no doubt 
 serves to greatly increase the area over which absorption of 
 the food takes place, for it is in the intestine that that pro- 
 cess especially goes on. The numerous folds and saccula- 
 
238 Elementary Biology. 
 
 tions of the intestinal walls over and above the typhlosole 
 fold must also serve to greatly increase the area of absorp- 
 tion, an end of great importance when we bear in mind the 
 relatively small quantity of food matter which must be pre- 
 sent in a large amount of humus swallowed. The intestine 
 is almost enveloped in a yellowish granular tissue, which is 
 probably of use in rendering % the food capable of absorption. ' 
 The intestine terminates at the anus, the external opening 
 which we have already noted in the examination of the ex- 
 ternal characters. 
 
 When we examine microscopically the constituent tissues 
 of the alimentary canal we find them to consist of a layer 
 of columnar cells internally, surrounded by connective 
 tissue, composed of delicate fibres and branched cells, in 
 which are embedded numerous blood-vessels followed by an 
 inner layer of circular, and an outer layer of longitudinal 
 muscular fibres. 
 
 Nutrition. The physiology of nutrition is extremely 
 simple, for the worm, living as it does on decaying vegetable 
 matter, takes into its alimentary canal food already partially 
 digested, or at least rendered very easily digestible by the 
 natural processes of decomposition taking place in the en 
 vironment. No doubt the salivary and other secretions 
 which the woim mixes with the food accelerates this process 
 or has a similar decomposing effect on the vegetable matter 
 which has not already undergone disintegration. The 
 nitrogenous and other compounds are absorbed by the cells 
 lining the alimentary canal, although the part played by the 
 yellow cells, which, as has been already mentioned, line the 
 outer surface of intestine, is still looked upon by some as 
 doubtful. The excreta, consisting of the earth from which 
 the food matters have been extracted, are ejected from the 
 anus and accumulate often in large piles at the mouth of 
 the worm's burrow. The nutritive substances having been 
 
 1 By some biologists this tissue is considered as vasifactive, or blood - 
 producing. 
 
MetazoaLumbricus. 239 
 
 absorbed by the columnar cells lining the intestine find 
 their way into the vessels which lie in the connective tissue 
 beneath. 
 
 The circulatory system. The vessels which lie in the 
 wall of the intestine have extremely thin walls, often in- 
 deed only one cell thick, and are known as capillaries. 
 These capillaries, however, lead into a very complete system 
 of larger vessels, some of which run longitudinally, others 
 circularly round the intestine and in the body-wall. The 
 principal longitudinal vessels are three in number, one 
 large vessel running dorsally along the top of the alimentary 
 canal, and two ventrally placed, one immediately beneath 
 the intestine (subintestinal), the other close to the ventral 
 body-wall, and separated from the subintestinal vessel (as 
 we shall afterwards find) by the nervous system. From its 
 position it has been named the subneural vessel. From 
 all of these vessels numerous smaller branches are given off, 
 which run in a circular manner round the intestine or the 
 body wall. The arrangement of these circular vessels is 
 very complicated, and it will not be necessary for us to 
 do more than glance at the moie important branches. 
 Moreover the manner in which they are arranged differs 
 according to the part of the body examined. If we select 
 a somite near the middle, we find that from the dorsal vessel 
 a large vessel is given off on either side, which splits up into 
 numerous smaller vessels, ultimately becoming continuous 
 with the capillaries already mentioned as forming a net- 
 work in the walls of the intestine. The dorsal vessel also 
 gives off on either side a smaller branch, which, after giving 
 off capillaries on its way, opens into the lowest of the two 
 ventral vessels. The other longitudinal vessel also gives off 
 branches to the wall of the intestine and the organs in its 
 vicinity. 
 
 In the anterior region of the body the arrangement is 
 somewhat different. In the first half dozen-somites the 
 arrangement is quite irregular, all the longitudinal vessels 
 
240 Elementary Biology. 
 
 breaking up into a close network of capillaries, spreading 
 over the pharynx, body-wall, and buccal cavity. In the 
 succeeding six somites the lateral hoops, instead of being 
 delicate tubes and opening into the sub neural vessel, are 
 large, swollen, tapering sacs which unite the dorsal to the 
 sub-intestinal vessel. These vessels are known as { hearts,' 
 although, as we shall presently see, they are functionally 
 not quite equivalent to the heart of higher animals. 
 
 The vessels contain a red coloured fluid, with a few 
 corpuscles suspended in it. Although some uncertainty 
 still exists as to the real nature of this haemal fluid, there 
 can be little doubt but that it is in most respects compar- 
 able to the blood of the higher animals. Some biologists 
 are inclined however to consider another fluid found in the 
 spaces between the intestine and the body-wall, and known 
 as the ccelomic fluid, as equivalent to true blood. The 
 coelomic fluid is colourless and contains amoeboid cor- 
 puscles, and is distinctly albuminous in its chemical com- 
 position. Probably both the haemal and the coelomic 
 fluids together perform the functions ascribed to true 
 blood. 
 
 Circulation. There seems to be no very definite 
 mechanism for distributing the food matters absorbed from 
 the intestine through the body. It is true that the swollen 
 circular vessels in the anterior part of the body exhibit 
 tolerably definite and rhythmic contractions from above 
 downwards, but the impetus so given to the haemal fluid 
 cannot be sufficient to send it through the complicated 
 series of vessels described above. If the ccelomic fluid 
 is to be considered as the true medium employed for 
 the distribution of nourishment, then no mechanism 
 exists at all for circulation beyond the continuous wrig- 
 gling movements of the body, movements which, con- 
 sidering the spongy character of the tissues of the earth- 
 worm, may serve very effectually in lieu of the force-pump 
 commonly called a heart. 
 
Metazoa L umbricus. 24 1 
 
 The purificatory system. For the removal of the pro- 
 ducts of decomposition two kinds of purificatory organs 
 are necessary, viz. those which have for their duty the 
 removal of gaseous products, and those whose function it 
 is to get rid of the nitrogenous waste. We shall first of alt 
 consider the former. 
 
 The respiratory system and respiration. No special 
 organ exists in the earthworm for getting rid of the gaseous 
 excreta, unless the haemal fluid be looked upon as the 
 agent in the process. Respiration is always a double 
 process, and consists of internal respiration, a gaseous 
 interchange between the tissues and the blood on the 
 one hand, and external respiration, a gaseous interchange 
 between the blood and the atmosphere on the other. 
 Blood may in fact be defined from this point of view as the 
 medium by which oxygen is conveyed to the tissues from 
 the air, and by which carbonic acid is transferred from the 
 tissues to the exterior. The haemal fluid is very plentifully 
 distributed to all parts of the body, and thus serves as an 
 agent for the collection of gaseous waste, whilst at the same 
 time it gives up to the tissues oxygen obtained from the 
 external air. Again, the distribution of the fluid in the body- 
 walls admits of its absorbing oxygen from the atmosphere, 
 whilst at the same time it facilitates the excretion of carbonic 
 acid. If this be the case then the two blood fluids of the 
 earthworm would together correspond to the blood of the 
 higher animals the one being respiratory, the other nutri- 
 tive. True blood performs both functions, an obvious 
 economy when the animal type becomes more complicated, 
 as it enables two operations to be performed by one agent, 
 and avoids the necessity of a double pumping mechanism. 
 
 The renal system and excretion. There is no doubt 
 whatever as to the organs for the purification of the body 
 from the nitrogenous excreta. The renal system consists of 
 a large number of coiled tubes complicated in structure and 
 distributed in pairs, one pair in each somite of the body 
 
 R 
 
242 Elementary Biology. 
 
 (fig. 122). Each tube is known as a nephridium (rape's, a 
 kidney), and consists of three distinct portions, a much con- 
 voluted thin portion terminated by a funnel-shaped opening, 
 a thick walled but less convoluted glandular portion, and 
 lastly a still thicker walled muscular portion which opens to 
 the exterior by a minute aperture between the outer and 
 inner rows of setae on either side and close to the inner. 
 The arrangement of the various portions of the nephridium 
 in the somite is sufficiently important to merit description. 
 Each somite, as has been already noted, is separated from 
 the neighbouring somites on either side by an incomplete 
 partition running from the body-wall inwards to the intes- 
 tine, and known as a septum or mesentery. To this septum 
 the nephridium is closely related, lying on it in a series of 
 loops, the muscuiar parts of the tube being external, and the 
 various other folds arranged on the septum in a definite 
 order. The nephridium as a whole lies. on the posterior 
 side of the septum, but the funnel-shaped inner end pierces 
 it and opens on the anterior surface, that is to say, into the 
 cavity of the somite in front of that in which the main body 
 of the nephridium lies. The cells lining the mouth of the 
 funnel are ciliated. The septum is plentifully supplied with 
 blood-vessels, many of which are intimately connected with 
 the folds of the nephridium. Doubtless the nitrogenous 
 waste is absorbed by the glandular portion of the tube, and 
 ejected by the contractile part to the exterior. 
 
 We have now to glance at the locomotory and protect- 
 ing organs, for a skeleton or supporting system is absent 
 We have already referred to the subdivision of the body 
 into somites, separated from each other by incomplete septa. 
 We have also noted the existence of large annular spaces 
 surrounding the alimentary canal, spaces which we have 
 seen contain a (probably) nutritive fluid. It will be seen 
 that the several somitic cavities taken together really form 
 one large broken-up cavity lying between the alimentary 
 canal wall on the one hand, and the body- wall on the other ; 
 
Metasoa Lumbricus. 243 
 
 whilst the septa are merely plates serving for the attachment 
 and support of the nephridia and the alimentary system. 
 This large space, or rather series of spaces, is known as the 
 body cavity, or, better, ccelom. Glancing backwards at the 
 
 FIG. 122. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE MIDDLE OF THE 
 LOLiY OF Liimbricus terrestris. (Milnes Marshall.) 
 
 A, cuticle ; B, epidermis ; c, coelom ; CM, layer of circular muscles ; D, seta ; 
 DV, dorsal vesssl r E, cavity of intestine ; F, cavity of typhlosole filled wuh 
 hepatic cells ; G, epithelium of intestine ; H, hepatic cells clothing intestine ; 
 I, ventral nerve-cord ; J, nerve ; L, ciliated funnel of the nephridium ; LM, 
 layer of longitudinal muscles ; M, point where nephridium perforates the 
 septum ; N, o, p, loops of the nephridium ; R, external aperture of the 
 nephridium ; sv, subneural vessel ; v, lateral vessel ; vv, ventral vessel. 
 
 structure of the types of the Hydrozoa discussed in the last 
 section we see that that space is entirely absent. The space 
 in the interior of Hydra or Obelia is not a coelom, but an 
 alimentary canal, or enteron, although one without an anus 
 and without differentiation into stomach and intestine. 1 
 
 1 It is of the utmost importance that students should, from the very 
 commencement, guard against the misuse of the two terms ccelom, or 
 body-cavity, and enteron, or alimentary cavity. If a clear distinction 
 be not drawn between these two spaces, endless confusion is apt to be 
 created in the minds of beginners in the subject. The enteron may be 
 defined as a cavity always nutritive in function and enclosed by hypo- 
 
 R 2 
 
244 Elementary Biology. 
 
 Here, however, both cavities are present. The relation of 
 the one to the other, and the structure of the body-walls, 
 will be best understood by the examination of a transverse 
 section of the middle of the body. 
 
 Externally covering the entire surface, save where the 
 nephridial, alimentary, and reproductive apertures occur, lies 
 a very thin and tough membrane, not formed of cells, but of 
 a cuticular substance known as chitin, chemically related 
 to keratin (p. 27), This membrane or cuticle is easily re- 
 moved, and is probably secreted by the layer (ectoderm) 
 immediately beneath it. That layer is cellular and corre- 
 sponds to the outer or ectoderm layer of Hydra. The cells 
 forming it are columnar, and have scattered amongst them 
 many glandular cells, whose duty it is no doubt to secrete 
 the cuticle. The ectoderm covers the double muscular 
 layer. The outer circular muscles are arranged in the form 
 of an almost continuous sheet from one end of the body to 
 the other, but the inner longitudinal layer is broken up into 
 bands which are separated from each other by the rows of 
 bristles and the openings of the nephridia. The septa are 
 thin, partly muscular, partly fibrous plates originating from 
 and continuous with the body-wall. 
 
 On the ventral surface of each somite four pairs of minute 
 pits occur, from each of which projects a long curved bristle, 
 or seta, thicker towards the middle and tapering to either 
 end. These bristles are chitinous in their nature, and are 
 of cuticular origin. The ectoderm retreats into and lines 
 each pit, whilst there are minute strands of muscle fibre 
 springing from the body-wall and attached to the sunk end 
 of the bristle, by means of which the seta can be projected 
 and withdrawn at will. 
 
 The nervous system and nervation. In connection 
 
 blastic, or endoderm cells ; whilst the coelom is a space between two 
 layers of mesoblast, containing organs derived, it may be, from all 
 three layers. In animals like Hydra, in which the mssoderm is 
 wanting, manifestly no coelom can occur. 
 
Metazoa Lumbricus. 245 
 
 with the maintenance of individual life we have yet to treat 
 of the system which regulates the action of the various 
 tissues and organs of the body, and maintains communica- 
 tion between the external world and the internal economy. 
 In the worm the nervous system has arrived at a much higher 
 stage of differentiation than that attained in the Hydrozoa. 
 The nerve fibres are not irregularly diffused, but collected 
 in definite strands or nerves, and the cells, in which, pre- 
 sumably, the nervous changes originate, are grouped to- 
 gether into ganglia. In the last section we saw that probably 
 the first stage in the development of a definite nervous system 
 consisted in the gathering of the nerve fibres into two lateral 
 cords, with branches to the different organs, and the aggre- 
 gation of the nerve cells on either side of the body into a 
 mass at the anterior end of the cord of that side. This was 
 followed by the approximation of the two ganglia to form 
 a brain. Lastly the two cords also approached each other 
 on the ventral side and united to form a ventral chain with 
 lateral branches. When segmentation of the body ensued, 
 ganglia were formed on the cords in each segment to serve 
 as centres for local nerve impulses. Hence we find the 
 nervous system of Lumbricus shows a pair of closely united 
 ganglia above the oesophagus, and therefore named supra- 
 cesophageal, from which spring two circum-oesophageal 
 commissures, which surround the gullet and unite under- 
 neath it in two sub-cesophageal ganglia. From the sub- 
 cesophageal ganglia the double cord, with its pairs of 
 ganglionic swellings, passes backwards along the ventral 
 surface, piercing each successive septum, and giving off a 
 nerve to it on either side in its passage. The two united 
 ganglia, which occupy the centre of the ventral wall of each 
 somite, also give off nerves to the organs in the neighbour- 
 hood. 
 
 Sense organs and sensation, The earthworm can 
 scarcely be said to possess any sense organs, but it possesses 
 a generalised sensitiveness, due to the plentiful distribution 
 
246 
 
 Elementary Biology. 
 
 of nerve fibres through the body, and which, in many re- 
 spects, takes the place of a series of specialised organs, 
 corresponding to the senses of touch, taste, sight, hearing, 
 and smell. Its sensitiveness to touch and its dislike to 
 sunlight are well known ; and, though not possessed of organs 
 of sight or of smell, it is able easily to find its way to stores 
 
 of food and to retreat 
 
 Fir,. 123. DIAGRAMMATIC REPRESENTATION OF {-___ _._ ___ r j 
 
 THE ARRANGEMENT OF THK REPRODUCTIVE irOUl SOUTCCS OI Uan~ 
 
 ORGANS OF Liimbricus terrestris. (Hurst.) ^^ j n O a burrOW 
 
 The reproductive 
 system. The ovaria 
 
 are two extremely 
 minute organs situ- 
 ated in the thirteenth 
 somite and attached 
 to the ventral portion 
 of the posterior side 
 of the septum sepa- 
 rating that somite 
 from its predecessor. 
 The ova are typi- 
 cal, nucleated cells, 
 which, when ripe, are 
 shed into the ccelom, 
 and find their way, 
 by some means not 
 yet known, into the 
 wide ciliated mouths 
 of one or other of 
 
 A, n, c, the vesiculae seminales ; N, nerve-cord ; 
 o, ovary; OD, oviduct; s, spermathecae ; SF, 
 mouth of vas deferens, VD ; T, testis. 
 
 two tubes which open internally on the anterior face of the 
 succeeding septum, and, after piercing it, open to the exte- 
 rior by a small aperture on the ventral surface of the four- 
 teenth somite, near the outer row of setae. Each of these 
 tubes is known as an oviduct. 
 
 The earthworm is hermaphrodite, but, as in the case of 
 most, of the hermaphrodite higher plants, it does not fertilise 
 
Metazoa Lumbricus. 247 
 
 its own ova. The fertilising sperms are obtained from 
 another worm, and stored in four small spherical sacs, or 
 spermathecae, which open one pair between the ninth and 
 tenth, and the other pair between the tenth and eleventh 
 somites. 
 
 Passing now to the male organs we find that the sperm- 
 aria are, like the ovaria, microscopic bodies attached to 
 the mesentery. There are two pairs of spermaria, and they 
 lie on the posterior sides of the septa, separating the ninth 
 and tenth, and the tenth and eleventh somites. The sperms 
 are, however, not sexually mature when they leave the sperm- 
 aria. They are known as spermospores from which sperms 
 are afterwards formed by a process of budding. This further 
 development takes place in large irregular organs known as 
 vesieulae seminales, or seminal reservoirs (fig. 123). The 
 vesiculse seminales are four in number, two on either side 
 of the ventral middle line, and occupy the cavities of the 
 tenth and eleventh somites, growing forwards and backwards 
 when fully developed into the ninth and twelfth somitic 
 spaces. The spermaria are found fully developed only in 
 young worms, and the vesiculae seminales, though present 
 along with the spermaria, attain a large size only in the adults. 
 
 The fully developed sperms, each consisting of along, rod- 
 shaped head and flagellum, are conveyed to the exterior by 
 four wide- mouthed delicate sperm -ducts, or vasa deferentia. 
 The mouths of the vasa deferentia open on the anterior 
 faces of the septa immediately behind those to which the 
 spermaria are attached. The ducts in continuation with the 
 funnels run backwards, the two vasa deferentia on either 
 side fusing and opening by one aperture on the ventral wall 
 of the fifteenth somite, the somite immediately behind that 
 on which the oviducts open. 
 
 Fertilisation and development. The ripe ova, on their 
 escape from the oviducts, are fertilised by sperms (of another 
 worm) squeezed out of the spermathecal openings. The 
 fertilised ovum, or embryo, is then enclosed in a chitinous 
 
248 
 
 Elementary Biology. 
 
 shell, formed by the glandular area of skin before mentioned 
 as occurring, roughly speaking, between the twenty-fourth 
 and thirty- sixth somites, and named the clitellum. 
 
 After a short period of rest, during which, no doubt, 
 molecular rearrangements are taking place in the protoplasm 
 of the embryo, segmentation commences. The result of 
 segmentation is the formation of, what was termed in the 
 
 FIG. 124. PRIMARY STAGES IN THE SEGMENTATION OF THE FERTILISED 
 OVUM OF Lnmbrictts. (Schiifer.) 
 
 preceding section, a blastosphere, but in this case of a 
 peculiar type. In the worm the blastosphere is flattened, and 
 the cells forming the sac-wall are of unequal size, being on 
 one side large and clear, on the other, small and more 
 granular. Further there is nothing corresponding to a seg- 
 mentation-cavity between the two layers. But owing to I he 
 more rapid growth of the smaller cells, the larger clear cells 
 become enclosed, so that the embryo is composed at this 
 stage of an outer epiblast and an inner hypoblast layer en- 
 closing a small cavity. The worm is now in the gastrula 
 
Metazoa Lumbricus. 249 
 
 stage of its development, and is comparable to the planula 
 formed by Hydra and Obelia. The small cavity in the in- 
 terior of the gastrula is the primitive alimentary canal, or 
 archenteron ; its slit-like mouth forms the blastopore, after- 
 wards to become the true mouth. 1 
 
 By this time the mesoblast has made its appearance by 
 the multiplication of two large cells, covered over by epiblast 
 and lying close to the blind end of the gastrula. It forms a 
 plate of cells on either side of the body from end to end of 
 the embryo. These plates become segmented into blocks, 
 known as mesoblastic somite?, each of which becomes 
 hollowed, forming a layer of mesoblast surrounding the 
 enteron and a layer applied to the epiblast. This is known 
 as metameric segmentation to distinguish it from the seg- 
 mentation of the egg. The two layers of mesoblast are 
 of great importance in embryology, and are named the 
 splanchnopleure, or visceral, and somatopleure, or body 
 layers respectively. Between the two layers there lies a 
 space, the ccelom, or body- cavity, subdivided into as many 
 small chambers as there are mesoblastic somites, and 
 bounded in front and behind by partitions formed by the 
 fusing of the anterior wall of each mesoblastic somite with 
 the posterior wall of that immediately in front. The anus 
 is formed late in development by a tucking in of the pos- 
 terior region of the body. The enteron is thus placed in 
 communication with the exterior at both ends. 
 
 The several organs found in the ccelom are formed from 
 the rapidly increasing mesoblast, save the nervous system, 
 which is formed out of epiblast and sinks into the mesoblast 
 in the course of subsequent development and growth of the 
 embryo. 
 
 In concluding our survey of Lumbricus we may note for 
 future reference the importance of the segmented condition, 
 
 1 It will be afterwards seen that the mouth is usually formed in an 
 entirely different manner (p. 262). 
 
250 Elementary Biology. 
 
 and especially the first appearance of that character in its 
 embryology. As we shall afterwards find, segmentation and 
 the formation of mesoblastic somites are features of the 
 highest importance in the morphology and development of 
 the higher types. Again, we may note the epiolastic origin 
 of the nervous system, also almost invariably the rule in the 
 animal kingdom. The origin of the coelom, the formation of 
 the enteron, and its differentiation into an alimentary canal 
 are also worthy of attention if any generalised view is to be 
 obtained of the mode of origin of the various morphological 
 types of animal organisation. 
 
Metazoa A mphioxus. 2 5 1 
 
 CHAPTER X. 
 
 METAZOA VERTEBRATA. 
 
 SECTION I.AMFHIOXUS. 
 
 IT has been already seen that it is possible to subdivide the 
 Animal Kingdom into the groups Protozoa and Metazoa, and 
 that the Metazoa may be again divided into those with and 
 those without a vertebral column Invertebrata and Verte- 
 brata, or more correctly Achordata and Chordata. Of the 
 former group we have examined two representatives, Obelia 
 geniculata and Lumbricus terrestris. These two forms may 
 be taken to represent the great classes Hydrozoa and 
 Vermes respectively. But between these and the Chordata 
 there come an immense series of forms, represented by such 
 animals as the sea-urchin and starfish (Echinodermata) ; 
 snail, mussel, and cuttlefish (Mollusca) ; cockroach and bee 
 (Insecta) ; lobster and crab (Crustacea) ; and many others 
 less familiar. To enter into the discussion of these groups 
 would necessitate the study of many different types, greatly 
 exceeding the limits of this text-book. 
 
 We may pass, however, from the study of the earthworm 
 to that of the Chordata the more easily and naturally inas- 
 much as zoologists believe that that group had in far past 
 ages worm-like ancestors. In discussing the structure of 
 the nervous system of Lumbricus it was noted that the gene- 
 rally diffused nervous system in the lower types, consisting 
 of nerve-fibres and nerve-cells, had become aggregated, 
 first, into two lateral cords, and then, by approximation, 
 
252 
 
 Elementary Biology. 
 
 FIG. 125. Amphioxiis lancco- 
 latns. (Milnes Marshall.) 
 
 A, buccal cavity; B, buccal ten- 
 tacles ; c, pharynx ; D, liver ; 
 E, stomach ; o, intestine ; H, 
 anus ; i, atrial pore ; K, noto- 
 chord ; L, spinal cord ; M, eye ; 
 N, septa ; o, dorsal fin ; P, ven- 
 tral fin ; R, transverse muscles 
 in floor of atrial cavity. 
 
 into the form of a nervous cord 
 lying along the ventral surface of 
 the body provided \vith a large 
 ganglionic enlargement anteriorly 
 and a series of smaller ganglia 
 posteriorly on the cord, one in 
 each somite. In the early 
 Chordata these lateral cords 
 seem to have become approxi- 
 mated on the dorsal side of the 
 body, and formed what are 
 known among the Vertebrata as 
 the spinal cord. Further, the 
 spinal cord became supported 
 and strengthened by the forma- 
 tion underneath it of a bar 
 known as the notochord. 
 
 It is indeed fortunate that we 
 possess in Amphioxus lanceolatus 
 a minute fish- like animal, found 
 abundantly burrowing in sand in 
 the Mediterranean and other re- 
 gions, a survival of some ancient 
 group of the Chordata, compara- 
 tively speaking not far removed 
 from this primitive type. We 
 shall give an account in the first 
 place of this form, and then point 
 out some of the conclusions 
 which may be drawn from its 
 study with regard to the various 
 and important differences which 
 we notice existing between the 
 vertebrate and the invertebrate 
 types. 
 
 External characters.^;;/- 
 
Metazoa A mphioxus. 253 
 
 phioxus lanceolatus is from i^ to 2 inches in length, and 
 when living is delicate, translucent, and fish-like. Its body 
 is compressed from side to side and pointed at both ends. 
 No limbs are present. A series of small and delicate fila- 
 ments may be seen protruding from the anterior end. A 
 narrow ridge or fold of skin, known as the dorial fin, runs 
 along the back. At the tail end this ridge becomes broader 
 and more distinct, and is there termed the caudal fin. 
 On the ventral surface the caudal fin is continued forwards 
 for a certain distance ; finally, however, in the anterior two- 
 thirds of the ventral surface it is replaced by two ridges 
 separated by a groove. 
 
 The mouth is an oval slit on the ventral surface of the 
 body, near the anterior end, from the margin of which spring 
 the filaments already referred to. Two other apertures are 
 to be found on the ventral surface : the anus, situated a 
 little to the left of the middle line, just in front of the caudal 
 fin, and an aperture known as the atrial pore, situated at 
 the point where the ventral continuation of the caudal fin 
 ends. The sides of the body are obliquely marked, the 
 lines indicating the outlines of muscle bands or myotoines, 
 and running dorsaily and ventrally from the median line 
 backwards. It is important to notice this point, as it exhibits 
 to us one of the points of evidence of the origin of the Chor- 
 data from segmented worm-like ancestors. 
 
