ai 
 
 Richard M. Holman 
 
 
-55*- 
 
BOTANY 
 
*Appleton\s Scientific ^Primers 
 Edited by J.^ynolds Green,Sc.T>.,F.R.S. 
 
 BOTANY 
 
 BY 
 
 J. REYNOLDS GREEN, Sc.D., F.R.S. 
 
 Downing College, Cambridge 
 
 
 ILLUSTRATIONS 
 
 New Tork 
 
 tAppleton and Company 
 
BIOLOGY 
 LIBfcARY 
 
 Apple ton's Scientific Primers 
 
 Edited by 
 
 J. Reynolds Green, Sc.D. 
 
 BIOLOGY. By Prof. HARVEY 
 
 GIBSON. 
 CHEMISTRY. By Prof. 
 
 W. A. TILDEN. 
 BOTANY. By J. REYNOLDS 
 
 GREEN, Sc.D:, F.R.S. 
 
 N\, 
 
PREFACE 
 
 IN writing this little introduction to the study of a plant 
 I have endeavoured especially to present it to the reader 
 as a living organism. Botany is now regarded as a 
 branch of biology, and is not satisfactorily studied by 
 gathering plants and, after ascertaining their names and 
 the natural orders to which they belong, drying them 
 and putting them away in a cabinet. I have tried to 
 present them as they are engaged in the struggle for 
 existence, and to call my readers' attention not only to 
 their form and structure but especially to what they do 
 in life, and why and how they do it. 
 
 I hope that those who study them by the assistance 
 of this little primer will try to have the living plant 
 under observation whiler they read it. I have not 
 written any detailed scheme of laboratory work, but I 
 hope my readers will be able to construct such a scheme 
 for themselves as they follow the directions for study 
 given in the text. 
 
 I should like to suggest that students should read the 
 Chemistry primer first, to gain some acquaintance with 
 the phenomena underlying the processes of construction 
 and decomposition going on in the plant. It would be 
 well to read the Biology primer also before beginning 
 Botany. 
 
 J. REYNOLDS GREEN. 
 
 CAMBRIDGE, 1909. 
 
 921904 
 
CONTENTS 
 
 CHAP. PAGE 
 
 I. INTRODUCTORY ...... -7 
 
 II. THE EARLY DEVELOPMENT OF A PLANT THE GERMINA- 
 TION OF A DICOTYLEDONOUS SEED 
 
 III. THE FORMATION OF THE ROOT SYSTEM 
 
 IV. THE STRUCTURE OF THE ROOT . 
 
 V. THE CHARACTERISTIC FEATURES OF THE SHOOT . 
 VI. THE CONSTRUCTION OF THE SHOOT SYSTEM 
 VII. THE STRUCTURE OF THE SHOOT . 
 VIII. THE MONOCOTYLEDONOUS PLANT 
 IX. THE FOOD OF PLANTS 
 
 X. THE RESPIRATION OF PLANTS ..... 
 XI. THE EVOLUTION OF THE FORMS OF PLANTS ALG.E 
 
 XII. THE DEVELOPMENT OF THE REPRODUCTIVE PROCESSES IN 
 THE ALG.E ... .... 
 
 XIII. THE ORIGIN OF TERRESTRIAL PLANTS EVOLUTION OF 
 
 MOSSES AND FERNS ... . . 
 
 XIV. REPRODUCTION OF FLOWERING PLANTS VEGETATIVE 
 
 PROPAGATION ....... 
 
 XV. THE INFLORESCENCE AND THE FLOWER 
 XVI. POLLINATION AND ITS MECHANISMS FERTILISATION 
 
 XVII. FORMATION OF THE SEED AND ITS MIGRATION THE 
 FRUIT ........ 
 
 VI 
 
BOTANY 
 
 CHAPTER I 
 
 INTRODUCTORY 
 
 OF all the things we see about us as soon as we escape 
 from the life and surroundings of the town, none is more 
 familiar to us than the common green plant. We 
 tread upon grass and other plants which clothe the 
 earth's surface, we walk under trees, around bushes, and 
 by the sides of hedges, or we wander through more 
 cultivated scenes, enjoying the beauty and fragrance of 
 the well-cared-for garden. In all this wealth of vegeta- 
 tion perhaps, however, one fact sometimes escapes our 
 notice. These plants, trees, shrubs, weeds, or what not 
 are alive. We do not deny this when we hear it said, 
 but the idea is hardly a prominent one in the view we 
 take of things in general. It is based probably on the 
 fact that we do not see the plants move, except as their 
 slender twigs and branches or their numerous leaves are 
 swayed to and fro by the wind, for to our own some- 
 what narrow experience life is so closely connected with' 
 restless change of position or locomotion. Yet if we 
 wish to study plants to learn something more about 
 them than a casual glance can tell us, we must bear in 
 mind these two facts on which their whole story turns : 
 first, they are living creatures ; second, they spend their 
 lives in the same place in which they commenced them. 
 This is true of the greater number of plants we see 
 around us, though there are some exceptions, chiefly 
 
 7 
 
8 BOTANY 
 
 plants which, living in water, are passively moved about 
 by the currents of the stream. 
 
 The fact that a plant is alive and conducts itself as a 
 living organism implies certain things. It must receive 
 suitable and sufficient nourishment; it must possess a 
 certain power of adjusting itself to its surroundings, 
 defending itself against possible dangers and over- 
 coming definite difficulties which these surroundings 
 occasion, and taking advantage of such benefits as are 
 met with in them. It must possess, to at any rate a 
 limited extent, a power of appreciating its relations to 
 such surroundings, of realising variations in certain of 
 them, such as light, moisture, and temperature, that it 
 may adapt itself accordingly. 
 
 The second fact, that it cannot alter its position by 
 moving freely about, makes those requirements more 
 essential. It also demands that it shall be possessed of 
 such a safe attachment to its situation as shall secure 
 it an appropriate position and shall enable it to enjoy 
 undisturbed such advantages as the surroundings offer. 
 Further, it calls for a certain power of adjustment of its 
 various parts to the air above it and the earth in which 
 it is fastened, as changes in both of them are frequent 
 and sometimes violent. As the only sources of nourish- 
 ment possible to it are the air and the soil, together 
 with the water which both contain in constantly vary- 
 ing amount, its construction must be such that the same 
 parts which secure anchorage or support shall be capable 
 of securing supplies of the various materials which ulti- 
 mately become the medium of nourishment. 
 
 A further requirement of every living organism is the 
 need of possessing the means of bearing offspring which 
 shall succeed it in the great scene of nature. To a 
 stationary organism this introduces difficulties from 
 which the readily moving animal is free, but these diffi- 
 culties have been overcome by adaptations to the habit 
 
INTRODUCTORY 
 
 of life which are among the most complicated and the 
 most perfect that nature shows us. 
 
 So the life of a plant shows us conflict and struggle 
 waged against disadvantages of a very formidable 
 nature; a power of appreciating difficulties and of 
 struggling against them ; further, it exhibits a capacity 
 of seizing upon such advantages as present themselves, 
 not only in the air and in the soil, but in relative 
 association and competition with each other. 
 
 We are familiar with the fact that part of an ordinary 
 green plant is embedded in the soil. Such a part is 
 commonly known to us as its 
 root, and we distinguish it in 
 several ways from the part 
 which rises into the air (Fig. i). 
 In the case of plants which live 
 in water we find much the same 
 division of the plant body. 
 There is in their case also a root 
 part, which is not green and 
 which is buried in the soil or 
 mud at the bottom of the 
 water ; there is a part which 
 stretches up into the water, in 
 some cases extending into the 
 air above the surface. We 
 often express this fact by say- 
 ing that the plant is differenti- 
 ated into a root and a shoot. 
 
 This differentiation is a funda- FIG. i. Diagram showing the 
 mental one, for the two parts 
 behave very differently. They 
 always grow in opposite directions, and as these direc- 
 tions are generally upwards and downwards they are 
 spoken of as the ascending and the descending axes of 
 the plant. 
 
 general structure of 
 dicotyledonous plant. 
 
io BOTANY 
 
 We need not at present consider very fully the case 
 of the water-plant, and will therefore examine the 
 relations between the root and shoot in general and 
 the surroundings in which each finds itself. 
 
 The anchorage of the plant is secured by the penetra- 
 tion of the soil by the roots. The advantage thus 
 secured is not obtained without difficulty and even 
 danger. To become fixed in the soil the plant must 
 penetrate it, a process which it can only carry out by 
 its gradual growth. The composition of the soil offers 
 certain difficulties to this penetration: it may be too 
 dense or too powdery, too dry or too wet ; it may be 
 slimy like clay, or very hard and strong. The amount 
 of water in the soil and the degree in which it contains 
 air are also factors which must be taken into account 
 in considering this growth. After a plant has once 
 established itself and secured firm anchorage, it still has 
 to deal with varying conditions of a similar nature, for 
 the character of the soil is very liable to changes, depend- 
 ing on conditions of temperature, weather, and so on. 
 
 Besides the advantage of a firm anchorage, the root 
 depends upon the soil for the supply of certain materials 
 which ultimately aid in some way in its nutritive pro- 
 cesses. Certain minerals are necessary to every green 
 plant, many others are advantageous, some are dele- 
 terious. We are here face to face with dangers and 
 advantages which need adjustment to the plant as it 
 is growing in the soil. Such a struggle can be easily 
 observed. While all plants need compounds of nitrogen, 
 some will only flourish on soil which contains as well a 
 certain, often a large, proportion of chalk, others fail 
 entirely if the chalk is plentiful. It is much the same 
 with other constituents of the soil. 
 
 If a plant is growing in uncongenial surroundings it 
 has but little power of adjustment to them. It conse- 
 quently dies out more or less rapidly. If on the other 
 
INTRODUCTORY n 
 
 hand its environment suits its constitution, it has to 
 adapt its structure to the duty of absorbing from the 
 soil what the latter will afford. So the two duties of 
 anchorage and absorption exist together, and the 
 differentiated root system necessarily discharges both. 
 
 If we turn to inquire what dangers beset the part of 
 the plant we have called the shoot, which grows up 
 into the air and forms a head that is frequently of 
 large size, we find them taking shape in the various 
 atmospheric changes incident to every climate. First 
 of these we may place wind or tempest. As the shoot 
 body grows it must offer more and more resistance to 
 air currents, a resistance which may easily culminate in 
 a violent uprooting of the plant. This involves such a 
 subdivision of the plant body as will allow the wind to 
 penetrate through it without serious disturbance. Here 
 we see one meaning of the tapering boughs and twigs, 
 which become more and more flexible as they become 
 increasingly slender. In the central part of the shoot 
 system they are rigid and can resist the storm; where 
 by their dimensions resistance becomes impracticable we 
 find flexibility, enabling them to bow to the wind often 
 so completely as to place their long axes parallel to the 
 direction in which it is blowing. 
 
 Yet another reason for this continued subdivision of 
 the plant body is found in its relation to the absorp- 
 tion from the soil which we have found associated with 
 the root. The latter is continually absorbing the water 
 of the soil ; after separating from such water the mineral 
 constituents it contains, a very large part indeed of the 
 water is evaporated, and so passes away to the exterior 
 again. To favour such evaporation it is advantageous 
 that the ratio between surface and bulk shall be a large 
 one, and so the great subdivision of the subaerial part 
 of the plant is concerned in solving the problem of its 
 nourishment. 
 
12 BOTANY 
 
 Indirectly the composition of the above-ground part 
 of the plant has a direct application to a danger to which 
 the underground region is exposed. The pressure of 
 the wind upon an unyielding surface in the air would be 
 attended by great danger to the anchoring root, which 
 might be violently pulled from the ground by the 
 leverage exerted by such pressure. The great subdivi- 
 sion of the shoot system and the flexibility of its ultimate 
 twigs minimises this danger, but even as it is, it is not 
 unusual after a tempest to find even sturdy trees up- 
 rooted and thrown down. 
 
 The distribution of the water of rain-storms presents 
 another problem which must be solved by the shoot 
 system. The water can be led either towards or away 
 from the centre of the plant. Should the root system 
 be one which spreads considerably and extends to long 
 distances below the surface of the soil, it is of great 
 importance that the rainfall collected on the central 
 mass of shoots shall be distributed widely so as to reach 
 as far as the extremities of the roots, watering in this 
 way a large area of ground. If the root system consists 
 of a strong main root with comparatively few branches, 
 this arrangement would largely deprive it of water. 
 Hence in plants with roots distributed in this way we 
 find arrangements to conduct the water into the centre 
 of the mass of shoots. 
 
 In some rare cases the duty generally discharged by 
 the root as an anchoring organ falls upon the shoot, 
 which then is partly developed underground. Such a 
 stem bearing in its turn appendages has a special name 
 it is called a rhizome. 
 
 If we pass to a closer study of the much divided or 
 branched shoot we find almost invariably that its ulti- 
 mate twigs put forth certain regularly arranged flattened 
 expansions. In cases where there is much exposure to 
 currents of air, these flattened portions are furnished 
 
INTRODUCTORY 13 
 
 with stalks of variable length which are extremely 
 flexible and allow the flattened organs to sway freely 
 backwards and forwards as the wind blows upon them. 
 These flattened portions, further, are usually of a vivid 
 green colour; they are then known as leaves, or, pre- 
 ferably, foliage leaves. 
 
 As almost all plants possess leaves we may inquire 
 why these organs should so uniformly be thin and flat. 
 
 There are several reasons of almost equal importance. 
 The leaf or other winged part of the shoot portion is in 
 contact or relation with the air only. Interchanges of 
 gases between the air and the leaf are continually going 
 on, and these interchanges are effected most easily and 
 fully with a large extent of surface. No form gives so 
 much surface in proportion to its bulk as a thin flat 
 plate, just such a form indeed as the flattened portion or 
 blade of the leaf. The interchanges include the absorp- 
 tion of particular gases from the air, and the giving out 
 of gases and water vapour. As we shall see later, the 
 internal structure of the leaf-blade is arranged largely 
 with a view to the carrying out of these exchanges. 
 
 A second reason for the flattening of the leaf is 
 concerned with the manufacture of the plant's food. 
 A particular gas known as carbon dioxide, which is taken 
 in from the air, is ultimately built up into a true food 
 material, a kind of sugar. Though the formation of 
 sugar in the plant is not fully understood, it is known to 
 depend upon the presence of the green colouring matter 
 and its being properly illuminated. The flattened form 
 helps to secure the arrangement of the green colouring 
 matter in such a way that the light, either of direct 
 sunshine or of the less bright diffused daylight, may 
 reach it with the least obstruction. 
 
 Yet a third reason may be given. The leaves are 
 very frequently so placed that they extend outwards 
 from the plant and lie nearly parallel to the surface of 
 
14 BOTANY 
 
 the ground. In this way they present their edges to 
 the wind, and offer as little obstacle as possible to its 
 passage through the tree, so making as small as possible 
 the risk of being torn off when the force of the wind is 
 strong. As the wind passes between them they are 
 made to rise and fall, but they offer much less resistance 
 to its force than they would if they were not flattened. 
 
 The arrangements of the plant and its parts so far as 
 we have studied them are such as to secure its firm 
 attachment to the soil, its stability in storms, with rela- 
 tion both to wind and rain. They also make possible 
 the absorption of liquid, containing mineral matters, 
 from the soil ; the evaporation of the excess of water so 
 absorbed ; the free interchange of gases between it and 
 the air; the needed facilities for the manufacture of 
 sugar from the gases absorbed from the air and the 
 water from the soil. They are, in fact, suitable to 
 support and nourish a stationary living organism and 
 to furnish defences against the most evident dangers to 
 which it is exposed. 
 
 The establishment of such a position by the plant 
 is carried out by means of growth alone. It is a gradual 
 process, therefore, and must be accompanied by the 
 nutritive processes which enable growth to take place 
 First among these comes the supply of the material for 
 the increase of size which we associate with growth. 
 We have seen that the plant absorbs from the soil cer- 
 tain mineral compounds dissolved in water, and from 
 the air certain of its constituent gases. The most 
 important of the materials which the earth yields are 
 nitrates of potassium, calcium, and other metals, phos- 
 phates of the same, traces of compounds of iron, a little 
 silica in some combination, together with the water in 
 which they are dissolved ; carbon dioxide is supplied by 
 the air. When the absorption of these substances is 
 possible, and when light is sufficient and temperature 
 
INTRODUCTORY 15 
 
 moderate, the healthy plant is found to increase in size, 
 and gradually to show all the phenomena of growth. 
 Hence these various compounds have been regarded as 
 its food. This is not, however, a correct view, for they 
 should be considered only as raw materials from which 
 the green plant can make the food it needs. This is 
 effected by the agency of the green colouring matter, 
 the so-called chlorophyll, but only when it receives an 
 appropriate amount of light. In the absence of chloro- 
 phyll or in insufficient light the supply of all these 
 various compounds does not afford any nourishment to 
 the plant. Plants without chlorophyll are not far to 
 seek; we find them in the mushrooms, in the moulds 
 that grow so readily on decaying matter, the mildews of 
 corn and other crops, and so on. These cannot develop 
 at all when supplied only with the inorganic compounds 
 mentioned. 
 
 The plant then in order to grow and to establish itself 
 has to be provided with suitable food. If it has chloro- 
 phyll and is properly illuminated it makes this food for 
 itself from the inorganic materials the soil and air pro- 
 vide. Plants which cannot make their food have to 
 obtain it from living or dead organic matter. Though 
 this is difficult it is not impossible, for such matter 
 abounds almost everywhere not only in the soil but in 
 the numerous manufactured products which we meet 
 with all around us. Living organisms also are often 
 made to yield food to these non-green plants. The 
 chlorophyll-containing plants are continuously making 
 the organic substance which constitutes their food as 
 long as light shines upon them. We find them growing 
 at its expense and accumulating large quantities of such 
 substances as sugar, starch, proteins, and fats in their 
 own bodies. As they in their turn, or many of them, 
 ultimately become the food of animals, we may see 
 their importance in the role of nature. The fact is that 
 
16 BOTANY 
 
 the green plant is the only organism which has the 
 power of forming organic substance from the inorganic 
 material of the earth and air. As all living beings are 
 dependent on this organic substance for the maintenance 
 of life, we see how the continuation of life itself upon 
 the earth depends on the activity of the green plant. 
 
 The establishment of the position of the plant and its 
 defence when so established may be seen, therefore, to be 
 subordinate to the manufacture of organic food. 
 
 The food so made is complex in character and will be 
 dealt with in greater detail in a subsequent chapter. 
 It comprises chiefly three classes of substance: carbo- 
 hydrates, of which sugar and starch are representatives, 
 fats, and proteins, which are much more complex in 
 composition, and are represented by the white of egg 
 and by the chief constituent of meat and fish. The 
 proteins are held to be the organic material which 
 most resembles the living substance itself. 
 
 As it is the process of growth at the expense of this 
 newly constructed food, or of a small supply derived 
 directly from its parent, by 
 which the young plant makes 
 its way into its appropriate 
 position, it is clear that this is 
 the action of a living organism 
 and becomes probable that the 
 surroundings of the plant affect 
 FIG. 2. Geotropic curvature j 1 in other ways than by afford- 
 in root and shoot of ing it the material from which 
 
 (After r Gibson.f Ufal ^ to make its food - Careful ob- 
 servation shows that this is the 
 
 case. The root of the plant at even its first appear- 
 ance grows downwards in the direction of the soil. 
 If it be made to point in another direction, its plan 
 of growth slowly changes and it gradually curves 
 till its tip is pointing downwards again (Fig. 2). If 
 
INTRODUCTORY 17 
 
 light reaches it, it bends slowly away from the illu- 
 minating ray; if anything comes into contact with its 
 tip, growth causes it to curve so as to leave the obstacle 
 on one side. The young root shows in these ways 
 certain sensitivities, reacting to the incidents of its 
 environment, and behaving as if it were possessed of 
 rudimentary perceptions of direction, illumination, and 
 contact. Other features of the environment also affect 
 it, particularly moisture. The shoot in its behaviour 
 shows similar phenomena, but its conduct when influ- 
 enced, or, as it is generally called, stimulated, by gravity, 
 light, or other disturbing causes, is as a rule the opposite 
 of that of the root. It grows upwards against gravity; 
 it curves towards and not away from light ; its behaviour 
 with regard to contact is not always uniform. Both 
 parts, however, show what we claimed at the outset as 
 one of its primal necessities, the power of adjusting 
 itself to changes in its surroundings. 
 
 Consideration of the fourth requirement, the power to 
 reproduce itself, must be deferred for the present. 
 
 We may now with advantage turn to the composition 
 of the plant and ask what is the distribution in it of the 
 living matter to which this behaviour is to be attributed. 
 
 It is best to begin the study of this point by examin- 
 ing quite a young plant, or preferably the seed of a 
 plant, as the structure is then simple, 
 while it becomes very complex as the plant 
 grows. If we take a seed (Fig. 3) we find 
 it contains a young plant or embryo, in 
 which bv careful dissection we can make J^****^ ^* 
 
 J , ,, FIG. 3. Section 
 
 out a young root and a young shoot. The O f a seed, 
 shoot consists of a short axis, to which > embryo, 
 are attached either one or two leaves 
 known as cotyledons, with perhaps traces of more 
 leaves above them (Fig. 4). When we cut such a 
 young root or young shoot, we find that it is made up of 
 
 B 
 
i8 
 
 BOTANY 
 
 a large number of very small pieces of living substance, 
 or protoplasm, each separated from its neighbours by a 
 thin membrane or cell wall which 
 surrounds it (Fig. 5). Very fine 
 connecting threads of protoplasm 
 extend through the cell walls and 
 so join the little pieces of proto- 
 plasm together, but these are so 
 delicate that it is not possible 
 
 t0 See the With ut Vef y Skilful 
 
 preparation. 
 
 The living substance thus extends throughout the 
 plant in complete continuity, though it is apparently 
 divided into a number of separate pieces by cell walls or 
 membranes. These serve at the outset only for pur- 
 poses of support, and form a kind of 
 skeleton. Each little piece of proto- 
 plasm contains a small highly organ- 
 ised portion called its nucleus; the 
 whole piece is called a protoplast ; it 
 is approximately cubical in shape and 
 has a diameter of about i-30Ooth of 
 an inch. 
 
 As the little protoplast absorbs 
 water and gets larger, entering into 
 active life, it finds itself in need of 
 constantly renewed supplies of water. 
 Here is its first individual difficulty, 
 for it is only the external cells 
 which can come into contact with 
 the water outside the plant. To 
 overcome this difficulty the proto- 
 plast gradually forms a central cavity 
 in its own substance, in which it 
 holds a store of water. This cavity is known as a 
 vacuole ; it is of the greatest importance in the 
 
 FIG. 5. Vegetable cells. 
 h, celr.wall; p, proto- 
 plasm; k, k, nucleus; 
 s, vacuole. X7oo. 
 (After Sachs.) 
 
GERMINATION OF A SEED 19 
 
 maintenance of the life and the nutrition of the 
 protoplast. 
 
 As the plant gets older and larger a considerable 
 amount of differentiation of its internal substance 
 becomes necessary. This we shall study later. Mean- 
 time we may say that these changes are accompanied 
 by the death of some of the protoplasts. The mem- 
 branes or skeleton of these protoplasts are left in the 
 interior and subserve certain important purposes ; but 
 the protoplasts remain in full vigour towards the 
 exterior and particularly towards the extremities of 
 both shoots and roots, where new formation of them is 
 continually taking place. 
 
 The living substance is thus situated in greatest 
 amount towards the outside of the plant and at its 
 extremities, where its contact with the environment can 
 be most easily maintained. The subordinate mechanisms 
 of its life, which are concerned with its mechanical sup- 
 port and with the efficient working of its body and the 
 co-ordination of its various forces, are hidden away more 
 deeply in its interior. 
 
 CHAPTER II 
 
 THE EARLY DEVELOPMENT OF A PLANT THE GERMINA- 
 TION OF A DICOTYLEDONOUS SEED 
 
 THERE is a great variety in degree of development 
 among the plants which exist upon the earth. The 
 most highly organised of these are the so-called flowering 
 plants, to which most of the terrestrial forms belong. 
 
 These plants have a certain feature in common which 
 distinguishes them from all others. They form seeds, 
 which become separated from the parent and after a 
 period of rest develop into new plants. A seed is essen- 
 tially a very young plant in a dormant or resting 
 
20 BOTANY 
 
 condition, clothed with a separable protective coat, and 
 supplied with a certain quantity of food stored in it or 
 around it by the parent from which it came. In its 
 quiescent condition this young plant is called an embryo. 
 It consists of a young root and a young shoot, the latter 
 being composed of a stem on which are borne a certain 
 number of leaves. These parts are known as the 
 radicle and plumule respectively, the first-formed leaves 
 being called cotyledons. The number of cotyledons 
 varies ; in most cases there are two, in others one, while 
 in others again there may be several. The number of 
 cotyledons is constant throughout large groups of plants 
 and is associated with differences of structure of the 
 other parts of the plant. The first two groups referred 
 to are called Dicotyledons and Monocotyledons. In 
 another group called the Gymnosperms we find a 
 variable number, sometimes as many as fifteen. 
 
 The young embryo is fitted to bear separation from 
 the parent and transport to different situations by the 
 fact that its life is in a dormant state and that it is 
 protected by the skin or testa of the seed. Under 
 appropriate conditions it can resume active life and 
 grow into an adult plant, provision having been made 
 for its nutrition during the early stages of its develop- 
 ment and until it acquires the power of making its own 
 food. This necessary food is prepared by the parent 
 plant and is originally deposited as a relatively bulky 
 mass around the embryo in its early development in a 
 particular cell known as the embryo sac. This food con- 
 stitutes what is known as the endosperm, a collection 
 of cells which fill up all the space in the embryo sac 
 which is not occupied by the embryo. 
 
 The cells of the endosperm with their contents are all 
 provided for the nourishment of the embryo. In some 
 cases the embryo feeds upon this store while very 
 immature and before it assumes its quiescent state. In 
 
GERMINATION OF A SEED 
 
 21 
 
 others its quiescence takes place very early, so that the 
 endosperm remains unabsorbed around it and is not 
 used till the resumption of active life and growth takes 
 place. The difference in the time of this absorption 
 influences the size of the embryo, which is naturally 
 much larger when it has absorbed the endosperm. The 
 food so absorbed is always deposited again in some part 
 of the young embryo, very frequently in the cotyledons 
 which become large and fleshy. Occasionally, as in the 
 Brazil nut, it is stored in the axis of the embryo. . 
 
 When the endosperm persists till the resumption of 
 life by the embryo the process known as 
 the germination of the seed the latter is 
 said to be an albuminous seed (Fig. 6). If 
 the embryo alone is present inside the skin 
 (Fig. 7) it is called exalbuminous. 
 
 It is best to begin the study of these 
 seedbearing plants with the largest group, 
 the Dicotyledons. 
 
 They furnish us with examples of both classes of 
 seeds which are easily accessible and which germinate 
 readily. We may take first the 
 common bean. To examine the 
 seed it is well to soak it for 
 several hours in water, which 
 is absorbed by the skin, so that 
 the whole seed swells and its 
 
 FIG. 7. Embryo of pea parts can be easily separated 
 magnified, r, radicle; nt, f rom one another. The seed is 
 
 plumule; c, cotyledons. . . . , . 
 
 somewhat kidney - shaped, and 
 
 bears on the concave part a scar at the point at which 
 it was attached to the fruit from which it came. A 
 little way from one end of this scar is found an 
 aperture through the skin, known as the micropyle, 
 through which the radicle emerges on germination. It 
 can be localised by gently squeezing the soaked seed, 
 
 a, embryo. 
 
22 BOTANY 
 
 when a drop of water will ooze out of it. On removing 
 the testa the body of the seed is found to consist of 
 a very bulky embryo. The two cotyledons are large 
 masses placed face to face and easily separated from 
 each other. On gently moving them apart each is found 
 to be attached to a very short axis which lies between 
 them and is almost hidden when their faces are in con- 
 tact. The lower end of the axis is the radicle and is 
 bluntly pointed ; the upper end, the plumule, which curls 
 inwards between the cotyledons, bears two minute leaves. 
 
 We may compare with the bean a seed of about the 
 same size, that of the castor oil plant. It must be 
 soaked until it swells, when the hard coat it possesses 
 will crack. On removing the latter a fleshy mass will 
 be seen which cannot be separated into two portions 
 without splitting it. If it is divided into two it will be 
 iound that the embryo plant consists of two very thin 
 flat cotyledons lying in the centre face to face, with the 
 very short axis (plumule and radicle) between them. 
 The fleshy part of the seed surrounds the whole and 
 adheres firmly to the backs of the delicate cotyledons. 
 This mass is the endosperm, which has not been absorbed 
 by the embryo during its early growth. 
 
 If the seed is soaked in alcohol this dissection is easier, 
 as the parts do not then adhere so closely together. 
 
 After a period of variable length the embryo awakes 
 from its quiescent or resting state and develops into a 
 seedling, which goes on to become an adult plant. The 
 quickening into this renewed activity, which is techni- 
 cally called its germination, is only possible when the 
 external conditions become favourable. The process 
 demands moisture, a moderate degree of warmth, and 
 the presence of oxygen. It may be studied easily with 
 a little care, as it can take place in an ordinary room. 
 The -absence of light is not essential, although seeds are 
 usually buried in the soil before they germinate. 
 
GERMINATION OF A SEED 23 
 
 Having soaked a bean for several hours till it has 
 become swollen, remove it from the water and keep it 
 on damp boiled sawdust or in some moist situation in 
 an ordinary room for some days. After a short time the 
 young radicle will be found to protrude from the micro- 
 pyle and to grow downwards. The cotyledons swell 
 and the testa cracks and begins to slip off. The plumule, 
 which was seen to be curved inwards, elongates ; the 
 curvature becomes more marked and forms a loop 
 which emerges from between the cotyledons ; it finally 
 straightens itself and thenceforward grows vertically 
 upwards. This loop is formed from the part just 
 above the cotyledons which is known as the epicotyl. 
 The cotyledons remain much as they were, but as the 
 seedling grows their contents are gradually absorbed 
 by the axis and they shrivel away. In their normal 
 development when the seed is below the surface of the 
 earth the cotyledons remain buried. The advantage of 
 the looped epicotyl is seen as it presses upward through 
 the layer of soil above the seed, for the delicate leaves 
 of the plumule are saved from the injury which they 
 would suffer if they had to force their way through the 
 earth. The epicotyl in fact opens a passage for them. 
 
 Some seeds whose structure is almost identical with 
 that of the bean behave a little differently in germina- 
 tion. The part of the axis which elongates and brings 
 the plumule through the soil is a region a little below the 
 cotyledons, and it is consequently called the hypocotyl. 
 The lengthening of this part causes the cotyledons also 
 to be carried up into the air, and after a short time they 
 turn green, and take on the work of the foliage leaves 
 which are developed as the plumule grows. 
 
 When the castor oil seed germinates the early stages 
 are much the same as in those of the bean. The seed 
 swells and the radicle grows through the micropyle, and 
 very_soon the young root branches freely. The endo- 
 
24 BOTANY 
 
 sperm swells and the flat cotyledons which remain in 
 contact with it begin to absorb the contents of its 
 cells. The face of the endosperm becomes very slimy 
 or mucilaginous and it continues to swell for some 
 days, ultimately cracking and being loosely attached 
 to the absorbing cotyledons. The hypocotyl grows up 
 in the form of a loop and drags the cotyledons out of 
 the soil with the endosperm clinging to them. They 
 very quickly change colour, becoming yellow and ulti- 
 mately green, and as the last traces of the endosperm 
 are used up they grow out laterally and take on the 
 appearance and the function of foliage leaves. 
 
 CHAPTER III 
 
 THE FORMATION OF THE ROOT SYSTEM 
 
 THE seeds just described are very useful for observing 
 also the growth and development of the seedling. Even 
 better material for this purpose is supplied by the seeds 
 of the common cress. If several of these seeds are 
 soaked in water and then scattered over the inside of a 
 damp flower pot they will germinate very freely if the 
 pot is kept moist and moderately warm, putting out 
 their roots in a few hours. As they will have been sown 
 quite indiscriminately, their positions will be irregular 
 and the young rootlets will emerge at first in very 
 different directions. If they are allowed to remain un- 
 disturbed as they elongate they invariably manage to- 
 direct their apices downwards, effecting sometimes 
 curious curvatures to do so. This strange uniformity 
 of behaviour suggests that the young seedling has a kind 
 of appreciation of its position or the direction of its 
 growth. We can test this suggestion by taking several 
 of them from the positions they have assumed and 
 placing them so that their roots are at different angles 
 
THE FORMATION OF THE ROOT SYSTEM 25 
 
 with the vertical. So long as they are intact, they 
 gradually modify their growth so as to make their apices 
 again point vertically downwards (Fig. 8). 
 
 If we study the behaviour of the roots under various 
 conditions we soon find that they manifest other forms 
 of sensitiveness, all of which are brought to bear upon 
 the problem of establishing themselves in the soil. 
 When a root enters the latter and passes between the 
 particles which compose it, it must sooner or later 
 come into contact with some of them, and not improb- 
 ably such contact will hinder the advance of the root 
 in a straight or nearly straight 
 line. The growth of the root is 
 achieved by its advancing in a 
 kind of corkscrew fashion, the 
 tip describing a spiral rather 
 than a straight line. This no 
 doubt tends to push aside slight 
 
 obstacles which may meet the FIG. 8. Geotropic curvature in 
 
 advancing tip. If we experi- root and shoot of mustard. 
 
 ment upon a seedling bean, size '> (After 
 
 which we have seen can be culti- 
 vated in moist air, we can imitate the conditions met 
 with in the soil by attaching some small piece of a hard 
 substance to one side of the root tip, using a little gum 
 as the attaching medium. By this treatment we can 
 ensure that the contact shall be prolonged, and hence 
 the struggle between the root and the obstacle will be 
 carried to such a point as to exhibit very striking effects. 
 After a short time the growing region of the root, which 
 is some little distance behind the apex, will be observed 
 to curve in such a way as to turn the tip from the object 
 touching it. As the pressure is not removed under the 
 conditions of the experiment, this curvature will become 
 very pronounced and after a day or two the root will be 
 curled into a loop. In the soil so pronounced a curva- 
 
26 BOTANY 
 
 ture is not met with, as a slight change in the direction 
 of growth causes the root to grow past the obstructing 
 body, and then the downward direction is resumed. 
 
 We can thus show that the young root has not 
 only an appreciation of direction, but it can in some 
 way recognise when it is in contact with some solid 
 obstacle and that it can modify its 
 growth with a view to getting past such 
 a body and penetrating further into 
 the soil. 
 
 The root further appreciates the in- 
 cidence of a lateral light. If the seed- 
 ling is cultivated in a glass vessel and 
 so placed that light reaches it only on 
 one side it very quickly modifies its 
 growth so that the apex becomes turned 
 away from the light. In the soil this 
 behaviour brings it closer to the par- 
 ticles of the soil, especially a little way 
 behind the tip. These three rudimen- 
 tary senses or sensitivities are supple- 
 nted by a fourth. It shows an 
 evident appreciation of the presence 
 of moisture, and grows towards the dampest parts of the 
 medium in which it is placed. 
 
 If we revert for a moment to the young cress seedling 
 we find that when it has attained the length of about 
 half an inch a number of long delicate outgrowths of 
 its surface may be seen arranged in a broad band all 
 round the root at a little distance behind the apex (Fig. 
 9). So long as the root grows this band of outgrowths, 
 which are known as root hairs, is maintained. New ones 
 are formed on the side of the apex while the older ones 
 die and disappear on the hinder margin of the band. 
 As the root advances in the soil these hairs become so 
 closely attached to its particles that they cannot be 
 
THE FORMATION OF THE ROOT SYSTEM 27 
 
 separated mechanically. While they thus aid materi- 
 ally in attaching the root to the soil, they carry on the 
 absorption of the water of the soil with the mineral 
 compounds dissolved therein. 
 
 It is customary to consider the influences we have 
 spoken of, gravitation, contact, light, and moisture, as 
 stimuli, and to speak of the behaviour of the root as 
 response to stimulation. The power of receiving stimu- 
 lation indicates the possession of special sensitiveness, 
 and its response is to a large extent under the control 
 of the living root. The movements or alterations of 
 growth are purposeful, and lead us to look upon the 
 latter as a living sensitive organism engaged in the task 
 of making the best of its surroundings and varying its 
 behaviour as the surroundings change. 
 
 Seeing the very purposeful behaviour of the root we 
 may pause to ask what is the most potent factor in the 
 growth, or what is the determining influence which 
 causes it to penetrate the ground. The fact that 
 stability of position is secured strikes us at once, but it 
 is doubtful if this is the first consideration. 
 
 We may dismiss the responses to the stimulation of 
 light and contact. They are accessory to the effort of 
 the plant to come into close relatio'n with the soil, but 
 they by themselves do not minister to any of its needs. 
 The behaviour of the root suggests that it is seeking 
 something which the long experience of the race has 
 shown to be advantageous and which has now become 
 hereditary in the plant. The object of this search is 
 the water which the soil contains, which is present as 
 delicate films surrounding the particles of which it is 
 composed. Inherited experience has shown to the 
 vegetable organism that the soil is the source of water, 
 and its instinctive efforts are directed to the securing of 
 a position leading to an adequate supply. 
 
 The stimulus of gravity, therefore, or the perception 
 
28 
 
 BOTANY 
 
 of direction, indicates to the root the whereabouts of the 
 water which it needs. The perception of water aids that 
 of direction and under normal conditions the two co- 
 operate. If, however, there be no water in the soil, the 
 inherited instincts of the plant lead it to penetrate even 
 the driest sand. 
 
 If the plant is in such a position that the two stimu- 
 lations do not co-operate, but are antagonistic to each 
 other, the chief instinct of the plant becomes evident, 
 and it can be shown that its great object is the coming 
 into relationship with water rather 
 than with soil. 
 
 If some seedlings are allowed to 
 grow on a sieve which is covered 
 by a layer of moss they will at the 
 outset put out their roots through 
 the holes of the sieve and grow 
 downwards in a normal way, seek- 
 ing as their inherited instincts tell 
 them the soil which should nor- 
 mally be situated below them. If 
 the sieve is suspended over a pan of water, so that moist 
 air is below the roots, they keep on growing downwards as 
 if growing into earth. But if the conditions be changed 
 after the roots have attained a length of, say, half an 
 inch, the air below them being made very dry by artificial 
 means, while the moss in the sieve is kept well wetted, the 
 roots soon curve upwards and growing in opposition to 
 gravity turn towards the water (Fig. 10). They appear 
 to recognise that their original instinct is deceiving 
 them and that the true habitat for them is for some 
 reason above and not below them. If after they have 
 established this new direction of growth the conditions 
 be again changed and moisture be restored to the air 
 below them while the moss is allowed to dry, another 
 reversal of the direction of growth takes place and 
 
 FIG. 10. Hvdrotropism 
 (After Gibson.) 
 
THE FORMATION OF THE ROOT SYSTEM 29 
 
 again the position of the water determines this direction. 
 
 The behaviour of the root thus shows it to possess 
 certain tendencies which are based upon inheritance of 
 the accumulated experience of the race to which it 
 belongs, but which are controlled by certain sensitivities 
 which are its own personal possession. These sensi- 
 tivities are no doubt hereditary also. 
 
 The power of appreciating the influence of these 
 various stimulating influences has been found to be 
 confined to a very small region of the root, extending 
 about one-tenth of an inch from the apex. This region, 
 which may be called the root tip, may consequently 
 be regarded as a rudimentary sense organ. There 
 is, however, nothing in its structure to mark it off 
 from the region further back. The part receiving 
 the stimulus is not the part which becomes curved 
 in the act of responding. The latter is the region of 
 active growth, where the cells are undergoing elonga- 
 tion. The cells at the tip only retain their sensitiveness 
 for a short time. When new cells are formed in front of 
 them in the process of elongation these are found to be 
 sensitive, and the original ones, passing into the region 
 of active growth, lose the power of appreciating stimula- 
 tion. There is thus no permanent sense-organ in the 
 root. The protoplasm is sensitive at some particular 
 stage of its development, and, having passed that stage, 
 loses its power of appreciating these stimulating changes. 
 
 The way in which the stimulus received at the tip 
 causes a modification of the growth of the cells some 
 little distance farther back is not at present understood. 
 Something in the nature of a nervous impulse is thought 
 to be transmitted from the one region to the other, 
 passing along the delicate threads of protoplasm which 
 extend through the separating walls of the cells and put 
 all the cells in communication with one another. 
 
 We noticed in studying certain seedlings, especially 
 
30 BOTANY 
 
 those of the castor oil plant, that the root does not 
 remain single but very speedily begins to give off 
 branches. By this process of branching a very large 
 root system is made possible. The main root of Di- 
 cotyledons usually persists and remains longer and 
 stronger than its branches. Such a main root is called 
 a tap root. The branches in turn branch and we get 
 roots of the second, third, and higher orders. If we 
 trace the formation of these roots, as we can do by 
 cultivating a seedling in water, or a dilute solution of 
 the necessary mineral compounds, we find that they 
 arise in constant succession as the main root grows, 
 the youngest thus being always nearest the growing 
 point of the main root. Each branch root has the same 
 appearance as the one from which it springs, and 
 similarly bears near its apex a band or zone of root 
 hairs. The branches orginate in the interior of the old 
 root and bore their way outwards. They arise in 
 definite positions, in relations to certain internal 
 structures which will be discussed a little later. 
 
 The branches are sensitive to the same stimuli as 
 the main root, but they respond rather differently to 
 the action of gravity. Instead of growing vertically 
 downwards, the first branches stand out nearly at right 
 angles to the main root and persist in growing in this 
 direction. The branches which in their turn they bear 
 do not grow in such definite positions, but extend 
 symmetrically round the one from which they spring. 
 If by accident the main root is killed, its place is taken 
 by one of its strongest branches, which alters its response 
 to gravity and grows vertically downwards. 
 
 By this course of development the root system of a 
 plant comes to occupy considerable space in the earth 
 and to fill the interstices of the latter very completely. 
 Two advantages are thus secured: a very firm grip of 
 the soil is secured by the attachment of the root hairs 
 
THE STRUCTURE OF THE ROOT 31 
 
 of the numerous rootlets, spread through so much of 
 the earth, aided very conspicuously by the large net- 
 work which the branches form; and a very large area 
 of water-covered particles is tapped by the absorbing 
 root hairs the rootlets bear. 
 
 As the system gets older, not only is it continually 
 enlarged by the increased branching, but the individual 
 roots and branches increase in girth and press more and 
 more firmly into the soil. They penetrate very deeply 
 and extend laterally very widely, so that with the in- 
 creasing size of the above-ground portion of the plant a 
 firmer and firmer anchorage is afforded, securing the 
 needed stability. 
 
 CHAPTER IV 
 
 THE STRUCTURE OF THE ROOT 
 
 THE internal structure of the root can be properly 
 understood only when it is studied from the point of 
 view of the work which the root has to do. At its first 
 emergence from the seed its substance is composed of a 
 large number of the vegetable cells which we have 
 described, each a little mass of protoplasm separated 
 from its neighbours by delicate cell walls. They are in 
 close contact with each other at all points and have no 
 cavities in them. The chief difference in the mass is 
 that the external cells at the apex form a kind of cap 
 over the tip of the radicle, so that its actual apex is not 
 exposed. This cap protects the true apex from damage 
 as it penetrates into the soil. When the radicle has 
 begun to elongate changes in the cells are set up. If a 
 longitudinal section of it (Fig. n) is examined these 
 changes will be seen to separate the young root into 
 roughly three areas. The cap can be seen in front, a 
 short region behind it shows the cells small and actively 
 
BOTANY 
 
 dividing, so increasing their number, and a longer part 
 still further back is marked by the enlargement of the 
 
 cells in all directions, but 
 most notably longitudinally, 
 while their vacuoles are being 
 formed. These regions are 
 known as the root cap, the 
 region of cell division, and 
 the region of cell growth. 
 Little more can be distin- 
 guished at this stage. 
 
 A little later, when the ex- 
 ternal band of root hairs 
 appears, preparation for the 
 discharge of particular duties 
 by the different parts begins 
 to be indicated, while the 
 requirements of the life of 
 the organ involve further 
 adaptations. The first of 
 these is the admission of air 
 to the interior to supply the 
 oxygen all living substance 
 needs to breathe. The com- 
 mencement of the formation 
 of an aerating mechanism 
 can be traced all through the 
 young embryo, even at this 
 age ; as seen in an older root 
 it consists of the splitting of 
 the cells apart from one 
 another at some point of 
 each, frequently at the angles 
 their walls make with each 
 other (Fig. 12). These little splittings make a number 
 of spaces between particular cells, and as growth goes on 
 
 FIG. ii. Longitudinal section of 
 young root. Xao. 
 
THE STRUCTURE OF THE ROOT 33 
 
 these separate spaces become united, so that intercellular 
 passages run among the cells of every region, being of 
 different dimensions in different areas. As we shall see 
 later these passages become open to the exterior in the 
 upper portion of the plant and so enable air to enter and 
 circulate in the interior of the tissues. 
 
 enpeph x p x px 
 
 PIG. 12. Section of central part of root. In the outer region the cells 
 are separated in places by the intercellular spaces. en, endo- 
 dermis; pe, pericycle; ph, phloem strand; p, pith; x, xylem 
 strand; px, protoxylem. Xioo. (After Kny.) 
 
 A longitudinal section of the root taken at this age 
 will show that beside the longitudinal areas or regions 
 already remarked the internal tissue is beginning to be 
 differentiated in another direction. The section of the 
 root is almost conical, but the apex of the cone can be 
 divided into three layers, each of which is continued 
 backwards alone: the axis. At the apex each layer can 
 
 c 
 
34 BOTANY 
 
 be recognised in the zone of cell division. The cells of 
 these layers can divide and they are called in conse- 
 quence meristematic layers. The outermost, which is 
 known as the dermatogen, forms the root cap, and ex- 
 tending backwards gives rise also to the outermost 
 layer of the root from which the root hairs grow. The 
 central one forms a more or less well-marked cylinder 
 or core, which is known in the meristematic region as 
 the plerome, while the intermediate one is called the 
 periblem, and forms the part of the root that lies 
 between the central cylinder and the external layer. As 
 we trace these further backwards we find that the 
 central cylinder becomes very clearly marked off from 
 the rest by a peculiar layer called the endodermis. 
 