 Ihe alimentary and respiratory systems. As in the 
 worm, the alimentary canal is straight and even simpler than 
 it was in that type. The mouth opens into a buccal cavity 
 lined by ciliated epithelium. The buccal cavity is separated 
 posteriorly from the pharynx by a circular fold of membrane 
 or diaphragm. This diaphragm is pierced by an aperture 
 fringed by numerous ciliated lobes. The buccal cavity 
 opens into the pharynx, or anterior portion of the alimentary 
 canal, and it is here that we meet with a great point of dif- 
 ference between the worm and ^nftj$tp$us. Unlike that of 
 the worm, the pharynx ijS^fmfely anm^ntary in function 
 
 v 
 
254 Elementary Biology. 
 
 but is also the organ of respiration. It will be necessary for 
 us, therefore, to study the alimentary system in connection 
 with the respiratory system. 
 
 The pharynx is a wide tube or sac, and extends fully 
 half the length of the body. Its lateral walls are pierced by 
 a series of oblique slits, putting the cavity of the pharynx 
 in communication with a space which lies round the ali- 
 mentary canal known as the atrial cavity. These slits are 
 lined by columnar ciliated cells, the cilia of which during 
 life are constantly in motion. The slits are known as bran- 
 chial slits, and the columns between them as branchial 
 arches. Each arch is strengthened by having developed in 
 its interior a chitinous rod. The long oblique bars are 
 connected to each other by short crossbars, so that the slits 
 rather resemble an open meshwork than anything else. 
 Dorsally and ventrally the pharynx has developed on its 
 wall a couple of ridges, the hollow between which is known 
 as the hyperbranchial groove and hypobranchial groove 
 respectively. 
 
 Posteriorly the pharynx rapidly narrows into a short 
 straight intestine. The intestine is not of the same calibre 
 throughout. Immediately after leaving the pharynx it is 
 very narrow ; it then widens considerably, giving off for- 
 wards a diverticulum or sac known as the liver, again 
 narrowing gradually unul it ends at the anus at the base of 
 the caudal fin. 
 
 It will not be possible to understand the mechanism of 
 alimentation and respiration until we have considered the 
 chamber the so-called atrial cavity into which the bran- 
 chial slits open. The relation of this cavity to the cavity 
 of the pharynx on the one hand, and to the exterior on the 
 other, may be best understood by studying their mode of 
 origin. In the very young Amphioxus there is no atrial 
 cavity, and the branchial slits open directly to the exterior 
 as do the branchial slits of an ordinary fish. Late in the 
 course of development two ridges appear along the sides of 
 
Metazoa A mphioxns. 255 
 
 the body above the level of the branchial slits. These folds 
 of skin grow downwards until they have met on the ventral 
 surface of the body. They then fuse all along the middle 
 line, save opposite the posterior end of the pharynx, where 
 they leave a small aperture already mentioned the atrial 
 pore. In front these folds form the side -walls of the buccal 
 cavity. The cavity thus enclosed the atrial cavity must 
 not be confused with the body-cavity, or ccelom, afterwards 
 to be mentioned. 
 
 Alimentation and respiration, The primary stages 
 in these two processes take place simultaneously, and by 
 means of the same organ. The sea- water containing minute 
 organisms enters the buccal cavity, being induced to do so 
 by the incessant action of the cilia on the cells lining the 
 cavity. The food particles are then collected from the 
 water and enter the intestine. The water meantime escapes 
 through the branchial slits, and in streaming over the bran- 
 chial arches carries fresh oxygen to, and removes carbonic 
 acid from, the blood circulating in spaces (lacunae) in the 
 branchial arches. The water escapes from the atrial chamber 
 by the atrial pore. The food matter, after being mixed with 
 the liver secretion and undergoing digestion in the wider 
 portion of the alimentary canal, is absorbed into the circu- 
 lation in the intestine proper. 
 
 The circulatory system. Just beneath the floor of the 
 hypobranchial groove of the pharynx there lies a large blood- 
 vessel, the cardiac aorta, which gives off a series of lateral 
 branches, which enter into the branchial arches, and there 
 break up into smaller vessels. On each lateral branch, just 
 before it enters the arch, there is a small dilatation, which 
 has the power of contracting rhythmically, known as a bran- 
 chial heart. The blood, which is colourless and contains 
 very few corpuscles, is collected again from the branchial 
 lacunae into two vessels, one on either side, situated close to 
 the sides of the ridges of the pharynx which form the walls 
 of the hyperbranchial groove. These are the two dorsal 
 
256 Elementary Biology. 
 
 aortae. Posteriorly to the pharynx the two aortse unite into 
 one dorsal aorta, which gives off secondary branches or 
 arteries as it goes, each of these in turn breaking up into a 
 series of lacunae in the several organs of the body. 
 
 In the intestinal wall there are a number of vessels, 
 known as portal veins, carrying impure blood and food 
 material from the intestine to the saccular dilatation from 
 the wide portion of the intestine the so-called liver. In 
 the liver the portal veins form a capillary network, and from 
 it the blood is again collected and carried to the cardiac 
 aorta to be distributed to the branchial arches. The cardiac 
 aorta also receives impure blood from the muscles and other 
 organs of the body. 
 
 Circulation. We have seen that there is no distinct 
 heart in Amphioxus, its place being taken by the cardiac 
 aorta and the small branchial hearts at the origin of the 
 branchial vessels. 
 
 The cardiac aorta contains impure blood that is, blood 
 laden with gaseous and liquid excreta from the various organs 
 of the body. The impure blood passes into the branchial 
 arches, there, as we have seen, coming in contact with fresh 
 oxygen dissolved in sea-water, whilst at the same time the 
 gaseous waste matters are got rid of. The blood collects in 
 the two dorsal aortae, which may be said now to contain so 
 far pure blood, and is distributed to the body generally. 
 The blood is again collected by veins from the intestine 
 and is carried to the liver, and from the liver again and body 
 generally to the cardiac aorta, thus comoleting the circuit. 
 
 The excretory system. It is doubtful whether Amphi- 
 oxus possesses any organs comparable to the nephridia of 
 the earthworm. Two short secretory tubules, with pig- 
 mented walls which open into the atrial cavity, opposite the 
 end of the pharynx, are looked upon by many as renal 
 organs. 
 
 It will be remembered that in the earthworm the ali- 
 mentary canal, nephridia, and reproductive organs lay in 
 
Metazoa A mpJiioxus. 
 
 257 
 
 a large cavity the coelom which was further subdivided by 
 the various mesenteries or transverse plates. In Amphioxus 
 the coelom is also present, although much more irregular in 
 in its nature. In transverse section it can be seen in 
 various places surrounding the alimentary canal, and ante- 
 riorly as a much reduced space above the pharynx. The 
 
 FIG. 126. TRANSVERSE SECTION THROUGH ANTERIOR PHARYNGEAL REGION 
 OF Amphioxus lanceolatus. (Milnes Marshall.) 
 
 A, dorsal fin ; B, spinal cord ; c, notochord ; D, connective-tissue sheath ; 
 E, pharynx ; F, hyperbranchial groove ; G, hypobranch : al groove ; 
 H, atrial cavity ; j, transverse muscle-bands ; M, p, coelom ; R, left 
 dorsal aorta ; s, cardiac aorta ; x, myotome ; Y, fold of pharynx ; z, 
 branchial bar. 
 
 compressed character of the body and the addition of the 
 atrial cavity, which it is, however, to be noted is really a 
 part of the outer world, render the limits of the coelom more 
 difficult to define. If, however, the primary stages in the 
 development of the atrial cavity be borne in mind, no diffi- 
 culty will be experienced in understanding the relationships 
 of the two cavities. We have now to glance at the 
 
258 Elementary Biology. 
 
 Locomotory, protective, and supporting systems. 
 
 Protection is afforded rather by the habits of the animal 
 than by any hardened integument such as we find among 
 fish or Crustacea. Amphioxus live, as a rule, buried in 
 sand below water ; a skin specially suited to perform a 
 protective function is therefore unnecessary. The body is 
 well provided wich muscles. They are in the form of 
 myotomes, or long thick plates, separated by fibrous septa. 
 The muscle fibres themselves are arranged horizontally in 
 the long axis of the body, so that the characteristic fish like 
 motion is brought about by alternate contraction of the 
 muscle bands, first on one side, then on the other. A sheet 
 of transversely placed muscle fibres also stretches along the 
 floor of the atrial cavity, the duty of which is to expel the 
 water from the atrial cavity. 
 
 It is in Amphioxus that we first meet with rudiments of 
 the skeleton in the interior of the body,- which becomes so 
 fully developed in the higher forms of animal life. It 
 could not well be simpler than it is in Amphioxus, for it 
 consists only of a delicate rod of clear thin-walled cells lying 
 immediately above the pharynx and alimentary canal. It 
 has been called the notochord. The notochord is enclosed 
 by a sheath of fibrous tissue, and from its nature is highly 
 elastic. The notochord, it is important to note, extends 
 throughout the entire length of the body. In addition to 
 this axial rod the lips of the buccal cavity are strengthened 
 by delicate bars, from which arise yet finer bars to support 
 the tentacular processes. The branchial bars may also be 
 looked upon as part of the internal skeleton ; whilst the 
 entire body is strengthened by a considerable development 
 of dense fibrous tissue which surrounds the notochord, the 
 nervous system (immediately dorsal to the notochord), 
 separates the muscle plates from each other, and strengthens 
 the bases of the fins. 
 
 The nervous system. The nervous system is dorsally 
 placed, and supported beneath by the notochord. It con- 
 
Metazoa A mphioxus. 259 
 
 sists of a hollow rod of nerve-fibres and cells surrounded by 
 connective tissue. 
 
 The spinal cord, as the rod may be termed, is not so 
 long as the notochord, ending a short distance behind its 
 anterior termination. When we come to examine the frog 
 in the next section we shall find that this relation of the 
 spinal cord to the notochord is of great importance. From 
 the spinal cord are given off many nerves in two series ; 
 a dorsal series consisting of nerves which spring by only one 
 root from the cord, and a ventral series, each of which has 
 many roots. The nerves of the dorsal series alternate with 
 those of the ventral series. The fibres are distributed to 
 various organs and other parts of the body, and apparently 
 differ in function, the dorsal nerves being concerned in 
 carrying nervous impressions to the spinal cord from the 
 exterior (sensory nerves), the ventral nerves corresponding 
 to what are known as the motor and secretory nerves of 
 higher animals i.e. whose duty it is to convey nerve im- 
 pulses to the muscles, glands, and other organs of the 
 body (p. 318). 
 
 The sense-organs are very rudimentary. The sense of 
 touch is, of course, fully developed, and the sensations are 
 conveyed to the central nervous system by means of the 
 sensory spinal nerves just referred to. A minute sac-like 
 hollow in the skin, lined by ciliated epithelium, situated 
 close to the anterior end of the spinal cord, is believed to 
 represent the olfactory organ, whilst a pigment spot placed 
 on the anterior extremity of the cord itself may do duty for 
 an eye. 
 
 The details of the nervous system in the Chordata, and 
 how it performs its functions, will be more fully dealt with in 
 the two succeeding sections. 
 
 The reproductive system. Lastly we have to discuss 
 the reproductive organs, which, like most of the organs in 
 AmphioxitS) are very simple. The animals are male and 
 female. In both sexes the reproductive glands are situated 
 
 S 2 
 
260 Elementary Biology. 
 
 in the ccelomic space, which is continued down into the 
 outer walls of the atrial cavity. There are no reproductive 
 ducts nor openings. The ova and sperms are shed by the 
 rupture of the inner wall of the atrial fold into the atrial 
 cavity, the former probably escaping by the buccal cavity to 
 the exterior after passing through the branchial slits, the 
 latter probably by the atrial pore. 
 
 Fertilisation and development. The ova are fertilised 
 by the sperms after leaving the body of the female. The 
 ovum is enclosed in a delicate cell-wall. The embryo, or 
 fertilised ovum, immediately segments, and by successive 
 division the originally unicellular ovum becomes a multi- 
 cellular hollow sac or ball termed a blast o sphere. When 
 fully formed one side of the blastosphere becomes invagi- 
 nated, or pushed inwards, so that a double-walled cup is 
 formed, the shell membrane being meantime cast off. The 
 cup elongates into a flask, and the originally wide open mouth, 
 where the inner and outer walls were continuous, becomes 
 narrowed to a small aperture the blastopore. While in 
 this condition the outer wall of the embryo is ciliated, and 
 the body rotates freely in the water. The embryo is now a 
 gastrula, composed externally of a layer of ciliated ecto- 
 derm, and internally of long columnar endoderm cells. 
 
 At this point there follow some very important changes. 
 First, the embryo becomes flatter on one side. This 
 flattened area, known as the medullary plate, becomes 
 converted into a groove or valley, partly by the formation of 
 two ridges, which rise up along the sides of the plate, and 
 partly by the downward curving of the centre of the plate 
 itself. The folds, or dorsal laminae, gradually enclose the 
 blastopore, and unite together behind it. The blastopore 
 is thus made to open on what will become the dorsal sur- 
 face. The laminae unite above the medullary groove from 
 behind (the blastopore end) forwards, roofing over a tubular 
 cavity formed partly by the laminae as a roof, and partly 
 by the curved medullary plate as sides and floor. This 
 
Metazoa A mphioxus. 
 
 261 
 
 tube is still open in front, however, and is in communication 
 with' the gastrular cavity, or archenteron, by the blastopore. 
 The narrowTopen limb will become the spinal cord of the 
 adult, and will become separated from what is destined to 
 be the enteron, or alimentary canal below. 
 
 Meantime from the walls of the archenteron there have 
 
 FIG. 127. DEVELOPMENT OF Amphioxus lanccolatiis. (After B. Hatscheck.) 
 
 A, blastosphere ; B, gastrula ; c, transverse section of the embryo ; D, 
 longitudinal section of the embryo ; a-, ectoderm ; />, endoderm ; c, 
 enteric cavity ; ef, neural canal ; e, mesoblastic somites ; f, neurenteric 
 canal ; g, anterior opening of the neural canal ; h t notochord. 
 
 been forming a series of saccular outgrowths, which ere long 
 become disconnected from the archenteron, and lie along 
 its sides as cubical bodies, known as the mesoblastic 
 somites. Each block becomes subdivided into a dorsal 
 and a ventral portion. The dorsal part becomes transformed 
 into the muscle plates, while the ventral portion splits into 
 two layers : the inner, or splanchnic layer, forming the 
 
262 Elementary Biology. 
 
 muscular and other layers of the alimentary canal (save the 
 cell-layer lining the canal which is formed by endoderm) ; 
 whilst the outer or parietal layer applies itself to the body- 
 wall, and becomes transformed into a muscular layer. The 
 space between these two layers is the coelom. 
 
 The origin of one other important organ may be alluded 
 to, namely, the notochord. That structure is formed by 
 the folding off of a ridge from the dorsal wall of the 
 enteron. This band afterwards becomes distinct and en- 
 veloped, as does also the nerve-cord, by cells developed 
 from the mesoblastic somites, which become modified to 
 form the fibrous sheath above alluded to. The embryo 
 meanwhile becomes elongated, and the communication be- 
 tween the neural and enteric canals is interrupted, the 
 notochord growing backwards between the two canals. The 
 anterior end of the neural canal now becomes closed, and 
 the walls of the canal thicken to form the spinal cord of the 
 adult. It will be noted that at this time the alimentary 
 canal is a tube closed at both ends. It is put in communica- 
 tion first anteriorly and, later on, posteriorly, by the wall of 
 the tube and the wall of the body growing together, and 
 by the subsequent absorption of the tissue at the point of 
 union. In this way are formed an anterior depression, 
 or stomodseum, and a posterior depression, or procto- 
 daeum (anus). 
 
 In a quite similar way the gill-slits are formed in the 
 pharynx, or anterior portion of the enteron. We have 
 already seen how the atrial folds arise along the sides ,of the 
 body and enclose the atrial cavity. The further changes 
 which take place need not be dealt with. They consist in 
 the gradual moulding of the body of the embryo into the 
 shape it ultimately assumes : the formation of a liver as a 
 diveiticulum from the mesenteron as the alimentary canal 
 between the proctodaeum and stomodaeum is termed and 
 of reproductive and circulatory organs from the mesoblastic 
 somites. 
 
Metazoa Rana. 263 
 
 It will have been seen, so far, that the great point of 
 advance in organisation which Amphioxus shows over Lum- 
 bricus is the possession by the former of an endoskeltton 
 supporting a dorsally placed nervous system. Further, as a 
 consequence of the mode of formation of the nervous 
 system, a transverse section of the body of Amphioxus 
 exhibits two tubes a large ventral y placed ccelom, and a 
 small dorsally placed nervous canal. Lumbriats, on the 
 other hand, in transverse section, exhibits only one tube 
 the alimentary canal the nervous system lying free in the 
 coelom. 
 
 Finally, although the alimentary system of the worm is 
 scarcely less complicated than that of Amphioxus, yet the 
 respiratory system in the latter is greatly in advance of the 
 simple dermal respiratory mechanism in Lumbricus. 
 
 We have now completed our study of this very simple 
 chordate, and we have now to consider a more highly de- 
 veloped type of the class, so as to obtain some idea as to 
 how the simple organs of Amphioxus become modified in 
 these higher forms. 
 
 SECTION II. AMPHIBIA RANA. 
 
 In the preceding section we noted that the great point 
 of advance in the structure of the higher animals was the 
 possession of a backbone, or vertebral column, above which 
 (i.e. dorsally) lay the nervous system in the form of a tube 
 developed from the outer layer of the body of the embryo, 
 and beneath which (i.e. ventrally) was situated the body- 
 cavity, a space containing the chief organs of alimentation, 
 circulation, purification, and reproduction. In Amphioxus 
 the vertebral column we found to be of very simple structure, 
 being merely a rod of cellular tissue, or notochord, but en- 
 closed by a tough membrane continuous with the septa 
 separating the myotomes from each other, and present also 
 as a sheath round the dorsally placed nerve-cord. In forms 
 
 *S 4 
 
264 Elementary Biology. 
 
 higher in the scale than Amphioxus this notochord becomes 
 much modified. The originally cylindrical rod is encroached 
 upon by the fibrous sheath which covers it, and the sheath 
 itself becomes transformed into or replaced by cartilage, or 
 'gristle' (in the lower fishes, e.g. skate and shark), and 
 by bone (in bony fishes, e.g. cod, and all the higher 
 animals). Moreover the flexibility of the primitive axial 
 rod, which would otherwise have been lost in the firm 
 osseous cylinder, is still obtained by the segmentation of 
 the osseous column into separate short cylinders, or ver- 
 tebrae, capable of a limited amount of movement on each 
 other. 
 
 In accordance with the plan adopted in the preceding 
 sections, we may begin our study of one of the higher 
 animal types by a brief summary of the external characters. 
 In regard to the choice of a suitable type for the discussion 
 of the structure and general physiology of the higher Chor- 
 data there is much to be said in favour of the newt or the 
 salamander. The Amphibia, to which these examples 
 belong, is a remarkably interesting group in many respects. 
 Among them, for example, we find the first appearance of 
 true lungs, the characteristic respiratory organs of the 
 reptiles, birds, and mammalia, and the life-history of many 
 members of the class presents us with stages where the 
 transition from the fish-like condition respiration by gills, 
 or branchiae to the true air-breathing condition can be 
 easily demonstrated. Again, the skeleton in the Amphibia, 
 and especially in those members of the group cited above, 
 is found in a very typical condition, and the modifications 
 which have taken place in birds and mammals can be more 
 easily understood after a careful study of the system in its 
 relatively simple state. 
 
 The difficulty of obtaining specimens of the newt or 
 salamander is a serious drawback, and we are accordingly 
 compelled to fall back on the common frog (Rana tempo- 
 raria) for dissecting purposes, although the skeleton of that 
 
Metazoa Rana. 265 
 
 form is so much specialised as to render it a less suitable 
 subject for the preliminary study of that system. We will 
 therefore adopt the frog as the subject for consideration, 
 save. in the case of the skeletal system, where the skeleton of 
 the salamander will be taken as our type. 
 
 External characters. A very cursory examination of 
 the external characters of the frog enables us to distinguish 
 a head, neck, trunk, and fore and hind limbs (fig. 128). The 
 wide mouth, nostrils, eyes are easily seen on the head ; and 
 the ears may be identified as two small rounded patches 
 a short distance behind the eyes. There is no external ear, 
 but the drum, or tympanic membrane, is itself exposed. 
 Posteriorly only one opening can be made out, the opening 
 of the cloaca, a small chamber into which the ducts of the 
 renal and reproductive glands open together with the ali- 
 mentary canal. 
 
 The animal is covered externally by a soft, moist, pig- 
 mented skin, which is very loosely attached to the body 
 proper. The underlying space is occupied by a colourless 
 fluid, called lymph, which will call for a more detailed de- 
 scription presently. 
 
 The alimentary system. We have already seen that 
 the alimentary system in the animal consists essentially of a 
 tube of variable length, running from mouth to anus, into 
 which the food is introduced, and in which it is subjected 
 to the action of certain fluids or secretions which render it 
 capable of being absorbed through the walls of the ali- 
 mentary canal, directly or indirectly into the terminal vessels 
 (capillaries) of the circulatory system. In Lumbricus and in 
 Amphioxus that canal is straight and simple, but in the frog 
 it has become much modified. In the first place it is much 
 longer than the body of the animal, and consequently is 
 coiled up inside the body-cavity. In the second place, it 
 has become differentiated into several distinct regions : a 
 buccal cavity, where (though not in the frog) food is mas- 
 ticated ; an oesophagus, or tube for the carriage of the 
 
266 
 
 Elementary Biology. 
 
 food down to a stomach, or store-chamber, where the food 
 is mixed with digestive secretions ; an intestine, or narrow 
 canal, where further digestive changes are effected, and 
 where the nutritive part of the food is chiefly absorbed into 
 the circulation ; and lastly a rectum where the effete or 
 useless portions are collected, and from which they are 
 periodically got rid of through the cloaca. 
 
 FIG. 128. RANA TEMPORARIA. (Milnes Marshall.) 
 
 Again, the secretions with which the food is mixed, and 
 which render it capable of being absorbed, are produced in 
 glands which are either minute and sunk in the wall of the 
 alimentary canal itself, or are large and distinct from it, but 
 opening into it by ducts at various points. These glands, 
 
Metasoa Rana. 267 
 
 as \ve shall afterwards see, are formed as outpushings from 
 the alimentary canal itself. The two chief glands of this 
 nature are the liver and pancreas. 
 
 It will be necessary now to describe the course of 
 the alimentary canal and the structure of that organ and of 
 the glands which open into it. It will be convenient to 
 subdivide it into four regions, viz. the buccal cavity, the 
 oesophagus, the stomach, and the intestine and to describe 
 in connection with these several regions the glands which 
 are specially related to them. 
 
 The buccal cavity. The mouth is proportionally of large 
 size, the gape being very wide. The upper jaw is fringed 
 with a row of small teeth, and additional patches of teeth 
 may be felt on the roof of the mouth behind the jaw-teeth. 
 These are known as maxillary, premaxillary, and vomerine 
 teeth respectively from the bones of the skull on which they 
 are supported (p. 301). The lower jaw bears no teeth, but 
 has attached immediately behind its most anterior portion 
 the tongue, a flat bilobed organ of large size, free behind 
 and capable during life of being everted with great rapidity. 
 The large bulgings due to the eyeballs may also be noted on 
 the roof of the mouth. 
 
 Six openings may be easily made out on the walls of the 
 buccal cavity. Most noticeable of all of course is the wide 
 pharynx, or upper part of the oesophagus, and close to it 
 the small slit-like glottis, or aperture of the respiratory 
 system. Two other apertures are visible in the hinder 
 portion of the cavity. These are the eustachian tubes 
 leading into the ears. Lastly, in front and on the outer side 
 of the vomerine teeth are two small apertures, the posterior 
 nares, or inner nostrils. 
 
 The entire buccal cavity is lined by a continuation of the 
 outside skin termed a mucous membrane, since it contains 
 many small glands whose function it is to secrete mucus 
 a sticky, more or less watery fluid mainly of service in 
 keeping the membrane moist and facilitating the act of 
 
268 
 
 Elementary Biology. 
 
 FIG. 129. VERTICAL SECTION SKIN OF 
 RAN A. (Owen ) 
 
 swallowing. The microscopic structure of skin proper and 
 mucous membrane may be best considered together. The 
 skin consists of a series of layers of cells, the superficial 
 layers being composed of flat plates or squames and the 
 deeper layers of more rounded or polygonal cells. To 
 this portion of the skin is given the name of epidermis. 
 
 The epidermis rests on, 
 and is organically con- 
 nected with the dermis, 
 which is composed chiefly 
 of fibrous tissue, blood- 
 vessels, and nerves, 
 amongst which are im- 
 bedded, especially next the 
 epidermis, a large number 
 of irregular pigment cells. 
 The dermis is elevated 
 into a series of vascular 
 papillae which project into 
 the epidermis. In the 
 dermis, and with their 
 ducts piercing and opening 
 on the surface of the epi- 
 dermis, are many subcu- 
 taneous or mucous glands, 
 already referred to. The 
 subcutaneous tissue is 
 separated from the under- 
 f . , . lying muscles of the bodv- 
 
 A, vertical section; e, superficial layer of i O 
 
 epidermis; b, deep layers of epidermis ; \yall by a larSje lymph 
 
 a, layer of pigment cells : c, dermis d, 
 
 mucus gland ; g, connective tissue ; f, SpaCC. 
 
 The mucous membrane 
 
 of the buccal cavity does not differ essentially from the 
 skin in structure ; the layers of cells are, however, fewer in 
 number, the pigment cells are absent, and the vascular 
 supply is more abundant. We shall find, later on, that the 
 
Metazoa Rana. 269 
 
 FIG. 130. VERTICAL SECTION OF A TOOTH. (Quain.) 
 
 c, pulp cavity ; i, enamel, with radial and concentric markings ; 2, den- 
 tine, showing tubules and concentric lines of growth ; 3, cement (bone) ; 
 4, periosteum or bone-sheath ; 5, bone of jaw. 
 
 character of the mucous membrane is considerably modified 
 in other 'portions of the alimentary canal 
 
Elementary Biology. 
 