 The root hairs are long slender outgrowths of the cells 
 of the outer layer, which when past the meristematic 
 region is known as the pili/erous layer, or epiblema. 
 Each hair has a thin wall of cellulose, which is brought 
 into close contact with particles of soil as it grows in 
 among them. On coming into contact with these 
 particles the outer layers of its walls become changed 
 into a kind of mucilage, which makes the hair adhere 
 very closely to the soil. The film of water which sur- 
 rounds the particles is then absorbed by the root hair. 
 As there are enormous numbers of these hairs on the 
 young root, there is soon a great increase in the water 
 which the root contains. This water passes on from 
 the hairs into the second region of the root, now called 
 cortex instead of periblem, and gradually makes its cells 
 extremely swollen or turgid thereby. 
 
 The special mechanism for carrying this water 
 from the root to the upper parts of the plant is by this 
 time beginning to appear. It lies in the central region, 
 now partly shut off from the rest by the endodermis. 
 Here the growth of the cells is such as to cause them 
 to become elongated. Certain special areas of these 
 
THE STRUCTURE OF THE ROOT 
 
 35 
 
 elongated cells form a definite number of columns of 
 cells which can be traced separately upwards. They 
 are fitted especially to transport water by changes in 
 the constitution of their cell walls, which become 
 gradually changed from cellulose to lignin, the latter 
 enabling water to pass through it in all directions with 
 great ease. At the same time the horizontal walls of 
 
 st st h TV C' 
 
 FIG. 13. Longitudinal section through a vascular bundle of a stem. 
 5, s', p, p. different types of wood vessel; w, wood fibres; st, sieve 
 tubes; ph, bast fibres; p', pith; c, cambium. 
 
 these cells in great part disappear, so that the columns 
 of cells become changed into hollow tubes, or vessels, 
 while their side walls are irregularly thickened by the 
 deposit of more cell-wall substance upon them in 
 particular areas. On account of the presence of these 
 vessels, the collections are known as vascular strands or 
 vascular bundles (Fig. 13). In the root they are com- 
 posed, entirely of lignified cells and are therefore called 
 wood or xylem bundles, to distinguish them from other 
 vascular strands lying near them. The number of these 
 
36 BOTANY 
 
 strands varies in' different roots ; it is very common to 
 find four, but two is not an infrequent number. They 
 may extend completely to the centre and all unite there 
 to form a solid cylinder. If the number is large they 
 .generally fuse together before extending so far, leaving 
 a small-celled column as a core. This is known as a 
 -pith. In form the bundles are wedge-shaped, the apex 
 of the wedge pointing outwards. 
 
 If we trace these conducting strands towards the tip 
 of the root they can be distinguished among the soft 
 cells of the plerome by their narrow diameters and their 
 tendency to elongation. The area of each embryonic 
 strand can be seen distinctly in a transverse section, 
 their small size and a certain density of their protoplasm 
 marking them off from their neighbours. The gradual 
 change from these cells to the mature forms can be 
 traced; the alteration of the wall and its thickening 
 appear first along the outer edge of the wedge, known 
 consequently as the protoxylem, and extending thence 
 towards the centre of the root. 
 
 If these vascular bundles are traced along the root in 
 the direction opposite to the tip they are seen to be 
 continuous with similar structures in the stem. In this 
 way a path is made throughout the plant for the trans- 
 port of the water after its absorption. 
 
 These strands are chiefly concerned with the func- 
 tion of the root. Others which also are traceable 
 throughout the plant can be seen to lie one between 
 each pair of them in the central cylinder. These are 
 chiefly concerned in the nutrition of the root. They 
 are equally well defined and lie side by side with the 
 wood strands, separated from them by a few packing 
 cells. They differ in texture, their walls remaining 
 cellulose. They are known as bast or phloem ; and are 
 made up of vessels known as sieve tubes from . their 
 terminal walls being somewhat thickened and perforated 
 
THE STRUCTURE OF THE ROOT 
 
 37 
 
 by a number of holes, so that their protoplasm is con- 
 tinuous (Fig. 13). With the sieve tubes are a certain 
 number of slightly elongated cells of the ordinary type. 
 The bast and wood strands are thus seen to occupy, 
 with a little supporting tissue, almost the whole central 
 cylinder of the root (Fig. 14). There is always an outer 
 
 FIG. 14. Section of central part of root, b, bast strands; w, wood 
 bundles. Xioo. (After Kny.) 
 
 continuous sheath over the whole, one cell thick as a 
 rule, which is called the pericycle. Outside the peri- 
 cycle comes the endodermis. 
 
 The endodermis forms a sheath, one cell thick, round 
 the central cylinder. Its walls in some cases become 
 uniformly thickened and lignified. In others the 
 outer and inner walls remain thin, while the side walls 
 become changed in a different way. The cellulose is 
 replaced by another material which resists r the passage 
 
38 BOTANY 
 
 of water through it, so that the water of the cortex can 
 pass directly to the wood strands, but cannot pass from 
 one endodermal cell to another, being prevented by 
 bands of a cuticularised substance that pass round the 
 radial walls (Fig. 15). By their interlocking together 
 they make the endodermis separate the intercellular 
 passages of the cortex from those of the cylinder, so that 
 air cannot penetrate directly to the latter. 
 
 As the root grows older and larger and the upper part 
 or shoot system of the plant develops to a corresponding 
 extent, this primary structure becomes insufficient for 
 
 its requirements. They call 
 for a greater amount of 
 conducting tissue as the 
 branches and leaves of the 
 shoot multiply, for all the 
 latter need a supply of 
 water. The stability of 
 the whole structure needs 
 
 pericycie. strengthening, in view of the 
 
 greater size being acquired 
 
 above ground. There is, as we have seen, a great growth 
 in thickness of the root and the development of a 
 system of branches, each behaving like the parent root. 
 In the stage we have examined the young root shows 
 no provision for this increase of thickness. It can take 
 place only by the formation of new cells, and such forma- 
 tion is not going on except at the apical meristem. A 
 new departure has accordingly to be made (Fig. 16). 
 It begins by a curved band of cells of the supporting 
 tissue lying in front of each strand of bast becoming 
 meristematic, beginning to divide by walls which are 
 parallel in direction with the circumference of the root. 
 These tangential divisions cause the formation of several 
 rows of cells, one of which, the nearest to the bast, 
 retains the power of division and is called cambium. 
 
THE STRUCTURE OF THE ROOT 
 
 39 
 
 The newly formed cells become converted into wood, 
 so that a strand of wood, called secondary wood, is 
 formed inside each bast bundle. The cambium layer 
 
 FIG. 1 6. Thickening of root ; px, primary wood ; .SAT, secondary 
 wood; c, cambium; en, endodermis. X8o. (After Kny.) 
 
 extends laterally round the bast bundle, so that it tends 
 to pass up towards the outer edge of the wood bundle on 
 each side. By the time a little mass of wood has thus 
 been formed between each bast strand and the centre 
 of the root, the cells of the pericycle outside the 
 wood bundles divide by similar tangential walls, so 
 
40 BOTANY 
 
 that the pericycle at these points becomes several cells 
 thick. The innermost of these cells, lying in contact 
 with the protoxylem, become cambium, and soon extend 
 to unite the two bands of cambium approaching them 
 from the two bast strands between which the bundle of 
 wood is lying, so that a complete ring of cambium is 
 formed. At first it is necessarily sinuous or wavy, but 
 as more and more wood is formed inside the bast masses 
 it is pushed further and further outwards there, till the 
 waviness of the ring disappears. This cambium ring 
 then continues to add more and more wood in the same 
 way to the secondary wood already formed. Behind 
 the protoxylem groups, which form the outer edge of 
 the primary wood bundles, no secondary wood is formed, 
 but only rows of thin-walled cells; consequently the 
 secondary wood is divided into separate masses by 
 these rows of cells, which are known as medullary rays. 
 They are formed with a view to the transport of food 
 substances from the bast into the interior of the wood. 
 
 The cambium produces a little secondary bast out- 
 side the ring in the same way as it forms wood inside it, 
 but the quantity of bast is much less than that of wood. 
 This is natural, as the bast has only to provide a path 
 of transit for the actual food of the root cells, while the 
 wood has to furnish a continually increasing amount of 
 water-transporting tissue. 
 
 This woody formation in the centre of the root is dis- 
 posed very advantageously for maintaining its stability. 
 A structure with a hard central core is the most suitable 
 to resist such a vertical pull as would cause uprooting. 
 This vertical pull is continually being made by the 
 movement of the storm-tossed upper region of such a 
 structure as a tree. 
 
 The young root as it increases in thickness in the soil 
 encounters two dangers, one internal, the other external. 
 The process of thickening compresses very severely 
 
THE STRUCTURE OF THE ROOT 41 
 
 its more external layers and in time ruptures them- 
 The pressure of wet soil against its epiblema is not 
 unlikely to set up decay. The cortical tissues and the 
 epiblema are therefore inadequate to protect the gradu- 
 ally thickening central cylinder. But these difficulties 
 become obviated as the growth proceeds. By the time 
 the central cylinder has become only slightly thickened 
 the zone of the root hairs has been removed to some 
 distance in advance, by the continuous elongation of the 
 root. The cortex of the thickened part is consequently 
 not supplied with water as before, and ceases to play its 
 original part in transporting the water upwards. The 
 hairs having disappeared from that region too, the 
 epiblema has not its first importance there. The pres- 
 sure of the gradually increasing girth stimulates the 
 cells of the pericycle and they again show the power of 
 increasing by tangential divisions. The pericycle be- 
 comes uniformly several cells thick, one layer of which 
 remains meristematic. It cuts off repeatedly bands or 
 shells of cells which remain very regular in shape,, 
 appearing in transverse sections like rows of bricks. 
 The outermost ones lose their contents and their walls 
 are transformed into suberin, a substance closely re- 
 sembling the cuticularised material of the endodermis. 
 This band of cells forms what is known as a cork layer. 
 It extends completely round the root and forms a 
 strongly protecting sheath. It is perforated here and 
 there by little rounded masses of cells loosely arranged 1 
 so that air can pass between them. These are known as 
 lenticels ; they serve to admit air to the interior of the 
 root. It is quite impervious to water except at these 
 spots, and hence preserves the root from loss of water by 
 outward leakage. The cells of the cortex and epiblema 
 may now rot away without causing any damage to the 
 root. The latter acquires, in fact, a fresh exterior of 
 a more resistant and permanent character than the 
 
BOTANY 
 
 PIG. 17. Transverse section of root to show 
 a rootlet at two stages of development. 
 rh, root hairs ; ec, cortex ; d, cells in pro- 
 cess of absorption; en, endodermis; pe, 
 pericycle; co, conjunctive tissue; ph, 
 bast ; g, cambium ; x, wood ; c, derma- 
 togen of rootlet ; p, its periblem ; pi, its 
 plerome. (After Scott.) 
 
 original one. This 
 corky formation con- 
 tinues as long as the 
 root lives and adapts 
 itself to its increasing 
 girth. Its outer part 
 is composed of dead 
 cells, and together 
 with the remains of 
 the layers originally 
 outside it, constitutes 
 the bark of the root. 
 The cortex and epi- 
 blema continue for a 
 very short time, so 
 that in an old root the 
 bark consists of peri- 
 cycle tissue and layers 
 of cork. 
 
 We must again re- 
 turn to the young root 
 to trace the manner 
 of formation of its 
 branches. The latter 
 originate when it is 
 quite young, as we 
 have seen already. 
 They arise in the peri- 
 cycle, in very many 
 cases opposite to the 
 protoxylem of each 
 wood bundle, gener- 
 ally before the strands 
 are lignified through- 
 out. There are con- 
 sequently usually as 
 
THE SHOOT 43 
 
 many rows of lateral roots as there are wood strands. 
 A little group of the cells become marked out by be- 
 coming meristematic, and dividing chiefly by tangential 
 walls, so that soon a little mass seems to be growing 
 outwards. It can shortly afterwards be seen to have a 
 central plerome covered by a periblem and dermatogen, 
 which behave just like those of the parent root. The 
 cells of the cortex which lie in front of the new root 
 branch are gradually digested and eaten by the latter as 
 it grows outwards and finally penetrates to the exterior 
 (Fig. 17). 
 
 The cells of the root cap are continually being worn 
 away by contact with the soil. The cap is added to all 
 the while by the dermatogen behind it. 
 
 CHAPTER V 
 
 THE CHARACTERISTIC FEATURES OF THE SHOOT 
 
 THE work which falls upon the shoot portion of the 
 plant is very different from that discharged by the roots, 
 being very largely the construction of the organic sub- 
 stance which serves as food, not only for the plant itself 
 but for the world in general. To understand this con- 
 struction we must consider the absorption of carbon 
 dioxide, the utilisation of certain amounts of the water 
 and mineral constituents furnished by the roots, and 
 the evaporation of the surplus water. The work in- 
 volves certain minor or subordinate duties connected 
 with the distribution of the food after its formation. 
 
 The important questions of the breathing of the plant 
 and the maintenance of a suitable temperature in its 
 different parts must also engage our attention. 
 
 The form and composition of the shoot need careful 
 study from these points of view, but these are not all. 
 The relation of its structure, internal as well as external, 
 
44 BOTANY 
 
 to its stationary position, and the difficulties and 
 dangers which the latter presents, must be considered. 
 The adaptations which it shows and the changes of 
 climate which it meets are of great importance. 
 Finally, we have the relation of the shoot system to the 
 processes of reproduction. 
 
 When the young shoot has emerged from the seed and 
 made its way into the air in the ways already described, 
 the bent or hooked form gradually changes till an up- 
 right position is attained. We have already examined 
 the behaviour of the young root, noting its perception of 
 direction and its modification of its growth if necessary, 
 till it can make its way vertically downwards. The 
 same appreciation of direction is exhibited by the young 
 shoot and its behaviour is very similar, with the im- 
 portant difference, however, that it seeks the light and 
 air and hence grows vertically upwards. We cannot 
 explain this difference except by recognising the pur- 
 poseful character of its response to the influence of 
 gravity. There is no difference in the growing cells, so- 
 far as we can see, for they have all practically the same 
 structure whether they are in root or shoot. We see in 
 this behaviour really a living organism trying in a. 
 limited way to make the best of the circumstances in 
 which it finds itself. As we continue to study it we 
 shall be able to ascertain that it possesses the same 
 sensitivities and powers of response to changes in its 
 surroundings that we have found exhibited by the root. 
 
 The growth of the shoot, however, is a much more 
 complicated process than that of the root, in conse- 
 quence of its more manifold duties, which have called 
 for a more complicated structure. 
 
 The young plumule when it has emerged from the 
 seed coats consists of a very delicate axis, at the apex 
 of which a number of minute outgrowths are to be seen. 
 These are folded in various ways, the outermost covering 
 
THE SHOOT 45 
 
 those internal to them. Their number is not uniform, 
 nor is their method of folding, nor their arrangement, 
 but they all arch over the apex of the shoot. The 
 latter does not bear any protective cap, such as is seen 
 over the root. It is a delicate conical tip, which bears 
 its outgrowths in regular succession, the latter being 
 continually developed by the apex as it elongates, so 
 that the youngest are always nearest to the tip. 
 
 These outgrowths are borne upon the axis at definite 
 points, which show a remarkable difference of behaviour 
 from the spaces between them, in that they do not 
 elongate during the processes of growth. All the growth 
 in length is carried out by these spaces. The points at 
 which the outgrowths are borne are called nodes, the 
 spaces between them internodes. 
 
 The behaviour of these parts can be studied advan- 
 tageously on a shoot a little older than the plumule. 
 
 It is well to select a tree of some few years' growth and 
 to examine some of the ultimate endings of its branches. 
 If from such a tree in the early spring we take a twig 
 we shall be able to observe that during the previous 
 summer its internodes elongated, causing the out- 
 growths to be separated from each other by some little 
 distance. The year's growth may have caused the shoot 
 to become perhaps three or four inches long. If we 
 examine the nodes closely we shall find that between 
 the original outgrowths and the axis certain small knob- 
 like bodies occur, almost hidden between the others. 
 These several parts can always be observed with greater 
 or less facility on all shoots. The axis is called the 
 stem, the first-formed outgrowths are the leaves, and the 
 little knob-like bodies between the two are known as 
 buds. The angle between the stem and its leaf in which 
 the bud arises is the axil of the leaf. The apex of the 
 stem will be seen in the spring to exhibit also the form 
 of a bud, rather larger than the lateral ones in the axils 
 
46 BOTANY 
 
 of the leaves. The plumule is really the first bud of the 
 seedling, and it shows fundamentally the same structure 
 as the others appearing later on the stem. 
 
 As the seasons of the year in our climate render 
 growth intermittent, confined to little more than half 
 the time, and as the growing shoots are exposed to very 
 unfavourable conditions during the remainder, it is easy 
 to understand that special precautions are called for, 
 
 A B 
 
 FIG. 1 8. Buds of lilac. A, shows the external appearance; 
 B, a slightly magnified section ; C, the bud-scales are reflexed 
 and the leafy shoot has begun to elongate. (After Marshall 
 Ward.) 
 
 that they may develop. If we cut a longitudinal 
 section through one of these buds in the spring before 
 growth is resumed we shall find evidence of such (Fig. 
 1 8 B). The delicate growing cone in the centre will be 
 found to be surrounded by a varying number of leaves, 
 each of which arches over it and is in turn arched over 
 by the next one external to it. The most internal ones 
 are extremely delicate and almost unformed, while the 
 cone itself if magnified will be seen in many cases to bear 
 upon its surface small swellings which indicate that 
 other leaves are in course of formation there. Over 
 
THE SHOOT 47 
 
 these delicate leaves are others more sturdy, while the 
 exterior ones are frequently quite dry and hard and in 
 many cases covered over by a sticky substance. Some 
 of those in the interior are in many cases covered with 
 thick coatings of hairs, forming a downy pad of material 
 calculated by its non-conductivity to keep out the cold. 
 
 If the bud is small, it will be found to contain only a 
 few leaves, perhaps only two or three; even in this 
 case, however, the general arrangements are the same. 
 
 If we compare the apices of stem and root, we see 
 how the surroundings in each case have influenced the 
 structure. The root apex is specially protected against 
 damage from contact with hard or rough materials while 
 penetrating through the soil ; the stem is exposed to no 
 such danger, but shows a careful protection from frost 
 and wet, and undue evaporation. 
 
 The young leaves are thus merely flattened boat- 
 shaped expansions curling over the apex of the stem. 
 Later, when their protective powers are no longer 
 called for, their adult forms are assumed. 
 
 The leaves bring about their curving over the apex of 
 the stem in the bud by an irregularity of growth. When 
 the little swelling first appears on the growing cone it 
 is itself rounded or conical; it soon becomes laterally 
 flattened and for a time, so long as it is in the bud, its 
 under surface grows faster than its upper one, so that it 
 is made to curve forwards. When it escapes from the 
 bud later it reverses this distribution of growth and 
 grows more rapidly on its upper face, so becoming flat. 
 
 Buds always terminate the ends of normal growing 
 shoots ; indeed the bud-form is always assumed by the 
 apex of the shoot as soon as its growth is suspended by 
 unfavourable conditions. The buds which appear in 
 the axils of the leaves lower down on the stem are the 
 commencements of the secondary shoots or branches, 
 which will elongate in due course. 
 
48 BOTANY 
 
 In many cases the bud is the foreshadowing of the 
 growth of the stem or branch of the next year. It has 
 "been formed by the shoot as its last effort for the year, 
 and its development during the succeeding year will 
 only involve the elongation of the internodes, the 
 assumption of the adult forms of the leaves, and the 
 preparation of the buds for the year following. In 
 other cases it is not so simple. During the growing 
 period more leaves will be produced than the bud in its 
 resting state exhibits, and growth will be prolonged 
 accordingly. But even in these cases as soon as growth 
 in length stops, the development of another terminal 
 bud with its potentialities can be noticed. 
 
 The growth of the shoot thus shows considerable 
 differences from that of the root. In the case of the 
 latter it is not at all easy to say what are the limits of 
 the year's elongation, while in that of the shoot they 
 may be fairly accurately determined. 
 
 When the next growing season sets in, the bud begins 
 to swell owing to the upward pressure of the elongating 
 axis. The outer leaves are loosened and pressed apart, 
 so that the bud bursts open at the apex. When the 
 external leaves are hard scales they are generally cast 
 off entirety, and the internal leaves emerge. The 
 ^elongation of the several internodes rapidly follows and 
 the shoot takes on its proper form. 
 
 As this change proceeds certain other facts can be 
 determined. The external scales have no buds in their 
 axils, nor do all the leaves develop into foliage leaves. 
 The external ones, and often some just internal to them, 
 do not change their form, and frequently only persist 
 lor a short time, soon falling away. All these are 
 classed together as bud-scales ; they really represent 
 only the bases of leaves (Fig. 18 C). 
 
 As growth goes on other differences appear. The 
 internodes between the bud-scales do not elongate, so 
 
THE SHOOT 
 
 49 
 
 that while the scales persist the young shoot seems sur- 
 rounded by a number of small leaves at its base (Fig. 
 1 8 C). When the bud-scales fall off, they leave the base 
 of the shoot surrounded by scars, which mark the places 
 of their original attachment. At the close of growth on the 
 onset of winter, the 
 shoot, now become 
 what is technically 
 called a twig (Fig. 
 19), shows these 
 scars closely placed 
 together round its 
 base. In the winter 
 it is easy to recog- 
 nise the amount of 
 growth of a twig 
 during the preced- 
 ing year, by noting 
 the distance be- 
 tween this collec- 
 tion of scars and 
 the apex. In an 
 older twig or young 
 branch several 
 such collections of 
 scars can be de- 
 tected, and so the 
 limits of each year's growth can be easily ascertained. 
 With the opening of the bud and the expansion of its 
 leaves as its stem elongates we can trace the sensitive- 
 ness of the shoot to the various influences that sur- 
 round it. We have pointed out the response its axis 
 makes to the influence of gravity and lateral light. We 
 have also incidently mentioned the change of the cur- 
 vature of the leaves which sets in as soon as the bud 
 opens. The change is a response to the access of light 
 
 D 
 
 FIG. 19. Twig of 3 years' growth, bs, scars 
 of bud-scales of each year. The twig shows 
 racemose branching. (After Ward.) 
 
50 BOTANY 
 
 which accompanies this opening. The water in the cells 
 of the leaf was in the early stages of development dis- 
 tributed mainly to those of the under side, making them 
 most turgid and causing them to grow most freely. 
 The access of light disturbs this relationship and the 
 cells of the upper side become most turgid; the conse- 
 quent growth causes them to lose the concavity of their 
 upper sides; they become flat or sometimes slightly 
 curved in the other direction. 
 
 If light is not allowed access to them this growth of 
 the upper side is very much interfered with, and the 
 leaves show but little change of curvature, lying close 
 to the stem as it elongates, and in some cases not 
 becoming even flat. 
 
 CHAPTER VI 
 
 THE CONSTRUCTION OF THE SHOOT SYSTEM 
 
 THE behaviour of the plumule in elongating and ex- 
 panding gives rise to the primary shoot. Every succes- 
 sive bud and branch which spring from it increases its 
 dimensions by multiplying the number of twigs it bears. 
 As the number of such buds upon each twig is fairly 
 large, we see that the young branches increase in a kind 
 of geometrical progression, causing the formation of a 
 large shoot system, which constitutes the body of a 
 shrub or the head of a tree. We must next study the 
 construction of such a head. 
 
 To understand it we must inquire what are the 
 purposes for which it exists, and what are the dangers 
 against which it must protect itself. 
 
 We have already drawn attention to the fact that the 
 functions of the root and the shoot are fundamentally 
 different. That being so, it seems clear that the mode 
 of arrangement of the parts of the one need form no 
 
CONSTRUCTION OF THE SHOOT SYSTEM 51 
 
 rule for the other. There is nevertheless a general 
 agreement between the two, though careful observa- 
 tion shows that similarity of arrangement subserves 
 very different purposes. The arrangements of the shoot 
 all bear a certain relationship to life in air and its conse- 
 quent requirements, and show further a co-ordination 
 with the needs which are cared for by the roots. 
 
 We have seen that one of the primary objects of the 
 latter is to secure a firm anchorage for the plant that it 
 may be able to maintain its erect position. The de- 
 velopment of a large head or upper part makes against 
 such anchorage, by offering a large area to the pressure 
 of wind and the beating of rain forces likely to lead 
 to uprooting from the soil. 
 
 We may ask why such a risk should be undertaken 
 why the sub-aerial portion of the plant need attain the 
 large dimensions it possesses. What are the advantages 
 which are afforded by a widespreading head rearing 
 itself into the air? Are they commensurate with the 
 risk, and what are the precautions which protect the 
 plant in face of the dangers it involves ? 
 
 In seeking answers to these questions we must look a 
 little more closely at the chief features of the upper 
 portions of the shoot system. We soon see that one of 
 the objects secured by the method of development which 
 it follows is the great amount of surface in proportion to 
 bulk which the shoot presents. The twigs are thin, the 
 leaves flat. We have indeed, as we have in the root, 
 and as we notice in the case of the large seaweeds, the 
 bringing of the structure of the plant into relationship 
 with as large a portion of the environment as possible. 
 Here is clearly an indication or suggestion of an inter- 
 change of material between the two. 
 
 We have already assumed that there is such an inter- 
 change, and may now examine more closely its nature. 
 A few simple observations will enable us to prove it. 
 
52 BOTANY 
 
 Let us remove a twig with its expanded leaves to a con- 
 fined space, so that we examine the conditions of the air 
 around it to see what changes, if any, take place. Let 
 us shut it up in a well-dried bottle and keep it at its 
 accustomed temperature. We shall find after a short 
 time that the sides of the bottle become bedewed with 
 moisture, and a little later we shall see that the leaves 
 upon the twig and at least its upper part become wilted 
 and drooping. Part of the work of the shoot is clearly 
 to exhale watery vapour from its surface. 
 
 FIG. 20. Section of leaf showing intercellular spaces and stomata. 
 The cells contain chloroplasts. X8o. 
 
 If careful measurements are made of the total water 
 a plant gives off, it is found to be very considerable in 
 amount, and to be given off during the whole of the day 
 in quantities varying with the changing conditions sur- 
 rounding the plant. The structure of the leaf, to which 
 we must give later some careful attention, shows us 
 that the intercellular spaces which we observed in the 
 body of the root exist in even greater degree in the leaf 
 (Fig. 20) and yield an evaporating surface much larger 
 than the external surface of the twigs and leaves. 
 These internal channels communicate with the exterior 
 through small openings in the limiting membrane of the 
 
CONSTRUCTION OF THE SHOOT SYSTEM 53 
 
 leaves and the more delicate parts of the twig. These 
 openings, which are known as stomata, are themselves 
 co-ordinated with the regulation of this exhalation of 
 vapour, the width of the opening being capable of 
 variation according to different conditions. We must 
 associate the evaporation of so much vapour by the 
 leaves with the very large absorption of water we 
 observed in the root, and we can see that the structure 
 of the leaf is as well adapted to evaporation as that of 
 the root is to absorption. Further structural adapta- 
 tions to this maintenance of a stream of water through 
 the plant will become evident later, but in the mean- 
 time we can see in the features already alluded to a 
 definite relation to this particular interchange between 
 the plant and its surroundings or environment. 
 
 Still pursuing our inquiry, we may notice that while 
 the general colour of the shoot is green, the depth of the 
 green tint is not uniform. The flattened parts or leaves 
 are of a brighter green than the cylindrical axes, and in 
 general it soon appears that the more exposed any part 
 of the shoot is in its young and most delicate condition, 
 the more prominent the green colour becomes. We 
 have consequently a suggestion of some co-ordination 
 between exposure and colour. Comparing two shoots 
 growing in different places we can soon associate the 
 optimum brightness with the best illumination, and we 
 are led to infer that one reason for the flatness of certain 
 parts of the shoot is the desirability of exposing as much 
 of their surface as possible to light. 
 
 We have already called attention to the fact that the 
 greater part of the plant's food is manufactured in the 
 leaves, and that the green colouring matter chlorophyll 
 is chiefly concerned in making it. The chlorophyll is 
 not diffused throughout the living substance, but is 
 confined to a number of small ovoid bodies which are 
 embedded in it, and these green bodies are placed very 
 
54 BOTANY 
 
 little below the surface of the leaves, being thus covered 
 only by a thin transparent layer of cells. The dis- 
 tribution of these green bodies, which are known as 
 chloroplasts, so bears a very definite relation to the 
 incidence of the light, and suggests to us that while one 
 duty of the leaf is to exhale watery vapour, another 
 is to secure the illumination of a definite part of its 
 mechanism, which is concerned with the most intimate 
 questions of nutrition. 
 
 As the two functions thus suggested are found upon 
 further inquiry to be intimately bound up with the well- 
 being of the plant, we must examine them a little more 
 closely before looking for the ways in which they in- 
 fluence the form and position of the shoot system. 
 
 There are two reasons for the copious evaporation of 
 water which we have pointed out. The first is con- 
 nected with the problem of feeding, as we noticed in 
 our introductory chapter. Certain constituents, either 
 entering into the composition of the food itself or 
 necessary factors in its construction, are only to be 
 found in the soil and are procured therefrom by the 
 roots. These compounds are absorbed from the soil in 
 solution in the water entering the root hairs, and the 
 solutions are necessarily very dilute to facilitate their 
 passage through the living substance of the hairs. As 
 with a rapidly-growing plant continuously increasing 
 quantities of these substances are needed for nutritive 
 purposes, it follows that large quantities of the solution 
 must be absorbed. In the plant these mineral com- 
 pounds are taken from the water, and the great bulk of 
 the latter is evaporated into the intercellular passages 
 and the vapour subsequently passed out of the stomata. 
 Hence, speaking broadly, the more water that is taken 
 up and subsequently evaporated, the more mineral 
 matter is secured for the use of the organism. 
 
 But there is another and equally important function 
 
CONSTRUCTION OF THE SHOOT SYSTEM 55 
 
 that this evaporation discharges. In the time of sun-, 
 shine a great deal of the sun's energy in the form of heat 
 and light is falling on the plant. It has been computed 
 that the amount is so great that it would raise its 
 temperature to such a dangerous extent that if no 
 counter-influence were at work it must speedily perish. 
 Now the evaporation of water always requires the 
 expenditure of a considerable amount of heat, and we 
 find that the greater part of the heat reaching the 
 plant from the sun is devoted to the vaporisation of 
 the water in the intercellular passages of the leaves and 
 other parts. The normal temperature of the plant is 
 thus maintained in the face of the enormous absorption 
 of solar heat which its exposed and often unprotected 
 position renders inevitable. 
 
 The study of the behaviour of the chloroplasts shows 
 us that their position is definitely associated with the 
 duty which we have attributed to them. Not only is 
 their colour dependent on their exposure to light, but 
 the part they play in the construction of food is equally 
 related to the illumination they receive. We have 
 already spoken of the work done by the chloroplasts, 
 and have seen that they construct organic food in the 
 form of sugar and .similar compounds from the carbon 
 dioxide of the air, together with a portion of the water 
 supplied them by the root. Carbon dioxide is present 
 in very small proportion in the air, only some 3 or 4 
 parts in 10,000. The construction of food from such 
 antecedents is only possible in the presence of light; 
 two things therefore must be secured a wide surface 
 and preferably a copiously subdivided one, to bring as 
 much air as possible into contact with it, and as com- 
 plete an exposure as possible of the chloroplasts to 
 light to enable the construction of the sugar to go on. 
 
 The form and disposition of the shoot system must 
 be regarded from the point of view of these require- 
 
56 BOTANY 
 
 ments. True, at first sight they seem a little antagonistic 
 to other needs. The evaporation of the water and the 
 illumination of the chloroplasts demand a large and 
 increasing shoot-body, but its increase in size brings 
 with it a distinct danger to the stability which we have 
 seen is one of the first necessities of the plant as a whole. 
 The reconciling of these demands must add to the 
 interest with which we study the form and distribution 
 of the members of the shoot system. 
 
 We have seen that the axis of the latter is very much 
 subdivided, the ultimate divisions, the branches, taper- 
 ing to points, in some cases extremely gradually, in 
 others more abruptly. These cylindrical or conical 
 divisions bear a number of flattened organs, the leaves, 
 which are usually attached to the axis by flexible stalks 
 or particles. We can now see the reason for this sub- 
 divided conformation. It secures strength by the 
 cylindrical form of the twigs, surface by the flattened 
 form of the leaves. The winds can blow freely through 
 the mass of twigs, while the long leaf stalks allow of 
 sufficient displacement of the flattened parts when the 
 pressure of the wind is brought to bear upon them. 
 Moreover, the parts concerned are all extremely flexible 
 and elastic, so that they can yield to pressure and 
 regain their positions as soon as it is removed. 
 
 The form of the shoot system of a plant will depend 
 upon the manner of its branching, and the number, size, 
 and arrangement of the leaves its branches bear. 
 
 The branching will be affected by two main factors : 
 firstly, the number of branches produced at a node ; 
 secondly, the relative degree of growth of each main 
 branch and those to which it gives rise. 
 
 The first of these does not show as much variety as 
 might be expected. Usually one branch, not infre- 
 quently two, appear at a node, but seldom more. 
 
 The second factor, however, plays a much more 
 
CONSTRUCTION OF THE SHOOT SYSTEM 57 
 
 prominent part in the construction of the head. If the 
 first axis grows more vigorously than its branches a 
 behaviour we found to lead to the formation of a tap 
 root in the root system and if each of the branches in 
 turn is longer and stronger than those arising from it, 
 the ultimate form of the head is pyramidal, for the 
 successive branches arise nearer and nearer to the apex, 
 and so long as the growth is regular the lowest will be 
 generally the most widespreading. This is true of the 
 series of branches which each of them bears. This type 
 of branching is said to be indefinite or racemose (Fig. 19), 
 and it is illustrated by such trees as the spruce fir. 
 
 If, on the other hand, the growth of each axis or 
 branch is soon checked and so its development becomes 
 exceeded by the growth of the daughter-axes to which 
 it gives rise, the head will be sub-globular or rounded. 
 The exact shape, however, will largely depend on the 
 number of branches springing from below the apex of 
 each in turn, for these all arise at the same node. They 
 are therefore on the same level, and do not grow in 
 what is called acropetal succession, as in the first case. 
 A very common form is that in which each branch is 
 solitary. This form of branching is called definite or 
 cymose. Examples are afforded by the elms, oaks, and 
 many other forest trees (Fig. 21). 
 
 Another factor in the shape which the branching 
 helps to give to the shoot system is the non-develop- 
 ment of some of the buds. We have seen that a bud is 
 produced in the axil of every foliage leaf. It often 
 happens, however, that a twig cannot adequately feed 
 all the buds it bears. Hence some perish and others 
 remain dormant for some time, circumstances which 
 cause a good deal of irregularity. 
 
 Before we study the influence of the arrangement of 
 the leaves upon the form of the shoot and the shoot 
 system we must look a little more closely at the peculi- 
 
58 BOTANY 
 
 arities of their flattened form. We have seen wherein 
 lie its advantages, but we must consider also the diffi- 
 culties and even dangers which it involves. Difficulties 
 arise from the certainty that the leaves must encounter 
 rough weather in the course of the year. Rain may 
 soak them through, wind may tear them apart, or even 
 strip them from the twigs. How are these perils met ? 
 
 FIG. 21. Diagram of forms of cymose branching. 
 
 There are two reasons why rain falling upon them 
 does not affect them seriously for a long time. Gener- 
 ally the shape of each is such that there is a longitudinal 
 groove all along its upper surface, running from apex to 
 base in the centre of the flattened blades. This con- 
 ducts the water away as fast as it falls upon the leaf, 
 either towards the apex or towards the base. In the 
 latter case the groove is continued along the leaf stalk 
 so that the water is taken to the ground. The second 
 reason is that the outer layers of the walls of the cells 
 
CONSTRUCTION OF THE SHOOT SYSTEM 59 
 
 FIG. 22. Venation of 
 leaf. 
 
 of the upper surface become almost impermeable by 
 water. It is only after long soaking, therefore, that 
 any can find entrance. 
 
 The danger from wind is perhaps greater than that 
 from rain. The leaf-blade, however, 
 though delicate and thin, is never- 
 theless very strong and not easily 
 torn. Running through it are the 
 ultimate endings of the vascular 
 strands we have already noted in the 
 root, the conducting tissue (Fig. 22). 
 These strands form the so-called veins 
 of the leaf and they constitute a net- 
 work of very tough fibrous bands 
 upon which the delicate tearable 
 tissue is supported. They generally 
 strengthen particularly the margins 
 and the apex of the leaf-blade and protect it from being 
 torn. The blade, therefore, when acted on by wind is 
 seldom either bent or curled, but is made to play as a 
 single rigid piece moving up and down without losing its 
 flatness for a moment. 
 
 The danger of stripping from the twig is dealt 
 with differently. When the plant is of a sturdy, rigid 
 habit, the leaves are usually attached to the stem very 
 strongly, and are bent upwards so that the direction 
 of the wind must drive them towards the stem, and its 
 force cannot be felt between the latter and the leaf's 
 upper surface. More frequently, however, we find that 
 the blade of the leaf is attached to the twig by means of 
 a tough, flexible stalk, capable of movement in almost 
 every direction on its point of attachment. The elasti- 
 city is so great and so readily called into play that with 
 even the lightest breezes the leaves of most trees are 
 seen to swing to and fro with the greatest freedom. 
 
 The form of the head of the tree is influenced by the 
 
6o 
 
 BOTANY 
 
 shape as well as the arrangement of the leaves. Usually 
 leaves consist of three regions, a flattened part or blade, 
 a leaf-stalk or petiole, and a leaf base by which it is 
 attached to the stem. If we regard it as an outgrowth 
 from the stem, we find that it assumes 
 its flattened form by developing a wing 
 on each side, the outgrowth itself also 
 becoming flat. If the outgrowth 
 branches and only its branches develop 
 wings we have what is commonly 
 termed a compound leaf (Fig. 23). 
 
 The leaf-stalk is the lower part of the 
 axis of the leaf and it is continued 
 forwards to the tip, the part which has 
 become winged being called the mid-rib. 
 In some cases the whole of the axis of 
 the leaf becomes winged. The leaf is 
 said then to be sessile or to have no 
 
 stalk> M ^ bage of the kaf are yery 
 
 frequently two small outgrowths, of the 
 nature of leaf branches. These are known as stipules. 
 They vary a good deal in shape and size. 
 
 The object aimed at in the distribution of leaves on a 
 tree is the covering of the framework of its head as com- 
 pletely as possible by a thin curtain of leaves, as free 
 from unoccupied gaps as possible ; the leaves themselves 
 must be so arranged that little shading of one part by 
 another shall occur. If we stand under a tree and look 
 up through its branches we find the leaves are not dis- 
 tributed all about the interior of the space occupied by 
 the boughs and branches ; they are seen to be a more or 
 less complete covering to the head. In a humbler type, 
 such as a thistle or a sunflower, the leaves overlap very 
 little, so that practically the whole leaf-surface is ex- 
 posed to the light during at any rate some part of the day. 
 
 The leaves are arranged in various ways upon the 
 
 FlG ' ^Compound 
 
CONSTRUCTION OF THE SHOOT SYSTEM 6r 
 
 stem, but always occur in vertical or nearly vertical 
 rows. Sometimes only one leaf originates at each node, 
 sometimes two, or occasionally more. When only one 
 occurs the leaves are found to be arranged spirally or 
 alternately up the stem. When more than one there is 
 said to be a whorl of leaves at each node. Frequently 
 the whorl consists of two leaves only. Successive 
 whorls, whatever the number of leaves, have their 
 separate leaves placed opposite the spaces between 
 the leaves of the whorls above and below them. 
 
 The number of the vertical rows is correlated with the 
 shape and size of the leaves which compose them. 
 Leaves with very broad bases, often indented, and 
 tapering fairly rapidly to a pointed apex, known techni- 
 cally as ovate or cordate leaves, generally occur opposite 
 to one another on the stem, there being only two rows. 
 Sometimes they have short stalks, sometimes none. 
 When the leaf has its broadest part near the middle and 
 tapers to both apex and base it is termed an oval or ellip- 
 tical leaf; such are generally arranged in three rows. 
 When still narrower, becoming what are known as 
 lanceolate leaves, the number of rows increases to five or 
 eight. Still narrower leaves occur in greater numbers of 
 ranks still. We see thus a co-ordination between the 
 shapes and dimensions of the leaves and their mode of 
 attachment to the stem, just such a co-ordination as we 
 should expect when we remember the disadvantages 
 which would arise from a crowding together of large 
 ovate leaves in several ranks, or the sparse scattering of 
 linear or narrow leaves in few rows. 
 
 When we study in this way the shoot systems of 
 different plants we find them to be in harmony with 
 their surroundings as fully as are the root systems. The 
 surroundings influence the plant very forcibly while it 
 is developing, and many of the results of its development 
 can only be understood by observing that they are 
 
62 BOTANY 
 
 essentially purposeful. The only mode of securing this 
 adjustment with the environment which is possible is 
 that of regulating its growth. 
 
 During the early development and growth the plant 
 exhibits in its shoots as in its roots powers of purposeful 
 response to certain features of the environment which it 
 is capable of appreciating. If we examine the plumule 
 or young bud of the seedling as soon as it begins to grow, 
 we shall notice the same perception of direction as we 
 observed in the root. As the latter would persist in 
 growing downwards, curving itself if its apex pointed 
 in any other direction, so the shoot persists in growing 
 upwards. The sensitive part is not so easy to localise 
 as in the root, but careful experiments made on various 
 plants have proved that the perceptive part of the 
 shoot is the tip and that the sensitive zone does not 
 extend far downwards. The response to the stimulus is 
 brought about in the same way in the two cases, viz., by 
 a modification of the growth, and it is clearly purposeful, 
 to plant the root in the earth and the shoot in the air. 
 There is a close resemblance again in their behaviour 
 between the primary branches of the stem and root. 
 None of them grows in the same direction as the axis 
 from which it springs, but usually they stretch out nearly 
 at right angles to it. This is a response to the stimulation 
 of gravitation in both. 
 
 Another factor which is of much greater importance 
 in the case of the shoot than in that of the root is the 
 incidence of lateral light, which helps to determine the 
 position of the branches as well as of the leaves. If the 
 light falls on a shoot more intensely on one side than 
 another, the rate of growth very speedily changes so as 
 to cause the growing region of the stem to bend or curve 
 till its apex is directed towards the point from which the 
 strongest light is coming. 1 The plant exhibits a power 
 
 1 A figure illustrating this is given in the Biology primer, p. 71. 
 
THE STRUCTURE OF THE SHOOT 63 
 
 of perceiving or appreciating differences of intensity o^ 
 illumination. This sensitiveness is of the greatest value 
 io the shoot, for as the stem bends towards the source of 
 the light the leaves which are expanded nearly at right 
 angles to it are exposed to the rays which they need for 
 the manufacture of sugar. 
 
 The leaves also manifest an independent sensitiveness 
 to light. They are generally so expanded as to expose 
 their upper surfaces to the sunshine. If this position 
 cannot be attained without a movement of the leaf this 
 movement is effected and supplements the other. The 
 leaf-blade twists on its petiole, or the petiole twists in 
 such a way as to expose the surface of the blade. 
 
 With the same sensitiveness to light we see thus that 
 the different members of the plant respond differently, 
 but always purposefully, to it. The root grows away 
 from the incident rays, penetrating into the deeper 
 crevices of the soil ; the stem grows towards them, 
 while the leaf places itself across their path. 
 
 The positions assumed by the stem, branches, and 
 leaves are greatly influenced by the various stimulations 
 they receive ; some respond more actively to one, others 
 to another; but all show both perception and response 
 as they adapt themselves to their environment. 
 
 CHAPTER VII 
 
 THE STRUCTURE OF THE SHOOT 
 
 WE must now examine what are the arrangements of 
 the internal structures of the shoot which enable it to 
 carry on these duties. Though the shoot is to be 
 regarded as a single system comparable with the root, 
 the duties discharged by its cylindrical and its flattened 
 parts are so far distinct that it will be well to consider 
 them separately from our present point of view. 
 
64 BOTANY 
 
 When we examine the plumule we find it to be com- 
 posed of cells resembling those of the radicle. They are 
 at first all alike, and only slowly do differences become 
 apparent. At the apex we find them meristematic, 
 that is, each cell has the power to divide into two. A 
 little farther back they increase in size and become 
 vacuolated. If we take a longitudinal section at this. 
 d pe pi a e > we fi 11 ^ that, as in the 
 
 root, we can distinguish 
 three regions which are 
 faintl Y indicated (Fig. 24). 
 The central strand or 
 plerome is visible, appear- 
 ing conical in shape as in 
 the root. Outside it lies 
 a periblem, and this is 
 covered by a dermatogen, 
 a layer of a single cell in 
 thickness. These two are 
 not conical, but are thrown 
 into irregularity by the 
 outgrowth of the leaves. 
 The leaves and branches 
 FIG. 24 . Growing point of stem of diff er in their origin from 
 
 Dicotyledon, d, dermatogen ; pe, the branches of the root as 
 
 young the y begin with the Ollt " 
 
 growth of the periblem, 
 which pushes the dermatogen before it. The plerome 
 takes no part in their formation. As the plumule gets 
 older its elongation proceeds by the continued formation 
 of new cells and their subsequent growth. This goes on 
 for some time, and extends as a rule further back than it 
 does in the root. The growing region is a little more 
 complex in the stem than in the root, because the cells 
 do not all grow alike, those of the nodes, or places 
 .where the leaves arise, elongating scarcely at all, while 
 
THE STRUCTURE OF THE SHOOT 
 
 those of the internodes are very vigorous. The leaves 
 on the nodes elongate from the first, but the branches 
 in their axils appear much later. 
 