 FIG. 131. SECTION OF A PORTION OF DEVELOP 
 i.xG TOOTH. (Quain.) 
 
 The teeth, 1 already described as present en the upper 
 jaw and palate, next claim some attention. They are by 
 no means so distinct and elaborate in structure as those of 
 the higher animals, such as those of the rabbit or dog ; yet 
 the several layers distinguishable in the teeth of those latter 
 forms are to be seen even in the frog's teeth. Each tooth 
 consists essentially of a papilla of the dermis, capped by 
 epidermis, which has become considerably modified. In 
 the frog these teeth are constantly being renewed as the 
 older teeth became worn away. A typical tooth, say of a 
 dog, consists of a crown and neck above the level Ot the 
 
 jaw, and a fang sunk 
 in a cavity, or alve- 
 olus, in the jaw itself! 
 In the frog the tooth 
 is simply united to a 
 process of the bone 
 beneath. The sur- 
 face of the exposed 
 portion is covered by 
 an exceedingly hard 
 dense substance 
 termed enamel, 
 covering a hard but 
 sensitive core of dentine a substance not unlike bone 
 which again contains in its interior a papilla of submucous 
 tissues. The enamel consists of long closely packed prisms 
 of phosphate of lime, while the dentine is partly organic, 
 being composed of long branching tubules, imbedded, 
 
 1 The student is recommended to study the figures of the histology 
 of the frog in Howes' Atlas of Biology. The figures introduced in the 
 text are mainly taken from QuaMs Anatomy , and illustrate the histo- 
 logical structure of the mammal. Good woodcuts of the microscopic 
 structure of the lower vertebrata are still a desideratum, but those em- 
 ployed in the text, as being in most points applicable, may serve the 
 purpose in the present instance. 
 
 ;, ou'er layer of dentine, fully calcified ; b, un- 
 calcitied layer ; , dentine-forming cells ; d, 
 pulp. 
 
Metazoa Rana. 271 
 
 however, in a matrix consisting chiefly of phosphate of 
 lime. 
 
 The cavity of the tooth (pulp cavity) is occupied, as has 
 already been stated, by a papilla of submucous tissue. Over 
 the surface of the papilla there lies a number of branched 
 cells, from each of which proceeds an especially long branch 
 which enters a dentinal tubule. When we come to speak 
 of the minute structure of bone, the meaning and function 
 of these fibres will become apparent. 
 
 The only other organ we need mention in the buccal 
 cavity is the tongue. It has already been stated that, 
 contrary to the usual rule, the frog's tongue is free behind 
 and attached to the jaw in front. The organ is composed 
 of bands of muscle separated and at the same time bound 
 together by connective tissue. The surface of the tongue 
 is covered by mucous membrane, which is, however, elevated 
 to form papillae. There are also special, papillae, which 
 have to do with the sense of taste. These will be referred 
 to later on when the senses come to be discussed. 
 
 We must now endeavour to gain some general idea of 
 the nature of the alimentary canal and the modifications 
 met with on its course. The pharynx opens into a wide 
 dilatable tube, the oesophagus, opening in its turn into a 
 thick- walled stomach (fig. 153). The stomach together 
 with the lower part of the oesophagus is partly hidden by 
 the liver, a large, brown lobed organ, lying ventral to the 
 alimentary canal, and attached to its wall just anterior to 
 the proximal or cardiac end of the stomach. On leaving 
 the stomach the alimentary canal narrows, and forms a tube 
 of uniform bore, the small intestine, which lies coiled on 
 the right-hand side of the lower portion of the abdominal 
 cavity. The small intestine bends forward along the wall of 
 the stomach, and encloses in the fold a long whitish organ, 
 the pancreas. Lying between the lobes of the liver is a 
 small round sac. of greenish colour if full, the gall-bladder. 
 From the gall-bladder a delicate tube, the bile-duct, passes 
 
2/2 
 
 Elementary Biology. 
 
 away, traversing the substance of the pancreas and receiving 
 in its course ducts from that organ. The bile-duct finally 
 opens into the small intestine about midway along the re- 
 current fold. This first portion of the intestine is known as 
 the duodenum. The intestine thereafter makes a series of 
 
 coils and finally en- 
 
 FIG. 132. VERTICAL SECTION OF THE HUMAN -, , , . 
 
 CESOPHAGUS. (Quain.) larges abruptly mtc 
 
 a large short tube, 
 the rectum, which 
 opens into the cham- 
 ber already referred 
 to as the cloaca. 
 
 The entire outer 
 surface of the ali- 
 mentary canal and 
 its glands is covered 
 by a shining mem- 
 brane continuous 
 with a similar mem- 
 brane covering the 
 inner wall of the 
 abdominal cavity, 
 and known as the 
 peritoneum. The pe- 
 ritoneum rises from 
 tfte dorsal wall of 
 the abdomen and 
 becomes continuous 
 with the layer re- 
 flected over the 
 viscera, forming na- 
 turally a double layer, which attaches the alimentary canal 
 and its glands to the abdominal wall, and forms the delicate 
 membrane known as the mesentery. The mesentery, as we 
 shall afterwards see, serves to support not only the alimentary 
 canal but also blood-vessels, ducts, and nerves. 
 
 a, connective-tissue layer; b, longitudinal muscles, 
 cut transversely; c, circular muscles, cut longi- 
 tudinally ; d, layer containing mucous glands ; e, 
 layer of muscle fibre ; f, subepithelial tissue 
 (dermis) ; g; epithelium layer (epidermis). 
 
Metazoa Rana. 273 
 
 The alimentary canal, therefore, consists of a tube for 
 conveying the food to a storing and digesting organ, from 
 which it is passed into another tube, in which it is mixed 
 with and further acted on by digestive fluids secreted by 
 certain organs opening into the canal, through whose walls 
 absorption of nutriment takes place. The refuse is col- 
 lected in the rectum and ejected to the exterior periodically. 
 The wall of the oesophagus is composed of three layers, 
 internally (i) mucous membrane with mucous glands (fig. 
 132, d, e, f, g) surrounded by (2) a layer of muscle fibres 
 (, c) and (3) of connective tissue (a). It will be most con- 
 venient to refer briefly at this point to the essential charac- 
 ters of the muscular and connective tissues, since we shall 
 meet with them in almost every part of the body. 
 
 Muscle. Like every tissue in the organism muscle is 
 composed of cells. The cells are much elongated in one 
 direction, and have the function of contractility specially 
 developed. There are two varieties of muscle, striped and 
 non-striped. The protoplasm of the cell or fibre of striped 
 muscle is greatly modified, being transformed into a bundle 
 of exceedingly delicate fibrillae, each of which appears to 
 consist of alternate dark and light segments. The cell-wall 
 is exceedingly thin, and is known as the sarcolemma. 
 Nuclei may be observed scattered here and there through 
 the fibre, or immediately below the sarcolemma of the 
 muscle-fibres of higher forms. The alternate light and dark 
 segments of the individual fibrillae give the fibre as a whole 
 a transversely striped appearance, whence this variety of 
 muscle derives its name. Striped muscle is also known as 
 * voluntary ' because it is the variety met with in muscles 
 under the direct command of the will. There are, however, 
 exceptions to this rule, as, for instance, in the heart, where 
 the fibres are striped, but where the muscles are independent 
 of the influence of the will. The voluntary muscles of the 
 frog are sometimes branched as in the tongue (fig. 135). 
 
 The other variety of muscle, termed non-striped or 
 
 T 
 
274 
 
 Elementary Biology. 
 
 involuntary from the fact of its occurring in situations 
 where movement is outside the dominion of will, consists 
 of cells less far removed from their typical condition. The 
 protoplasm is longitudinally striated, suggesting the differ- 
 entiation into nbrillae which we have studied in the striped 
 muscle-cell, and a delicate envelope is present correspond- 
 
 FIG. 133.- STRIPED MUSCLE FIBRES. (Quain.) 
 
 B 
 
 A, portion of a muscle fibre ; n, part of the same teased, showing groups 
 of fibrils, a, b, c, and ultimate fibrillse, d, d. 
 
 ing to the sarcolemma. The cell possesses an elongated 
 nucleus lying centrally in the protoplasm. 
 
 In the case of both striped and non-striped muscle the 
 cells are bound together by an intercellular cement into 
 elementary bundles, or fasciculi, which are again collected 
 into larger bundles. Both large and small bundles are held 
 
Metazoa Rana. 
 
 275 
 
 together by connective tissue, to which we may now briefly 
 
 vpfpj- 
 
 ' . . FIG. 136. FIBRES 
 
 Connective tissue, which performs, as OF NON- STRIPED 
 
 . .. . , . , MUSCLE. (Quain.) 
 
 its name indicates, a purely mechanical 
 function, namely, that of binding other 
 tissues together or sheathing special 
 organs, consists typically of three elements 
 white fibres, elastic fibres, and cells 
 (figs. 137-9). The white fibres are pale, 
 extremely fine and wavy, and unbranched; 
 the elastic fibres, on the other hand, are 
 
 FlG. 134. A MUSCLE- 
 FIBRE TORN SO AS TO 
 SHOW SARCOLEMMA 
 
 AND NUCLEI. (Quain.) 
 
 FIG. 135. BRANCHED MUSCLE 
 
 FIBRE FROM THE TONGUE OF 
 
 THE FROG. (Quain.) 
 
 sharply defined, branched, and show a disposition to curl 
 up at their free ends. The connective-tissue cells are 
 
 T 2 
 
276 
 
 Elementary Biology. 
 
 typical branched nucleated cells, of quite irregular form 
 and size. These three elements are present in variable 
 amount in different varieties of connective tissue. 
 
 FIG. 137. ELASTIC FIBRES 
 
 OF CONNECTIVE TISSUE. 
 
 Quain.) 
 
 FIG. 138. WHITE FIBROUS ELEMENTS OF CONNECTIVE 
 TISSUE. (Quain.) 
 
 In the wall of the oesophagus the muscles are for the 
 most part non-striped. In the upper portion only are the 
 
 FIG. 139. CONNECTIVE TISSUE CORPUSCITES. (Quain.) 
 I 
 
 fibres striped. The fibres are arranged in two layers (fig. 
 132), an inner circular and an outer longitudinal. Their 
 
Metazoa Rana. 
 
 277 
 
 FIG. 140. PORTION OF THE WALL OF 
 THE ALIMENTARY CANAL AT THE 
 JUNCTION OF THE STOMACH AND 
 
 SMALL INTESTINE. (Quain.) 
 
 function is to assist by their contraction (and consequent 
 narrowing of the oesophagus) in pushing the food down 
 into the stomach. The wave of contraction that follows the 
 food is known as peristalsis. 
 
 The stomach may be divided into a proximal or cardiac 
 (nearest the heart) portion and a distal or pyloric end. 
 Its wall from within outwards consists of (i) a mucous coat 
 with special glands ; (2) a submucous layer of connective 
 tissue, non- striped muscle, blood-vessels, nerves, and ab- 
 sorbing ducts known as lymphatics ; (3) a non-striped 
 muscular coat, whose fibres 
 are arranged in two layers 
 similarly to those of the 
 oesophagus ; and (4) a con- 
 nective tissue coat formed by 
 the peritoneum. 
 
 The intestine similarly 
 consists of four layers 
 mucous, submucous, muscu- 
 lar, and fibrous (fig. 140). 
 
 A detailed study of the 
 last three of these layers is 
 unnecessary in the present 
 instance, but the principal 
 characters of the mucous layer merit more careful considera- 
 tion. In the case of the stomach it is thick, relatively to 
 the other layers, from the presence of closely packed tubular 
 glands, not unlike the test-tubes used in chemical manipu- 
 lations. The glands (gastric glands) are arranged vertically 
 to the surface of the stomach, and consist of a wall formed 
 of columnar or cubical cells capable of secreting into the 
 cavity of the tubule a digestive fluid known as gastric juice, 
 having for its function the rendering soluble of certain sub- 
 stances in the food chemically known as proteids (p. 26). 
 The glands are surrounded by fine connective tissue, capil- 
 laries, and lacteals prolonged upwards from the submucous 
 
 peritoneum ; me, muscular layer ; mi t 
 layer of submucous tissue ; g,g mucous 
 layer ; /, p } pyloric valve ; z, villi of 
 small intestine. 
 
278 Elementary Biology. 
 
 layer. It is from the blood circulating in the capillaries 
 that the secretory cells lining the glands obtain the materials 
 for the formation of gastric juice, the most important con- 
 stituent of which is a ferment called pepsin. In the intestinal 
 mucous membrane there are similar glands, known there by 
 the name of Lieberkiihnian follicles, and having a similar 
 function to those in the gastric wall. In addition, however, 
 
 FIG. 141. VERTICAL SECTION OF THE SMALL INTESTINE. (Qualn.) 
 
 /, /, submucous layer containing blood-vessels d, c, and large lacteal, a, a; 
 ~, g", mucous layer with Lieberkiihnian follicles ; e, e : , epithelial layer; 
 a, lacteal in the interior of a villus ; b, network of blood-capillaries in 
 the subepithelial tissue (/) of the villus. 
 
 the mucous membrane is elevated between the mouths of 
 the follicles into long papillae, or villi, which are, structurally 
 speaking, follicles turned inside out. Each villus is covered 
 by columnar cells resting on a basement membrane, and 
 covering a core of submucous tissue, consisting of lymphatics, 
 or, as they are here termed, lacteals, nerves, blood-capilla- 
 ries, and connective tissue. Among the columnar cells are 
 found many wide-mouthed goblet-shaped cells, whose func- 
 
Metazoa Ran a. 279 
 
 tion it is to secrete mucin to keep the surface of the intestine 
 moist, and to dilute the intestinal digestive juice as it flows 
 from the follicles. 
 
 At the junction of the . intestine and the stomach the 
 circular muscular coat is very much thicker, and forms a 
 sphincter valve (fig. 140). When it is contracted no food 
 can pass from the stomach into the intestine ; relaxation takes 
 place only when the food in the stomach has been sufficiently 
 acted on by the gastric juice. 
 
 In the rectum both follicles and villi are absent, and 
 the mucous membrane consists simply of stratified squamous 
 cells with a few mucous glands. 
 
 The liver is the largest organ in the body, and consists 
 
 FIG. 142. GOBLET CELLS. (Quain.) 
 U. 
 
 of a single right and a double left lobe. The two lobes are 
 united by a transverse commissure of liver substance. 
 The liver lobes are composed of an immense number of 
 polygonal lobules of small diameter (^ to ^ of an inch). 
 Each lobule is composed of many liver-cells, also polygonal 
 in form. The cells have extremely fine cell-walls, with 
 protoplasmic contents and nucleus, in which intracellular 
 and intranuclear networks can be readily made out. The 
 intercellular spaces open into minute ducts, lined by cubical 
 epithelium, which in turn communicate with the duct 
 already referred to as the bile-duct. The gall-bladder, 
 already mentioned, is practically an enlargement on the 
 course of the bile-duct, in which the surplus bile is stored. 
 
280 
 
 Elementary Biology. 
 
 In reality the duct from the liver opens into the gall- 
 bladder, from which in turn the true bile-duct arises, open- 
 ing into the alimentary canal in that region known as the 
 duodenum. 
 
 Bile is, in the frog, a greenish-yellow viscid fluid, highly 
 antiseptic in character, and for that reason of service in pre- 
 venting putrefaction of the intestinal contents. It performs 
 the additional function of serving to assist in rendering 
 
 FIG. 143. T\VO LIVER LOBULES (SEMI-DIAGRAMMATIC). (Qualn.) 
 
 h, h, intralobular vein ; /, interlobular (portal) veins ; .?, sublobular 
 (hepatic) vein. The arrows represent the course of the blood. Liver- 
 cells are represented in one part of each lobule. 
 
 fatty substances in the intestine capable of being absorbed 
 through the intestinal wall. The formation of bjle is, how- 
 ever, by no means the only function which the liver has to 
 perform. Probably its chief function is to act as a manu- 
 factory and storehouse for glycogen, or animal starch. The 
 physiology of the so-called glycogenic function of the liver 
 is, however, not yet fully understood. 
 
 The arrangement of blood-vessels in the liver is of the 
 
Metazoa Rana. 
 
 281 
 
 highest importance, and will be described in connection 
 with the circulatory system. 
 
 One Other digestive FlG - T 44- FOUR HEPATIC CELLS WITH 
 
 f COMMENCEMENTS OF A BILE-CAPIL- 
 
 gland may be referred to, LARY. (Quain.) 
 viz. the pancreas. This 
 organ lies in the fold of the 
 duodenum, and is the 
 agent in the formation of 
 pancreatic juice. It con- 
 sists of a series of closely 
 packed tubules, lined by 
 cubical glandular cells 
 which have the power of 
 abstracting the constituents 
 cf the pancreatic juice 
 from the capillary blood- 
 vessels distributed in large 
 numbers through 
 its tissue. The 
 secretion differs 
 from that of the 
 stomach in being 
 alkaline (that of the 
 stomach is acid), 
 but, like the gastric 
 juice, it has the 
 power of rendering 
 proteid substances 
 soluble which have 
 escaped the action 
 of the gastric se- 
 cretion. It also 
 assists in the ab- 
 
 . r r ^ j a > secret ory tubule ; d, origin of a pancreatic duct. 
 
 sorption of fat, and, 
 
 in addition, transforms staj-h intp .sugar a function in the 
 
 higher animals mainl^g^faled by "the secretion of the 
 
 FIG. 145. SECRETORY TUBULES OF THE PANCREAS. 
 (Quain.) 
 
282 Elementary Biology. 
 
 salivary glands. These latter are absent, however, from 
 the Amphibia. 
 
 By the action of these various secretions the contents of 
 the alimentary canal are thus rendered capable of absorption. 
 That process takes place through the walls of the small 
 intestine chiefly, where there are very many special vessels 
 known as lacteals and blood-capillaries in readiness to re- 
 ceive the soluble products. The lacteals are extremely 
 delicate ducts, whose walls are composed of a single layer 
 of thin plate-like cells (squames). They abound in the villi 
 of the small intestine and in the wall of the alimentary canal 
 generally. These lacteals communicate with larger trunks, 
 which ultimately pour their contents into the general blood 
 circulation. The food matters also enter the capillaries 
 and so pass directly into the circulation. 
 
 The circulatory organs are in the frog, and, indeed, in all 
 the Vertebrata, very highly differentiated. It will not be 
 possible to give more than a brief outline of the plan of these 
 organs. Further details must be obtained from zoological 
 treatises. 
 
 Generally speaking, the circulatory system of any one of 
 the higher animals, and of the frog in particular, consists 
 fundamentally of a pumping organ or heart and two sets of 
 vessels, one passing from the heart (arteries) and one passing 
 to the heart (veins). Blood, the contents of these various 
 organs", is, as we have previously seen, a fluid which has 
 two all-important duties to perform, viz. (i) to convey to the 
 various tissues of the body the nutriment absorbed from 
 the alimentary canal, and (2) to act as the medium for the 
 carriage of a supply of oxygen to the tissues whereby that 
 disintegration of organic compounds may be effected which 
 we have seen to be a sine qua non in the manifestation of 
 life, and incidentally to act as the medium for the con- 
 veyance of the products of disintegration from the tissues to 
 the exterior. 
 
 The blood of the frog is a red, slightly viscid fluid, which 
 
Metazoa Rana. 
 
 283 
 
 on being examined under the microscope is found to consist 
 of an almost colourless medium or plasma in which floats an 
 immense number of blood-corpuscles of two kinds the one 
 irregular in shape, colourless, and not unlike Amotbce in 
 
 FIG. 146. BLOOD-CORPUSCLES OF THE FROG. (Ranvier.) 
 
 a, red corpuscles seen on the flat ; v, vacuoles in a corpuscle ; t>, c, red 
 corpuscles seen in profile ; n, k, colourless corpuscles at rest ; in, colour- 
 less corpuscle showing pseudopodia ; /, coloured fusiform corpuscle. 
 
 general appearance, and exhibiting when living that 
 indefinite motion which has already been described and 
 termed amoeboid ; the other definite and elliptical in shape 
 and reddish yellow in colour. It is from these latter that 
 the blood derives its red colour. 
 Blood after death, or if with- 
 drawn from the living body, does 
 not long remain liquid. In 
 the plasma extremely delicate 
 branched filaments (fibrin threads) 
 make their appearance, in the 
 meshes of which the corpuscles 
 become entangled. The clot (coagulum) subsequently con- 
 tracts, with the result that the watery element (serum) of 
 
 FIG. 147. FIBRIN THREADS. 
 (Schafer.) 
 
 The coloured corpuscles have 
 been removed by washing. 
 
284 Elementary Biology. 
 
 the plasma is expressed. We might therefore classify the 
 various constituents of blood thus : 
 
 Blood 
 
 Corpuscles Plasma 
 
 Colourless Red Fibrin Serum 
 
 Coagulum 
 
 The two most important functions of blood have been 
 mentioned above ; it will be sufficient to add here that the 
 plasma performs the double duty of carrying nutritive 
 material to the various tissues, and of removing from them 
 many of the waste products always being formed there 
 during life, while on the colouring matter of the red cor- 
 puscles, haemoglobin (p. 21), devolves the duty of carrying 
 oxygen gas from the exterior to the tissues. 
 
 Naturally, therefore, a considerable difference exists 
 between blood passing to the tissues from the heart (ar- 
 terial blood) and blood passing from the tissues to the 
 heart (venous blooi). Arterial blood contains about 17 % 
 of oxygen gas (by volume), while venous blood contains 
 only 6 %. Similarly arterial blood contains about 30 % of 
 carbonic acid gas (by volume), while venous blood may have 
 as much as 40 % to 45 %. There are also other points of 
 difference between arterial and venous blood, the most 
 noticeable of which is the bright scarlet colour of arterial, 
 contrasting in this respect with the dull red of venous blood. 
 
 The red corpuscles are derived from the white in a manner 
 not yet accurately determined. The white in their turn are 
 developed in a variety of situations in the body, chief amongst 
 which are the so called blood-glands. 
 
 The heart, or pump which drives the blood through the 
 vessels, is a hollow, muscular, pyramidal sac, lying in the 
 anterior part of the body-cavity known in the higher animals 
 as the thorax, where indeed it forms a distinct subdivision 
 
Metasoa Rana. 
 
 28 S 
 
 FIG. 148. ARRANGEMENT OF THE CHIEF ARTERIES 
 
 AND VEINS IN THE FROG. (Owen.) 
 
 of the ccelom. The heart is enclosed in a double-walled 
 sac, or pericardium, between the walls of which there is a 
 space filled with a colourless nutritive fluid known as lymph. 
 From and to the heart a number of large blood-vessels 
 pass, dividing 
 afterwards into 
 smaller branches. 
 The heart itself 
 is subdivided into 
 three chambers, a 
 ventricle, which 
 composes the apex 
 and aconsiderable 
 portion of the 
 body, and two 
 auricles, which 
 occupy the base 
 of the pyramid. 
 The heart lies 
 close to the ven- 
 tral body- wall in 
 such a way that 
 the apex points 
 backwards and the 
 large vessels arise 
 from near the base 
 and pass forwards. 
 Above or behind 
 the heart . lies a 
 large membranous 
 sac known as the 
 sinus venosus, 
 into which all the 
 impure blood is poured in the course of the circulation ; 
 below or in front of the heart and springing from the 
 ventricle is a large muscular vessel known as the truncus 
 
 H, heart ; A, above the origin of the common carotids ; 
 p, the left lung ; /, pulmonary vein ; p', pulmo- 
 cutaneous artery ; o, right precaval vein ; /, hepatic 
 vein ; L, portal vein ; v, postcaval vein ; A, union 
 of the two aortae ; u, right aortic arch ; k, renal vein ; 
 /', renal artery ; , anterior abdominal vein ; A', divi- 
 sion of the aorta into two common iliac arteries going 
 to the hind limbs. Compare this figure with fig. 53. 
 
286 Elementary Biology. 
 
 arteriosus, by which pure blood leaves the heart to pass to 
 the tissues. It very soon branches into a number of large 
 arteries afterwards to be specified. Lastly, one vessel opens 
 directly into the left auricle, while the sinus venosus mani- 
 festly is connected with the right auricle. 
 
 Before entering into further details with regard to the 
 circulatory system it will be necessary for us to understand 
 clearly the nature of the three important sets of vessels con- 
 cerned in the transport of the blood, namely, the arteries, 
 veins, and capillaries. 
 
 Arteries and veins have fundamentally the same struc- 
 ture, but they differ in the relative amounts of the 
 
 FIG. 149. TRANSVERSE SECTION OF THE WALL OF A TYPICAL ARTERY. 
 (Quain.) 
 
 c 
 
 & ?". 
 
 "' -' : '" \^ 
 
 a, epithelial layer ; b, elastic membrane ; c, tunica media ; d, tunica adventitia. 
 
 separate elements entering into the composition of their 
 walls. The wall consists of an internal epithelial lining of 
 flat squames followed by a sub- epithelial membrane com- 
 posed of elastic fibres : this is known as the tunica intima. 
 The tunica media consists of a thick layer of circularly 
 arranged muscle-fibres of the non-striped variety, plentifully 
 mingled with delicate elastic fibres. The whole is strengthened 
 and attached to adjacent parts by a tunica adventitia com- 
 posed of connective tissue in which all three elements of 
 that tissue cellular, fibrous and elastic are well represented. 
 The veins differ from the arteries chiefly in two points ; 
 first, in that the various coats are thinner ; and secondly, 
 
Metazoa Ran a. 
 
 28; 
 
 in the presence (in most veins) of valves or pocket-like flaps 
 formed by the tunica intima and tunica media. These 
 valves are so arranged that a current of blood flowing in the 
 normal direction into the heart encounters no opposition, 
 
 FIG. 150. TRANSVERSE SECTION OF WALL OF A TYPICAL VEIN. (Quain.) 
 
 a, epithelial layer ; l>, elastic membrane ; c, tunica media ; d, tunica adventitia. 
 
 whilst a regurgitation from the heart causes the pockets to 
 open and press against each other, and so prevent the back- 
 ward passage of impure blood to the tissues (fig. 151). 
 
 FIG. 151. -VALVES IN A VEIN. (Quain.) 
 
 A, vein cut open ; n, longitudinal section of vein showing the valve in 
 action ; c, the same seen from outside. 
 
 Capillaries are exceedingly delicate vessels, which form 
 the connection between the artery and its corresponding 
 vein, and whose walls have been reduced to a tunica intima of 
 
288 
 
 Elementary Biology. 
 
 FIG. 152. CAPILLARY NETWORK 
 UNITING AN ARTEK>' AND A VEIN 
 IN THE WEB OF THE FROG'S FOOT. 
 (Quain.) 
 
 epithelial cells only. Every artery subdivides into capillaries ; 
 every vein is formed by the reunion of capillaries ; the 
 current of the circulation is from the artery to the vein 
 through the capillary network so formed. Capillaries thus 
 form the ultimate subdivisions of the circulatory system, and 
 
 as such bring oxygen and 
 nutritive fluid into close rela- 
 tion with the tissues, and at 
 the same time extract the 
 waste products produced in 
 the tissues as a consequence 
 of metabolic changes taking 
 place there. 
 
 Having now briefly noticed 
 the general character of the 
 constituents of the vascular 
 system it will be necessary to 
 sketch the course of the chief 
 blood-streams through the 
 body. A little consideration 
 will show us that there must be three important currents, 
 viz. (i) a blood-stream from the alimentary canal which 
 passes through the liver to the heart, carrying the products 
 of assimilation into the general circulation ; (2) a blood- 
 stream from the heart to and from the various organs of the 
 body ; and (3) a blood-stream to and from certain purifi- 
 catory organs the lung and the kidneys. 
 
 The first current is conveyed by a large vein, the portal 
 vein, which on the one hand has its terminal capillaries 
 diffused in the walls of the intestine, and on the other 
 breaks up into a similar capillary network in the substance 
 of the liver, in which organ the constituents of bile are ab- 
 stracted from the blood, while at the same time glycogen is 
 removed and stored in the liver-cells. This blood-stream 
 is known as the portal circulation. 
 
 The second or systemic circulation is more elaborate. 
 
 The arrows indicate the direction of 
 the blood-flow. 
 
Metazoa Rana. 289 
 
 Starting from the truncus artericsus the blood is pumped by 
 the ventricle into a number of vessels springing from the 
 truncus. These vessels are six in number, three on either 
 side : nearest the origin of the truncus, a pair of vessels we 
 may know as pulmo-cutaneous ; next two aortse ; and lastly 
 the two common carotids. We have already seen that the 
 heart consists of three chambers, a ventricle and two auricles. 
 The sinus venosus opens into the right auricle, while it has 
 been noted that one vessel, to be known now as the pul- 
 monary vein, opens into the left auricle. The auricles in 
 turn open into the ventricles, and the auriculo- ventricular 
 openings are guarded by valves which prevent regurgitation. 
 Further it will be noted that the truncus arteriosus springs 
 more from the right than the left side of the ventricle. 
 When the ventricle contracts, therefore, the blood occupying 
 the right side of the ventricle will be the first to enter the 
 truncus. Now we have already learnt that the sinus venosus 
 receives all the impure blood from the systemic veins ; while 
 the pulmonary vein, on the other hand, contains pure blood 
 from the respiratory organ, the lung. Under these circum- 
 stances, when the auricles contract and pour their contents 
 into the ventricle, the ventricle will contain pure blood on 
 its left side, impure blood on its right side, and mixed blood 
 in the middle. Therefore the ventricular contraction will 
 drive into the truncus first of all impure blood, which will find 
 its way into the first of the branches of the truncus, namely, 
 the pulmo-cutaneous arteries, by which vessels the blood 
 is conveyed to the lung and skin (the two great respiratory 
 organs in the frog) -to be purified. The blood immediately 
 following will pass into the two aortae, and the last and 
 purest blood will enter the carotids. This process is greatly 
 assisted by a special valve in the interior of the truncus, 
 which closes the entrance to the pulmo-cutaneous arteries 
 after they have been filled. By the aortas (which afterwards 
 unite) the blood (mixed) is carried to all the chief organs of 
 the trunk, to the limbs, stomach, intestine, &c. supplying 
 
 u 
 
Elementary Biology. 
 
 FIG. 153. VENOUS SYSTEM OF THE FROG. (Milnes Marshall) 
 
 ST. JV. 
 
 ar.T. 
 
 , stomach ;".!/., anterior abdominal vein ; b., bladder ; b.v., brachial vein ; 
 cl., cloaca ; c.v., cardiac vein ; d, rectum ; e, liver ; e.v., external jugu- 
 lar vein ; f.v. femoral vein ; g., gall bladder ; h, sp'een ; i.e., inferior 
 vena cava ; i.v., innominate vein ; jv, internal jugular ; /./., left pelvic 
 ; m.v., musculo-cutaneous vein; o, kidney; p.v., hepatic portal 
 ; r.p., right pelvic vein ; r.v., right renal-portal ; s., sinus venosus ; 
 
 ven 
 vein 
 
 sc., sciatic vein; s.v., subclavian vein; t., tongue; t.a., truncus arteri- 
 osus ; a., right auricle ; z/., ventricle ; v.v., vesical veins. 
 
 FIG. 154. ARTERIAL SYSTEM OF THE FKOG. (Milnes Marshall.) 
 
 2 S.a 
 
 c.a 
 c.a | 
 
 b, nostril; c, small intestine; c.a., carotid artery; e.g., carotid gland; 
 c.m., coeliaco-mesenteric artery; en, cutaneous artery; d.a., dorsal 
 aorta ; /., femur ; h.a., hepatic artery ; z, right lung ; l.a., lingual artery ; 
 tn, spermarium, or testis ; o.a,, occipito-vertebral artery ;/>.#., pulmonary 
 artery ; r, pelvic girdle ; s., sternum ; s.a,, subclavian artery ; j c., sciatic 
 artery; u.a., urino-genital arteries; i, carotid arch; 2, systemic arch ; 
 3, pulmo-cutaneous arch. 
 
Metazoa Rana. 291 
 
 these viscera with nutriment and oxygen. The purest blood 
 passes by the carotids to the head region, where naturally 
 the purest blood is wanted. 
 
 Similarly the impure blood is returned to the heart by 
 special veins from each organ, uniting ultimately into four 
 large vessels, two anterior or precaval veins and one posterior 
 or postcaval vein which run direct to the sinus venosus, 
 and one anterior abdominal, which, after traversing the 
 ventral body-wall goes to the liver. 
 
 The details of the arrangement and distribution of the 
 smaller branches though of great importance cannot be dealt 
 with here. Figs. 148, 153, and 154 will, however, show 
 many of these details, which can be made out without diffi- 
 culty in the animal itself. (The names of the various veins 
 and arteries indicate sufficiently the origin and distribution.-) 
 One point must, however, be briefly noticed with regard to 
 the renal circulation. The common aorta gives off a number 
 of branches to each of the two kidneys lying in the dorsal 
 portion of the ccelom, while the renal veins open into the 
 postcaval : this is the normal arrangement of artery and 
 vein. But in the frog an additional means of circulation is 
 present in the shape of a large vessel known as the renal- 
 portal, which carries venous blood from the hind-limb and 
 lumbar region to the kidney. The renal-portal on either 
 side gives off a branch to the anterior abdominal, so that 
 blood coming from the hind-limbs may pass to the heart 
 by two channels, through the liver by the anterior abdomi- 
 nal, or through the kidney by the renal-portal. 
 
 Respiratory system, Before venous blood can be made 
 use of again as a carrier of oxygen to the tissues it must be 
 purified by the removal from it of the carbonic acid, water, 
 and other waste products collected in its passage through 
 the systemic capillaries. We shall consider first the re- 
 moval of the carbonic acid. This duty is performed by the 
 lungs, two semi-transparent sacs of large size situated 
 ventrally to the oesophagus. At the posterior end of the 
 
 u 2 
 
292 
 
 Elementary Biology. 
 
 buccal cavity, and immediately beneath the opening ol the 
 oesophagus, may be seen a slit-like aperture, already men- 
 tioned, known as the glottis, the walls of which are strength- 
 ened by cartilage. The glottis leads into a cylindrical 
 chamber from which the two lung-sacs open. Each lung 
 is a hollow tapering bag, externally covered by a layer of 
 
 FIG. 155. SECTION OF INJECTED LUNG. (Quain.) 
 
 a, a, free edges of the pulmonary sacs (alveoli); c, c, partitions between 
 neighbouring alveoli in section ; b, small artery giving off numerous 
 ca i laries which form network over the alveolar wall. 
 
 the same membrane which has already been described as 
 covering the viscera generally, here, however, known under 
 the special name of pleuron. The inner surface of the 
 lung is thrown into folds and ridges, giving the wall the 
 appearance of being honeycombed. Manifestly the super- 
 ficies of the lung-wall is thus greatly increased. The pul- 
 
Metazoa Rana. 293 
 
 monary artery already mentioned in considering the branches 
 of the truncus arteriosus here subdivides again and again, 
 and its- ultimate capillaries ramify in the lung-wall, separated 
 from the air which normally fills the lung by a single layer 
 of squamous epithelium. It is manifest that a gaseous ex- 
 change between the atmosphere and the blood circulating in 
 the capillary terminations of the pulmonary artery is thus 
 made possible. (It must be noted that the pulmonary 
 artery, despite its name, contains venous blood.) The wall 
 of the limg is exceedingly elastic, and readily collapses if the 
 glottis be kept open or the wall be punctured. 
 
 In principle the lung of the frog does not differ from the 
 gill of the fish, for in both the end to be gained is the 
 exposure of a maximum amount of blood to the atmosphere. 
 In the fish the blood is carried outside the body in capillaries, 
 which ramify in processes supported by a connective tissue 
 framework, and thus meets with the oxygen dissolved in the 
 water, while in the frog the air is sucked into the interior of 
 the body to the blood, a method by which risk of injury to 
 so delicate and important a system is reduced to a minimum, 
 If space permitted, it would be interesting to trace the 
 manner in which the lungs have arisen as a modification of 
 a curious organ -the swim-bladderdeveloped in many 
 fish. By altering the quantity of air contained in the swim- 
 bladder, fish are able to increase or decrease their buoy- 
 ancy. In higher animals, such as the rabbit or the dog, the 
 lung, instead of being a single hollow sac, is composed of an 
 enormous number of extremely minute sacs closely packed 
 together (fig. 155), and communicating with the external 
 world by means of a series of branched tubes (bronchi), 
 which ultimately unite to form one large tube (trachea), 
 of which the sole representative in the frog is the small 
 cylindrical chamber into which the glottis opens. The gain 
 in the extent of respiratory surface in the mammalian lung 
 by this arrangement must be at once evident, whilst at the 
 same time every particle of air inhaled is made use of. In 
 
294 
 
 Elementary Biology. 
 
 the frog, on the other hand, only the air nearest the wall of 
 the lung can be employed for respiratory purposes ; a loss, 
 however, not of much importance, since pulmonary respira- 
 tion is in the frog greatly assisted by secondary respiration 
 through the skin. The mode of inspiration in the frog 
 differs from that of the higher animals, since, instead of 
 
 FIG. 156.- DEVELOPMENT OF THE LUNGS. (Wiedersheim.) 
 PD 
 
 ff 
 
 AT' 
 
 FD, primitive intestine ; S, S', lung-sacs ; /, trachea ; b, bronchus ; 
 Lg, seconda y pulmonary sacs. 
 
 employing the muscles of the lung and the abdominal 
 wall, &c. to bring about the distension of the lung, the frog 
 inhales by the nostrils, so filling the buccal cavity, then, 
 shutting the oesophagus and nostrils by means of the 
 muscles of the buccal wall, it forces the air into the lungs, 
 from which it is e.xpelled by the natural elasticity of the 
 lung itself. 
 
Metazoa Rana. 
 
 295 
 
 FIG. 157. KIDNEYS AND SPERMARIA OF 
 FROG. (Wiedersheim.) 
 
 CrAo 
 
 FK 
 
 Renal system. It will now be necessary to glance at 
 the mechanism by which the water and soluble nitrogenous 
 waste are removed from the blood and got rid of. That duty 
 is performed by two organs, the kidneys, which have already 
 been mentioned in connection with the circulatory system 
 as lying dorsally one on either side of the dorsal aorta, 
 and receiving branches 
 from that vessel. The kid- 
 neys are closely attached 
 to the dorsal body-wall, 
 and are covered on the 
 ventral aspect by a layer of 
 peritoneum. Each kidney 
 is a flat elongated body of 
 a dull red colour. Enter- 
 ing the outer edge of each 
 is the renal-portal vein, 
 and leaving the same edge, 
 a little more than half-way 
 down, is a delicate tube 
 known as the ureter, by 
 which the renal excreta are 
 got rid of. The ureters open 
 into the common chamber, 
 or cloaca, behind the open- 
 ing of the rectum. 
 
 In structure the frog's 
 kidney is essentially an im- 
 mense number of nephridia ^ fkidne3?; Ur> Ur , ur eters ; t, their 
 
 Closely packed together P int , of ongin; S,S', their opening imo 
 
 1 ' the cloaca, Ll\ HO, spermaria ; FA, fat 
 
 Supported by Connective body; AO, aorta; Cv, vena cava ; Vr t 
 
 . . . efferent veins. 
 
 tissue and permeated by 
 
 the capillary endings of the renal artery and renal and 
 renal-portal veins. Each nephridium consists essentially 
 of a terminal double-walled sac, containing a tuft of capil- 
 laries communicating on the one hand with a small artery 
 
296 
 
 Elementary Biology. 
 
 FlG. 158. TUBT OF CAPILt.ARIKS 
 IN A GLOMERULUS WITH RELATED 
 
 BLOOD-VESSELS. (Quain.) 
 
 and on the other with a small vein. The tuft and its cap- 
 sule together form a glomerulus. The space between the 
 walls is continuous with the tube of the nephridium. The 
 tube-wall is lined by secretory epithelium, which differs in 
 
 character in different regions. 
 It pursues a very convoluted 
 course through the substance 
 of the kidney, and is in in- 
 timate relation throughout 
 with the renal capillaries. It 
 ultimately unites with other 
 tubules, and all finally open 
 into the ureter above men- 
 tioned. In passing through 
 the capillaries of the glome- 
 rulus, part of the water in the 
 blood is squeezed out into the 
 space between the two walls 
 of the sac, whence it trickles 
 down the tube, washing out 
 in its course the soluble ni- 
 trogenous waste abstracted 
 meanwhile by the secretory 
 cells from the renal capillaries 
 surrounding them. The water 
 and the nitrogenous and other 
 salts contained in solution in 
 it go by the name of urine, 
 the most important constitu- 
 ent of which is a complex 
 nitrogenous compound known 
 as urea. 
 
 Urine does not escape 
 directly from the ureter to the exterior. It is collected in a 
 large bilobed and very extensible sac, the urinary bladder, 
 which opens on the ventral surface of the cloaca, just in 
 
 a, artery ; af, afferent brand i ; /, 
 capi lary tuft; ef, efferent vein; b t 
 capillaries of the efferent vein. 
 
Metasoa Ran a. 297 
 
 front of the rectal aperture. From the bladder the urine 
 escapes to the exterior periodically. 
 
 So far we have followed the course of the blood only to 
 and from special organs. A very large amount of the blood 
 is devoted, however, to the nourishing and oxygenating of 
 the muscular and skeletal or supporting system. At p. 273 
 we have glanced at the histological characters of muscle, so 
 that we may now proceed to the consideration of the hard 
 parts or, in other words, the skeleton, or skeletal system. 
 
 The only representative of the skeletal system we have 
 yet had to deal with was the notochord of Amphioxus, a 
 structure, however, of supreme importance ; for, as indicated 
 at p. 258, we find that this rod forms the basis on which 
 the chief part of the skeleton of the frog and of higher 
 animals as well is built. 
 
 The skeleton is composed of two substances, bone and 
 cartilage. The histological character of these are of some 
 importance. 
 
 Cartilage is, like most other tissues, composed of a 
 matrix in which are embedded numerous cells. The matrix 
 is relatively very abundant ; it consists of a homogeneous 
 intercellular substance very elastic in its nature. The cells 
 are spherical or polygonal, and frequently occur in small 
 groups which result from the fissiparous division of a single 
 cell. By studying the process of division it may be made 
 out that the matrix is produced by modification of the old 
 cell- walls, which are successively cast off as new walls are 
 formed round the daughter-cells. 
 
 The matrix is not always homogeneous. In one variety 
 of cartilage there may be formed in the matrix delicate 
 elastic fibrils (yellow fibro-cartilage), in another wavy fibres 
 of white fibrous tissue (white fibro-cartilage) (fig. 16). 
 Cartilage whose matrix is free of either form of fibril is 
 known as hyaline cartilage (fig. 159). 
 
 Bone similarly consists of a matrix and embedded cells, 
 although here both cells and matrix undergo considerable 
 
298 
 
 Elementary Biology. 
 
 modification. The cells are repeatedly branched, and lie 
 in spaces (lacunae), which are moulded to the form of the 
 contained cell. The branched processes of the cells lie in 
 delicate canals (canaliculi), and these again communicate 
 with the canaliculi of neighbouring lacunae. Moreover the 
 
 FIG. 159. HYALINE CAHTTLAGE. (Quain.) 
 
 a, group of two cells ; b, group of four cells ; ft, protoplasm of cell, with 
 fat granules, ; , nucleus. 
 
 lacunae themselves are arranged in circles round larger 
 spaces (Haversian canals) which contain one or more blood- 
 vessels. The Haversian canals anastomose and form a com- 
 plete network in the bone and act as the channels by which 
 nutrient blood-vessels pass to every part of the bone. 
 
Metazoa Rana. 
 
 299 
 
 The skeleton, looked at as a whole, consists of an axial 
 vertebral column terminated anteriorly by the skull, and 
 having attached to it two girdles, each bearing a pair of 
 appendages, which constitute the appendicular skeleton. It 
 will be advisable to compare the skeleton of the allied sala- 
 mander with that of the frog, since the skeleton of the latter 
 deviates considerably from the typical condition. 
 
 FIG. ifo. TRANSVERSE SECTION OF TYPICAL LONG BONE (HUMERUS) (Quain.) 
 
 The section shows three Haversian canals, with three concentric rings of 
 lacuna;, from each of which spring a number of canaliculi. 
 
 The axial vertebral column is composed of a series of 
 vertebras articulated to each other, and all more or less re- 
 sembling each other. A typical vertebra, say from the 
 middle of the back, consists of a short cylindrical body 
 (centrum), from the dorsal side of which an arch arises, the 
 apex of the arch being prolonged upwards as a spinous 
 process. The arches of the successive vertebrae, when 
 placed together, thus form a canal in wfiich lies the spinal 
 
300 
 
 Elementary Biology. 
 
 FlG. 162. A TYPICAL VERTEBRA 
 
 (HUMAM). (Quain.) 
 
 cord, and which therefore goes by the name of the spinal or 
 neural canal. From either side of the arch there projects 
 a longer or shorter process known as the transverse process. 
 It is to this process and to the body of the vertebra that the 
 ribs are articulated in those forms which possess ribs ; here, 
 however, ribs are absent. In addition we must note the 
 existence of facets or the points 
 of articulation of the successive 
 vertebrae on each other. There 
 are two facets in front and two 
 behind, also borne on the sides 
 of the arch. Vertebrae differ 
 from each other in size, in the 
 length and direction of the 
 
 FIG. 161. A HAVERSIAN SYSTEM. 
 (Schafer.) 
 
 v, vein ; a, artery ; /, lymphatic. 
 
 :, centrum ; 2, arch ; 4, neural 
 canal ; 5, spinous process ; 6, 
 transverse process bearing facets, 
 f> 7, 7 1 ; c, c f , facets on centrum. 
 ( I he upper figure shows a vertebra 
 seen from before backwards, the 
 lower is a lateral view of the same ) 
 
 transverse processes, and in other less important points. A 
 comparison of the vertebral column of the newt with that 
 of the frog shows that in the latter the tail vertebrae are 
 absent and that the last few vertebrae have coalesced into a 
 long fluted bone, the urostyle, in which, however, evidence 
 of the union can still be made out. 
 
Metazoa Rana. 
 
 301 
 
 l63 _ DoRSAL VIEW OF THE FROG>S SKULL> 
 
 (Milnes Marshall.) 
 
 Nat 
 
 The several vertebrae are bound together by connective 
 tissue, but in such a maner as to permit of a limited amount 
 of movement. 
 
 The skull is too complicated a structure to allow of de- 
 tailed treatment here, and therefore only the essential 
 features will be referred to. It is to be noted that the neural 
 canal communicates with the cavity of the skull by the 
 foramen magnum, a large aperture in the posterior wall 
 of the skull. The skull is hinged to the first vertebra by 
 two condyles, or 
 
 SmOOth processes be- 
 
 tween which a peg- 
 like projection of the 
 first, or atlas, vertebra 
 fits. 
 
 The skull itself 
 may be said to con- 
 sist of three parts, a 
 cylindrical box, the 
 cranium, or skull 
 proper, to which are 
 attached two pairs of 
 sense-organs, the ol- 
 factory and auditory 
 
 Capsules I a frame- 
 
 WOrk attached tO the 
 .j _ . T i 
 
 sides of the cylinder 
 and forming the upper jaw, the maxilla ; and a lower jaw, 
 or mandible, articulating on either side with the posterior 
 part of the upper framework. The cranium itself is com- 
 posed partly of cartilage, partly of bone. Posteriorly, i.e. 
 in the occipital region, it consists of a floor and two side 
 walls of bone (the basi- and two ex-occipitals) and a roof of 
 cartilage, the ring thus formed bounding the foramen mag- 
 num. The floor of the middle portion of the box is formed 
 mainly of cartilage strengthened by a dagger-shaped bone, 
 
 -, premaxilla ; Na., nasal ; S.e., sphenethmoid ; 
 Sr., parieto-frontal ; Pr.O., prootic ; E.G., ex- 
 occipital ;Q. 7., quadrato-jugal ; Sg,'', squamosal ; 
 Pt'i pterygcid ; Pa., palatine. 
 
3<D2 Elementary Biology. 
 
 the para-sphenoid ; the walls are composed of cartilage and 
 the roof of two narrow plates closely united, the parieto- 
 frontal bones. Anteriorly the walls of the cylinder are 
 composed of a bony girdle, the sphenethmoid, while above 
 that bone and in front of the parieto-frontals, are two small 
 bones known as the nasals. Attached to the occipital ring, 
 one on either side, are the ear or otic capsules, composed 
 partly of bone, partly of cartilage, while the nasal bones 
 shelter the nasal capsules. The eye is not attached to the 
 
 FIG. 164. -LATERAL VIEW OF THE FROG'S SKULL. (Milnes Marshall.) 
 FP L 
 
 A, para-sphenoid ; AS, angulo-sphenial ; B, i, anterior and posterior horns 
 of hyoid ; H, body of hyoid ; c, columella ; D, dentary ; E, exoccipital 
 F, nostril ; FP, parieto-frontal ; L, exit of optic nerve ; M, maxilla 
 MM, mento-meckelian, or chin bone ; M', exit for fifth and seventh nerves 
 N, nasal ; o, prootic ; p, pterygoid ; P.M, premaxilla ; Q, quadrato 
 jugal ; R, exit for ninth and tenth nerves ; s, squamosal ; SE, sphen- 
 ethmoid. 
 
 skull, but lies free on either side, midway between the ear 
 and nose. 
 
 The upper framework attached to this box consists of a 
 number of narrow bones passing from the ear-capsule on the 
 one hand to the nose-capsule on the other, and forming 
 with the skull a bony ring round the eye. In front and on 
 the under surface of the skull lie four small bones, two rod- 
 like and stretching from the sphenethmoid outwards, the 
 palatines, and two triangular bones lying in front of the 
 palatines, the vomers, which have already been referred to 
 
Metazoa Rana. 303 
 
 as carrying teeth. In front of the two nasals are two small 
 
 FIG. 165. SKELETON OF FROG (DORSAL VIEW, WITH LEFT SCAPULA AND 
 SUPRA-SCAPULA REMOVED). (Milnes Marshall.) 
 
 a, astragalus (tibiale) ; r, calcaneum (fibulare) ; d, suprascapula ; e, ex- 
 occipital ; f, femur ; fp, parieto-frontal ; g, metacarpals ; h, humerus ; 
 z", ilium; k, metatarsals ; /, carpus; ;, maxilla; , nasal; 0, prootic; 
 p, pterygoid ; /;//, premaxilla ; q, quadrato-jugal ; r, radi -ulna ; .<, 
 squamosal ; se, sphenethmoid ; sv, sacral vertebra ; t, tibio-fibula ; 
 , urostyle. 
 
 bones, the premaxillge, which form the most anterior part 
 of the upper jaw. The premaxillse articulate with two bent 
 
304 Elementary Biology. 
 
 rod-like bones, the maxillae, which pass backwards along 
 either side. Both maxillae and premaxillae carry teeth. 
 The hoop is completed by a triradiate bone, the pterygoid, 
 which, with the assistance of a small bone, the quadrato- 
 jugal, unites the posterior end of the maxilla to the skull, the 
 whole being steadied by a hammer- shaped bone, the squa- 
 mosal, which passes from the point of junction of the 
 quadrato-jugal and pterygoid to the upper portion of the 
 ear- capsule. 
 
 The mandible, or lower jaw, consists of two halves or 
 rami, each of which again is made up of four smaller bones. 
 The rami articulate with the quadrato-jugal on either side. 
 
 There is also an arch (the hyoid), composed, partly of 
 cartilage, partly of bone, supporting the tongue and throat. 
 
 The skull, and indeed the entire skeleton, of the embryo 
 is at first composed of cartilage only, part of which after- 
 wards becomes altered into bone, part remaining cartilaginous 
 during life. Not only in the amphibian skull, but also in 
 the skull of the higher animals, certain bones are developed 
 from the skin, membrane bones, either over the parts which 
 have remained cartilaginous or in addition to the cartilage 
 bones already formed. Many of the bones forming the 
 framework of the jaws are membrane bones. 
 
 Turning now to the appendicular skeleton we note 
 first that it consists, us already stated, of two hoops or 
 girdles to which are attached the fore and hind limbs re- 
 spectively. These are known as the pectoral and pelvic 
 girdles ; both are composed essentially of six bones or their 
 cartilaginous representatives. The fore and hind limbs are 
 also built on the same type. 
 