 As the seedling grows, it prepares for the discharge of 
 the duties which devolve upon it. What we are about 
 to describe of its structure corresponds almost exactly 
 with the structure of each year's 
 twigs of the tree or shrub into 
 which ultimately it develops. 
 
 The two main duties of the 
 stem we have seen to be the 
 support of the head or leaf-bear- 
 ing part of the shoot and the 
 transport of the water and 
 mineral compounds absorbed 
 by the roots to the seat of con- 
 struction of organic substance. 
 Both these objects are carried 
 out by the arrangements in the 
 central cylinder, and both de- 
 pend upon the development of 
 vascular strands connected with 
 those of the root. If we look 
 at a longitudinal section of a 
 whole plant we find that these FlG - 
 strands are continuous through- 
 out it though they are arranged 
 differently in its different regions. In the root we found 
 the strands of wood lying sometimes separately in a 
 central ring, sometimes joined to form a solid cone of 
 wood. Other strands, soft in nature, known as bast, lie 
 between them or between their outer limbs when they 
 are fused in the centre. As we pass upwards we find 
 that in the region just below the cotyledons a certain 
 rearrangement of the strands takes place. The bundles 
 shift their relative positions and the wood strands come 
 
 25. Diagram showing 
 the general structure of a 
 dicotyledonous plant. 
 
66 BOTANY 
 
 to lie exactly inside the bast strands, the two being 
 separated only by a layer of meristematic cells known as 
 cambium. The bundles in the shoot are known as con- 
 joint bundles from this association of the wood and bast. 
 The wood strands, further, become twisted on their long 
 axes in this same region, so that the protoxylem, which in 
 the root is on the outside, is in the stem on the inner face. 
 The bundles are wedge-shaped much as they are in the 
 root. Each seems thus to have turned completely round 
 so as to face in the opposite direction. Instead of the 
 cylinder being solid in the centre, the conjoint strands 
 always stand round its periphery so that there is a large 
 unoccupied space in the centre, known as the pith. 
 
 Following them to the growing end of the stem we 
 find that they do not terminate in its growing cells, but 
 can be traced into the leaves through their petioles. 
 In the latter they usually form a half cylinder open on 
 the upper side, instead of a complete hollow cylinder as 
 in the stem. From the petiole they can be traced into 
 the flattened portion of the leaf, where they form the 
 network which we call the veins of the leaf. 
 
 While the leaf and stem are very young we find in 
 them the first traces of the origination of these strands. 
 They appear in the growing point a little way back, 
 as separate strands in the plerome, made up of small 
 cells, longer than broad, defined from the rest chiefly by 
 their smaller transverse diameters. They are all meri- 
 stematic and only slowly lose the power of dividing. A 
 transverse section (Fig. 26) of the plerome shows these 
 little strands as wedge-shaped areas, the procambial 
 strands, arranged in a circle near the outside of the 
 plerome, separated by narrow areas known as medullary 
 rays. As they get older the cells become changed into 
 their adult form. The change in the wood cells is asso- 
 ciated with growth in diameter and irregular thickening 
 of the walls, making them appear as if marked out into 
 
THE STRUCTURE OF THE SHOOT 67 
 
 FIG. 26. Diagram of sections of stem of dicotyledon at three ages. 
 A, young condition, showing commencement of differentiation of 
 the plerome and its vascular strands: a, strand; b, limits of the 
 plerome; c, periblem; m.r., medullary ray; pi, pith. B, a little 
 older stage: p, bast; x, wood; c, cambium; i.e., interfascicular cam- 
 bium: (one of the strands has been shaded). C, older stage, after the 
 commencement of secondary thickening: px, protoxylem or first- 
 formed wood; x, secondary wood; ph, secondary bast. (After Sachs.) 
 
68 BOTANY 
 
 curious patterns ; with substitution of lignin for cellu- 
 lose as the material of which they are composed; and 
 with the disappearance of many of the transverse 
 separating walls, causing a vertical row of cells to be- 
 come a vessel. The cells to show the change first are 
 those on the inside of the wedge-shaped strand the 
 protoxylem. In these the thickening of the walls is 
 laid down in the shape of a spiral band, or a series of 
 rings. These vessels remain of small diameter. The 
 other wood cells and vessels are thickened more irre- 
 gularly and are called reticulated ; in some cases when 
 the thickening deposit leaves only very small thin spots 
 they are known as pitted elements (Fig. 13, p. 35). 
 
 The bast of the strand begins to be differentiated on 
 the side nearest the periphery, where the cells are called 
 the protophloem. The vessels of the bast are sieve tubes 
 (Fig. 27) as in the root. The other elements are mainly 
 elongated cells with thin cellulose walls. 
 
 As the differentiation begins at the front and back of 
 the bundle and advances in each direction the wood and 
 bast are not very long in meeting. In plants that only 
 live for a few weeks or months they come into actual 
 contact, but to those whose lives are longer provision is 
 made for further development by the last layer left 
 between them remaining meristematic or capable of con- 
 tinuous dividing. This is the cambium layer of which 
 we have spoken. It is only a single cell in thickness. 
 
 This arrangement of the supporting tissue is very 
 strong and most economical. The hollow cylinder or 
 tube is one of the strongest forms of support that a 
 structure can possess. It has, too, a certain flexibility, 
 for while the strands are gradually hardening they can 
 bend freely without breaking. The young stem thus 
 shows itself built for toughness and elasticity, so pos- 
 sessing a power of bending to wind and recovering as 
 the force of the air passes it. The continuity of the 
 
THE STRUCTURE OF THE SHOOT 
 
 69 
 
 vascular strands throughout the plant ensures the proper 
 distribution of the water absorbed from the soil. 
 
 Certain other features of the framework of the plant 
 next call for attention. If we examine the outer layer, 
 called in the stem and leaf the epidermis, we find it as a 
 continuous sheet over the whole, and in most cases a 
 
 FIG. 27. Sieve tube from stem of Cucurbita. A, transverse; B, longitu- 
 dinal section ; s.p., sieve plate ; c, companion cell. (After Strasburger.) 
 X5oo. 
 
 single cell in thickness. A delicate structure like a seed- 
 ling, whose cells are filled with water, is exposed to a 
 general evaporation at the surface. This, if not guarded 
 against, would lead to a loss of water beyond the control 
 of the plant, and would interfere with the proper con- 
 duction of the water to the places where the construction 
 of sugar takes place. We find a very simple but very 
 effective protective mechanism. The outer walls of the 
 cells of the epidermis become thickened and their 
 external layers are changed into a very impermeable 
 
70 BOTANY 
 
 material called cutin. These external layers can be 
 stripped off from large pieces of the surface in a kind of 
 pellicle, which is known as the cuticle. It is developed 
 more freely over the leaves than over the stem. 
 
 This layer serves too as a protection against cold. 
 For this purpose many plants have an additional safe- 
 guard, in the shape of hairs, or outgrowths of the 
 epidermal cells, forming a fine felt work over their sur- 
 faces, clothing them indeed in a kind of cotton garment. 
 
 Both cuticle and hairy coating serve also to protect 
 the delicate surface from injury by rain. 
 
 The outer coating or cuticle, covering as it does the 
 whqle exterior, would be a source of danger to the plant 
 by preventing almost all evaporation, if it were altogether 
 intact. The epidermis is pierced by small apertures, 
 which are the openings of the system of intercellular 
 spaces or passages we saw to be developed in the root 
 and which we now find to extend throughout the whole 
 of the shoot as well. These stomata, as they are called, 
 are more numerous in the leaf than in the stem, but they 
 are present in the latter so long as it is young. The 
 aperture or stoma is surrounded by two cells called 
 guard-cells, which are attached together at their ends 
 but not in their centre. They are kidney-shaped in 
 appearance, and when filled with water they stretch so 
 as to draw apart in the centre, opening the stoma (Fig. 
 28). When the water is withdrawn from them they fall 
 together and close the aperture. This arrangement 
 thus allows the necessary evaporation of water to take 
 place. The vapour is formed in the intercellular 
 passages and passes out through the stomata, the 
 width of the apertures being regulated by the amount of 
 water in the guard-cells, which in turn depends on the 
 amount of water in the plant. 
 
 The layer of cells between the central cylinder and 
 the epidermis, which is the continuation backwards of 
 
THE STRUCTURE OF THE SHOOT 71 
 
 the periblem, is the cortex. Its composition is very 
 varied as the plants grow older. In the young condition 
 it is only noteworthy because its outer layers of cells 
 contain the green bodies we have called chloroplasts. 
 The great development of branching which takes 
 
 FIG. 28. Epidermal cells of leaf showing three stomata in 
 various stages of opening. 
 
 place necessitates a considerable enlargement of this 
 primary structure. The increase in number of the 
 leaves makes it important to increase the means of 
 transport of water; the slender cylindrical tube of the 
 young stem soon becomes unable to support the weight 
 resulting from its greater size and the number of its 
 branches. The transport of food to its different parts 
 makes increasing demands upon its bast. We must 
 examine the way in which these necessities are supplied. 
 
72 BOTANY 
 
 As the stem grows we find additional vascular strands 
 continually being developed, a change directed especially 
 to the strengthening of the stem, as the new strands are 
 not directly connected with the leaves. The original 
 strands also are much enlarged and strengthened. 
 
 All this work is done by the cambium layer. Part of 
 the original bundles, as we have seen, consists of this 
 tissue, which is hence called fascicular cambium. By 
 continual division of its cells, mainly in a direction 
 parallel with the outside of the stem, masses of cells are 
 produced between the wood and the bast. One layer of 
 these remains cambium those on the inside of it are 
 changed into wood, those on its outside into bast. 
 
 Very soon after this process has been set up in the 
 bundle the cells which lie between the strands, known as 
 the medullary rays, are the seat of change. No doubt the 
 multiplication of the cells of the cambium sets up a 
 strain in the ray cells adjoining them, stretching them, 
 or dragging upon them. The stimulus of this strain 
 makes certain of these cells, extending across the ray, 
 begin to divide in their turn, and soon the rays are all 
 crossed by layers of meristematic cells, joining up into 
 a ring the isolated cambiums of the bundles. These 
 new portions, which complete the ring, are known as 
 inter fascicular cambium. The whole ring now behaves 
 as the original cambium of the bundles does, and soon a 
 ring of wood is formed in front of and a ring of bast behind 
 it. The parts of the ring formed by the interfascicular 
 cambium have no connection directly with the leaves. 
 
 As new leaves arise at the apices of the twigs the 
 vascular strands belonging to them are connected with 
 this vascular cylinder as were the primary ones, for the 
 structure of the young twigs resembles in all points that 
 which we* have described for the seedling. 
 
 At the end of each year, in our climate, the growth of 
 most trees ceases, owing to the fact that the leaves fall 
 
THE STRUCTURE OF THE SHOOT 
 
 73 
 
 off. When it is resumed on the putting out of the 
 leaves of the next year this process recommences. 
 There is a difference in appearance between the wood 
 
 
 FK 
 
 . 29. Section of twig of lime-tree, 3 years old. ix, 2x, 
 the successive annual rings of wood. (After Kny.) 
 
 formed at the end of the season and at the beginning, so 
 that the formation of each year stands out distinct from 
 that of the next, when a transverse section of a twig or 
 branch is examined. Each year's formation is spoken 
 of as an annual ring. The rings of wood are easily seen, 
 but those of bast are not so conspicuous (Fig. 29). 
 
 
74 
 
 BOTANY 
 
 When a tree gets old the central part of its wood 
 usually dies and becomes very hard. The only living 
 wood is a narrow area close to the cambium. This is 
 known as the sap-wood, or alburnum, the dead centre 
 being the heartwood or duramen. 
 
 This substitution of a solid core for a hollow cylinder 
 of wood is necessary for the strengthening of the trunk 
 and branches. In this well-developed form the mass of 
 the body of both shoot and root is made up of hard wood. 
 As the trunk and branches gradually thicken, their 
 outer regions are strengthened by the production of bark. 
 This begins in the stem as in the root by the forma- 
 tion of sheets of cork. In 
 the root these layers arise 
 in the pericycle ; in the stem 
 they begin in the cortex just 
 below the epidermis (Fig. 
 30). As the years pass, 
 more and more layers of 
 cork are formed deeper and deeper in this region. The 
 outer ones are pierced by lenticels (Fig. 31). They are 
 all formed in the same way, by the formation of meri- 
 stematic layers, which 
 produce cork in their out- 
 side and add to the cortex 
 on their inner faces, be- 
 having in much the same 
 way as the cambium, 
 though the cells they 
 form do not give rise 
 to bast and wood. The n 
 
 , , FIG. 31. Section of a lenticel, /; per, 
 
 cork is impermeable to cork layer. 
 
 water and consequently 
 
 all the cells outside the innermost layer of it die ; the 
 
 tissue thus formed constitutes the bark (Fig. 32). As the 
 
 years go on it becomes thicker and thicker and much 
 
 FIG. 30. Commencement of cork 
 formation in stem. 
 
THE STRUCTURE OF THE SHOOT 
 
 75 
 
 crinkled and split up through the action of the weather 
 and the storms the tree is exposed to. 
 
 We must now examine the interior arrangements of 
 the leaf. We have already learnt that its special work 
 is mainly twofold. It is the chief agent in transpira- 
 
 FIG. 33. Section of bark of Quercus. pe, cork layers arising at 
 different depths in the cortex. (After Kny.) 
 
 tion or the evaporation of the water which the plant 
 does not permanently retain; while it also is the chief 
 seat of the construction of organic food substance. 
 These two duties are discharged mainly by the cells of 
 the lower and of the upper halves of the leaf respectively. 
 The petiole or stalk of the leaf has a general structure 
 not unlike that of the stem, except that its vascular 
 strands do not form a complete cylinder, but only half 
 
7 6 
 
 BOTANY 
 
 a one, being open on the upper side (Fig. 33). The 
 petiole is generally not cylindrical in shape, but flattened 
 on the upper face. It is continued with little change of 
 structure into the blade, where it appears as the mid-rib. 
 The blade of a stalked leaf is the ultimate portion of 
 the outgrowth with the flattened wing which has been 
 
 " ep 
 
 -co 
 
 FIG. 33. Section of petiole of Primula sinensis. ep, epidermis; co, 
 cortex; en, endodermis ; pe, pericycle; ph, bast; x, wood. 
 
 developed along its two edges. The part between the 
 two wings is generally known as the mid-rib. If we cut 
 a section through the blade (Fig. 20) we find its structure 
 adapted to the work for which the flat part was de- 
 veloped. On both surfaces we find an epidermis, the 
 lower one especially pierced with stomata. Under the 
 upper epidermis the cells are long and narrow and 
 arranged side by side much like the vertical railings 
 of a fence. They touch each other along nearly their 
 
THE MONOCOTYLEDONOUS PLANT 77 
 
 whole length, intercellular spaces being small and not 
 numerous. There may be only one layer or several 
 layers of these cells, which constitute what is known as 
 the palissade tissue. The cells contain numerous chloro- 
 plasts, which are embedded in their protoplasm. Each 
 has as usual a central vacuole filled with water. These 
 chloroplasts are capable of a little movement in the celL 
 It is in this layer, exposed to the light most freely, that 
 the sugar is constructed. (See Fig. 20, p. 52.) 
 
 The lower half of the leaf is made of cells which are 
 spherical, cubical, or oblong, and are arranged so as to- 
 touch each other only at few points ; consequently the 
 intercellular passages between them are very large, 
 taking up sometimes more space than the actual cells. 
 This is often called the spongy tissue of the leaf. The 
 cells contain some chloroplasts, but not nearly so many 
 as the palissade cells. This layer is the layer in which 
 evaporation occurs. The veins generally run in the 
 centre of the blade, between these two layers of cells. 
 
 All the structures of both petiole and blade thus show- 
 exact adaptation to the two main duties of the leaf. 
 
 CHAPTER VIII 
 
 THE MONOCOTYLEDONOUS PLANT 
 
 OUR attention has been mainly directed so far to the 
 peculiarities of the dicotyledonous plant. We must 
 now turn for a little while to study another form, in 
 which the embryo has only one cotyledon. The plants 
 of this type are not so numerous as the former class, but 
 they are still very widespread. The most easily acces- 
 sible of them in this country are the grasses and the 
 group which is represented by the common white lily. 
 
 If we take a grain of wheat we have what is very 
 generally spoken of as the seed of the plant. This is 
 
78 BOTANY 
 
 not strictly accurate ; it is really the fruit and contains 
 the seed, but the testa of the seed and the wall of the fruit 
 are so closely united that we cannot separate them. The 
 grain of wheat is a small ovoid body with one side flattened 
 and grooved down its length. At the back, 
 quite at the lower end of the grain, is a little 
 . wrinkled area, which marks the position of the 
 embryo, above which and forming the greater 
 part of the grain is the endosperm, filled with 
 food for the young plant during its early growth 
 or germination. A section of the grain is shown 
 in Fig. 34. 
 
 The grass embryo possesses a single large 
 cotyledon which is at first terminal and con- 
 tinuous in a straight line with the radicle, 
 while the plumule grows out laterally some 
 p lG little distance below the apex. As it grows 
 
 Longitudi- the cotyledon becomes forced over to one side, 
 n ^ s?" ^ and the plumule and radicle come to lie in a 
 oat grai ^ straight line, as in the dicotyledons. The 
 cotyledon then develops along the side of the 
 rest of the embryo, separating it from the endosperm. 
 The side of it which is in contact with the latter is the 
 part which absorbs the food in the endosperm cells. In 
 other seeds the cotyledon remains in a line with the radicle, 
 the whole embryo being surrounded by the endosperm. 
 In germination, however, the upper end of the cotyledon 
 is the last to leave the seed coat, remaining there and 
 absorbing the endosperm as long as any persists. 
 
 If we soak some grains of wheat or barley and keep 
 them warm germination soon begins. The radicle pro- 
 trudes as a little white body from the micropyle ; looking 
 along the back of the grain we can notice a little pointed 
 prominence gradually making its way in the other 
 direction under the skin and ultimately emerging at the 
 other end ; this is the plumule. As it gets larger it dis- 
 
THE MONOCOTYLEDONOUS PLANT 79 
 
 places the rest of the grain, which comes to lie on the side 
 of the young plantlet. The grain remains underground. 
 
 The further development is differ- 
 ent from that of the dicotyledon. The 
 root does not form a tap root, but 
 branches almost at once; indeed in 
 the grasses generally it begins to do 
 so before it escapes from the grain. 
 The main root grows scarcely at all, 
 but a number of branches arise behind 
 its apex, making in the grasses a 
 cluster of delicate fibrous rootlets. 
 The growth of the young stem is seen 
 more advantageously in a larger grass 
 the maize. At first it is very 
 slender, but as development proceeds 
 its growing point becomes continu- 
 ally larger and more vigorous, so that 
 each node and internode become 
 larger than the preceding ones. The 
 young stem has thus the form of an 
 inverted cone (Fig. 35). This goes 
 on till the plant reaches a certain 
 height, when this continuous enlarge- 
 ment ceases and the later parts of the 
 stem are cylindrical. Several roots 
 are developed from the nodes of the 
 lower part of the stem in the case 
 of most monocotyledons ; as they 
 arise out of the normal order they are called adventitious 
 roots. 
 
 The stem of the monocotyledon produces as a rule 
 one leaf at each node. This leaf has a very broad base, 
 which encircles the stem in large part, or sometimes 
 entirely. The leaves are said to be sheathing. 
 
 The general requirements of the plant are not very 
 different from those of the dicotyledon and need not 
 
 G. 35- Diagram of 
 mode of growth of 
 stem of monocoty- 
 ledon (Zea). (After 
 Sachs.) 
 
8o 
 
 BOTANY 
 
 therefore be discussed at any length. The distribution 
 of the conducting tissues is, however, materially different 
 so far as the stem is concerned. The root has its strands 
 placed like those of the dicotyledon, but it never increases 
 in thickness and does not show, therefore, any develop- 
 ment of cambium or secondary woody elements. The 
 strands in the stem are confined to the central cylinder, 
 but each conjoint strand of bast and wood passes up the 
 
 stem separately. Each is 
 surrounded by a protecting 
 sheath of hardened cells 
 and never contains any 
 cambium. The strands 
 are numerous and are 
 arranged in a series of 
 circles. When the number 
 is very great this circular 
 arrangement cannot easily 
 be seen and the strands 
 appear to be scattered 
 thickly through the central 
 cylinder (Fig. 36). They 
 are found to be continuous 
 with similar strands in the leaf, which as before are called 
 the veins. These run in the main parallel to one another, 
 and do not form the complex network which is seen in 
 the leaf of a dicotyledon. 
 
 The relations of the leaf to the stem are a little 
 different from those of most dicotyledons. The bases of 
 the leaves sheathe the stem and do not as a rule fall off 
 in autumn. The leaf, however, sooner or later dies, but 
 its base remains where it was. As the stem grows 
 older, as in the case of many palms, these leaf bases 
 cover nearly the whole of the trunk, causing the latter 
 to appear much thicker than it really is. 
 
 The leaf has palissade parenchyma in a narrow layer 
 under both surfaces. 
 
 FIG. 36. Diagram of transverse sec- 
 tion of stem of monocotyledon. 
 
THE FOOD OF PLANTS 81 
 
 CHAPTER IX 
 
 THE FOOD OF PLANTS 
 
 WE have in our introductory chapter considered in out- 
 line the important question of the nutrition of plants, a 
 subject which has been treated of also in Chapter VI. of 
 the primer of Biology. It is necessary now to return to 
 this subject and examine it a little more fully. 
 
 The substances that are taken in by a plant are the 
 carbon dioxide which is present in the air, and the water 
 and dissolved mineral matters which the roots obtain 
 from the soil. We must again emphasise the fact that 
 these materials are not capable of serving as food in the 
 condition in which they are absorbed, but that a great 
 deal of work has to be carried out to convert them into 
 nutritive material. It is only the green plant which can 
 build them up into such compounds as the living substance 
 can incorporate into itself, the work being effected by 
 the chloroplasts, the little ovoid bodies which are the 
 seat of the green colouring matter. Further, the chloro- 
 plasts can only work when they are properly illuminated. 
 
 The carbon dioxide exists in very small proportion in 
 the air, not more than about 3 parts being present in 
 10,000. The gas enters the plant by way of the stomata, 
 and so makes its way into the intercellular spaces 
 whence it obtains access to the cells in which the chloro- 
 plasts are present. The water from the soil is conducted 
 in the way we have described to the same cells, continu- 
 ally replenishing the supply in their vacuoles. We 
 have thus present in the cells of the parenchyma of the 
 leaf a supply of carbon dioxide, water, and the chloro- 
 plasts themselves. When sunlight shines upon the 
 leaves in appropriate intensity the constructive action 
 commences. The stages are not yet fully understood, 
 but there appears to be no doubt that in some way the 
 chloroplasts cause a certain chemical action to be set 
 
 F 
 
82 BOTANY 
 
 up ; the carbon dioxide and some of the water disappear 
 and are replaced by a simple organic substance known 
 as formaldehyde, while a quantity of oxygen, equal in 
 volume to the carbon -dioxide, is set free. The oxygen 
 finds its way from the cells into the intercellular spaces 
 and passes out of the plant by way of the stomata. 
 Formaldehyde is thus the first organic product which is 
 formed; it is a gaseous body and probably is never 
 present in any but very small quantities, for it is almost 
 immediately transformed into a kind of sugar. The 
 manufacture of sugar is thus the first stage in the 
 preparation of the food of the plant. 
 