 Pectoral girdle and fore-limb. The pectoral girdle 
 consists of a median ventral bar of bone, the sternum, ter- 
 minated posteriorly by a plate of cartilage known as the 
 xiphi-sternum. Springing on either side from the median 
 line anteriorly to the sternum are a pair of strong bones, 
 the coracoids, and anterior to these another pair of more 
 
Metazoa Ran a. 305 
 
 delicate rod-like bones, the clavicles. Attached to the outer 
 ends of the clavicle and coracoid of either side is a broad 
 flat bone, the scapula, also tipped by a plate of cartilage 
 known as the supra-scapula ; while in the middle line and in 
 front of the inner union of the clavicles there lies a 
 small osseous rod, the omo-sternum, continued forward 
 as a delicate cartilaginous plate. The six bones above 
 referred to as forming the essential elements of the pectoral 
 girdle are the two clavicles, two coracoids, and the two sca- 
 pulae, which form a half-circle with the cartilaginous supra- 
 scapulse arching over the back. At the junction of the 
 coracoid and scapula there is a circular depression, or glenoid 
 cavity, into which the upper end of the fore-limb fits. 
 
 A typical fore-limb consists of three parts : the arm, 
 composed of one bone, the humerus ; the fore-arm, of 
 two bones jointed to the lower end of the humerus, the 
 ulna externally, and the radius internally ; and a hand, 
 composed of many bones, forming proximally the wrist and 
 distally the fingers, or digits. Typically, the wrist, or carpus, 
 consists of an ulnare articulating with the distal end of 
 the ulna, a radiale articulating with the distal end of the 
 radius, and a small intermediate bone, the intermedium. In 
 front of the intermedium is a larger nodule, the centrale, and 
 beyond it five small bones, the carpals. To these five 
 carpals are articulated the five jointed digits, each made up 
 of a metacarpal and several phalanges. In the frog con- 
 siderable modification has taken place. In the first instance 
 the radius and ulna have fused to form one bone, though the 
 distinction is still visible at the extremities, while the nine 
 bones of the typical carpus has been reduced to six and the 
 first digit is rudimentary. 
 
 Pelvic girdle and hind-limb. In the pelvic girdle there 
 is nothing corresponding to the sternum, though the other 
 elements of the pectoral girdle are represented. The scapula 
 is represented by a long curved bone on either side, the 
 ilium, placed dorsally and having its free end attached to 
 
306 Elementary Biology. 
 
 the transverse process of the last free vertebra. The ilium 
 is fused posteriorly to the ilium of the other side, and the 
 two ilia together with the representatives of the clavicles and 
 coracoids of the pectoral girdle form a disc, the whole 
 structure being not unlike the 'merrythought' of a fowl. 
 The posterior part of the disc is composed of two fused 
 ischia, the homologues of the coracoids, while the anterior 
 and ventral portion is formed of the two pubes, or the 
 homologues of the clavicles. A union such as this between 
 two bones typically free by means of cartilage is known as a 
 symphysis. As in the pectoral girdle, at the junction of 
 the ilium and ischiurn there is a concavity into which the 
 upper end of the hind-limb fits. Here, however, the pubes 
 take part in the formation of the cavity, which is known as 
 the acetabulum. 
 
 In the typical hind-limb the same arrangement of parts 
 is maintained as in the fore-limb, though the bones are 
 known by different names. The leg is formed by the femur, 
 whose proximal end rotates in the acetabulum and whose 
 distal end articulates with the proximal end of the fore-leg". 
 The fore-leg like the fore-arm is formed of cwo long bones, 
 the fibula externally and the tibia internally. These in 
 turn articulate with a fibulare and a tibiale, between which 
 lies an intermedium. A centrale and five tarsals complete 
 the ankle, or tarsus. To the five tarsals are articulated five 
 metatarsals, which with a number of phalanges form the 
 five digits of the hind-limb. As in the case of the fore- 
 limb, so also the hind-limb of the frog deviates considerably 
 from the typical limb just described. The tibia and fibula 
 are fused together, as were the radius and ulna ; the tibiale 
 and fibulare are long bones and are fused at their extremi- 
 ties, their relatively great size accounting for the extent of the 
 sole of the frog's foot. The other bones of the tarsus are 
 only present in the form of two partly cartilaginous, partly 
 osseous nodules. The five metatarsals and their phalanges 
 are, however, well developed. 
 
Metazoa Rana. 
 
 307 
 
 Having briefly sketched the essential features in the 
 structure of the skeleton we must now pass to the nervous 
 system, which has been already described as lying in the 
 brain-box and the canal formed by the apposition of the 
 neural arches of the vertebrae. 
 
 The nervous system. The nervous system of the frog 
 shows a very great advance in differentiation upon that of 
 
 FIG. 166. -BRAIN or FROG, A, FROM ABOVE ; n, FROM BELOW. (Ecker.) 
 
 01 
 
 HO --i- 
 
 i, olfactory nerves ; Ol, olfactory lobes ; CH, cerebral hemispheres ; L T, 
 lamina terminalis ; Th, thalamencephalon ; PG, pineal gland; OpL, 
 optic lobes ; Ci, cerebellum ; MO, medulla oblongata ; 2, optic nerves ; 
 OT, optic chiasma ; H, pituitary body ; 3-10, cerebral nerves. 
 
 AmpMoxus. In the amphibian brain, indeed, we have 
 represented all the most important parts of the brain of the 
 mammal (dog, rabbit, &c.). The nervous system consists of 
 a spinal cord enlarged anteriorly to form the brain, the former 
 being lodged in the neural canal of the vertebral column, 
 the latter in the cavity of the skull, or cranium. From both 
 
 x 2 
 
3OS Elementary Biology. 
 
 are given off many nerves, known as cranial and spinal 
 nerves, according as they arise from the brain itself or from 
 the spinal cord. 
 
 The brain. The brain consists essentially of three 
 portions a fore-brain, a mid-brain, and a hind-brain. 
 
 The fore-brain is composed of two portions, the pros- 
 encephalon and the thalamencephalon. The former consists 
 of two pear-shaped masses, narrowed anteriorly and known 
 as the cerebral hemispheres. From these there project two 
 smaller lobes, the olfactory lobes. The thalamencephalon 
 is much smaller and consists dorsally of a thin plate uniting 
 the two laterally placed optic thalami, from which in turn 
 on the ventral surface spring two broad bands of nerve-tissue 
 known as the optic tracts. The optic tracts cross each 
 other and form what is known as the optic chiasma. Pos- 
 teriorly from the dorsal surface of the thalamencephalon 
 there springs a small vascular prominence known as the 
 pineal gland. This structure has recently been shown to 
 be in all probability the rudiment of an eye. Ventrally also 
 a similar structure, the infundibulum, terminated by a small 
 mass of tissue, the pituitary body, arises from the posterior 
 portion of the thalamencephalon. The pituitary body is, 
 however, of totally different origin from the pineal gland, 
 and is indeed a bud from the roof of the mouth, which 
 becomes separated off and united to the base of the brain 
 in the course of development. 
 
 The mid-brain is composed dorsally of two large 
 prominences the optic lobes, connected ventrally by a 
 broad belt of nerve-tissue. The mid-brain is also known 
 as the mesencephalon. 
 
 The hind-brain, like the fore-brain, consists of two 
 regions the metencephalon, or cerebellum, and the 
 myelencephalon, or medulla oblongata. These two parts 
 cannot be easily distinguished from each other on the 
 ventral aspect; but dorsally the metencephalon is coincident 
 with a ridge of nerve-tissue, the cerebellum, which overlaps 
 
FIG. 167.- NERVOUS SYSTEM OF FROG (VENTRAL ASPECT). (Ecker) 
 
 N 
 
 F, facial nerve; c. 
 gang ion on x ; 
 H e, cerebrum ; 
 Lr, optic tract ; 
 L0/$, optic lobe ; 
 M, boundary be- 
 tween medulla 
 and spinal cord ; 
 MI- 10, spinal 
 nerves; MS, con- 
 nection between 
 4th spinal nerve 
 and sympathetic 
 chain ; N, nasal 
 sac ; NZ, sciatic 
 nerve ; i^ o, crural 
 nerve ; ff, eyeball ; 
 s, sympathetic 
 trunk ; s i - 10 
 .sympathetic gan- 
 glia; ^ s/, con- 
 tinuation of sym- 
 pathetic into 
 head. The num- 
 bers refer to the 
 cranial nerves 
 (see table on p. 
 
 5? 
 
Elementary Biology. 
 
 the broad triangular-like area of the myelencephalon, or 
 medulla. 
 
 The medulla is continued directly into the spinal cord, 
 or myelon. It is relatively of considerable size, and for 
 the greater part of its length is a thick broad belt, with a 
 well marked groove running down its upper or dorsal sur- 
 face. Its latter third is much thinner and narrower than 
 the rest and terminates in a delicate thread, the filum 
 terminale, which lies within the anterior end of the urostyle. 
 
 The cranial nerves. There are ten pairs of cranial 
 nerves, all save the first springing from the ventral aspect 
 of the brain, and, after escaping from the cranium, breaking 
 up into finer branches and becoming distributed to the 
 various organs which it is their duty to supply. They 
 may be tabulated as follows : 
 
 Nerve 
 
 Origin 
 
 Distribution 
 
 I. Olfactory 
 II. Optic . 
 
 Olfactory lobes 
 Optic chiasma. 
 
 Nasal sacs (sensory) 
 Eyes (sensory) 
 
 III. Oculi-motor 
 
 Floor of mesencephalon . 
 
 Muscles of eye (motor) 
 
 IV. Pathetic 
 
 Floor of mesencephalon . 
 
 Muscle of eye (motor) 
 
 V. Trigeminal 
 
 Floor of myelencephalon 
 
 Mandible and maxilla 
 
 
 
 (sensory and motor) 
 
 VI. Abducens 
 
 Floor of myelencephalon 
 
 Muscle of eye (motor) 
 
 VII. Facial . 
 
 Floor of myelencephalon 
 
 Palate and face (sensory 
 
 
 
 and motor) 
 
 VIII. Auditory 
 IX. Glosso-pharyngeal 
 
 Floor of myelencephalon 
 Floor of myelencephalon 
 
 Ear (sensory) 
 Tongue (sensory and 
 
 
 
 motor) 
 
 X. Vagus, or Pneumo- 
 gastric 
 
 Floor of myelencephalon 
 
 Lungs, heart, stomach, 
 and other viscera (sen- 
 
 
 
 sory and motor) 
 
 From the spinal cord also ten pairs of spinal nerves 
 are given off. Each nerve arises from the cord by two roots, 
 one dorsal or posterior, the other ventral or anterior. The 
 two roots unite before leaving the neural canal, and the 
 nerve formed by their union passes through one of the 
 intervertebral foramina, as the spaces between the several 
 vertebrae are termed. The posterior root, which is known 
 as the sensory root, for a reason to be afterwards explained 
 
Metasoa Ran a. 311 
 
 (p. 317), has a ganglionic swelling upon it. The first spinal 
 nerve is known as the hypoglossal, from the fact that it is 
 distributed to the muscles of the tongue ; the next two 
 (brachial plexus) supply the fore-limbs ; the fourth, fifth, 
 and sixth are distributed to the muscles of the body-wall ; 
 the next three go to the hind-limbs and parts in the vicinity ; 
 
 FIG. 168. -SPINAL CORD (ox). (Quai-i.) 
 
 A, antero-posterior v'ew; B, lateral view; c, transverse section ; D, uni^n 
 of the two roots of a spinal i.erve. i, anterior fissure ; 2, posterior 
 fissure ; 3, origin of anterior root (5) ; 4, origin of posterior root (6) ; 
 6'. ganglion on the posterior root ; 7, 7', sensory and motor nerves. 
 
 while the tenth and last supplies the region of the urostyle 
 and cloaca. 
 
 In addition to the cerebro-spinal nervous system thus 
 briefly sketched there is another visceral or sympathetic 
 nervous system, consisting of a double chain of ganglia 
 connected by commissures to each other and to the cerebro- 
 spinal nerves, and lying in the dorsal region of the ccelom, 
 
312 
 
 Elementary Biology. 
 
 just beneath the vertebral column, close to the aorta, This 
 system has under its control more especially the intestine 
 and blood-vessels in relation thereto. 
 
 Histologically it will be necessary to glance at the dis- 
 tribution of the three elements of the nervous system the 
 
 FIG. 169. TRANSVERSE SECTION OF THE SPINAL CORD. (Quain.) 
 
 * 
 
 a, b, c, multipolar nerve-cells in the anterior horn of grey matter ; </, pos- 
 terior horn of grey matter ; e, anterior fissure ; _/, central canal ', g, 
 posterior commissure of grey matter ; h, exit of the posterior horn of 
 grey matter ; /, anterior commissure ; k, canal in grey matter ; /, /. 
 nerve-fibres. 
 
 nerve-fibres, nerve-cells, and connective tissue ; and in 
 
 order to do so it will be sufficient to examine a section of the 
 
 cerebrum, a section of the spinal cord and of a spinal nerve. 
 
 Nerve-tissue with a preponderance of nerve- cells has, 
 
Metazoa Rana. 3 1 3 
 
 from its appearance under the naked eye, been called 
 grey matter ; when the nerve-fibres are more abundant, 
 or alone present, the nervous tissue is known as white 
 matter. In both white and grey matter there is a consider- 
 able amount of connective tissue (neuroglcea) present. 
 
 A section of the cerebral hemisphere, or cerebellum, 
 shows it to be composed externally of a layer- of grey matter 
 that is, of nerve-cells and neuroglcea and internally of 
 white matter that is, of nerve-fibres and neurogloea. The 
 cerebellum, optic lobes, and other parts of the brain pre- 
 sent the same arrangement of elements. A careful examina- 
 tion of the sections of the frog's brain shows that there is a 
 complicated series of spaces and channels in the interior, all 
 of which are continuous with a canal which runs down the- 
 centre of the spinal cord. 
 
 If a section of the spinal cord be now examined the 
 arrangement of elements will be found to be somewhat dif- 
 ferent. Here the grey matter occupies the interior and 
 has roughly the form of an H, the central canal just alluded 
 to piercing the middle of the cross-bar. The grey matter is 
 said to have two anterior and two posterior horns, and if 
 the section be taken at the origin of a spinal nerve it will 
 be found that the two roots of the nerve are connected 
 with the anterior and posterior horns respectively. The 
 central canal is lined by ciliated epithelium. Surround- 
 ing the grey matter on all sides lies the white matter of the 
 cord, consisting of nerve-fibres and connective tissue and 
 surrounded in turn by the sheath of delicate connective 
 tissue largely supplied with blood- vessels, which is continuous 
 over the entire brain and spinal cord, and which is known 
 as the pia mater. The pia mater dips down into the cord 
 anteriorly and posteriorly almost to the transverse band of 
 grey matter, thus nearly dividing the cord into two parts. 
 These two clefts are known as the anterior and posterior 
 fissures, and it is the mouth of the posterior fissure that 
 gives the grooved appearance to the dorsal aspect of the 
 
Elementary Biology. 
 
 cord. Externally to the pia mater is a strong protective 
 sheath, which does not follow the pia mater into the fissures : 
 to this the name of dura mater is given. 
 
 A nerve consists of a large number of ultimate nerve- 
 fibres bound together by connective tissue. The connec- 
 
 FIG. 170. TRANSVERSE SECTION OF A LARGE NERVE. (Schaftr.) 
 
 ?', ?', blood-vessels ; end, endoneurium ; per, perineurium ; ep, epineurium. 
 
 tive tissue receives different names according as it binds 
 together ultimate nerve-fibres (endoneurium), simple bundles 
 of nerve-fibres (perineurium), or compound bundles (epi- 
 neurium). An ultimate nerve-fibre consists of an axial 
 cylinder of nerve-fibrillse, surrounded by one or two 
 sheaths. The inner sheath, composed of a fatty substance. 
 
Metazoa Rana, 
 
 FlG. 171. NON-MEDtTLLATED NERVE-FIBRES. 
 
 (Schafer.) 
 
 is spoken of as the medullary sheath, 
 and fibres which possess it are 
 spoken of as medullated fibres. ' 
 Many nerve- fibres, however, are non- 
 medullated, and in that case possess 
 only the outer sheath, or neuri- 
 lemma, which corresponds to the 
 sarcolemma of the muscle-fibre, and 
 with which it is homologous. 
 
 Nerve-cells are extremely vari- 
 able in size and shape. They are, 
 however, usually branched, and are 
 termed unipolar, bipolar, or multi- 
 polar, according to the number of 
 poles or branches they possess. The 
 poles are continuous 
 
3i6 Elementary Biology. 
 
 cylinders of nerve-fibres. The cells are naked, and the 
 sheaths of the nerve cease before uniting with the nerve- 
 cell. 
 
 Before leaving the nervous system it will be necessary to 
 indicate briefly the functions of the various parts of that 
 system. The brain, as a whole, may be looked upon as 
 the grand centre for the originating and governing of the 
 
 FIG. 173. MULTIPOLAR NERVE-CELL. (Schafer.) 
 
 various actions of the body, as well as for the reception of 
 all impressions derived from the body itself or from the 
 external world ; whilst the nerves are the organs for trans- 
 mitting these impulses and impressions from and to the brain, 
 or sensorium. A little reflection will enable us to analyse 
 the nervous messages into at least two chief classes 
 centrifugal or motor impulses, and centripetal or sensory 
 impulses. To these, however, we must add impulses of at 
 
Metazoa Rana. 3 1 7 
 
 least two other kinds, viz. impulses governing the action of 
 glands, and therefore called secretory, and those which 
 regulate the phenomena of growth and nutrition of tissues 
 generally ; to these latter the name of trophic has been given. 
 It is as yet doubtful whether secretory and trophic impulses 
 pass by special nerves devoted to the transmission of such, 
 or travel by nerves which are under other circumstances 
 intrusted with the carriage of motor impulses. We know, 
 however, that sensory impressions do have special nerves 
 devoted to their transport, while motor nerves carry im- 
 pulses which stimulate to action the muscles to which they 
 are distributed ; hence the names of sensory and motor 
 applied to the posterior and anterior roots of the spinal 
 nerves to indicate that messages from the outer world travel 
 to the central nervous system by nerve-fibres which enter 
 the cord by the posterior root, and that messages from the 
 central nervous system pass outwards to the muscles (glands 
 and tissues generally) by the anterior roots. Sensory fibres 
 are sometimes spoken of as afferent, and motor fibres as 
 efferent. 
 
 From analogy with the higher animals it is probable 
 that the cerebrum has to do with the originating of volun- 
 tary actions and with the analysing of sensory impressions. 
 The cerebellum is probably the centre for the co-ordinating 
 of muscular movement, while the optic lobes and the 
 olfactory lobes have to do with the two important sense- 
 organs which the nerves originating from them supply, 
 namely, the eye and the ear. 
 
 The spinal cord in the higher animals is, in all prob- 
 ability, chiefly a path for the transmission of impulses from 
 the brain outward or from the nerves inwards, though it no 
 doubt also has to do with the government of local move- 
 ments or simple organic changes not requiring the special 
 interference of the brain. These are known as reflex 
 actions, and consist essentially of three separate phenomena 
 viz. (i) the transmission of a sensory impression along an 
 
Elementary Biology. 
 
 afferent nerve to a nerve-centre ; (2) a metabolic change 
 taking place in the centre, which results in (3) the trans- 
 mission of a motor or secretory impulse along an efferent 
 fibre to the muscle or gland concerned. For example, the 
 olfactory nerve is a sensory nerve ; stimulations of the ter- 
 mination of that nerve in the organ of smell, say by means 
 of the fumes of acetic acid, produces an effect on the nerve- 
 centre (in this case the olfactory lobes) which we term a 
 sensation something perceived. In ordinary circumstances, 
 i.e. when the brain proper does not interfere, an impulse 
 
 FIG. 174. DIAGRAM TO ILLUSTRATE REFLEX ACTION. (Landois and Stirling.) 
 
 A 
 
 s, sensitive surface ; G, ganglion on sensory nerve ; af, afferent or sensory 
 nerve ; ef, efferent or motor nerve ; M, muscle fibre ; N, nerve-cell ; 
 A, P, anterior and posterior aspects of the spinal cord. The arrows in- 
 dicate the direction of the nerve-impulse. 
 
 originating involuntarily in the nerve-centre is carried along 
 a secretory neive (the glosso pharyngenl) to the salivary 
 glands, causing them immediately to secrete a large quantity 
 of saliva. Numerous instances might be given of reflex 
 actions in different parts of the body ; that just given may 
 suffice to illustrate what is meant by the term. That these 
 reflex actions are possible without the intervention of the 
 brain is proved by the fact that even in a decapitated Irog 
 stimulation of the sensory-nerve terminations in the foot 
 is at once followed by the withdrawal of the foot from the 
 source of irritation. 
 
Metazoa Rana. 
 
 319 
 
 We must now briefly touch on the terminations of the 
 special sensory nerves connected with the senses of touch, 
 taste, smell, hearing, and sight. First of all we may begin 
 with the general statement that the ultimate fibrillae of all 
 these nerves are in the long run in direct connection with 
 more or less modified epithelial cells. 
 
 Touch. The organ of touch is the skin, and in it fine 
 nerve-fibres are abundantly distributed, and their ultimate 
 
 FtG. 175. -ClRCUMVALLATE PAPILLA FROM THE TONGUE OF THE CAT. 
 
 (Schafer.) 
 
 a, M, transverse and longitudinal sections of nerves in the dermis ; 7>, lym- 
 " phatic in a papilla of the dermis, / ; , gustatory body, or taste-bulb. 
 
 fibrillge lie in close relation to the epithelial cells which 
 form the outer layer of the skin, although in the frog no 
 special organs of touch have been discovered. 
 
 Taste. The organ of taste, at least in the higher 
 animals, lies in the tongue, and more especially in its 
 posterior third, where there is a limited number of papillae 
 of very peculiar character. Each consists of a broad 
 columnar projection of dermis and epidermis, surrounded 
 
3 20 
 
 Elementary Biology. 
 
 FIG. 17*?. SENSORY EPITHELIUM FROM 
 THE NOSE. (Schafer.) 
 
 by a groove or ditch : hence the name of circumvallate 
 applied to these papillae. On the sides of this groove are 
 situated numerous flask-shaped depressions, formed of long 
 stave-like epithelial cells, whose outer ends project freely 
 into the vallum, while their inner ends are in communica- 
 tion with the axis cylinder of the glosso-pharyngeal nerve. 
 
 Soluble substances introduced 
 into the groove must of ne- 
 cessity stimulate the nerve- 
 terminations through the epi- 
 thelial cells. 
 
 Smell. The sense of 
 smell is located in the rose. 
 The interior of the nasal 
 capsule is ridged in such a 
 manner as to largely increase 
 the surface .of the chamber- 
 wall. The terminations of 
 the nerves of smell are dis- 
 tributed over the mucous 
 membrane covering the 
 ridges, and their axis-cylin- 
 ders end in long spindle- 
 shaped cells, whose pointed 
 free ends project from the 
 surface of the membrane. 
 Air bearing odoriferous par- 
 ticles enters the anterior 
 nares and stimulates the 
 nerve - terminations in its 
 passage over the ridges projecting from the walls of the 
 nasal capsule. 
 
 Hearing. The organ of hearing is much more compli- 
 cated, though fundamentally the same in principle as the 
 sense-organs already described. Without entering into detail, 
 it will be sufficient to say that the auditory or periotic 
 
 and 2, varieties of olfactory epithe- 
 lium and supporting cell-; 3, termi- 
 nation of the olfactory nerve. 
 
Metazoa Rana. 
 
 321 
 
 capsule of the frog contains a cavity, lined by a mem- 
 branous capsule, which fits it exactly. The membranous 
 capsule, which takes the form of a partially divided vestibule 
 with semicircular canals opening into it, is composed of a 
 framework of connective tissue carrying modified ciliated 
 epithelial cells. These cells are connected with the termina- 
 
 FIG. 177. THE MEMBRANOUS LINING 
 OF THE RIGHT INTERNAL EAR OF 
 THE FROG FROM THE OUTER ASPECT. 
 (Millies Marshall.) 
 
 a, anterior vertical semicircular canal ; 
 b, ampulla or swelling on it ; h, hori- 
 zontal canal and its ampulla, i ; p^ 
 posterior vertical canal and its am- 
 pulla, r '\ s, sacculus and 7/, utriculus, 
 the two subdivisions of the vestibule. 
 
 FIG. 178. SENSORY EPITHELIUM 
 OF THE EAR. (Quain.) 
 
 c, c, sensory cells ; f, f, support- 
 ing cells ; , nerve ; h, /t', cilia. 
 
 tions of the auditory nerve. The cavity of the capsule is 
 filled with lyrnph into which the cilia of the cells project. 
 The wall of the cavity is perforated at one point, termed the 
 fenestra ovalis, which, however, is closed by a delicate 
 membrane, to which is fastened on its outer side a small 
 rod-like bone, known as the columella, whose outer end 
 abuts against the membrana tympani already described as 
 visible on either side of the skull behind the eyes. The 
 intermediate chamber (or tympanic cavity) containing the 
 columella communicates with the buccal cavity by a tube, 
 known as the Eustachian tube, whose opening has already 
 
322 Elementary Biology. 
 
 been referred to (p. 267). Vibrations of the air cause the 
 membrana tympani to tremble in unison, and those trem- 
 blings are, by means of the columella and the membrane 
 covering the fenestra ovalis, communicated to the lymph 
 filling the inner ear, which in its turn stimulates the epithelial 
 cells in connection with the terminations of the auditory 
 nerve. 
 