 This construction of sugar cannot be carried out 
 without the application of energy. We are familiar, 
 from our ordinary experience of things, with the fact 
 that a machine cannot be made to do work without a 
 supply of energy. A steam engine cannot work without 
 the expenditure of a certain amount of fuel. Whence 
 then does the chloroplast obtain the energy which it 
 applies to sugar-making? The answer to this question 
 explains the necessity for the proper illumination which 
 we have spoken of as a condition of its activity. The 
 rays from the sun, which we speak of as the rays of 
 light, are absorbed by the green colouring matter of the 
 chloroplast. We can prove this by the use of an instru- 
 ment known as the spectroscope, which is an arrange- 
 ment of glass prisms. If we let a beam of white light 
 from the sun fall upon such a prism the rays of which it 
 is composed are bent or deflected unequally on entering 
 and leaving the glass, so that if they are allowed on 
 emerging to fall upon a plane surface they appear as a 
 broad band of light showing a series of colours ranging 
 in order from red to orange, yellow, green, blue, and 
 violet. If now we place a thin film of a solution of 
 chlorophyll between the source of light and the prism 
 we find all the rays do not reach the glass, so that the 
 
THE FOOD OF PLANTS 83 
 
 coloured band which emerges is not continuous, certain 
 parts of it being blotted out. The spectrum, as the 
 band is called, is consequently crossed vertically by a 
 number of dark bands, corresponding to the position of 
 the missing rays. In the living cell these rays are 
 absorbed by the chlorophyll exactly as they are by the 
 solution used in the experiment, and it is from them the 
 plant derives the energy which is used. The rays which 
 are most active are a certain number of the red ones; 
 these correspond in position with a broad black band 
 which is one of those described. 
 
 This process of sugar construction can only take 
 place at a moderate temperature. 
 
 Another very important constituent of the plant's 
 food is protein, which differs from the group of food- 
 stuffs to which sugar belongs by containing nitrogen in 
 combination. Very little is at present known of the 
 processes by which protein is made. Compounds of 
 nitrogen, preferably nitrates of potassium, calcium, 
 magnesium, or ammonia, are absorbed by the roots, dis- 
 solved in the water which they take in. The changes 
 they undergo lead in some still unexplained manner to 
 the formation of much more complex nitrogen com- 
 pounds, which are generally though not strictly accur- 
 ately described as amides. Among them may be 
 mentioned asparagin, leucin, and tyrosin. 
 
 By still further changes these are converted into 
 proteins, but the chemistry of the process is still obscure. 
 
 A third constituent of the food of plants is fat or oil. 
 This is less widely distributed and appears for the most 
 part only in places of storage. It is formed directly 
 from the living substance there and does not appear to 
 be built up from simple substances in the plant. 
 
 Sugar is a member of a group of substances which are 
 called carbohydrates. It is formed in large quantities 
 when the chloroplasts are properly illuminated; much 
 
84 BOTANY 
 
 more is made than is needed for immediate use. The 
 surplus is immediately deposited in the chloroplasts in 
 the form of grains of starch. Starch is another carbo- 
 hydrate which very readily becomes transformed into 
 sugar by the action of dilute mineral acids or of a 
 peculiar body known as diastase, which is a digestive 
 ferment or enzyme. Probably an excess of protein is 
 also made while the conditions are favourable. 
 
 The cells in which these foodstuffs are made have to 
 manufacture sufficient food not for themselves only, but 
 for the whole plant, many of whose cells, as we have 
 seen, are set apart for other purposes altogether. It is 
 necessary accordingly for the food to be made in large 
 quantities and for the surplus to be transported from 
 the cells in which it is made to the other cells of the 
 organism. Part of it is devoted to the nutrition and 
 growth of the body of the plant, and a large amount is 
 stored in various places to nourish the reproductive organs. 
 
 There is consequently a continuous movement of the 
 manufactured food substances about the plant. The 
 sugar temporarily deposited as starch grains in the 
 chloroplasts is taken away as soon as darkness falls. 
 The ferment or enzyme diastase, which is present in the 
 cells, converts the starch into another form of sugar, 
 which diffuses outwards. A stream of such sugar is thus 
 leaving the leaf during the night and passing to other 
 cells of the plant, either to be consumed in the growth 
 processes or to be stored up till wanted. Probably this 
 stream of sugar is passing also during the day, but while 
 the manufacture is going on it is not easy to detect it. 
 
 When proteins leave the cell they are first converted 
 by a similar ferment into the amides we have spoken of 
 and pass through the plant's tissues as such. 
 
 Sugar and either proteins or amides are taken up by the 
 living protoplasm and incorporated into its substance. 
 This is the true assimilation, or nutrition of the plant. 
 
THE RESPIRATION 'OF PLANTS 85 
 
 The foodstuffs that are stored instead of being 
 directly consumed, undergo a transformation which is 
 the opposite of that to which the enzymes give rise. 
 The sugar is converted again into starch, a process 
 carried out by certain plastids much like chloroplasts, 
 but without any green pigment. These leucoplasts, as 
 they are called, are present in the cells in which the 
 storage takes place. The amides are built up again into 
 proteins and deposited in the cells. In seeds they appear 
 as peculiar grains of protein matter, which have long been 
 known under the name of aleurone grains. They are formed 
 by the protoplasm of the cell and not by any plastid. 
 
 The transport of these streams of food material is 
 effected chiefly by those soft parts of the vascular 
 strands of which we have spoken as bast or phloem. 
 
 CHAPTER X 
 
 THE RESPIRATION OF PLANTS 
 
 IT is a matter of common experience with us all that a 
 certain process called breathing must take place. We 
 know that we are continually taking air into our bodies 
 and passing it out again. What we are not perhaps 
 aware of is that the air we give out differs in two im- 
 portant particulars from that which we take in it has 
 gained some carbon dioxide and it has lost some oxygen. 
 What is true of ourselves is true also of plants. 
 During the whole time of their lives they are absorbing 
 oxygen and giving out carbon dioxide, two processes 
 which constitute the beginning and the end of another 
 very complex internal one which is known as respiration. 
 So long as active life lasts this interchange of gases can 
 be detected by appropriate methods, though it is 
 observed with difficulty during the daytime on account 
 of the absorption of carbon dioxide and liberation of 
 
86 BOTANY 
 
 oxygen, which we have seen to be associated with the 
 manufacture of sugar. It can, however, be detected by 
 experiments. To prove the absorption of oxygen, take 
 a flask and fit it with an india-rubber stopper through 
 which a glass tube bent twice at right angles can be 
 passed; let the end of the tube dip into mercury in a 
 small vessel standing side by side with the flask. Fill 
 the flask with healthy leaves. In addition to the leaves 
 place in the flask a test tube containing a solution of 
 caustic potash and cork it up so that the outlet tube 
 dips into the mercury. Keep it at a constant tempera- 
 ture for some hours. The mercury will rise gradually 
 into the tube, and will continue to do so for some time, 
 so showing that a diminution of the volume of the air in 
 the flask has taken place. Analysis of the air then left 
 in the flask will show that the volume of nitrogen is un- 
 changed, while that of oxygen has diminished. Exami- 
 nation of the caustic potash will show that it has 
 absorbed an amount of carbon dioxide. 
 
 The reason for employing the potash is that the 
 carbon dioxide which is exhaled in respiration is about 
 equal in volume to the oxygen absorbed, so that unless 
 it is removed the diminution of the volume of the 
 oxygen will be compensated for by the addition of the 
 carbon dioxide and the mercury will not rise in the tube. 
 
 We can show the exhalation of carbon dioxide by the 
 living plant by means of another experiment. Place a 
 number of leaves in a flask through the stopper of which 
 air can be made to enter by one glass tube and can be 
 removed by another. Make the air as it enters pass 
 through a bottle containing a solution of caustic potash 
 which will free it from all traces of carbon dioxide. 
 Pump the air through the flask by means of some form 
 of air pump or aspirator and lead it through a bottle of 
 lime-water. It will soon make the lime-water milky in 
 consequence of the formation of carbonate of calcium, 
 
THE EVOLUTION OF PLANTS 87 
 
 brought about by the reaction of carbon dioxide with 
 the lime. As all carbon dioxide was kept from entering 
 the flask, and as the lime-water shows by becoming 
 milky that some of this gas reached it in the stream of 
 air pumped through it, it becomes perfectly clear that 
 the plant has exhaled it. 1 
 
 Besides carbon dioxide^ certain amount of water vapour 
 is exhaled by the plant in the process of respiration. 
 
 To understand the purpose of respiration we must go 
 back a little and consider some of the features of sugar 
 manufacture. We saw that the energy for this work is 
 derived from the sun. The work it does is to build up 
 sugar and subsequently other compounds, which remain 
 in the plant. These substances retain in themselves the 
 energy which was expended in making them. When 
 they are decomposed or reduced to simple compounds 
 like those from which they were made, this energy can 
 be liberated again. If we burn them, for instance, we 
 get the liberation of a large amount of heat, which can 
 be made to do work in other directions. So we see 
 that the construction of the plant, both living and non- 
 living substance, involves the fixation of large amounts 
 of energy as well as of material. 
 
 The purpose of respiration, which involves the break- 
 ing down of the living substance incidental to its wear 
 and tear and the work it does, is in this way to liberate 
 the energy without which these operations could not 
 take place. 
 
 CHAPTER XI 
 
 THE EVOLUTION OF THE FORMS OF PLANTS ALG^ 
 
 THE considerations set out in the foregoing chapters 
 apply in great part only to the higher terrestrial plants. 
 But the whole range of vegetation is much more exten- 
 
 1 A form of the apparatus is shown in the primer of Biology, p. 56. 
 
88 BOTANY 
 
 sive than this. There are many other types of plants 
 which differ from the land plant we have considered 
 and differ too among themselves in many respects. 
 They live in various situations, they attain to very 
 different dimensions, and they show a great variety in 
 the details of their internal structure. A very large 
 number of plants, some very humble, others very 
 elaborate, in degree of development, live altogether or 
 almost entirely in water. Many others of simple struc- 
 ture occupy moist situations, such as rocks on the banks 
 of streams, damp earth, or trunks of trees near the 
 ground; others again dwell in hot, arid regions where 
 little water can reach them. Nearly all these forms 
 resemble the higher plants in possessing the green 
 colouring matter; but there are others which have lost 
 it, and which live, therefore, on decaying matter or in 
 the bodies of other plants or of animals. 
 
 If we consider the whole mass of vegetation and com- 
 pare the simple forms we find with others of much 
 complexity, we come to see that there is and has been 
 throughout it a continual advance, though a very slow 
 one, in the direction of greater complexity. This leads 
 us to believe that the most highly organised plants we 
 see to-day have been developed from extremely simple 
 ones during the long ages of the past. 
 
 If we try to determine how this has taken place we 
 are able to form some idea of its course by studying the 
 simple plants existing at the present time and the 
 gradual increase in complexity we can find among them. 
 Very probably the different forms show us the different 
 stages through which development has passed. 
 
 The simplest plant we find to-day is a very small 
 structure living in water. It consists of a single piece 
 of protoplasm with its nucleus ; it is clothed by a thin 
 cell wall and contains the green colouring matter. We 
 can suppose without much fear of mistake that the first 
 
THE EVOLUTION OF PLANTS 89 
 
 plant that existed was not very different from this, and 
 like it dwelt in water. Nor are we likely to be wrong 
 in thinking that the whole group of the seaweeds arose 
 by gradual development from such a form, perhaps 
 before any plants grew upon the surface of the land. 
 If we examine this group of seaweeds we find a 
 succession of forms which are step by step more 
 elaborate, and we see therein some justification for 
 thinking that development or evolution has proceeded 
 on similar lines. There are two points that may well 
 be insisted upon as a preliminary to thinking about 
 these changes: first, there is a predisposition in the 
 living substance to become more highly organised; 
 second, this organisation is brought about by the action, 
 direct or indirect, of the surroundings, the plant suc- 
 ceeding best which is in the most complete harmony 
 with them. We have already illustrated the second of 
 these points so far as the higher land plant is concerned. 
 
 It is possible that the original ancestors of the sea- 
 weeds were even simpler than the one we have described, 
 not possessing any cell wall. If so, the exposure of the 
 living substance to the changes in the water and to 
 contact with obstacles in it no doubt led to the formation 
 of this cell wall for purposes of protection. 
 
 The development of this membrane round it, how- 
 ever, introduced a difficulty in its relations with the 
 water, free access to which for so many reasons was very 
 necessary to its welfare. We can understand that the 
 development of the vacuole, with its little store of water, 
 was speedily effected to obviate this difficulty. 
 
 Increase of size of the protoplast was no doubt one of 
 the. early features of the life of the plant. This in the 
 condition of a cell clothed by a membrane was only 
 possible up to a certain point without weakening the 
 membrane and so disturbing the protection it had 
 secured. Growth was consequently followed by divi- 
 
90 BOTANY 
 
 sion into two and the gradual separation of the two from 
 one another. Here we see the simplest form of repro- 
 duction following growth, each of the two protoplasts 
 resulting from the division possessing the same proper- 
 ties as the original cell. When in some cases the 
 processes of growth and division became very rapid, a 
 second division might well take place in the two new 
 cells before they had become separated. There would 
 arise in this way by degrees a new form of plant, one in 
 which the processes of division continued, but the cells 
 did not separate at all, so that a chain or filament of 
 cells resulted. They would keep their individuality at 
 first at any rate, being almost as independent of each 
 other as if they had separated, their attachment being 
 mainly mechanical. We find a very large number of 
 water-weeds, or algae, existing in our ponds and streams 
 to-day which have just this structure. 
 
 The maintenance of this thread-like structure depends 
 on the cells always dividing in such a way that the new 
 cell walls arise across the length of the thread. It seems 
 certain that this course was soon departed from by some 
 of the plants and that some divisions arose at right 
 angles to the others. This led to the formation of a flat 
 plate of cells, still only one cell thick. A plant with this 
 structure would meet with very little greater difficulty 
 than the thread-like forms ; the requirements of the 
 cells would be the same, though the plant might easily 
 grow very much larger, the number of its protoplasts or 
 cells often amounting to many hundreds. Still the cells 
 would be all alike and probably all independent, for 
 water would have access to each and no further pro- 
 vision would be called for. Plants of this type of 
 structure are still common among marine seaweeds. 
 
 A great complexity, however, would arise if the 
 protoplasts began to divide in three planes, each at 
 right angles to the other two. No doubt this was not 
 
THE EVOLUTION OF PLANTS 91 
 
 long delayed, and plants began to possess a bulk or 
 mass, instead of being filaments or one-layered plates. 
 This was a most important change, for it altered the 
 relations between the cells or protoplasts of the plant 
 and the surrounding water. Only the external cells in 
 such a mass were able to absorb water and the inner ones 
 had to depend on them for a supply. The external 
 cells, too, were those which ran the greatest risks from 
 changes of temperature in the water and from contact 
 with particles in it, or the many dangers which the 
 environment brings to the plant. These dangers and 
 difficulties must have increased as the plant itself grew 
 larger. The difficulties of the internal cells were 
 different, but no less serious. They were compelled to 
 draw upon the external ones for the renewal of the 
 water in their vacuoles, for the oxygen necessary for 
 breathing, for their food or its constituents, and for the 
 removal of any waste products they might produce. So 
 the two became gradually less and less like each other, or 
 more specialised. The gradual change of structure and 
 form of plants that followed can be traced in this way to 
 the need for adapting them to their conditions of life. 
 
 We can follow with some probability the further 
 course of events. As the bulk increased the external 
 cells became still more devoted to protection and absorp- 
 tion ; the internal ones ceased to develop chlorophyll as 
 less light reached them, and the work of food construc- 
 tion which is the function of the chlorophyll was thus 
 thrown upon the outer layers. Among the internal 
 mass certain cells became concerned mainly in conduct- 
 ing the water to the rest, so supplying them with oxygen 
 and food. As the size became still greater the exterior 
 surface of the still symmetrical spheroidal or spherical 
 plant became inadequate to supply all that the inner 
 mass required; the spherical plant no longer had suffi- 
 cient surface in proportion to its bulk. This involved 
 
92 BOTANY 
 
 the restriction of growth mainly to certain regions of the 
 surface, which became what we now call growing points ; 
 here the multiplication of cells led to the formation of 
 conical outgrowths, and these in turn were soon recognis- 
 able as branches. The larger the plant became the 
 more necessary was it for it to become branched in view 
 of the dangers from storms, and currents in the water, 
 as it would oppose much less resistance to the move- 
 ments of the stream or tide. 
 
 The problem of the passage of the absorbed water, as 
 it made its way from exterior to interior cells, involved 
 the question of its being able to pass through the cell 
 walls that lay in its way. As the length of the body of 
 the plant increased with the putting out of branches, the 
 number of the cell walls became an inconvenient obstacle 
 to its passage. The flow being in a particular direction 
 caused the cells to stretch a little accordingly and so to 
 make them become a little longer than broad. This 
 elongation soon became advantageous, as with cells of 
 that shape there were fewer walls to pass in a given 
 distance. So gradually in the regions of transport 
 elongated cells became most usual. After a time the 
 end walls became perforated and later still dissolved 
 altogether, so that the water could pass easily along a 
 tract which was originally a number of columns of cells. 
 We can observe this change of internal structure taking 
 place in many plants to-day. Each column when it 
 has lost its end walls forms a vessel. We find them 
 fully developed only in terrestrial plants, but some of 
 the larger seaweeds contain structures much like them, 
 the end walls being perforated very freely. 
 
 During the time these changes were taking place 
 in the arrangements of the interior, other signs of 
 specialisation of exterior parts made themselves visible. 
 With increase of size and great flexibility of body as in 
 the filamentous forms it became advantageous to be 
 
DEVELOPMENT OF SEAWEEDS 93 
 
 attached to some substratum and no longer passively 
 floated about. So attaching organs were developed; 
 in some of the simple filaments they were only a modi- 
 fied cell at the basal end of the thread ; in the bulky 
 plants the whole of the unbranched end was often 
 specially modified to form a kind of grasping organ 
 capable of growing round stones or into crevices. 
 
 We find such anchorage mechanisms still in the larger 
 seaweeds of our coasts. These organs are not true 
 roots, for they anchor the plant only, and do not absorb 
 nutritive or mineral compounds for its use. 
 
 Such a development of the body of the plant was 
 sufficient for aquatic plants such as seaweeds. Even in 
 the largest of them there is only a very slight specialisa- 
 tion of structure compared with that which is necessary 
 for plants when they come to live on land. We can, 
 however, find in many present day forms three systems 
 of tissues, an external protective and absorbing coat, a 
 conducting system, and between them a system of cells 
 belonging to neither, whose function so far has not been 
 very apparent to us. We see that the division of the 
 body of the plant into members does not go so far as to 
 enable us to recognise what we call stem, leaf, or root. 
 It is in some cases undivided, in others very much 
 branched, but its structure is practically the same 
 throughout. We speak of such a form of plant as a 
 thallus, whether branched or simple, massive or minute. 
 
 CHAPTER XII 
 
 THE DEVELOPMENT OF THE REPRODUCTIVE 
 PROCESSES IN THE ALG^E 
 
 WHILE these changes were occurring in the form and 
 structure of the body of the plant, other developments 
 were taking place in the ways in which new individuals 
 
94 BOTANY 
 
 arose. While plants had only a unicellular body, the 
 division of the cell caused the appearance of a new in- 
 dividual. In fact, the individual was the same thing as 
 the cell. But when the cells failed to separate this 
 ceased to be the case, and the individual plant became 
 identified with the chain of cells. Division of a cell so 
 added another link to the chain, but it did not produce a 
 new individual. For a time the simple method of re- 
 production by cell division was replaced by the break- 
 ing of the chain into a number of segments, each of 
 which grew into a new filament like the first. This 
 method of reproduction is shown now by certain blue- 
 green algae and by many fungi. 
 
 These simple methods, however, led to the production 
 of comparatively few offspring. A larger number 
 became desirable if a species was to hold its own in the 
 competition with its neighbours for what the surround- 
 ings afforded, particularly as the life of any individual 
 was but short. So there arose in the plant body special 
 cells, usually in great numbers, which could be detached 
 and give rise to new individuals. Various forms of 
 these cells are still met with ; some with no membrane, 
 able to swim about by means of little protoplasmic 
 thread-like outgrowths, known as cilia, which by rapid 
 vibrations set up currents in the water ; some motion- 
 less and well protected by firm cell walls, so as to be able 
 to resist heat or cold, and even some degree of drying 
 up. Such little specialised reproductive cells are now 
 generally called gonidia. They are still produced in 
 various ways and usually in very large numbers by 
 many of the seaweeds and another group related to 
 them the fungi. This showed a great advance in the 
 spread of the species, as each of the large number 
 produced could give rise to a new plant. 
 
 But such rapid reproduction tends to weaken the race, 
 and no doubt it told its tale in the ancient times. It 
 
DEVELOPMENT OF SEAWEEDS 95 
 
 was supplemented by another method which gave rise 
 to a renewal of vigour in the offspring. How the new 
 method was first brought about it is hard to say. It 
 can still be seen to play its part among the free swim- 
 ming gonidia of the seaweeds. Two of them, instead of 
 developing into two plants, fuse completely together to 
 form a single protoplast, which after a period of rest 
 grows into a single new plant. This union of the two is 
 called conjugation and the fused product is a zygote. 
 
 We have in this process an indication of the way in 
 which sex in plants began to be developed. We cannot 
 speak of either of the two conjugating cells in this stage 
 as male or female, but the further development into 
 well-recognisable sexes can be easily traced. One of the 
 cells of the pair became larger and more sluggish than 
 the other ; then a stage was reached in which the larger 
 of the two was not motile at all. By and by, instead of 
 being developed from any cell of the plant they came 
 to be produced in particular cells or groups of cells 
 organs for their development. The smaller active cells 
 came to be recognised as male, the sluggish ones as 
 female. At first they were equally numerous, but as the 
 females became larger fewer of them were formed in the 
 specialised cells. The number varied indeed inversely 
 as the size. While the males continued to be produced 
 in large numbers the females became ultimately solitary 
 in the organs bearing them. Finally the female never 
 escaped from the organ but was joined there by a male. 
 The act of fusion of the two has come to be spoken of as 
 fertilisation. The female cell is now known as an ovum ; 
 the male cells are called sperms. There is an almost 
 infinite variety in the modifications which these lowly 
 plants have shown and show to-day in the ways they 
 develop their gonidia and their ova and sperms. We 
 cannot go more deeply into the matter here. 
 
 From the elaborate nature of its mechanism, this 
 
96 BOTANY 
 
 sexual process of reproduction led to the appearance of 
 fewer offspring than the method of gonidia production, 
 but those so formed were much more vigorous and better 
 qualified in every way to carry out their vital processes, 
 and hence this process has become almost universal. 
 