 Sight. The peripheral terminations of the nerve of sight 
 are more complicated than those of the other sense-organs. 
 The eye consists of a strong capsule of fibrous tissue, known 
 posteriorly as the sclerotic, anteriorly as the cornea. The 
 cornea is transparent, the sclerotic is white and opaque. . 
 The capsule is pierced behind by the optic nerve, and is 
 kept in its socket and at the same time moved by six bands 
 of muscle, four of which spring from the upper, under, and 
 two lateral margins of the eye, whilst the remaining two are 
 obliquely placed, one arising from the upper and outer 
 margin, the other from the under and inner margin. The 
 four muscles first mentioned are known as the superior, 
 inferior, exterior, and interior recti, while the oblique 
 muscles are spoken of as the superior and inferior oblique 
 respectively. It will be remembered that no less than three 
 out of the ten pairs of cranial nerves were distributed to the 
 muscles of the eye. Their distribution is as follows : 
 the superior, inferior, and interior recti, and inferior oblique 
 are supplied by the oculi-motor nerve (in.) ; the exterior rectus 
 is supplied by the abducens (vi.) ; while the pathetic (iv.) nerve 
 goes to the superior oblique muscle. Within the fibrous 
 capsule there are two chambers, a large posterior and a 
 small anterior. Lining the inner surface of the sclerotic is 
 a highly vascular pigmented membrane, the choroid, which 
 separates away from the sclerotic just where the sclerotic 
 becomes continuous with the cornea. It there forms a 
 diaphragm, ramed the iris, crossing the eye from side to 
 side, incomplete, however, in the centre, where a rounded 
 aperture is left, the pupil. Behind the iris lies a muscular 
 
Metasoa Rana. 
 
 FlG. I79.-~HuKIZO.NTAL SECTION OF THE RIGHT EVE. (Quaill.) 
 
 323 
 
 , a, antero-posterior axis ; <5, b, lateral axis ; i, cornea : 2, 2'. sclerotic ; 
 3, 3', choroid; r, r 1 , retina; 4, ciliary muscle; 5, ciliary process; 6, 
 
 10, 14, lymph canals ; 7, 7', iris; 8, 8', optic nerve; 8", point of acutest 
 vision ; 9, crinated zone marking the anterior termination of the retina ; 
 
 11, anterior or aqueous chamber ; 12, crystalline lens ; 13, posterior or 
 \ itreous chamber. 
 
 Y 2 
 
Elementary Biology. 
 
 FIG. I80.-VKRTICAL SECTION OF KET.NA. 
 
 zone, also continuous with the choroid, known as the ciliary 
 
 is continuous with a 
 ligamentous zone 
 which supports the 
 crystalline lens. 
 The lens thus occu- 
 pies the space im- 
 mediately behind the 
 pupil, and assists in 
 separating the an- 
 terior from the pos- 
 terior chamber of the 
 eye. Lining the 
 choroid is a very 
 delicate membrane, 
 composed partly of 
 nerve fibres, partly of 
 nerve-cells, arranged 
 in a series of definite 
 layers, supported by 
 very delicate con- 
 nective tissue. These 
 nervous elements are 
 the terminations of 
 the optic nerve, which 
 enters, as has already 
 been stated, at the 
 back of the optic 
 capsule. The greatly 
 modified epithelial 
 
 The layer of rods and cones is shown abutting Cells, which form the 
 against the pigment layer of the choroid. Nume- u -, j 
 
 rous granular layers follow, bounded by a layer of true peripheral end- 
 
 nerve-cells and fibres. The layer next the bottom : ncr c n f fV.p nPTVP lie 
 
 of the page is that which is nearest the centre of 1IJ fo b U1 luc 
 
 the e y e - between this nervous 
 
 layer and the innermost pigmented layer of the choroid, and 
 
Metasoa Rana. 
 
 325 
 
 point towards, or rather have their free ends abutting 
 against, the latter. The nervous layer and the terminal 
 epithelial cells (named the rods and cones from their ap- 
 pearance) form the retina. Lastly the posterior chamber is 
 filled by a gelatinous substance, the vitreous humour, while 
 the anterior chamber similarly contains a more watery fluid 
 allied to lymph, the aqueous humour. A ray of light trans- 
 mitted through the cornea and aqueous humour is focussed 
 on the retina by means of the crystalline lens. The cur- 
 vature, and therefore the focussing power of the lens, can 
 
 FIG. 181. ILLUSTRATION OF THE METHOD OF FOCUSSING RAYS OF LIGHT 
 ON THE RETINA. (Landois and Stirling.) 
 
 A, point of origin of rays ; B, cornea; c, iris; D, lens; E, point of acutest 
 vision ; F, scierotic. 
 
 be altered to suit rays coming from different distances by 
 means of the ciliary muscle. The iris is also provided with 
 radiating and circular muscle-fibres by means of which the 
 size of the pupil can be altered so as to allow of the entrance 
 of more or less light according to circumstances. The rays 
 pass through the vitreous humour and penetrating the retina 
 are reflected back from the choroid on the rods and cones. 
 The excitement produced there is carried by means of the 
 elements in the nervous layer of the retina to the optic nerve 
 itself, and by that means to the brain. 
 
326 Elementary Biology. 
 
 We have now briefly surveyed the chief points in which 
 one of the higher animals shows advance in organisation as 
 compared with the types already discussed, so far as indi- 
 vidual life is concerned. We have still left the organs con- 
 cerned in the maintenance of tribal life, viz. the reproductive 
 system. 
 
 This system is a comparatively simple one, though in the 
 course of development considerable modification has taken 
 place. The sexes are distinct. The spermaria (testes) in 
 the male consist of a pair of white elliptical masses lying 
 in close relation to the ventral surface of the kidneys and 
 connected to these organs by mesentery (mesorchium), and 
 bearing at their anterior ends lobed fatty masses, the corpora 
 adiposa (fig. 157). The spermaria of the frog differ in one 
 important point from the spermaria of the majority of animals, 
 viz. in that the vas deferens or special duct for the con- 
 veyance of the sperms to the exterior is also the ureter. 
 The mesorchium supports a large number of vasa efferentia, 
 or efferent vessels, which transfer the sperms from the sper- 
 maria to the kidney, whence they escape into the cloaca 
 by the ureter, which for that reason may be known as the 
 urine-genital duct. (From a developmental point of view, 
 however, the ureter is wanting, and the vas deferens carries 
 the products both of the kidneys and spermaria to the 
 exterior.) 
 
 In the female there are two ovaria, which at the breed- 
 ing season are often of very large size, connected to the 
 body-wall by peritoneum, and also provided with corpora 
 adiposa. The walls of the ovaria are very thin, and, when 
 the ova, which are of large size, are ripe, rupture readily, 
 shedding their contents into the ccelom. The oviducts 
 which are quite unconnected with the ovaria are of great 
 length, and are stowed away in the ccelom in complicated 
 coils. Each oviduct opens anteriorly beneath the lungs by 
 a thick-lipped aperture, into which the ova find their way in 
 a manner not understood. They are then forced down to 
 
Metazoa Rana. 
 
 327 
 
 the cloaca by the rhythmic contractions of the muscular 
 fibres in the wall of the oviduct, while at the same time 
 glands in the upper region of the duct coat the ova with an 
 albuminous substance which swells readily in water. The 
 ova, as they are shed from the cloacal aperture, are fertilised 
 by sperms from the 
 male. 
 
 Before describing 
 the subsequent de- 
 velopmental changes 
 which the fertilised 
 
 FIG. 
 
 182. FEMALE REPRODUCTIVE ORGANS OF THE 
 FROG. (Owen.) 
 
 ovum undergoes it 
 
 will be necessary to 
 glance at the origin 
 and development of 
 the sperms and ova 
 themselves, and also 
 at the changes which 
 take place in the 
 ovum before and in 
 the act of fertilisa- 
 tion. 
 
 Both kinds of 
 reproductive glands 
 are at first precisely 
 similar ; they both 
 originate from meso- 
 blast (p. 249), and 
 both consist of what 
 is known as germi- 
 nal epithelium. In the male this epithelium arranges itself 
 in the form of a mass of convoluted tubules, which are 
 bound together by connective tissue. The cells lining these 
 tubules are spermatospores, or cells capable of forming 
 sperms. Each sperm originates as a bud (spermatoblasl) 
 from the spermatospore, the bud or daughter-cell having its 
 
 a, oviduct ; o, ovaries ; 6, swollen end of the ovi- 
 duct with fertilised ova ; c, oviducal outlets into 
 the cloaca. 
 
328 
 
 Elementary Biology. 
 
 nucleus transformed into the head, while the cell-protoplasm 
 becomes the vibratile tail of the sperm. 
 
 No tubules aie formed in the ovarium ; the outer cells 
 of the germinal epithelium become transformed into the 
 ovarian wall, while the central cells become ova. The ripe 
 ovum differs from the germinal cell or primitive ovum in 
 the possession of a cell-wall, the vitelline membrane, and 
 in having a large development of oil-globules in one region, 
 the pure protoplasm (with the nucleus) tending to aggre- 
 gate towards one side of the ovum. Such an accumulation 
 of fat-granules is known as yolk, and it is much more 
 
 FIG. 183. DEVELOPMENT OF SPERM IN THE EARTHWORM. 
 (Blomfield ) 
 
 FIG. 184. 
 
 SPERM OF FKOG. 
 
 (Owen.) 
 
 .A, spermatospore.; B. eight young spermatoblasts ; c, numerous 
 spermatobLists ; D, spermatoblasts developing into sperms. 
 
 abundant in the ova of some forms (e.g. the fowl) than in 
 the frog. Before fertilisation takes place the nucleus of 
 the ovum undergoes karyokinesis (p. 78) and segments, one 
 half remaining as the nucleus of the ovum, the other half 
 being extruded and forming the so-called polar body (fig. 
 186). A second polar body is then extruded in the same 
 manner. Only one sperm fuses with the ovum, and its 
 nucleus (known as the male pronucleus) unites with the 
 new nucleus of the ovum (to which the term female pro- 
 nucleus has been given). When the fusion is complete the 
 ovum has been fertilised, viz. has become an embryo. Seg- 
 
Metasoa Rana. 
 
 329 
 
 *J>, 
 
 FIG. 185. TYPICAL OVUM. (Quain.) 
 vi V s 
 
 \ 
 
 mentation then commences, the entire ovum undergoing 
 
 division. Two cells, therefore, result, one half of each, 
 
 however, owing to the distribution of the yolk, being more 
 
 protoplasmic than the other. Occasionally (as in the bird) 
 
 the relative abundance of yolk and comparative absence of 
 
 protoplasm in that section of the ovum prevent the segmen- 
 
 tation being complete, that is to say, only the protoplasmic 
 
 part of the ovum undergoes segmentation, while the yolk 
 
 remains passive and acts as a store of food-matter for the 
 
 embryo which will de- 
 
 velop from the more 
 
 protoplasmic section. 
 
 When segmentation is 
 
 complete, as in the frog, 
 
 it is spoken of as holo- 
 
 blastic ; when it is in- 
 
 complete, as in the fowl, 
 
 it is spoken of as mero- 
 
 blastic. The yolk por- 
 
 tion of the frog's ovum 
 
 divides much more 
 
 slowly than the pro- 
 
 toplasmic, so that as a 
 
 final result of the divi- 
 
 cinn i micscs nf rplk ic 
 SlOn a maSS C 
 
 formed, small and very 
 numerous at the protoplasmic, large and fewer at the yolk 
 end. This will be best understood by reference to fig. 187. 
 The next change which takes place consists in the gradual 
 covering of the large slowly dividing cells by the rapidly 
 developing protoplasmic cells, until in the end the former 
 become entirely hidden save at one spot, the blastopore. 
 Although no actual invagination takes place as in Ampkioxus^ 
 yet it is not difficult to see the homologies of the parts of 
 the embryo, or the similarity of the phenomena in the two 
 cases. The ^mail-celled outer part is obviously epiblast 
 
 cell-wall ; w, protoplasm with fat-grermles 
 5^ nucleus ;**, nucleolus. 
 
330 
 
 Elementary Biology. 
 
 and comparable to the outer layer of the gastrula in Am- 
 phioxiis, the large enclosed cells being comparable to the 
 inner layer of the gastrula, though they do not take the 
 same share in the formation of the alimentary canal as in 
 the case of Amphioxus. The small segmentation cavity 
 also is left for a time between the two layers in the frog 
 embryo. This, however, soon becomes obliterated. The 
 blastopore is present in both, though formed rather as a 
 
 FIG. 186. FORMATION OF POLAR BODIES FROM THE UNFERTILISED OVUM 
 (Quain.) 
 
 i, segmentation of nucleus and formation of nuclear spindle, g.v. ; 2, 3, 
 extrusion of first polar body,/' ; 4, 5, extrusion of two polar bodies and 
 formation of female pronucleus, //. 
 
 consequence of the activity of the epiblast than as a result 
 of the sinking in of the hypoblast. It is plugged up in the 
 frog. 
 
 Before the epiblastic cells have enclosed the yolk-cells 
 a ridge or lip of epiblast arises at the margin of the blastopore, 
 and, pushing its way backward, roofs over a cavity whose 
 floor is formed by the yolk-cells. This cavity is the 
 mesenteron, and the layer of epiblastic origin which forms 
 
Metazoa Ran a. 
 
 331 
 
 its roof becomes, therefore, hypoblast. At the same time 
 between the epiblast and hypoblast above, and the epiblast 
 and yoke-cells on the other side, several layers of cells 
 are formed the mesoblast Lastly, the floor of the mesen- 
 
 FIG. 187. -STAGES IN THE DIVISION OF THK FROG'S OVUM. (Ecker.) 
 1248 
 
 FIG. 188. DIAGRAMMATIC LONGITUDINAL SECTION 
 OF EMBRYO. (Ow en.) 
 
 teron is formed partly of hypoblast cells derived from the 
 superficial cells of the yolk, and partly of an ingrowth of 
 epiblast. The only difference, therefore, at this stage be- 
 tween the embryo of Amphioxus and that of Rana lies in 
 the possession by the 
 latter of a quantity 
 of food-yolk (after- 
 wards absorbed), the 
 presence of which 
 to a certain extent 
 alters the shape of 
 the embryo and 
 renders the 
 in its development 
 rather more difficult to follow. 
 
 It is unnecessary to follow the various changes in the 
 development of the embryo in detail here ; we may rather 
 conclude this section by a brief consideration of the mode 
 of origin of the sstenth^?&^^vhich shows the 
 
 Sta CT eS "' neuro-vertebral axis; a, i, layers of the abdominal 
 wall ; r, heart ; hv, yolk. 
 
332 
 
 Elementary Biology. 
 
 FIG. 189. VENTRICLES OF 
 J HE BRAIN. (Jeffrey Bell.) 
 
 greatest amount of differentiation, viz. the nervous system 
 and by summarising the very remarkable changes undergone 
 by the embryo after it has been hatched. 
 
 The development of the brain. The nervous system 
 of the frog originates somewhat similarly to that of Am- 
 phioxus, save that in the former the neural canal is formed 
 entirely by the laminae dorsales. The anterior blind end 
 of this canal becomes swollen so as 
 to form three so-called cerebral 
 vesicles, the walls of which become 
 differentiated into the fore-, mid-, 
 and hind- brains, while their cavities 
 are represented by chambers already 
 referred to as present in the fully 
 developed brain. From the first- 
 cerebral vesicle a hollow bud is 
 given off on either side to form the 
 cerebral hemispheres, the vesicle 
 itself becoming the cavity of the 
 thalamencephalon ; its cavity is 
 known as the third ventricle. The 
 walls of the second cerebral vesicle 
 differentiate to form the optic lobes ; 
 its cavity becomes a narrow channel 
 T<nown as the iter. The walls of the 
 hind-brain develop into the cere- 
 bellum and medulla oblongata, while 
 its cavity, into which opens the iter 
 anteriorly and the central canal of 
 
 the spinal column posteriorly, is spoken of as the fourth 
 ventricle. 
 
 Metamorphosis. The young of the frog after it has 
 escaped from its vitelline membrane is known as a tadpole, 
 and differs very markedly from the adult frog in the general 
 contour of the body. The head is not well differentiated 
 from the body, and there is a long tail, by the rhythmic 
 
 213 
 
 in, third ventricle ; n, iter ; 
 CH, cerebral hemispheres ; 
 cv, their cavity ; FM, pass- 
 age from third ventricle to 
 cavity of cerebral hemi- 
 sphere ; FB, fore-brain ; MB, 
 mid-brain ; HB, hind-brain. 
 
Metazoa Rana. 
 
 333 
 
 contractions of which movements of the body as a whole 
 are effected. There are at first no limbs. The mouth is 
 round and sucker-like, and quite unlike the wide gape so 
 characteristic of the adult. From the sides of the neck 
 project three pairs of external gills (branchiae), prolonga- 
 tions, in short, of the vascular membrane covering the 
 branchial arches of Amphioxus. By them respiration is 
 effected just as in that type, for the lungs have not yet 
 been developed. Subsequently the gills become covered 
 over by a fold of skin, though a communication with the 
 exterior is still maintained late in embryonic life. Respira- 
 tion is carried on by means of the gills, which are now 
 
 FIG. 190. METAMORPHOSIS OF THE FROG. (Owen.) 
 
 much reduced in size and hidden beneath the fold in a sort 
 of branchial chamber comparable with the atrial cavity of 
 Amphioxus. The lungs are then developed as buds from 
 the alimentary canal, and the blood-stream is carried to 
 their walls by means of the pulmonary artery. The hind- 
 limbs are the first to appear, springing from the origin of 
 the tail. Concomitantly with the development of the hind- 
 limbs the tail atrophies (i.e. becomes absorbed) ; the fore- 
 limbs make their appearance from beneath the membrane 
 covering the branchiae. The branchial slits close, and the 
 gills atrophy also, the entire respiration being now under- 
 taken by the lungs. Finally the frog becomes a truly 
 
334 Elementary Biology. 
 
 carnivorous animal, for the tadpole is only partly so. This 
 change involves a certain amount of modification in the 
 length and character of the alimentary canal, for that organ 
 in the carnivora is relatively shorter than in the herbivora. 
 
 The entire series of changes thus briefly summarised 
 takes two to six months according to the temperature, being 
 longer in cold and shorter in warm weather. The first 
 fortnight or so of the embryonic life is spent within the egg- 
 membrane 5 the metamorphosis, however, is gone through 
 after hatching in fresh water. 
 
 It will be noted that that metamorphosis consists in the 
 gradual alteration of a herbivorous aquatic animal, breathing 
 of necessity by means of gills, into a carnivorous terrestrial 
 creature, breathing by means of lungs though capable of en- 
 during without injury prolonged immersion. The title of 
 amphibian is therefore fully justified, and the early life-history 
 of the frog thus forms one of the most interesting cases of 
 development known to us, although in many respects 
 special in its character and confined to the grojp of which 
 it is a representative. 
 
 An examination of the embryonic histories of higher 
 types, as we have already hinted (p. 53), shows many inter- 
 esting transition stages, pointing to genealogical relation- 
 ships with a primitive chordate type. The possession of 
 branchial arches and clefts, a tail more or less rudimentary, 
 certain transitory phases in the formation of the circulatory 
 system, and such like, are here alluded to. For a detailed 
 consideration of these and similar phenomena, and indeed 
 for many details with regard to the structure and life-history 
 of the frog itself, reference must be made to zoological text- 
 books. The above sketch of the organisation of the frog will 
 have served its purpose if it show how from such a simple 
 typical form as Amphioxus it is possible to derive an or- 
 ganism as complicated as that which we have been consider- 
 ing. We shall find in the next and concluding section that 
 the main principles we have already laid down are true, not 
 
General Physiology of Animals. 335 
 
 only morphologically, but physiologically as well ; that, in 
 short, the key to the understanding of the structure and life- 
 history of animals, as of plants, is morphological and phy- 
 siological differentiation of a primitively simple or generalised 
 type. 
 
 SECTION III. GENERAL PHYSIOLOGY OF ANIMALS. 
 
 In our study of plants we found it advisable to devote 
 a section to a general survey of the subject of physiology, 
 or the mode in which the various organs of the plant per- 
 formed their functions, and the part each played in the 
 general phenomena of life. In the preceding survey of the 
 chief points in the organisation of the representatives of 
 the Metazoa which we have chosen as typical, physiology 
 .has been incidentally dealt with in connection with mor- 
 phology. A section may, however, with profit be devoted 
 to a general summary of the principles of animal physi- 
 ology in particular, and the comparison of animal with plant 
 physiology. 
 
 We may begin our review by noting that the funda- 
 mental principles of plant and animal physiology are the 
 same. In both we have an organism essentially composed 
 of protoplasm, subdivided into cells which are again modi- 
 fied in various ways according to the special function or 
 functions they have to perform. Protoplasm, in short, as 
 already stated (p. 33), is a highly complex store of potential 
 energy, in virtue of the possession of which the plant or 
 animal is able to perform certain duties related to the main- 
 tenance, partly of tribal, partly of individual life. Since 
 the phenomena which characterise animal life are manifested 
 only as a result of the decomposition of some of these pro- 
 toplasmic compounds, so that a certain amount of potential 
 energy becomes kinetic, it follows that, as in the case of the 
 plant, the physiology of an animal has two aspects: first, the 
 anabolic or constructive aspect; and secondly, the katabolic 
 
336 Elementary Biology. 
 
 or destructive aspect. In the former the formation of new 
 protoplasm (nutrition and assimilation) has to be considered; 
 in the latter, the disintegration of protoplasm and the 
 various phenomena (locomotion, secretion, nervation, &c.) 
 which take place in consequence of that disintegration are 
 the subjects of consideration. 
 
 Anabolism. The first great point of difference which 
 may be noted between the physiology of the plant and that 
 of the animal is the nature of the food-supply in the latter. 
 In the plant (p 190,) we found that the food consisted of 
 chemical elements or exceedingly simple compounds, such 
 as carbonic acid, water, salts of nitric acid, c. ; in the 
 animal, though water and many simple salts are employed 
 as food accessories, yet the essential food-stuffs are derived 
 from already formed products of anabolism. In the long- 
 run all animals are dependent (p. 49) on the plant world 
 for food. Without the assistance of green plants the in- 
 organic constituents found in the environment could not be 
 built up, for the animal, as we have seen, has no power to 
 integrate such simple bodies into protoplasm. The most it 
 can do is to modify arid assimilate already formed com- 
 pounds, employing water and simple salts as accessories. 
 The popular distinction, therefore, of animals into car- 
 nivorous and herbivorous is only superficially correct, since 
 herbivors, on which the carnivors prey, are directly de- 
 pendent on the plant world for food. 
 
 Anabolism in the animal may be said to consist of the 
 following processes : 
 
 (a) Mastication. By the assistance of the teeth, tongue, 
 and muscles of the buccal cavity in the higher forms, or by 
 the gizzard or the masticatory organ of the lower animals, 
 the food, whether it be animal or veget?! in its nature, is 
 divided into small particles, enabling it thereby to be more 
 easily acted on by the juices which are subsequently mixed 
 with it. 
 
 (If) Digestion. The primary object of digestion is, of 
 
General Pliysiology of Animals. 33; 
 
 course, to render the food-particles soluble. For that purpose 
 various secretions are poured into the alimentary canal from 
 glands in its vicinity. The most important of these are 
 saliva, gastric juice, intestinal juice, pancreatic juice, and 
 bile. It may be convenient if we summarise the functions 
 of these fluids and the action they severally have on the 
 contents of the alimentary canal. 
 
 Saliva. Saliva, though absent from the Amphibia, is 
 so important a fluid that a notice of it cannot be omitted 
 in a general sketch of animal physiology. It is a colourless, 
 watery, but slightly viscid fluid, produced in abundance in 
 certain glands situated in or near the buccal cavity. The 
 essential constituent of saliva is a ferment, ptya^in, which 
 has the power of transforming starch into sugar. Starch, 
 being one of the substances already described as colloids, 
 is incapable of being absorbed through an animal mem- 
 brane, while sugar, being a crystalloid, may be so absorbed. 
 Ptyalin, by changing starchy substances in the food into 
 sugar, thus renders them soluble, while the water of the 
 saliva dissolves the sugar so formed, which thus is rendered 
 capable of subsequent absorption and assimilation. When 
 food is introduced into the mouth, and the masticatory 
 movements take place, the salivary glands are reflexly (p. 317) 
 stimulated to secrete a copious supply of saliva, which in 
 the process of mastication becomes thoroughly mixed with 
 the food. All the digestive glands act in the same way, i.e. 
 reftexly, the medium being a sensory or afferent nerve dis- 
 tributed to the mucous membrane of the alimentary canal, 
 and an efferent or secretory nerve going to the secretory 
 cells of the glands, the stimulation of which brings about 
 glandular activity and a flow of the secretion. 
 
 Gastric juice. The digestive fluid formed by the glands 
 in the mucous membrane of the stomach-wall also con- 
 tains one essential ferment, namely, pepsin, which in the 
 presence of a small amount of hydrochloric acid (0-2 per 
 cent.) transforms insoluble proteids into soluble substances 
 
 z 
 
33 8 Elementary Biology. 
 
 known as peptones. The involuntary churning movements 
 which take place in the stomach act in the same manner 
 as the voluntary movements of the buccal cavity in pro- 
 moting a perfect mixture of the gastric juice and the 
 food. 
 
 The action of these two fluids, therefore, is to render 
 two important constituents of the food, starch and albumin, 
 soluble and capable of being absorbed through the walls of 
 the intestine. 
 
 The food, or chyme, as it is termed, after being 
 acted on by gastric juice, now passes into the intestine, 
 where it meets with intestinal juice. This fluid is secreted 
 from the Lieberkiihnian follicles in the wall of the intestine, 
 and appears to have much the same action upon the 
 intestinal contents as the pancreatic juice has ; we may 
 simplify matters, therefore, by considering these together. 
 The pancreatic juice contains three ferments, all important 
 in their nature. The first and characteristic ferment of the 
 pancreas is trypsin, which has the power of transforming 
 proteids which have escaped the action of the peptic ferment 
 into peptones but here in an alkaline medium, while pepsin 
 only acts in an acid medium. Further, the pancreatic juice 
 contains a ferment which acts on fats or oils so as to 
 transform them into an emulsion, in which the oil occurs 
 in extremely fine particles ; it then decomposes them into 
 glycerin and their corresponding fatty acids (p, 28). 
 Lastly, the fatty acid forms soaps with the alkali present in 
 the juices of the pancreas and intestine. Pancreatic juice 
 is also able to change starch into sugar should any be left 
 over unacted upon by the saliva. Where salivary glands are 
 absent, as in the frog, the pancreas is the only agent in this 
 process. 
 