 This stage does not, however, mark finality among 
 plants. It involved a good deal of uncertainty as to 
 whether fertilisation would take place, as both the 
 parents of the sperm cells and the ova were unable to 
 move except as they were drifted about in the water. 
 The sperm retained the power of swimming when 
 liberated, but the ovum in the higher types remained 
 enclosed in the oogonium or cell in which it was formed. 
 This uncertainty led to a further development, by which 
 one act of fertilisation was made to produce several 
 young plants instead of one. This was brought about 
 by the zygote the fertilised female cell dividing up 
 into a number of cells, which were liberated by the 
 bursting of the zygote wall. These cells in most cases 
 were furnished with cilia and in general appearance and 
 behaviour were hardly if at all distinguishable from 
 the gonidia described before. Such reproductive cells 
 can be seen to-day in the alga (Edogonium. To distin- 
 guish them from the gonidia they are spoken of as 
 spores. One act of fertilisation results in this way in 
 the production of a number of new individuals. 
 
 A little further advance still was made among the 
 algae, which became, however, much more marked 
 among the plants which established themselves on land. 
 Instead of the fertilised cell dividing at once to produce 
 the spores, it developed into a relatively bulky multi- 
 cellular structure, which became differentiated so that 
 only part of it gave rise to the spores, while the rest 
 served to protect them and to minister to their nutrition. 
 Such a structure can be seen to-day in many of the red 
 seaweeds, in which it is known as a sporocarp. 
 
EVOLUTION OF THE LAND PLANT 97 
 
 In the land plants, as we shall see presently, this line 
 of development became much more extended. The 
 zygote gave rise to a much more highly organised sporo- 
 carp, in which the cells which formed the spores became 
 more restricted in their numbers and disposition. The 
 further progress of development led to much greater 
 differentiation of the sporocarp, which gave it the power 
 of living independently. The sterile part, or the region 
 which did not directly form spores, became much en- 
 larged and grew to dimensions much exceeding those of 
 the original thallus. Its body became differentiated 
 into root, stem, and leaves, and upon the sub-aerial 
 parts the spores were formed in structures known as 
 sporangia. This structure, known as the sporophyte, is 
 now the dominant form of all terrestrial plants. We 
 shall consider this change in the next chapter. 
 
 CHAPTER XIII 
 
 THE ORIGIN OF TERRESTRIAL PLANTS EVOLUTION 
 OF MOSSES AND FERNS 
 
 WE must now consider what has been perhaps the most 
 important step we find in the development of vegetation, 
 the transference of plants from water to land. At first 
 all were aquatic, but in the natural course of events no 
 doubt many were washed on to the moist earth by the 
 side of the water in which they were living. Being 
 adapted only to life in water no doubt most perished, 
 and it was only gradually that some established them- 
 selves. A comparison of aquatic with terrestrial forms 
 as we find them to-day shows how complete a change 
 in almost every respect took place. In the water the 
 direction of their growth was comparatively unimportant, 
 and a large number grew in a horizontal position. On 
 land we find this extremely rare; most plants, as we 
 
 G 
 
98 BOTANY 
 
 have seen, grow vertically upwards and downwards, and 
 show very different peculiarities in the parts which take 
 these opposite directions. The transference probably 
 did not take place till the habits of reproduction we 
 have briefly described had been acquired. Most likely 
 the first form which secured a footing in the soil was a 
 flattened thallus, consisting perhaps of only a few cells. 
 Its small size would enable it to touch the moist earth 
 over a large part of its surface and so to be able to 
 absorb the water it required. But with multiplication 
 of cells, and consequent increase of size, this became dim- 
 cult or impossible. It could only reach the moisture in 
 places and the supply so became insufficient. We find 
 such plants now on the earth, and we see that to secure 
 the water supply they have developed outgrowths of 
 their surface cells which resemble root hairs in struc- 
 ture; these the rhizoids adhere very closely to the 
 surface and gradually penetrate between the particles of 
 earth, so burying themselves in the soil. They become 
 very closely attached to its particles, so that they are 
 able to absorb the film of water by which each particle 
 is surrounded. They are produced in great numbers 
 and are continually being renewed. It is probable that 
 it was by such an arrangement that the early terrestrial 
 plants were enabled to establish themselves. 
 
 The difficulties which they encountered caused them 
 to produce as many and as vigorous offspring as possible, 
 and as in consequence of the scarcity of water fertilisa- 
 tion became more and more difficult, the sperms not 
 being brought into the neighbourhood of the ova, they 
 gradually came to develop an increasing number of 
 spores from each fertilised cell. This was advantageous 
 in another way as the spores were more capable of resist- 
 ing the adverse conditions than their parent, owing to 
 their simpler structure, their more moderate require- 
 ments, and their thicker outer membrane. On the 
 
EVOLUTION OF THE LAND PLANT 99 
 
 other hand, the multiplication of the young plants to 
 which the spores gave rise made competition much more 
 severe. The young plants were produced so close to 
 each other as to cause great overcrowding, making it 
 difficult for each to be properly illuminated and to 
 secure sufficient nutritive material from the soil. 
 
 The flattened form of plant under these conditions 
 became unsuitable, and each plant was compelled to 
 grow upwards or perish. The rhizoids in this way 
 tended to accumulate on the comparatively small part 
 of the thallus which was left nearest to the soil so that 
 the rest might raise itself into the air. Gradually thus 
 the plant came to show the descending portion adapted 
 to live buried in the soil and the ascending portion 
 freely exposed to light and air. We find thus an indica- 
 tion of the parts we have spoken of as the root and the 
 shoot. It must not be concluded, however, that these 
 early plants showed either in such a state of development 
 as we recognise in the land plants of to-day. 
 
 The methods of reproduction gradually underwent 
 much modification, as those of the aquatic plants were 
 but slightly suitable for erect plants, only part of which 
 could have free relations with water or moist earth. 
 The change of position rendered fertilisation uncertain, 
 and it became more and more difficult for the sperms to 
 get to the ova. To help the process to take place, more 
 elaborate structures were formed in which to develop 
 the sperms and the ova. These, known as antheridia 
 and archegonia, were produced in the neighbourhood of 
 moisture as far as possible, often on the under side of 
 the flattened thallus, or, in the case of an erect form, on 
 the parts liable to be moistened by rain or dew. The 
 result of the difficulties associated with fertilisation 
 was a great development of the structure to which the 
 fertilised ovum gave rise, which produced large numbers 
 of spores. Gradually this became more and more 
 
ioo BOTANY 
 
 elaborate, and instead of originating nothing beyond 
 the. spores and certain outgrowths to protect them, it 
 grew to be self-supporting, by producing cells or areas 
 of cells which contained the chlorophyll. Before it 
 succeeded in making chlorophyll for itself it was com- 
 pelled to derive all its nourishment from the plant which 
 bore the ovum that gave rise to it. This ovum never 
 left the archegonium in which it arose. The sporocarp, 
 as the spore-containing structure is called, originated 
 accordingly in the cavity of the archegonium and was 
 able thus to feed on the parent plant. 
 
 We find this stage in the evolution of the land plant 
 represented to-day by the mosses. The plants which 
 bear the ova and sperms are very small, seldom more 
 than an inch in height. They have a very slender stem, 
 bearing a number of delicate leaves, and are anchored 
 to the soil by a number of rhizoids which spring from 
 the bottom of the stem (Figs. 37, 40 B). The sperms 
 are produced in antheridia, which may be found at the 
 tops of some of the stems among the crowd of leaves 
 arising there (Fig. 38). The ova are similarly situated 
 at the tops of other stems ; each is developed singly in 
 an archegonium, a bottle-shaped body with a long neck 
 (Fig. 39). The sperm can only reach the archegonium 
 when the moss plants, which grow thickly together, are 
 wetted by rain or dew, as it must transport itself by 
 swimming. When it reaches the archegonium it makes 
 its way down the neck of the latter and fuses with the 
 ovum in the swollen basal part. The stem and leaves 
 of the moss plant are very simple in structure; the 
 former shows a protective outer layer or epidermis and 
 an interior mass of delicate thin-walled cells. In the 
 centre a strand of them is marked off from the rest by 
 their small size and in some cases by their altered cell 
 walls ; here we have the first indication of a conducting 
 system in the land plant. The leaves are flat plates of 
 
EVOLUTION OF THE LAND PLANT,,, ;*qj 
 
 cells. When fertilisation has been effected the ovum 
 becomes clothed with a cell wall and develops to form the 
 sporocarp. This is a small ovoid body which is 
 formed at the end of a long stalk-like structure 
 which grows out of the archegonium. It re- 
 mains attached in this loose way to the arche- 
 gonium and so appears to grow out of the 
 
 FIG. 38. Section of apex of stem of moss bearing 
 antheridia. 
 
 ordinary moss plant. The sporocarp is rather 
 complex in its structure (Fig. 40). It is not 
 all devoted to the formation of spores, but 
 contains a great deal of nutritive tissue, so 
 that it is capable of living for a long time. In 
 
 ,/ IG - p- its lower part it develops some chlorophyll- 
 Moss plant. , . . ,, ,, . r . . r , 
 
 containing cells, so that it can manufacture 
 its own food. The spores are developed in particular 
 bands of cells which arise in the interior and which are 
 usually found in the form of a hollow cylinder surround- 
 
IO2 
 
 BOTANY 
 
 ing a central core of ordinary cells (Fig. 40 C, s). The 
 moss sporocarp in most cases is provided with a special 
 mechanism to cause it to open 
 when the spores are ripe so that 
 u the latter may be discharged. 
 
 FIG. 40. Funaria. A, young sporo- 
 carp ; c, capsule or sporogonium. B, 
 moss plant with attached sporocarp 
 mature; s, stalk of sporogonium ; /, 
 capsule; g, leaves of the moss plant. 
 C, section of a sporogonium ; s, layer 
 of cells which develop the spores. 
 
 ,-, -r> When the spore germinates 
 
 , it produces a little filamentous 
 FIG. 39- ,4, section of stem of ___ __ Al _ ___^_._^ ,_ __^__ x ___.,__ 
 
 moss bearing archegonia ; B, outgro wth which branches freely 
 
 open neck of archegonium ; on the moist Soil, and which 
 
 resembles very closely a fila- 
 mentous alga. The moss plants 
 originate by the development of buds upon this out- 
 growth, which is known as a protonema. 
 
 C, archegonium highly mag- 
 nified. (After Sachs.) 
 
EVOLUTION OF THE LAND PLANT 103 
 
 The altered conditions of their life can now be seen to 
 lead to a very great change in the life history of plants. 
 The aquatic forms with their comparatively simple body 
 and the process of fertilisation depending on the power 
 of the sperm to swim to the ovum proved exceedingly 
 unsuitable for life on land, and very soon a great develop- 
 ment of the sporocarp began, and it gradually assumed 
 the form of a self-supporting plant the sporophyte. 
 From this point upward the tendency of evolution was 
 to diminish the plant which bore the ova and sperms till 
 there was hardly anything of it except these repro- 
 ductive cells, and at the same time to make the sporo- 
 phyte more and more important, till it became far the 
 most highly developed, and of infinitely larger size than 
 what was left to represent the original ancestor. 
 
 We find this change well established in the ferns and 
 the plants which are allied to them. The ova and 
 sperms are produced on a thin plate-like body of about 
 half an inch in diameter. This lies upon the soil and is 
 attached to it by rhizoids. The antheridia and arche- 
 gonia are borne upon its lower surface and fertilisation 
 is accomplished by free-swimming sperms. The plant 
 is known as the prothallus ; it is made up of cells which 
 are much alike throughout. The sporocarp arising from 
 it has been replaced by a large plant, showing root, stem, 
 and leaves. We do not know the stages by which so 
 large and well-organised a structure has been developed 
 from the original sporocarp, but it represents the latter 
 in its place in the life history of the fern. So important 
 has it become, and so great in comparison with the 
 prothallus, that it has come to be called the " plant," 
 leaving the ovum-bearing plant to be regarded as the 
 " prothallus of the fern." 
 
 We see that with the establishment of terrestrial 
 habit, the life history of the plants thus became quite 
 revolutionised. The large tree of the land flora does 
 
104 BOTANY 
 
 not correspond with the large seaweed, but with a parti- 
 cular reproductive structure to which the latter gave rise. 
 The fern plant differs a good deal from the seed-bear- 
 ing plants which we have examined in the earlier 
 chapters. It has- as a rule only underground stems 
 which are known as rhizomes; each bears very few 
 leaves at or near its apex. The stem grows horizontally 
 under the surface of the ground, reacting to gravitation 
 
 FIG. 41. Prothallus of fern. X5- 
 
 in a way unlike either stem or root of the flower- 
 ing plant. The leaves emerge from the soil generally 
 rolled up in the form of a shepherd's crook, in conse- 
 quence of the great growth of the under surface. They 
 soon straighten themselves as the growth of the upper 
 surface becomes vigorous, just as in the case of the leaf 
 in the bud of the flowering plant. They are then found 
 to be very much divided, except in a few cases. Roots 
 are given off from the rhizome in large numbers. 
 
 The structure of the fern rhizome differs in detail 
 from that of either type of flowering plant we have 
 
EVOLUTION OF THE LAND PLANT 105 
 
 described. There are the three systems of tissue, 
 dermatogen, periblem, and plerome, but they are not so 
 clearly distinguishable. They originate by divisions in 
 a single large pyramid-shaped cell at the apex the so- 
 called apical cell, which cuts off segments of itself by 
 walls parallel in turn to each of its sides except the 
 outside one. These segments divide later to make up 
 the mass of cells at the end, in which the differentiation 
 into the three regions spoken of takes place. 
 
 The plerome does not give rise to a single solid or 
 hollow cylinder, but to one in which the conducting 
 strands form a cylindrical network. The strands to 
 the leaves leave the network at the margins of the 
 meshes. The strands are composed of wood and bast 
 as in other cases, but in their arrangement the bast 
 usually surrounds the wood completely. There is no 
 cambium and no increase in thickness occurs. 
 
 The leaves are something like the leaves of a dicoty- 
 ledon in structure, but most of the internal tissue re- 
 sembles the spongy tissue rather than the palissade. 
 The epidermis contains chloroplasts. 
 
 The structure of the root resembles that of a dicoty- 
 ledon very closely, but there is no provision for any 
 increase in thickness. The pericycle is several layers of 
 cells in thickness. 
 
 The fern bears its spores in little cases known as 
 sporangia. In our common ferns these are grouped to- 
 gether on the under sides of the leaves in little patches called 
 sori (Fig. 42). Each sporangium contains a number of 
 spores. Each spore on germination produces a prothallus. 
 
 We found it impossible to say how such a plant as 
 this has come to be formed in the place of the sporocarp 
 of the mosses. We find almost as great difficulty in 
 tracing the formation of the flowering plants from 
 plants having the degree of development of the ferns. 
 
 We may well realise that with the gradual increase of 
 
106 BOTANY 
 
 complexity of the sporocarp it came to have an inde- 
 pendent existence, and that when it had achieved this, 
 it became an erect plant, the conditions we have already 
 alluded to making this necessary. At some stage or 
 other in the development a difference arose among the 
 prothalli arising from the spores, some giving rise to 
 sperms only while the others only produced ova. Gradu- 
 ally the change spread to the spores themselves, those 
 giving rise to sperm-bearing prothalli remaining small, 
 
 FIG. 42. Section of sorus of fern showing sporangia 
 covered by an indusium. X5o. (After Kny.) 
 
 those producing ova-bearing prothalli becoming much 
 larger. The prothalli also changed. Those from the 
 small spores became in some cases filamentous, and in 
 all extremely minute, consisting of little more than the 
 organ giving rise to the sperms. Those from the large 
 spores only protruded partly from the ruptured spore, 
 and came to be developed almost entirely inside it, the 
 spore splitting its coats but slightly. Such prothalli 
 developed very few archegonia. 
 
 These forms are still represented to-day by the 
 Selaginellas, a group classed among the fern-like plants. 
 
EVOLUTION OF THE LAND PLANT 107 
 
 As the spore-bearing plant or sporophyte grew larger 
 and by its erect position became carried away from the 
 ground, the separation of the two kinds of prothalli 
 developed from the spores rendered fertilisation by 
 means of free-swimming sperms increasingly difficult. 
 The larger spores, 
 too, tended to stay 
 longer and longer nrch 
 in their sporangia, 
 so that the time for 
 fertilisation became 
 shortened. This led 
 to the establish- 
 ment of a new 
 method of securing 
 fertilisation which 
 we find exhibited 
 by the flowering 
 plants. The pro- 
 thalli - bearing ova 
 became entirely en- 
 closed in the large 
 spore, and those 
 bearing sperms be- 
 came entirely fila- 
 mentous. Instead 
 of the sperm 
 travelling to the ovum, the small spore itself was brought 
 by various ways to the immediate neighbourhood of the 
 large one, so that the prothalli were developed in close 
 proximity to each other. The filamentous prothallus of the 
 small spore bored its way into the surroundings of the large 
 spore and the latter remained altogether in its sporangium. 
 Fertilisation consequently came about by means of a 
 tubular outgrowth of the little spore, instead of a free- 
 swimming sperm. If we compare the two methods, both 
 
 FIG. 43. Germination of megaspore of Selagi- 
 nella, showing prothallus almost entirely 
 inside the spore, which has opened at the 
 apex, arch, archegonia; oos, ovum; em, 
 young embryos at different stages of de- 
 velopment. 
 
io8 BOTANY 
 
 of which still persist, we find that in the latter all the 
 structures are exposed freely ; in the former the greater 
 part are embedded deeply in other parts of the plant. 
 
 We shall return to this subject in connection with 
 the mechanism of the flower in the higher plants. 
 
 CHAPTER XIV 
 
 REPRODUCTION OF FLOWERING PLANTS 
 VEGETATIVE PROPAGATION 
 
 THE last requirement of the plant we must consider is 
 the power of reproducing its species. 
 
 There is a good deal of variety in the ways in which 
 this is possible ; some methods consist in separating 
 certain parts of the ordinary plant body which after a 
 time grow into new plants ; in others special reproduc- 
 tive cells are produced. The first method is often 
 spoken of as vegetative propagation. The parts which 
 can be spared for it in different plants are a good deal 
 modified and have come to be considered as separate 
 organs. They include modified stems, leaves, or roots. 
 
 We find such structures include a part which is cap- 
 able of growing some kind of bud and a store of food 
 for the nutrition of the shoot which arises from it. 
 
 The most easily observed of the modified stems is 
 known as the tuber. This is a stem or branch which 
 grows under the ground instead of above the surface. 
 It consists of a few internodes, which become greatly 
 swollen and filled with starch and protein. The leaves 
 are only to be seen with the help of a lens; they 
 are minute scales, which never develop further. The 
 buds are in their axils and are generally several in 
 number, so that a tuber can put out several shoots, each 
 of which may grow into a new plant. The most familiar 
 instance of a tuber is afforded by the common potato. 
 
REPRODUCTION 109 
 
 Another form of underground stem which may be 
 included here is the rhizome of such plants as the iris. 
 It grows partly under the surface of the ground, but its 
 upper side often protrudes and becomes green. It 
 never grows vertically into the air, but its terminal bud 
 sends a shoot upwards which bears the foliage leaves 
 and flowers. It is swollen and filled with food like the 
 potato, but it does not become detached as the tuber 
 does. It is in fact the main stem of the plant and pro- 
 duces only a single bud, instead of a number. 
 
 Two other forms of propagating organ are the bulb 
 and the corm. The bulbs are very large buds, a rela- 
 tively small conical stem being covered with a large 
 number of scaly leaves, the inner ones becoming very 
 succulent and containing food, chiefly sugar, for the 
 young plant, into which the growing apex will develop. 
 The outer leaves remain dry scales and are only pro- 
 tective to the succulent interior. The onion is a good 
 example of the bulb. 
 
 The corm consists of a few internodes of an under- 
 ground stem. It is solid but resembles the bulb in 
 being clothed with dry scaly leaves. There is a bud at 
 its apex which is like the most internal part of the bulb 
 of the onion. An example is afforded by the crocus. 
 
 Roots which are modified for reproductive purposes 
 are shown by the dahlia. They swell and store food 
 after the manner of the tuber, but they do not develop 
 buds. They give up their contents to a shoot put out 
 from a portion of the stem of the original plant. 
 
 Vegetative propagation is very commonly employed 
 by gardeners, by means of cuttings. A piece of a young 
 stem, with a few leaves and buds, cut from the parent 
 will, when planted, often develop roots from the cut 
 surface and so establish a new plant. Some plants pro- 
 pagate themselves naturally in this way ; any detached, 
 injured portion, when left upon damp soil, will put out 
 
no BOTANY 
 
 adventitious roots and so grow into a plant. The power 
 of putting out adventitious roots is often used by plants 
 for this purpose. Other familiar examples are afforded 
 by the runners of the strawberry, the suckers of the 
 raspberry, the stolons of the gooseberry and other plants. 
 
 The runner is a lateral branch which grows along the 
 surface of the ground and puts out adventitious roots 
 at its nodes. In the case of the strawberry each runner 
 usually has two nodes, one terminal and one half way 
 along its length. Two new plants arise on each in con- 
 sequence, and when they are established the internodes 
 connecting them with the parent die. 
 
 The stolon is much like the runner ; arising as a lateral 
 branch above the ground it bends down and on reaching 
 the soil it puts out adventitious roots and becomes 
 detached from the parent. The sucker arises from the 
 underground portion of the stem and growing horizon- 
 tally for a time ultimately turns upwards and emerges 
 into the air becoming then detached as in other cases. 
 
 These methods are purely vegetative and do not 
 involve the production of any form of specialised repro- 
 ductive cell. 
 
 CHAPTER XV 
 
 THE INFLORESCENCE AND THE FLOWER 
 
 As we traced the development of the reproductive 
 organs from the seaweeds upwards we found that two 
 special kinds have become constant, the ova and 
 sperms on the one hand and the spores on the other. 
 We also learned that the plant form which produces the 
 spores has become the conspicuous plant, with its roots, 
 stems, and leaves ; while those which give rise to the ova 
 and sperms have dwindled away in dimensions till they 
 no longer have an existence apart from the spore itself. 
 We will study first the reproductive processes of the 
 
THE INFLORESCENCE AND THE FLOWER in 
 
 form which produces spores the so-called sporophyte 
 phase of the plant's life history. 
 
 We have pointed out that in the flowering plants two 
 kinds of spore are produced. The little ones resemble 
 the spores of the ferns and mosses ; they are small cells 
 with thick walls, usually spherical in shape. They are 
 liberated from the sporangia in which they are de- 
 veloped and germinate after their removal. The large 
 spores are produced singly in each sporangium and 
 never escape from it. The sporangium is a bulky struc- 
 ture and consequently the spore need not develop the 
 thick wall found on the little spore. 
 
 We have now to see where these spores occur and 
 what is the story of their behaviour. 
 