 Bile. The character and functions of this fluid have 
 been sufficiently referred to in connection with the liver 
 (p. 280). We may briefly summarise its uses by saying that, 
 while acting as a stimulant on the intestinal mucous mem- 
 
General Physiology of Animals. 339 
 
 brane, it also assists the process of absorption by moistening 
 the wall of the intestine and by acting as an antiseptic. 
 
 In the intestine also peristaltic movements (p. 277) per- 
 form the same function as the churning movements taking 
 place in the stomach, viz. that of mixing of the food-stuffs 
 and the digestive secretions. 
 
 (c) Absorption. The third process in anabolism is ab- 
 sorption, or the reception into the circulation of the pre- 
 pared food-substances. Absorption takes place partly in 
 the stomach, partly in the intestine. The mucous mem- 
 brane of the stomach and intestine contains an abundant 
 supply of capillaries ; the walls of these vessels, as has 
 already been pointed out (p. 287), are only one cell thick, 
 consequently the soluble peptones and sugar will diffuse 
 readily into their interiors. In the intestine the area of 
 absorption is largely increased by means of the villi, which, 
 in addition to absorbing the food-stuffs just mentioned, also 
 take into their interiors the emulsified fat. The oil-globules 
 pass into the lacteals, which in turn communicate with the 
 chief vessel of that system, the thoracic duct, which opens 
 into the left vena cava near the shoulder. 
 
 (d) Circulation. The food stuffs having been absorbed 
 are now circulated, and in their passage through the body 
 are altered in a variety of ways ; for instance, in the liver 
 the important starch-like body, glycogen, is abstracted from 
 the blood and stored temporarily in the hepatic cells ; the 
 elements of bile are likewise extracted and stored in the 
 gall-bladder or flow into the intestine directly, while prob- 
 ably urea and other products of metabolism are formed and 
 pass away in the blood-stream to the kidney to be got rid 
 of. In the lymph glands also the chyle, or alkaline nutritious 
 fluid absorbed by the lacteals, no doubt undergoes important 
 chemical changes. We have already (p. 288) referred in 
 sufficient detail to the course of the blood-flow, and general 
 physiology of the circulation. The action of the heart is 
 a purely muscular one under government of the nervous 
 
 z 2 
 
34 Elementary Biology. 
 
 system, while the muscular walls of the vessels assist in the 
 circulation, and by their elasticity regulate the flow. There 
 also the mot^r nerves (vasomotor) determine the degree 
 of contraction of the circular muscle-fibres, and so govern 
 the calibre of the vessel in question. 
 
 (e) Assimilation. The final processes of anabolism 
 which result in the formation of new protoplasm and the 
 repair of tissue-waste are as yet much involved in mystery. 
 No doubt the cells of the various tissues have the power of 
 abstracting or selecting from the blood and lymph supplied 
 to them the special substances which they require. Muscle- 
 cells are thus no doubt able to select the proteids that are 
 needed to form new muscle-cells, and, at the same time, the 
 carbohydrates on which the evolution of muscular energy 
 depends; saliva-secreting cells similarly seize on compounds 
 required to form ptyalin, mucin, and the other constituents of 
 saliva, and similarly for other glands. The nervous system 
 also undoubtedly depends on the blood (the purest in the 
 body) brought to it by the capillaries of the carotid arteries 
 for those compounds needed to repair nervous waste. In 
 the absence of detailed information with regard to the 
 anabolic processes we may summarise the probable nature 
 of these processes and those of katabolism by employing a 
 convenient and probably in the main correct diagram as 
 follows : 
 
 FIG. 191. 
 
 fc 
 
 o> 
 
 IS 
 
 ANABOLISM KATABOLISM 
 
General Physiology of Animals. 341 
 
 P represents the fully formed living protoplasm (animal) 
 occupying the centre of two sets of diverging lines. On the 
 left hand the ultimate food- stuffs are represented as being 
 built up into intermediate products, which may be known as 
 mesostates, while on the right hand protoplasm is repre- 
 sented as undergoing katabolism, where some of the inter- 
 mediate stages are of service in the body (secretions) 
 although ultimately they are all got rid of as katastates or 
 excretions. Of course the same diagram might apply to 
 protoplasm of green plants, although in that case the ulti- 
 mate food-stuffs would be inorganic only. Combining both 
 diagrams, and at the same time varying the character of 
 the figure itself, we might graphically summarise the meta- 
 bolism of both plant and animal in relation to the organic 
 world thus : 
 
 FIG. 192. 
 ANIMAL PROTOPLASM 
 
 INORGANIC WORLD 
 
 The diagram is, indeed, only a variation on that import- 
 ant scheme on which so much emphasis was laid at p. 49. 
 The separate steps stand for anastates and katastates, while 
 the anabolic stair for animal protoplasm is made to spring 
 from near the top of the katabolic vegetal stair as indicating 
 that the animal is dependent on the plant for food. 
 
 We must now glance for a moment at the results of the 
 metabolism of animal protoplasm. The more important of 
 these may be classified under growth, motion, nervation, 
 heat, light, and electricity. Reproduction or the separation 
 of sexual cells has already been sufficiently considered 
 
 <p 52). 
 
342 Elementary Biology. 
 
 Growth, in the animal as in the plant is due to the excess 
 .of repair over waste, of anabolism over katabolism. This 
 anabolic excess is proportionally very great in the young, 
 becoming less as maturity is approached. In the animal 
 this early period may be divided into an embryonic stage 
 spent inside the egg-membrane, and a postembryonic period, 
 during which, as in the frog, metamorphosis may occur. 
 Growth in the lower animals especially is frequently accom- 
 panied by ecdysis, or skin-casting, when the skin, whether 
 soft as in the frog, or stiffened by a deposit of keratin (insects), 
 or carbonate of lime (crustacea), is cast off and a new one is 
 assumed. Ecdyses are concomitant with periods of more 
 active growth. 
 
 Motion. Motion is brought about by the contraction 
 of muscle-cells in one direction ; it is therefore definite. 
 Indefinite motion in the higher animals is still represented 
 by the amoeboid movements of white blood-corpuscles and 
 lymph-cells, c. 
 
 A muscle contracts in response to a stimulus, usually 
 nervous. Muscles are themselves irritable, for if the peri- 
 pheral terminations of nerves be poisoned by curara, the 
 muscles are still able to respond to a non-nervous stimulus, 
 such as a prick from a needle or the application of certain 
 chemical compounds, for example acetic acid. As we have 
 already seen, the muscles of the body may be arranged in 
 two categories : muscles which are intrinsically parts of 
 organs, e.g. the muscles of the heart, alimentary canal, &c. ; 
 and muscles which are attached to hard parts, by the con- 
 traction of which motion of the limbs and trunk, or of the 
 body as a whole, is effected. A single muscular contraction 
 may be represented by a curve rapidly attaining a maximum, 
 and more slowly regaining the abscissa. If, however, the 
 muscular contraction be followed immediately by another 
 before the first contraction has reached its maximum point, 
 the total maximum contraction is considerably greater than 
 the maximum which would have resulted from one stimulus 
 
General Physiology of Animals. 343 
 
 alone. If a rapid succession of stimuli be applied to the 
 muscle, an absolute maximum is at length reached. The 
 muscle is then said to be in tetanus. The absolute maxi- 
 mum gradually falls as the muscle becomes exhausted. 
 
 Nervation. While the rate of muscular motion is com- 
 paratively slow the chemical changes which take place in 
 nerve-cells and nerve-fibres during nervous activity arc 
 extremely rapid. It has been calculated that a nerve 
 impulse travels along a nerve at the rate of 200 feet per 
 second. The rapidity of the metabolic processes in the 
 brain is proverbial. The essential nature of these processes 
 and of nerve motion is unknown. 
 
 Heat. The ultimate form into which all other forms of 
 energy become resolved is manifested more abundantly and 
 can be studied more easily in the animal than in the plant. 
 The temperature of the living body in the case of the higher 
 animals varies, but the majority exhibit a uniform tempera- 
 ture of about 100 Fahr. In birds it is slightly higher. In 
 the frog, however, the temperature of the body is that of the 
 surrounding air. Hence the old-fashioned classification of 
 animals into cold-blooded (e.g. fish, frogs), and warm- 
 blooded (e.g. birds, mammals). Heat, as has been already 
 stated, is the final results of katabolic processes taking 
 place in the body ; the more work done the more heat 
 evolved, and the more food required to repair waste. With- 
 out entering into detai's on the subject of animal heat, it 
 may be of interest to point out the important relationship 
 that exists between the evolution of heat and the metabolic 
 changes of the body viewed in relation to the constant 
 variation in atmospheric temperature. In the case of a hot- 
 blooded animal if the temperature of the air decrease, more 
 heat-producing food (fats) must be oxidised to keep up the 
 balance ; if the temperature increase, less food is of course 
 needed. If, on the other hand, the animal be cold-blooded 
 and the temperature of the air decrease, metabolism is 
 diminished \ if the temperature of the air increase, meta- 
 
344 Elementary Biology. 
 
 holism is also increased. It will be seen that this relation 
 depends entirely on the fact that the temperature of the 
 warm-blooded animal is nearly constant, while that of the 
 cold-blooded animal varies with the surrounding tempera- 
 ture within certain limits. 
 
 Light. Many animals have the power of giving out 
 luminous rays, which serve either to frighten away enemies 
 or to guide them in finding their prey in the dark. Many 
 fish, for instance, possess this power. This luminosity is 
 known as phosphorescence (p. 209). 
 
 Electricity. The existence of normal electrical currents 
 in certain animal tissues, notably, muscle and nerve, has 
 already been alluded to. These currents are particularly 
 noticeable if the muscle or nerve be injured in any way. 
 Further, a special electrical current, the so-called nega 
 tive variation, is developed in a muscle or nerve on the 
 application of a stimulus, and this electrical wave is the fort- 
 runner of the actual contraction, or of the nervous wave, 
 and shows that certain metabolic changes are taking place 
 preparatory to the actual contraction or the passage of the 
 nervous impulse. The whole subject is, however, still under 
 consideration, for the connection between the metabolic 
 changes in the tissues and the electric currents is by no 
 means clear. 
 
History of Biology. 345 
 
 CHAPTER XL 
 
 HISTORY OF BIOLOGY. 
 
 IN order to obtain a complete view, even though in outline 
 only, of any science, it is necessary that some effort should 
 be made to understand the chief stages in its history, the 
 more noticeable features in its development. In the pre- 
 ceding chapters we have considered a few of the more im- 
 portant types illustrating the structure and physiology of the 
 plant and animal kingdoms. No effort has been made even 
 to sketch the principles governing the distribution of living 
 organisms, nor have we entered at all into the great question 
 of their etiology or genealogy. Under these circumstances 
 it would be, of course, entirely out of place to mention any 
 of the multitudinous systems of classification of plants and 
 animals the knowledge of which was in old times, and is by 
 some even yet, considered the aim and object of zoological 
 and botanical study. We may, however, endeavour to ob- 
 tain some conception of the gradual evolution, so to speak, 
 of the science of biology out of a mass of isolated and 
 disconnected observations, the collecting of which, as we 
 have seen in the Introduction, forms the first stage in the 
 development of any science. 
 
 Aristotle has been often termed the ' father of natural 
 history,' and certainly he and his pupil Theophrastus in the 
 fourth century B.C. may be looked on as the first collabora- 
 teurs of that vast catalogue of species to which the scientific 
 expedition of H.M.S. Challenger (1872-76) has added only 
 the latest chapter. 
 
346 Elementary Biology. 
 
 To the Greek naturalists and the few Romans who, like 
 Pliny (23-79 A - D -)> followed in their steps, belongs the 
 honour of having laid that necessary foundation for future 
 generalisation the gathering of data. How great was the 
 work they achieved can scarcely be better shown than by 
 noting how men for centuries afterwards seemed to think 
 that Aristotle had probed the secrets of Nature to the 
 bottom ; that there was, in fact, nothing more to learn ; and 
 that what he said must be true simply because he said so. 
 
 After these early natural historians they can scarcely be 
 called biologists in the sense in which we now understand 
 the word there comes a gap of nearly five hundred years 
 before we meet with the founders of Arabian, medicine, 
 whose labours afterwards bore fruit in the great medical 
 schools of Bagdad and Cordova. 
 
 Wise men were not wanting in those early days -who saw 
 that the study of animals was one which was likely to throw 
 considerable light on the organisation of man himself, and 
 therefore must tend in great measure to advance the art of 
 medicine from what it was a mere collection ot recipes and 
 conclusions founded on empiricism to what it ought to be, 
 a well-digested and connected system of treatment based on 
 scientific generalisations arrived at by a careful study of the 
 comparative anatomy and physiology of man and the lower 
 animals. The work of the Arabian and Moorish doctors 
 consisted in the generalising of what was known for present 
 need ; and though necessarily very small absolutely, yet 
 relatively speaking the advance made was so great that it 
 took six hundred years more before the advent of the great 
 anatomists Vesalius (1514-1564), Fallopius (1523-1562), 
 Jabricius (1537-1619), and their pupils marked the begin- 
 ning of a new era of comparative morphology. 
 
 Nothing strikes one more forcibly in looking back over 
 the history of the science of those days than the paucity 
 of generalisations, which are at once the summation of work 
 done and the basis for future effort. There were doubtless 
 
History of Biology. 347 
 
 many reasons for this, but certainly one was the insufficiency 
 of data. Vesalius and Fabricius were too earnestly engaged 
 in mastering the details of mammalian anatomy to plunge 
 into tie sea of general Biology. It needed more students 
 of Nature moulded on the Aristotelian type for work such 
 as that ; and yet it was not till the sixteenth century that 
 Gesner (1516-1565) and Csesalpinus (1519-1603) rose to 
 supply the want. It was then that the lists of species com- 
 piled by Aristotle were found so defective, and the labours 
 of these men and others of their school did much to increase 
 the range of biological knowledge. But they did more ; 
 they attempted, and with wonderful success, to generalise 
 on the observations they had made, and from a careful 
 study of the material they had collected endeavoured to 
 establish a more or less rational classification of plants and 
 animals. 
 
 Biology was the last of the four great sciences to come 
 into being. Astronomy, the most general of them all, was 
 also the oldest. Physics and Chemistry were still in a tran- 
 sition state, and did not comprise a tithe of the array of 
 facts and conclusions which are now included under these 
 sciences. Yet just as Sociology, Anthropology, and the various 
 sub-sciences dealing with the relationship of man to man are 
 based on Biology which treats of him as an animal, struc- 
 turally and functionally, so Biology itself is, as we have 
 seen, based on the sciences of Physics and Chemistry. For 
 that reason Biology had to wait for Physics and Chemistry to 
 develop from the chaotic condition in which they were in 
 the sixteenth century into the comparatively orderly and 
 scientific department of knowledge which they became be- 
 fore the days of Cuvier, Haller, and Bonnet. Biology was 
 in the bud ; but it could not blossom until the physical and 
 chemical sciences were in a fit condition to supply the 
 necessary nourishment in the shape of suggestive ideas and 
 generalisations of wide application. 
 
 That preparation had now been made. Kepler and 
 
348 Elementary Biology. 
 
 Galileo, Geber and Van Helmont, and all the host of phy- 
 sicists and chemists that the alchemical movement of the 
 middle ages directly or indirectly gave birth to, had lived 
 and left their mark. The telescope was revealing what 
 was hidden in the sky beyond man's unaided vision, and 
 the microscope was preparing, under the fostering genius of 
 Leeuwenhceck (1632-1723), to tell him of the wonders that 
 lay unknown at his feet. Although in the end of the seven- 
 teenth and the beginning of the eighteenth centuries we 
 still meet with natural historians of the old school in the 
 form of Ray (1628-1705) and Willoughby (1635-1672), 
 Buffon, (1707-1788) and Linnaeus (1707-1778), yet these 
 famous writers were the last of their race, for a new epoch 
 was dawning in the history of our science. 
 
 As might be expected, no attempt worth refeiring to was 
 made in those early days to elucidate the nature of the 
 origin and development of living things, nor was it possible 
 in the condition of Geography and Geology to make even a 
 start, in the great subject of distribution. Even function 
 was not in those times treated separately from structure ; 
 no attempt was made to separate morphology from physiology. 
 The Histoire Naturelle (1749) of Buffon and the Systenia 
 Naturce (1736) of Linnceus were merely encyclopaedias 
 giving in the one case a popular, in the other a scientific, 
 account of the animals and plants then known. At this 
 point, however, we meet with a great advance, due perhaps 
 as much to the introduction of the microscope into biological 
 research as to anything else. Morphology and physiology 
 began to separate from each other, and eager workers 
 rapidly appeared in both subjects to raise the splendid 
 monuments of intellect that we are able to look upon 
 to-day. 
 
 Just as in the natural order of procedure in the exami- 
 nation of an animal or a plant we treat first of the organism 
 as a whole, its appearance, habits, and so on. and then pass 
 to a consideration of the structure and functions of the 
 
History of Biology. 349 
 
 organs of which it is made up, then of the tissues composing 
 these organs, of the cells, or component parts of the tissues, 
 and, lastly, of the protoplasm or ultimate physi -al basis of 
 life, so in the history of biology the progress has been in 
 all respects similar. 
 
 As, year by year, hundreds upon hundreds of new 
 species of animals and plants were added to the catalogues 
 already in existence, and as naturalists with the help of the 
 microscope and the scalpel dived deeper and deeper into 
 their structure, it soon became a sheer impossibility for one 
 man to master the entire range of biological knowledge a 
 feat easy enough in the days of Aristotle. Division of labour 
 became necessary. One naturalist devoted himself to Botany, 
 another to Zoology, still further specialising* in some par- 
 ticular line research, morphological or physiological, as taste 
 or circumstance dictated. 
 
 Thus Laurent de Jussieu (1748-1836) in Botany and 
 Baron Cuvier (1769-1832) in Zoology laid the foundation- 
 stone of morphology by their classic treatises, Genera Plan- 
 taruui (1789) and Rcgne Animal (1817). About the same 
 time Haller (1708-1777) and Bonnet (1720-1793) esta- 
 blished on a true scientific basis the subject of physiology. 
 With them Biology advanced from being a study of external 
 form and habit to that of internal organisation and function. 
 It was an easy step from that to the study of tissues, a step 
 greatly aided by the continued improvement of the micro- 
 scope. One prominent name, that of Bichat (1771-1802), 
 stands out as the founder of histology, or tissue study, and 
 histogeny, or tissue development, in his Anatomic Generate 
 (1801). 
 
 Research could not, however, rest there. As the physi- 
 cists improved their lenses, the biologists with their help 
 were enabled to analyse tissues into cells. Schleiden (1804- 
 1881) was the first by means of the improved technical 
 appliances to perform that analysis in plants ; a discovery 
 which he followed up by a generalisation to the effect that 
 
35 Elementary Biology. 
 
 all tissues were cohiposed of minute saccules, to which he 
 applied the name of 'cells.' Schwann (1810-1882) was 
 similarly the first to apply the same generalisation to the 
 tissue of animals, and so to aid in enunciating the cell 
 theory on which modern morphological research for the 
 most part rests. 
 
 Cells were found to vary greatly in size and shape, and 
 a collection of similar cells formed a tissue. Every plant 
 and every animal examined told the same tale. Organs 
 made up of different kinds of tissues, tissues composed 
 of collections of similar cells. Not only so, but every 
 organism in the earliest stage of its existence was found 
 to be composed of a single cell, which by division gradu- 
 ally became first a minute cellular mass, then by differen 
 tiation of cells a collection of tissues with different duties 
 to perform, and finally an adult plant or animal, as the 
 case might be, with tissues composed of different cellular 
 elements united to form organs concerned in the main 
 tenance of individual or of tribal life. 
 
 This takes us well into the nineteenth century, and its 
 early years bring us face to face with all those names that 
 we are most accustomed to associate with biological progress. 
 Geoffrey S. Hilaire (1772-1840) and Von Humboldt (1769- 
 1859) had begun their researches into the distribution of 
 living organisms, and Lamarck (1744-1829), working, on 
 the material collected by Cuvier and the anatomical schoo'l 
 of zoologists which he had founded, had made his guesses at 
 the origin of living things ; guesses which were destined in 
 after years, in Darwin's hands, to become organised into the 
 theory of evolution by natural selection. 
 
 Lamarck's generalisations were greatly in advance of the 
 age in which he lived. To write a book in those days in 
 support of the view that 'all species, including man, are de- 
 scended from other species ' was unlikely to lead to any other 
 result than to arouse suspicion as to the sanity of the author. 
 Lamarck, however, despite the risk he ran, published his views 
 
History of Biology. 351 
 
 and earned the ridicule and contempt of his contemporaries 
 and the posthumous applause of scientific investigators a 
 century after. In 1835 Dujardin had taken the final step in 
 the study of living things and had discovered the existence 
 of a substance within Schleiden's cell-wall to which he gave 
 the name of 'sarcode'; and Von Mohl (1805-1872) did 
 the like service for plant-cells and named the granular 
 gelatinous substance he found in the cell ' protoplasm.' 
 Lastly, Max Schultze (1825-1874) identified the two sub- 
 stances as being the same, and thus struck the keynote 
 of the science of Biology as we now understand it. For the 
 adoption of a new name for the science which has grown 
 out of, and has now absorbed the two older sub-sciences of 
 Zoology and Botany, signifies more than a merely nominal 
 union. It signifies the adoption of a unity of treatment in 
 investigations into the morphology and physiology of plants 
 and animals, a step parallel to and naturally following 
 from the discovery of the identity of Dujardin's animal 
 ' sarcode,' and Von Mohl's plant * protoplasm ' by Max 
 Schultze. 
 
 No sooner had the morphologists performed their final 
 analysis and discovered that all organisms were built out of 
 protoplasm, well named by Professor Huxley * the physical 
 basis of life,' than the physiological school led by Claude 
 Bernard (1813-1878), proceeded to study its functions, and 
 ere long elucidated the great general principle that living 
 protoplasm was constantly undergoing chemical changes ; 
 that these changes might be divided into two categories, viz. 
 constructive or anabolic changes, leading to the formation 
 of protoplasm, which thereby became a store of potential 
 energy, and destructive or katabolic changes, resulting in 
 the breaking down of the protoplasm and in the evolution 
 of the stored energy in a kinetic or active form which 
 manifested itself in the various phenomena of life. 
 
 Contemporary with these great leaders in biological 
 thought, were many who devoted themselves to some 
 
 ^. , ,,-r\a^ 
 
352 Elementary Biology. 
 
 special problem, or series of problems, such as the develop- 
 ment of the embryo in the plant (Robert Brown, 1773- 
 1858) and in the animal (Von Baer, 1792-1876); the 
 
 diseases to which cells are liable (Virchow, 1820 ) ; 
 
 the fertilisation of flowers (Sprengel, 1750-1816; ; and the 
 like. 
 
 Lastly, in our own day, comes the evolution school, led 
 by Darwin and Wallace, who, summing up the work of the 
 past in the light of the present, simultaneously formulated 
 an explanation of the structure and distribution of living 
 things based on their genealogical relationships (1858). 
 That the hypothesis of evolution by natural selection was 
 enunciated by either of these men in its final form would be 
 a statement both premature and unwarranted ; but that 
 their theory was a true and workable theory, and a key to 
 many, if not all, of the anomalies which had so puzzled the 
 workers of the past, and driven them often to grotesque 
 expedients by way of affording an explanation, is acknow- 
 ledged by most, and has been proved to be so by the 
 multitude of biologists who have spent or are spending their 
 lives in adding some fragment of the unknown to the 
 known. Foremost amongst these we may note the names 
 of Huxley, Agassiz, Balfour, Haeckel, and Gegenbaur in 
 Zoology, and von Sachs, Sir Joseph Hooker, Herman 
 Mttller, Hoffmeister, and De Bary in Botany. 
 
 It is of course quite impossible in this volume to give 
 a detailed account of the v/ork accomplished by these and 
 countless others in recent years. This summary will fulfil 
 its function if it serve to guide those who wish further in- 
 formation on the subject in selecting, out of the long cata- 
 logue of biologists who have made Biology what it is, those 
 who may, without prejudice, be considered, if not as the 
 leaders in the several subjects with which their names are 
 associated, at least as representative types of the different 
 schools in Biology ; which schools, however diverse their 
 opinions and varied their methods, have at least one thing 
 
History of Biology. 353 
 
 in common the advancement of the science of Biology 
 from a heterogeneous collection of isolated facts and ob- 
 servations to a homogeneous body of phenomena and laws, 
 with the gradual evolution of the higher type from the 
 lower, the more complex from the more general, as its 
 guiding principle. 
 
 AA 
 
INDEX. 
 
 ABS 
 
 A BSORPTION in the animal, 
 *"* 339 ; in the plant, 200 
 Acetabulum, 306 
 Afferent nerves, 317 
 Agassiz, 352 
 Albumin, 26 
 
 Albuminates, characters of, 26 
 Albuminoids, characters of, 26, 27 
 Aleurone, nature and uses of, 1^,6 
 Alkaloids, vegetal, 209 
 Alternation of geneiations, 100 
 Amides, 204 
 
 Amceba, life-history of, 73 
 Amoeboid motion, 33 
 
 stage in Protomvxa, 59 
 Amphioxus, alimentary system of, 
 
 253; atrial cavity, 254'; circula- 
 tory system, 255; coelom, 257; 
 development of the embryo, 260 ; 
 excretory system, 256 ; external 
 characters, 252 ; locomotory sys- 
 tem, 258 ; nervous system, 258 ; 
 protective system, 258 ; repr duc- 
 tive system, 259 ; respiratory sys- 
 tem, 253 ; sense organs, 259 ; 
 supporting system, 258 
 
 and Lumbricus compared, 263 
 Amyloids, characters of, 27 
 Anabolism, 34 ; in the plant, 197 ; 
 
 in the animal, 336 
 Analysis, 25 
 Anastates, 34 
 Anatomic Glntrale, 349 
 Angiospermse, 144 
 Annual rings in wood, 180 
 Anterior abdominal vein, 291 
 Anthotaxis, 160 
 Aorta, 289 
 Apheliotropism, 212 
 
 BIO 
 
 Apical cell, 120 
 
 Apogamy, 100 
 
 Aqueous humour, 325 
 
 Arabian medicine, 346 
 
 Archenteron, 249 
 
 Archesporium, 109 
 
 Aristotle, 345 
 
 Aromatic substances, 209 
 
 Artery, structure of an, 286 
 
 Asci, 97 
 
 Ascospores, 97 
 
 Asexual reproduction, 52 ; in Peni- 
 
 cillium, 95, 98 
 Ash of plants, analysis of the, 195 ; 
 
 constituent of the, 190 
 Aspaiagin, 204 
 Assimilation, conditions of, 203 ; 
 
 in the plant, 203 ; in the animal, 
 
 340 
 
 Astronomy, definition of. 3 
 Atmosphere, composition of the, 41 
 Atom, 9 
 
 Auricles of the heart, 285 
 Automatism of protoplasm, 34 
 Axial cylinders of nerves, 314 
 
 BAER, von, 352 
 Bagdad, medical schools of, 
 
 346 
 
 Balance of nature, the, 48 
 Balfour, 352 
 Bary, De, 352 
 Bast, 118, 147, 179 
 Bernard, Claude, 351 
 Bichat, 349 
 
 Bile, 280 ; functions of, 338 
 Biology, definition of, 4 ; position 
 
 among the sciences, 5 
 
 A A 2 
 
356 
 
 Elementary Biology. 
 
 BLA 
 
 DOR 
 
 Blastocoele, 231 
 B.astoderm, 173 
 Blastopore, 249, 290, 329 
 Blastosphere, 248, 260 
 Blastula, 231 
 Blood, 240, 255, 282 
 Bone, structure of, 297 
 Bonnet, 347, 349 
 Brachial plexus, 311 
 Brain, structure of the, 308 
 Bronchus, 293 
 Brown, Robert, 352 
 Buffon, 348 
 
 S, 247 
 
 Calcium, origin of and im- 
 portance to the plant of, 193 
 
 Calyptrogen, 174 
 
 Cambium, 150, 179 
 
 Capillary, sti ucture of a, 287 
 
 Carbohydrates, characters of, 27 
 
 Carbon, origin of and importance to 
 the plant of, 190 
 
 Carbonic acid, absorption of, by the 
 plant, 203 ; destiny of, 40 ; in the 
 atmosphere, 42 
 
 Carnivorous plants, 213 
 
 Carotid artery, 289 
 
 Carpel, structure of, 162 ; develop- 
 ment of, 166 
 
 Carpus, 305 
 
 Cartilage, 297 
 
 Causality, law of, 2 
 
 Cell, 29 ; cell-theory, 350 
 
 Cellulose, composition of, 61 
 
 Cell-wall, 31 ; thickening of, 149 
 
 Cerebellum, 308 ; structure of, 313; 
 function of, 317 
 
 Cerebrum, 308 ; function of, 317 
 
 'Challenger, 1 H.M.S., 345 
 
 Char a, 83 
 
 Chemical change, laws of, 22 ; affi- 
 nity, 19 ; compounds, classifica- 
 tion of, 21 
 
 Chemistry, definition of, 4 
 
 Chlorophyll, composition, structure, 
 and function of, 38 ; in Hydra 
 vtridis, 226 
 
 Choroid, 322 
 
 Chyle, 339 
 
 Chyme, 338 
 
 Ciliary muscle, 324, 325 
 
 Circulation, of water in the plant, 
 
 204 ; of sap, 206 ; in the animal, 
 
 339 
 
 Circumvallate papillae, 320 
 Classification of organisms, 54 
 Clavicle, 305 
 
 Coagulation of blood, 283 
 Coelom, 243, 249 
 Coenosarc, 221 
 Coleochaetese, 83 
 Coleorhiza, 177 
 Colour of flowers, 169 
 Columella, 321 
 Concentric fibrovascular strand, 
 
 120 
 
 Condyles, 301 
 
 Conjugation in Spirogyra, 80 
 Connective tissue, structure of, 275 
 Conservation of energy, law of, 
 
 15 
 
 Contractility of protoplasm, 33 
 Coracoid, 304 
 Corallineae, 91 
 
 Cordova, medical school of, 346 
 Cork, 181 
 Cornea, 322 
 Corpora adiposa, 326 
 Cotyledons, 173 
 Cranial nerves, 310 
 Cranium, 301 
 
 Cross-fertilisation, 139, 168 
 Crystals, composition and occur- 
 rence of, 157 
 Cuvier, 347, 349 
 Cystolith, 157 
 
 DARWIN, 352 
 Death, 36 
 Decay, 36 
 Decomposition, 22 
 Dehydration, 22 
 Deoxidation, 23 
 Dermatogen, 173 
 Development, 36 
 Diastole, 73 
 Diatomaceae, 72 
 
 Dicotyledonous type of plant, 178 
 Differentiation, 36 
 Digestion, 336 
 DiofKFa, 214 
 Dissociation, 22 
 Distribution of organisms, 54 
 Division of labour, 55 
 Dorsal laminae, 260, 332 
 
Index. 
 
 357 
 
 DRO 
 
 L)1S\J 
 
 Drosera, 214 ; structure of the leaf 
 
 of, 215 
 
 Dujardin, 351 
 Dura mater, 314 
 Duration of plants, 174 
 
 EARTHQUAKES, 18 
 Ecdysis, 342 
 
 Ec'otarpus, 91 
 
 Ectoderm, 221, 244 
 
 Ectoplasm, 29 
 
 Efferent nerves, 317 
 
 Electricity in plants, 210; in 
 animals, 344 
 
 Elements and compounds, 9 
 
 Embryo, 53 ; of Spirogvra, 81 ; of 
 Penicillium, 98 ; of Polyttichum, 
 1 08 ; of P ten's, 129 ; of Seligi- 
 nella, 142; of Lilium, 171; of 
 Obelia, 231 ; of Lumbricus, 247; 
 of Amphioxus, 260 ; of Ran a, 328 
 
 Embryo-sac, division of the 
 nucleus in the, 167 
 
 Encysted stage, 60 
 
 Endoderm, 221, 244 
 
 Endodermis, 116 
 
 Endogenous origin of roots, 177 
 
 Endoneurium, 314 
 
 Endop'asm, 29 
 
 Endosperm, 138, 172 
 
 Energy, n, 12; transformation of, 
 13; directly available sources of, 
 16 ; indirectly available sources 
 of, 19 
 
 Enteron, 228 
 
 Epiblast, 231, 248, 329 
 
 Epineurium, 314 
 
 Eustachian tube, 267, 321 
 
 Excretions, 3^ 
 
 Exogenous origin of branches, 177 
 
 Eye, 259, 302, 322 ; muscles of the, 
 322 
 
 HAL 
 
 , 346 
 Fallopius, 346 
 Fats, characters of, 27 
 Fatty acids, 28 
 Femur, 306 
 Fenestra ovalis, 321 
 Ferments in the plant, 206 ; in the 
 
 animal, 337 
 Fertilisation, 53 ; in the Angio- 
 
 spernue, 168 
 
 Fibrin, 283 
 
 Fibrovascular system, 102, 116, 146 
 
 Fibula, 306 
 
 Filum terminate, 310 
 
 Fission, 71 
 
 Flagellate stage, 61 
 
 Flondeae, 91 
 
 Flower, nature of the, 143; dia- 
 grams, 160; types, 144 
 
 Food of plants, 105 
 
 Food-supply of organisms, 47 
 
 Foramen magnum, 301 
 
 Formic aldehyde, 203 
 
 ' Fructification,' 96 
 
 Fruit, nature of the, 143 ; structure 
 of the, 174 
 
 Fucus, life-history of, 83 ; struc- 
 ture of the reproductive organs of, 
 85 ; structure of the thallus of, 
 84 ; germination of the embryo 
 of, 8 ; differentiation of tissues 
 in, 89 
 
 Fuel, 1 6 
 
 Fundamental tissue, 116, 134, 146, 
 178 
 
 Fungi, characters of, 91 
 
 Funicle, 123, 163 
 
 GALL-BLADDER, 279 
 Gamophyte, 132 
 Gases, solubility of, 42 ; absorption 
 
 of, by the plant, 202 
 Gastric juice, 277, 337 
 Gastruia, 260 
 Gtrgenbaur, 352 
 Genera plantarum, 349 
 Geotropism, 212 
 Gesner, 347 
 Glenoid cavity, 305 
 Globulins, characters of, 26 
 Glomerulus, 296 
 Glycogen, 280 
 Gonophore, 227 
 Gravity, 7, 19 
 Growth, 36; in he plant, 210 ; in 
 
 the animal, 342 
 Gymnosperma;, 142 
 
 TT /ECKEL, 352 
 A Haemoglobin, 21 
 Ha rs, forms of, 155 
 Hailer, 347, 349 
 
35* 
 
 Elementary Biology. 
 
 HEA 
 
 Hearing, structure of the organ of, 
 
 320 
 Heat, nature of, 14 ; mechanical 
 
 equivalent of, 16 ; in the animal, 
 
 343 ; in the plant, 209 ; earth's 
 
 internal, 19 
 Heliotropism, 212 
 Hilaire, S. , 350 
 Histoire Naturelle, 348 
 Hoffmeister, 352 
 Holoblastic segmentation, 329 
 Hooker, Sir J., 352 
 Hotsprings. 18 
 Humboldt, von, 350 
 Huxley, 352 
 Hydra, 219 ; nerve cells of, 229 ; 
 
 microscopic structure of, 224 
 Hydractinia, 221 
 Hydration, 23 
 Hydrogen, origin of, and importance 
 
 to the plant of, 191 
 Hydrozoa, 219 
 Hygroscopic water ^ 201 
 Hyoid, 304 
 Hyphae, 93 
 
 Hypoblast, 231, 248, 331 
 Hypocotyledonary axis, 174 
 Hypoglossal nerve, 311 
 
 TLIUM, 305 
 
 * Individual life, 50 
 
 Inertia, 10 
 
 Infundibulum, 308 
 
 Intervertebral foramina, 310 
 
 Intestinal juice, 338 
 
 Intraceliular digestion, 226 
 
 Inulin, nature and uses of, 156 
 
 Iris, 322 
 
 Iron, origin of, and importance to 
 
 the plant of, 193 
 Irritability of protop'asm, 33 
 Ischium, 306 
 Isomerism, 22 
 
 TUSSIEU, De, 349 
 
 KARYOKINESLS, 78 
 Katabolism, 34 ; products of, 
 in the plant, 208 
 Katastates, 34 
 Kidney, 295 
 Kinetic energy, 12 
 
 MAT 
 
 LACTEALS, 282 
 Lamarck, 350 
 
 Laminaria, 91 
 
 Latex, 182 
 
 Laticiferous vessels, 182 
 
 Leaves, arrangement of, 185 ; 
 types of, 185 
 
 Leeuwenhceck, 348 
 
 Lens of the eye, 324 
 
 Lenticels, 182 
 
 Lieberkuhnian follicles, 278 
 
 Light, nature of, 14 ; effect on orga- 
 nisms of, 45 
 
 Lilium, reproductive organs of, 
 157 ; s ructure of the flower of, 
 158 ; life-history of, 143 ; structure 
 of the root of, 150 ; structure of 
 the leaf of, 152 ; structure of the 
 stem of, 145 ; structure of the 
 floral axis of, 146 
 
 Linnaeus, 348 
 
 Liver, structure of the, 279 
 
 Lumbricus, nephrida of, 236 ; 
 clitellum of, 236 ; protective 
 system of, 242 ; n spiratory 
 system of, 241 ; nervous system 
 of, 244 ; reproductive system of, 
 246 ; ccelomic fluid of, 240 ; 
 structure of the body- wall of, 
 244 ; purificatory system of, 241 ; 
 cuticle of, 244 ; circulatory system 
 of, 239 ; sense organs of, 245 ; 
 circulation of, 240 ; alimentary 
 system of, 236; haemal fluid of, 
 240; differentiation of organs in, 
 234 ; locomotory system of, 242 ; 
 external characters and habits of, 
 233 ; development of the embryo 
 of, 247 ; nutrition of, 238 ; pro- 
 stomium of, 235 ; renal system of, 
 241 
 
 Lungs, structure of, 291 ; develop- 
 ment of, 293 
 
 MAGNESIUM, origin of, and 
 importance to the plant of, 
 
 193 
 
 Mandible, 301, 304 
 Manubrium, 228 
 Manuring, 195 
 Mastication, 336 
 Matter, states of, 10 ; fundamental 
 
 properties of, 7 ; constitution of, 9 
 
Index. 
 
 359 
 
 MAX 
 
 Maxilla, 301, 304 
 
 Medulla oblongata, 308 
 
 Medullary plate, 260 
 
 Medusoid, reproductive organs of, 
 230 ; structure of a, 227 ; nerve- 
 cells in a, 229 ; sense-organs of, 
 230 
 
 Membra na tympani, 321 
 
 Membrane bones, 304 
 
 Meristem, 120 
 
 Meroblastic segmentation, 329 
 
 Mesencephalon. 308 
 
 Mesenteron, 262 
 
 Mesentery ,242, 272 
 
 Mesoblastic somites, 249, 261 
 
 Mesoderm, 230, 244 
 
 Metabol sm of protoplasm, 34 ; in 
 the animal, 340 ; results of, in the 
 plant, 209 
 
 Metacarpals, 305 
 
 Metameric segmentation, 249 
 
 Metaphyta, 64, 65 
 
 Metastasis, 206 
 
 Metatarsals, 306 
 
 Metazoa, 64, 66 
 
 Metenceph^lon, 308 
 
 Mohl, von, 351 
 
 Molecular motion, it 
 
 Molecule, definition of, 8 
 
 Monocotyledon and Dicotyledon 
 compared, 144 
 
 Motion in animals, 342 
 
 Motor nerves, 317 
 
 Mucous membrane, 267 
 
 Miiller, Hermann, 352 
 
 Multicellular organisms, 64 
 
 Muscle, microscopic structure of, 273 
 
 Myelencephalon, 308 
 
 Myotomes, 258 
 
 Myxomycetes, 71 
 
 "^[ ASAL capsule, 302 
 
 *^ Natural law, i ; phenomena, i 
 
 Nectar, 209 ; glands, 184 
 
 Nectocalyx, 227 
 
 Nematocyst, 225 
 
 Nepenthes, 214 
 
 Nephridium, structure of a, 242 
 
 Nervation, 343 
 
 Nerve-cells, 315; structure of, 314; 
 tissue, elements of, 313 ; fibres, 
 medullated and non-medulteted, 
 
 PEC 
 
 Nervous system, function of, 316 ; 
 
 origin of, in Vermes, 232 
 Neurilemma, 315 
 Neurogloea, 313 
 
 Neuromuscular layer of Hydra, 225 
 Nitrogen, origin of, and importance 
 
 to the plant of, 191 
 Nitrogenous compounds, formation 
 
 of, in the plant, 204 
 Non-essential elements in the ash of 
 
 plants, 194 
 
 Notochord, 252 ; origin of the, 262 
 Nucleoli, 31 
 Nucleus, 30 
 Nutrition of the plant, 197 
 
 A, differentiation of zooids 
 in, 222 ; structure of, 220 ; de- 
 velopment of the embryo of, 231 
 
 Occipital segment of the skull, 301 
 
 Ocelli, 230 
 
 Oil glands, 184 
 
 Olfactory lobes, function of, 317 
 
 Omo-sternum, 305 
 
 Ontogeny, 53 
 
 Open fibro-vascular strand, 180 
 
 Optic tracts, 308 ; thalami, 308 ; 
 lobes, 308, 317 ; chiasma, 308 
 
 Organ, definition of an, 37 
 
 Organs, classification of, 51 
 
 Osmosis, 199 
 
 Otic capsule, 302 
 
 Otocysts, 230 
 
 Ovarium, 85 
 
 Oviducts of Rana, 326 ; of Lum- 
 bricus, 246 
 
 Ovules, varieties of, 163 
 
 Oxidation, 23 
 
 Oxygen, absorption of, by the plant, 
 202 ; origin of, and importance to 
 the plant of, 191 
 
 PALATINE, 302 
 Pancreas, structure of, 281 ; 
 secretion of, 338 
 Pandorina, 83 
 Parasitic Fungi, 100 
 Parasphenoid, 302 
 Parenchyma, 102, 148 
 Parieto-frontal, 302 
 Parthenogenesis, 53 
 Pectoral girdle, 304 
 
Elementary Biulogy. 
 
 PEL 
 
 Pelvic girdle, 305 
 
 Penicillium, mycelium of, 93 ; life- 
 history of, 93 ; reproductive 
 organs of, 96 ; compared with 
 Fucus, 97 
 
 Pepsin, 278, 337 
 
 Periblem, 173 
 
 Peri derm, 181 
 
 Perineurium, 314 
 
 Perisarc, 221 
 
 Peristalsis, 277 
 
 Peritoneum, 272 
 
 Phalanges, 305, 306 
 
 Phelloderm, 181 
 
 Phellogen, 181, 182 
 
 Phloem, 118,134, 147, 179 
 
 Phosphorescence in the animal, 344 ; 
 in the plant, 209 
 
 Phosphorus, origin of and import- 
 ance of, to the plant, 192 
 
 Phyllotaxis, 160, 185 
 
 Phylogeny, 53 
 
 Physics, definition of, 4 
 
 Physiology of protoplasm, 31 ; of 
 the plant, 189, 196 ; of the animal, 
 
 335 
 
 Pia mater, 313 
 
 Pineal gland, 308 
 
 Pituitary body, 308 
 
 Placenta, 123 
 
 Planula, 231 
 
 Plasmodium, 59 
 
 Plasmolysis, 200 
 
 Plerome, 173 
 
 Pleuron, 292 
 
 Pliny, 346 
 
 Polar body, 328 
 
 Pollen grains, structure of, 162, 166 ; 
 development of, 169 
 
 Polytrichum, life-history of, 101 ; 
 structure of the sporophyte of, 
 108 ; reproductive organs of, 104 ; 
 structure of the gamophyte of, 
 101 ; structure of the embryo of, 
 ic8 ; compared with Penicillium, 
 112 
 
 Portal circulation, 288 
 
 Postcaval vein, 291 
 
 Potassium, origin of and importance 
 to the plant of, 193 
 
 Potential energy, 12 
 
 Praefoliation, 121 
 
 Precaval vein, 291 
 
 Premaxilla, 303 
 
 RAN 
 
 Pressure, effect of, on organisms, 44 
 
 Proctodaeum, 262 
 
 Prosencephalon, 308 
 
 Prosenchyma, 149 
 
 Protamaoa, life-history of, 55 
 
 Proteids, 26 
 
 Protista, 55, 63 
 
 Protococcus, life-history of, 69 
 
 Protomyxa, life-history of, 57 
 
 Proton ema, 104 
 I Protophyta, 63 
 
 Protoplasm, morphology of, 29 ; 
 varieties of, 38 ; composition of, 
 25 ; physiology of, 31 
 
 Protozoa, 63 
 
 Kseudopodium, 33 
 
 Pteris, structure of the sporangium 
 o f , 122 ; structure of the leaves 
 of, 121 ; germination of the spores 
 of, 126 ; structure of the rhizome 
 of, 114 ; life-history of, 113 ; 
 structure of the roots of, 120; 
 structure and development of the 
 embryo of, 129 ; ovaria of, 128 ; 
 thallusof, 126 ; spermariaof, 127 ; 
 summary of the more important 
 conclusions in regard to, 130 
 
 Pterygoid, 304 
 
 Ptya'lin, 337 
 
 Pubis, 306 
 
 Pulmo-cutaneous artery, 289 
 
 Pulmonary vein, 289 
 
 Punctum vegetation is, 212 
 
 Pupil of the eye, 322 
 
 RADIUS, 305 
 I Kana, intestine of, 277 ; teeth 
 
 of, 267, 270 ; stomach of, 277 ; 
 tongue of, 271 ; skull of, 301 ; re- 
 spiratory system of, 291 ; circu- 
 latory system of, 282 ; alimentary 
 canal of, 265, 272 ; reproductive 
 system of, 326; blood of, 282; 
 heart of, 285 ; external characters 
 of, 265 ; renal system of, 294 ; 
 fore limb of, 305 ; deve-opment of 
 the brain of, 332 ; development of 
 the embryo of, 329 ; metamor- 
 phosis of, 332 ; sperms of, 327 ; 
 ova of, 328 ; spermarium of, 326 ; 
 ovarium of, 326 ; nervous system 
 of, 307 
 Ranunculus, stem of, 178 ; leaf of, 
 
Index. 
 
 RAP 
 
 185 ; root of, 184 ; flower of, 
 
 188 
 
 Raphides, 157 
 Ray, 348 
 Rectum, 279 
 Reflex action, 317 
 Regne Animal, 349 
 Rejuvenescence, 74 
 Renal-portal circulation, 291 
 Reproduction, 35, 52 
 Reproductive organs in Rana, 
 
 development of, 327 
 Resin canals, 183 
 Respiration, 35 ; in the plant, 208 ; 
 
 in the animal, 241 
 Retina, 325 
 Rhizoids, 103 
 Root cap, 120 ; hairs, 121 
 Rotation of crops, 195 ; of the 
 
 earth, 19 
 
 CACCHAROMYCETES, 72 
 
 J Sachs, von , 352 
 
 Saliva, 337 
 
 Salts associated with protoplasm, 
 
 28 ; absorption of, by the plant, 200 
 Saprophytic Fungi, 100 
 Sarracenia, 214 ; structure of the 
 
 leaf of, 216 
 
 S:alariform vessels, 119 
 Scapula, 305 
 Scent of flowers, 169 
 Schizomycetes, 72 
 Schleiden, 349 
 Schultze, Max, 351 
 Schwann, 350 
 Sc'ence, methods of, 2 
 Sciences, 2 ; classification of the, 2 
 Sclerenchyma, 102 
 Sclerotic, 322 
 Secretions, 35 
 Secretory nerves, 317 
 Seed, structure of the, 174 
 Segmentation, 61 ; cavity, 231 
 Sdaginella, leaf of, 134 ; root of, 
 
 135; sporangium of, 135, 141 ; 
 
 cone of, 136 ; spores of , 137, 139 ; 
 
 stem of, 134 ; sporophyte of, 132; 
 
 alternation of generations in, 142; 
 
 embryo of, 141 ; life-history of, 131 
 Self-fertilisation, 168 
 Sensitivity of plants, 213 
 Sensory nerves, 317 : . , 
 
 /^r'V^^" 
 
 TAR 
 
 Sexual differentiation, origin of, GO; 
 reproduction, 52 
 
 Sieve tubes, 119, 150 
 
 Sight, sensation of, 325 ; organ of, 
 322 
 
 Sinus venosus, 285 
 
 Skeleton, 297 
 
 Skin, 268 
 
 Smell, organ of, 320 
 
 Soil, composition of the, 196 
 
 Solar radiation, 19 
 
 Solubility of gases, 42 
 
 Somatopleure, 249 
 
 Somite, 235 
 
 Sorus, 123 
 
 Spectrum analysis, 4, 10 
 
 Sperm arium, 85 
 
 Spermathecae, 247 
 
 Sperms, development of, 247 
 
 Sphenethmoid, 302 
 
 Spinal cord, 252, 310, 313, 317 ; 
 nerves, 310 
 
 Spiral vessels, 119 
 
 Spi'Ogyra, life-history of, 76 
 
 Splanchnopleure, 249 
 
 Sporangium, 109 122, 162, 188 
 
 Spores, 95, no, 124 
 
 Sporophylla, 123, 136, 158, 188 
 
 Sporophyte, 132 
 
 Sprengel, 352 
 
 Squamosal, 304 
 
 Stamen, structure of, 162 ; develop- 
 ment of, 164 
 
 Starch, 155 
 
 Stem, increase in thickness of, 180 
 
 Sternum, 304 
 
 Stolon, 103 
 
 Stomata, 122, 153 
 
 Stomodaeum, 262 
 
 Sulphur, origin of and importance 
 to the plant of, 192 
 
 Supra-s' apula, 305 
 
 Suspensor, 173 
 
 Sympathetic nervous system, 311 
 
 Symphysis, 306 
 
 Synergidae, 170 
 
 Synthesis, 22 
 
 Systema Naturte, 348 
 
 Systemic circulation, 288 
 
 Systole, 73 
 
 TADPOLE, 332 
 Tapetal tissue, 126, 165, 167 
 T^sus, 3o6 
 
36? 
 
 Elementary Biology. 
 
 TAS 
 
 Taste, organ of, 319 
 
 Temperature, effect of, on organ- 
 isms, 43 
 
 Tentaculocyst, 230 
 
 Tetanus, 343 
 
 Thalamencephalon, 308 
 
 Thallus, nature of a, 77 
 
 Theophrastus, 345 
 
 Tibia, 306 
 
 Tides, 17 
 
 Tissue, definition of, 37 
 
 Tooth, structure of a typical, 270 
 
 Touch, sensation of, 319 
 
 Trachea, 293 
 
 Tracheae, 119 
 
 Transition substances associated 
 with protoplasm, 28 
 
 Transpiration, 205 
 
 Tribal life, 52 
 
 Trophic nerves, 317 
 
 -Truncus arteriosus, 285 
 
 Trypsin, 338 
 
 Typhlosole, 237 
 
 TTLNA, 305 
 
 V' Unicellular organisms, 63 
 
 Ureter, 295 
 
 Urinary bladder, 296 
 
 Urine, 296 
 
 Urino-genital duct, 326 
 
 Urostyle, 300 
 
 VACUOLE, 30 + 
 
 Vasa deferentia, 247; effer- 
 entia, 326 
 
 ZYG 
 
 Vegetative multiplication, 35 
 
 Vein, structure of a, 286 
 
 Velum, 228 
 
 Ventricles of the heart, 285 
 
 Vernation, 121 
 
 Vertebra, 299 
 
 Vertebral column, 299 
 
 Vertebrata, characters of, 263 
 
 Vesalius, 346 
 
 Vessels of the plant, 149 
 
 Villi of the intestine, 278 
 
 Virchow, 352 
 
 Vitreous humour, 325 
 
 Volcanoes, 18 
 
 Vomer, 302 
 
 WALLACE, 352 
 Water, absorption of, by the 
 plant, 200; currents, 18 ; per- 
 centage of, in protoplasm, 28 ; 
 power, 18 
 Willoughby, 348 
 Wind currents, 18 
 Wood, 119 
 Work, 12 
 
 VIPHI-STERNUM, 304 
 * Xylem, 119 
 
 VOOID, 221 
 
 ** ' Zygospore, 1 81 
 
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