 In most ferns we saw that the sporangia occur in 
 patches on certain places on the ordinary leaves. In 
 some, however, they are found on very specialised out- 
 growths, generally .spoken of as modified leaves. In the 
 flowering plant the sporangia are usually collected at 
 the end of special shoots which are known as flowers. 
 A special system of the branching is set apart for their 
 production. It may consist of very many branches and 
 may be very complicated, hence it is usually considered 
 separately under the name of the inflorescence. Its 
 ultimate branches are known as flowers, the leaves in 
 whose axils they are found are termed bracts. 
 
 A very common form of the inflorescence is that in 
 which the main axis continues to increase in length and 
 bears a succession of flowers as it grows on, so that the 
 youngest flower is nearest the apex. This corresponds 
 to the indefinite method of vegetative branching. The 
 inflorescence is called a raceme. There are many modi- 
 fications of it on all sides : if the flowers have no stalks 
 it is called a spike; if they all reach the same level 
 through the older stalks growing longer than the younger 
 it is a corymb ; if the axis is so short that all the flowers 
 
U2 BOTANY 
 
 appear to start at the same point it is an umbel (Fig. 44) ; 
 if it is itself branched it becomes a panicle. 
 
 A fundamentally different form is that in which the 
 main stalk is at once terminated by a flower and other 
 stalks grow out from under it to bear similarly a flower 
 
 FIG. 44. Umbellate inflorescence of ivy. (After Marshall Ward.) 
 
 at the end of each ; we have here what is called a cyme. 
 It corresponds to the definite method of branching of 
 the vegetative parts. As in the case of the raceme 
 there are many varieties of this kind of inflorescence. 
 
 A very noticeable modification of the raceme is the 
 form in which the apex is not elongated or conical, but 
 spreads out into a flat receptacle on which large numbers 
 of small flowers or florets are arranged so that the 
 youngest are in the centre and the oldest ones round the 
 
THE INFLORESCENCE AND THE FLOWER 113 
 
 circumference. This is known as a capitulum. It is 
 generally surrounded by a number of bracts, which form 
 what is called an involucre. 
 
 In its most primitive form the flower probably con- 
 sisted of an axis on which the sporangia were borne, and 
 the latter were of only one kind. In many cases, most 
 likely arising later, we find this axis bearing two sporan- 
 gial series of outgrowths, one for the production of each 
 kind of spore, and this arrangement gradually became 
 widespread, the large spores being developed in the 
 series nearest the apex and the small ones in the lower 
 of the two. As each series stood in a circle round the 
 axis, all its parts arising at the same node, they received 
 the name of whorls. 
 
 In the more perfect forms of flower which have been 
 developed as time has gone on (Fig. 45) we find, besides 
 these two whorls of spore-bearing leaves, two other 
 whorls growing below them. These two constitute the 
 perianth of the flower. A leaf lower down still, in the 
 axil of which the flower arises, is known as a bract. 
 
 The perianth of the flower is formed then by two 
 series or whorls of leaves. The outer ones are green 
 and often sturdy in their texture, and they protect 
 the young flower while it is in 
 the condition of a bud. They 
 are known as sepals, and the 
 collection of them forms the 
 calyx. The inner whorl is 
 usually made up of highly 
 coloured leaves which serve to 
 make the flower conspicuous. 
 They are called petals, and the FlG ' 45 ' ' 
 collection is termed the corolla. 
 These whorls frequently have their sepals or petals joined 
 together for part or even the whole of their length. 
 
 The whorls which bear the sporangia are distinguished 
 
H4 BOTANY 
 
 from the rest as the sporophylls. They are usually 
 considered to be modified leaves, as the name indicates. 
 The outer whorl consists of bodies very unlike leaves in 
 appearance. Each shows a slender stalk or filament 
 bearing at its top a swollen head or anther. The whole 
 club-shaped organ is called a stamen. Each anther 
 contains a group of four chambers, which are the 
 sporangia, and inside them are the small spores. They 
 are commonly spoken of as pollen sacs, containing pollen 
 grains. These were the old names applied to them 
 before their true nature was understood, and they are 
 retained still as a matter of convenience. 
 
 Like the sepals and petals the stamens may be joined 
 together in various ways, or they may be free. They 
 may grow from the flower stalk, or they may appear to 
 spring from the calyx or the corolla. These appearances 
 are due to curious irregularities in the growth of the 
 parts of the flower. 
 
 The sporophylls of the final whorl are known as 
 carpels, and together they form the pistil. In shape the 
 carpels resemble a leaf folded on the mid-rib till the 
 edges meet and become united, so that a cavity is 
 formed inside them. This cavity is the ovary. It is 
 more usual to find the carpels united by their edges to 
 form a large ovary, which is often divided up into several 
 chambers. The sporangia are found in the interior of 
 the ovary, attached usually to the edges of the carpels. 
 They were formerly called ovules, and the name is still 
 often used (Fig. 46) . The ovule or mega-sporangium is a 
 fairly substantial structure. As the spore never leaves 
 it but produces its prothallus in its own interior while 
 still within it, and as the young embryo plant is developed 
 on the prothallus inside the spore, it is clearly an organ 
 of the greatest importance. It consists of a mass of 
 small cells called the nucellus, and is covered by two 
 membranes or integuments which also are many-layered 
 
THE INFLORESCENCE AND THE FLOWER 115 
 
 and strong. At its upper end the integuments do not 
 cover it but leave a little aperture, the micropyle. Each 
 ovule contains a single thin-walled spore, which often 
 occupies a very large space in its interior. It is often 
 called the embryo sac, 
 from the fact that the 
 embryo is developed in 
 its interior. 
 
 The ovary is only the 
 basal part of the carpel 
 or the pistil. There is 
 always a sticky apex to 
 it, which is the stigma. 
 The stigma is usually 
 placed at the top of an 
 elongated part of the 
 pistil, seeming to arise 
 from the top of the ovary 
 this is the style. 
 
 The pistil is always the 
 terminal whorl of the 
 flower. Some flowers, 
 however, owing to a 
 curious mode of growth 
 of the axis, which be- 
 comes concave and comes to surround and often to cover 
 in the carpels, appear to bear their stamens and perianth 
 above the ovary. The latter is then termed inferior. 
 
 Many flowers are not provided with all these parts. 
 It is not at all uncommon to find that the corolla is not 
 developed. In such cases it frequently happens that 
 the calyx is not green but brightly coloured. In many 
 monocotyledons both calyx and corolla are coloured so 
 similarly that it is difficult to distinguish between them. 
 The flowers of some of our forest trees do not possess a 
 perianth at all, and in the flowers of others only one 
 
 Fig. 46. Section of an ovule, mac, 
 megaspore or embryo -sac; oos, 
 ovum; pt, pollen tube. 
 
n6 
 
 BOTANY 
 
 series of sporophylls is present. Those which bear 
 stamens only are known as staminate flowers ; those 
 which possess only carpels are called pistillate. Such 
 flowers are generally very small and inconspicuous. 
 
 Another type of flower altogether is found in the 
 group of plants called Gymnosperms, which is repre- 
 sented in this country by the fir trees and their allies. 
 
 FIG. 47. A, twig of fir tree bearing a young female cone; B, twig bearing 
 several male cones; C, ovaliferous scale from A showing two ovules 
 on the under surface. (After Scott.) 
 
 In most of these the sporophylls are arranged in a close 
 spiral round an axis and form the structures known as 
 cones. The fir tree itself bears two kinds of these cones 
 (Fig. 47) each bearing one kind of spore. The smaller 
 cones are composed of a large number of very small 
 leaves arranged spirally round the axis of the flower, 
 covering or overlapping each other very closely. On 
 the back of each leaf or sporophyll are two sporangia, or 
 pollen sacs, each containing a large number of micro- 
 spores or pollen grains. Larger cones are developed in 
 connection with the production of the megaspores. The 
 
POLLINATION 117 
 
 general arrangements of the cones are similar to those 
 of the smaller ones. There is a central axis round which 
 the leaves or sporophylls are spirally arranged. Each 
 sporophyll has on its inner face a flattened outgrowth 
 which in some cases becomes larger than the sporophyll 
 itself. On its upper side this so-called ovali/erous scale 
 bears two sporangia or ovules. There is no closing up 
 of the sporophyll to form an ovary hence the name of 
 the group gymnosperm, meaning naked seed. Each ovule 
 has much the same structure as that described above, but 
 there is only one integument and the micropyle is larger. 
 
 CHAPTER XVI 
 
 POLLINATION AND ITS MECHANISMS FERTILISATION 
 
 THE separation of the spores and the diminution of the 
 phase of the plant which results from their germination, 
 consisting of little more than the ova and the sperms, 
 have made it impossible for fertilisation to take place by 
 means of free-swimming sperms. To ensure its occur- 
 rence it has become necessary to secure that the two 
 spores producing ova and sperms respectively shall be 
 brought close to each other, so that, when they germinate, 
 their prothalli, now so rudimentary, shall be able to 
 meet, in order that the sperm may make its way to the 
 ovum. This has been effected by the transport of the 
 pollen grain the small spore to that part of the flower 
 which bears the large spore or embryo sac. This trans- 
 ference of the pollen is spoken of as pollination. In the 
 fully organised flowers of Dicotyledons and Monocoty- 
 ledons the pollen grain is deposited on the stigma in 
 the cones of the Gymnosperms it reaches the ovule or 
 megasporangium itself. The problem of pollination 
 presents many interesting features which we may now 
 briefly examine. 
 
n8 BOTANY 
 
 As in each perfect flower both spores are produced, the 
 problem at first sight appears to involve only the transfer- 
 ence of its pollen grains to its stigma. In many cases this 
 is all that takes place, but simple as the method is, its 
 occurrence is the exception rather than the rule, for much 
 stronger and healthier plants -are yielded when the pollen 
 from one flower is made to fall upon the stigma of another. 
 The former simpler process is termed self-pollination; it 
 was probably the most primitive, but became gradually 
 superseded by the latter, known as cross-pollination. 
 
 The necessity for pollination and the advantages 
 presented by the crossing have led to the development 
 of various mechanisms in flowers to bring it about. 
 Here, perhaps more than in any other case, do we 
 recognise the difficulties entailed by the stationary 
 situation of the plants. The small spores though set free 
 are non-motile and must be carried to the stigma by 
 some external agency which is not under the control of 
 either plant. Hence it is that we find perhaps more 
 adaptations or modifications of structure in connection 
 with this function than any other. 
 
 The transport of the pollen at the outset must have 
 been brought about by the physical agents of nature, 
 water in the case of aquatic flowering plants, wind in 
 that of those of terrestrial habit. In both these cases the 
 chances of the liberated pollen being floated or blown to 
 the stigma are not great. Hence the pollen in such 
 flowers is always produced in large quantities. Aerial 
 transport is further aided by the production of very 
 light, dry, relatively smooth pollen, in some cases aided 
 by mechanical modifications of the wall, expansion 
 into bladders, etc. The stigmas in plants which are 
 pollinated in this way are often divided a good deal, 
 becoming in the case of the grasses quite feathery, so as 
 to offer as much surface as possible to entangle the 
 pollen. The surface of the stigma often bears a velvet- 
 
POLLINATION 119 
 
 like pile of short hairs, and usually secretes a sticky, 
 sugary excretion features which subserve the same end. 
 
 This so-called anemophilous pollination is uncertain 
 and wasteful and has in many cases been superseded by 
 the intervention of insects. Hence we find developed 
 the colour, fragrance, and other attractions which 
 flowers so generally possess. Colour and fragrance cause 
 conspicuousness, and appeal to insect visitors. The 
 latter seek, of course, more substantial benefits than 
 these, regarding the flowers as the seats of supplies of 
 nutritive substances which they require. While they 
 rifle the flowers of both pollen and honey, they inad- 
 vertently serve their turn by transferring the pollen 
 from the stamens of one flower to the stigma of another. 
 
 The relations that have thus come to be so widely 
 established have probably grown up very gradually and 
 in an infinite variety of ways. Different insects visit 
 different flowers and the influence of the particular kind 
 of insect visitor explains the manner of modification 
 which the particular flower has undergone. The modi- 
 fications are almost infinite in variety and we can only 
 here deal with them in a very general way. 
 
 The earliest modifications, which were only slight, 
 were associated with the discovery by certain lowly 
 insects that pollen itself formed nutritious food. The 
 flower was widely open, the perianth leaves spreading 
 symmetrically round the axis below the stamens, and 
 the latter opened and let their pollen fall. A slight 
 change of colour, probably from green to yellow or 
 white, made the flowers sufficiently conspicuous, and 
 the visits of the insects followed. 
 
 But a further attraction was afforded later by the 
 formation and storage of honey in the flower. It is 
 impossible to discuss this subject in detail as it has 
 exhibited such infinite variety of modification. The 
 storage of honey led to the development of pocket-like 
 
120 BOTANY 
 
 bodies, slippers, or spurs, either on the receptacle or the 
 perianth leaves. Irregularity of form of the calyx or 
 corolla was thus caused. Markings on the individual 
 sepals or petals followed, which directed the attention of 
 the insect, usually a wasp or bee, to the hidden store. 
 The honey became hidden in such a way that to reach it 
 the visitor brought some definite part of its person into 
 contact with the stamens, and on visiting another 
 flower touched the stigma with the same region. The 
 influence of colour and of fragrance were brought to bear 
 upon the insect visitors with similar results, the whole 
 mechanism of any particular flower being correlated 
 with the habit of some appropriate insect. 
 
 The mechanisms of some flowers at the present time 
 are even more complete than this. As cross-pollination 
 is preferable to self-pollination, the flower is adapted 
 to some visitor to secure it; but as self-pollination is 
 better than none at all, if the insect mechanism should 
 fail to act, the stigma is in some way brought into 
 contact with the stamens of the same flower. 
 
 The apparent ease with which self-pollination of 
 almost every flower can be brought about has led in 
 many cases to a peculiar modification to secure the 
 advantages of crossing. This consists in the maturing 
 of the stamens and the stigmas of a flower at different 
 times, so that if self-pollination should take place there 
 would be no result therefrom. This condition is known 
 as dichogamy ; flowers whose stamens mature before 
 their carpels are said to be protandrous ; those in 
 which the condition is reversed are proterogynous. 
 
 It is of course obvious that cross-pollination is the 
 only possible method in the cases of those plants which 
 bear pistillate and staminate flowers. 
 
 The cones of the fir trees are pollinated by wind. 
 When the cone is ripe its leaves separate a little from 
 each other so that the pollen grains, blown in large 
 
POLLINATION 121 
 
 numbers, can be carried down into the heart of the cone 
 to the bases of the scales on which the ovules are seated, 
 as already described. There is no stigma and the 
 ovules are exposed; hence the pollen grain is dropped 
 upon the micropyle of the ovule, down which it is 
 drawn into a little space just above the body of the 
 ovule itself and inside its integument. This little 
 cavity is known as the pollen chamber. 
 
 In all cases these mechanisms bring the two spores 
 very near to one another. In the Gymnosperms they 
 are separated by only the substance of the upper part of 
 the ovule, whose spore or embryo sac lies quite near the 
 upper end. In the other flowering plants the two 
 spores, the pollen grain and the embryo sac, are separ- 
 ated by the length of the style, the chamber of the 
 ovary, and the upper part of the ovule. 
 
 The next steps in the life history are the germination 
 of these two spores. We have already seen that the 
 result of the germination is the production of little 
 besides reproductive cells, the vegetative parts of the 
 prothalli being very small indeed. In the Gymno- 
 sperm the megaspore or embryo sac becomes filled with 
 a cellular prothallus the endosperm at the upper 
 end of which several archegonia make their appear- 
 ance, each containing one ovum. The pollen grain or 
 microspore puts out a tube which bores its way through 
 the substance of the upper part of the ovule till it 
 reaches the embryo sac and comes into contact with its 
 wall. As it grows it produces two sperms in its interior ; 
 these are for the most part amorphous pieces of proto- 
 plasm, though in a few cases each bears a band of cilia. 
 When the tube reaches the embryo sac the walls at the 
 point of contact dissolve and the sperms pass through 
 and each can fuse with an ovum in one of the archegonia. 
 
 In the angiospermous plants the process is similar, 
 but there are characteristic differences. The tubular 
 
122 BOTANY 
 
 outgrowth of the pollen grain the pollen tube pene- 
 trates the style, reaches the cavity of the ovary, makes 
 its way to the micropyle of an ovule -(Fig. 46). During 
 its progress it develops two sperms, which are quite free 
 from cilia or motile organ of any kind. The embryo sac 
 meanwhile develops its prothallus, which consists of a 
 group of cells at either end and a large nucleus in the 
 centre. One of the cells at the apical end is the ovum. 
 The fusion of the walls of the pollen tube and embryo 
 sac takes place and the two sperms enter the megaspore. 
 One fuses with the ovum, the other with the large 
 nucleus in the centre of the sac. 
 
 These fusions constitute what is known as fertilisation. 
 
 CHAPTER XVII 
 
 FORMATION OF THE SEED AND ITS MIGRATION 
 THE FRUIT 
 
 AFTER a very short interval further development begins. 
 The ovum in each case clothes itself by a cell wall and 
 certain complicated divisions take place which we cannot 
 consider in detail here. They result in all cases in the 
 formation of an embryo, or young sporophyte, which 
 remains inside the embryo sac. In the Gymnosperm it 
 is surrounded from the first by the prothallar tissue; 
 in the Angiosperm its development is accompanied by 
 the formation from the fertilised nucleus of the embryo 
 sac of a similar mass of cells, also known as the endo- 
 sperm. These changes are accompanied by considerable 
 growth of the ovule. Its integuments not only increase 
 in size but become chemically altered, generally dry and 
 hard. The embryo sac usually grows at the expense of 
 the substance of the ovule till it has absorbed the whole 
 of the latter except the integuments. In some cases, as 
 the bean with which we began our study, the young 
 
THE FRUIT 123 
 
 embryo absorbs the contents of the cells of the endo- 
 sperm and so fills the embryo sac. In others, as in the 
 castor oil seed, a good deal of endosperm remains ; in 
 yet others part of the ovule may have escaped absorp- 
 tion by the growing embryo sac. In all cases there soon 
 comes a period when the growth and development of all 
 these parts stops and the resulting structure, now 
 become the seed, enters on a more or less prolonged 
 period of absolute rest. This is the period during which 
 alone the migration of the species is possible, as by 
 various means the seed is carried from the parent plant. 
 
 We called attention at the outset to the fact that the 
 life of the individual plant has to be spent absolutely in 
 one spot, that at which it is rooted to the ground. How 
 disadvantageous this is to the plant and what an endless 
 series of struggles against the difficulties it involves, has 
 been one of the principal lines of thought throughout 
 our study. The final difficulty meets us when we con- 
 sider the production of offspring. How can they 
 possibly flourish or even survive when their parent is 
 hampered in this way ? The solution of the difficulty is 
 found in the provision which is made for the wider dis- 
 persal of the reproductive bodies, mainly seeds. 
 
 The fact that most plants produce many seeds and 
 that each seed after a period of rest produces a new 
 plant makes it imperative that adequate means of dis- 
 persing seeds shall be found, or clearly the problem of 
 the maintenance of the species would not be solved. 
 We must turn, therefore, to consider this matter closely 
 and to study certain new structures which are immedi- 
 ately concerned with it. 
 
 The migration of seeds is brought about in an almost 
 infinite variety of ways, each species having its own 
 mechanism. In most cases it is associated with the 
 development of a new structure, the fruit. 
 
 While the changes are taking place by which the 
 
124 BOTANY 
 
 ovule is transformed into the seed, the stimulus to 
 growth, which the act of fertilisation of the ovum ad- 
 ministered to the latter, affects also the parts in its 
 neighbourhood. We have seen the embryo sac enlarg- 
 ing and the integuments of the ovule developing into the 
 testa of the seed. The ovary, too, resumes development 
 and increases in size, often enormously, by a large pro- 
 duction of succulent cells. When its full dimensions 
 have been attained the nature of the cells and their 
 contents changes. In some cases they become woody, 
 hard, and dry; in others succulent and charged with 
 sugar and various flavouring matters. These latter 
 changes make up the process known as ripening. The 
 enlarged dry or succulent wall is now known as a peri- 
 carp, and with the seeds which it contains it constitutes 
 the fruit. The latter is the modified wall of the ovary, 
 much changed and developed by the processes of growth 
 which have followed fertilisation. The fruit is thus a 
 mechanism which is concerned chiefly with the problem 
 of the dispersal of the seeds ; it plays also an important 
 protective function during the maturing of the seeds, 
 though this is not its main purpose. 
 
 In some plants the renewed development does not 
 stop at the ovary or the carpels. Other parts of the 
 flower are involved, generally the axis or receptacle, as 
 in the apple, strawberry, etc. In some cases the leaves 
 of the perianth also become succulent. 
 
 In yet other cases the development affects simul- 
 taneously all the flowers of a closely arranged inflores- 
 cence. All become succulent and fused together to 
 form a single fruit, as in the pine-apple and the fig. 
 
 There are thus many intricacies of development and 
 the construction of the fruit is often complicated. Its 
 purpose is to distribute the seeds over a wide area. 
 
 The lines of the development of the fruit are in 
 almost all cases the same at the outset. The parts 
 
THE FRUIT 125 
 
 which ultimately form it grow, and the new material 
 is at first succulent, being made of ordinary thin-walled 
 cells. When the full dimensions are attained this 
 succulent tissue changes and assumes the characters of 
 the ripe fruit. Two main departures may be noticed 
 in the first the succulence becomes more pronounced, 
 the cells more juicy, and their contents changed by the 
 deposition of sugar, the development of particular 
 flavours and fragrance, or of other less attractive 
 chemical substances. We get thus a class of fruits 
 which appeal to the animal world and whose fate is 
 probably to be eaten. The seeds which are developed 
 inside them are usually furnished with a hard testa or 
 skin, so that they may escape injury in passing through 
 the animal's body. Succulent and hard parts thus go 
 together, though their soft and their resistant parts 
 have not in all cases the same origin. 
 
 In the second departure from the original softness we 
 find a tendency to hardness and dry ness throughout. 
 Sometimes the fruit becomes woody more frequently 
 dry and papery, or resembling cork in its general pro- 
 perties. This type of fruit is associated with other 
 means of dispersal : often endowed with a kind of explo- 
 sive mechanism, so that rupture of its walls is followed 
 by a jerking of the seed for some distance; often 
 furnished with some means of transport, such as hooks, 
 that may attach themselves to passing animals, or 
 floats of various kinds that may buoy up the fruit in the 
 air and enable it to take advantage of currents of wind. 
 
 In some cases the seeds themselves are furnished 
 with one or other of these mechanisms. It is, indeed, 
 often difficult to distinguish between small fruits and 
 seeds when the latter are thus endowed. In such cases 
 they always escape from the fruit before dispersal. 
 
 Various methods of classifying fruits have been 
 adopted, and a somewhat ponderous nomenclature has 
 
126 BOTANY 
 
 arisen, which, however, is comparatively unimportant. 
 The important consideration is the need of the plant; 
 the various ways in which it is supplied may advan- 
 tageously occupy our thoughts rather than the duty of 
 finding a special name for each variety. It has, how- 
 ever, become the custom to speak of a fruit -which has 
 been developed from the carpel or carpels only of a 
 flower as a true fruit. One into whose composition some 
 other part of the flower enters, usually some part of the 
 floral axis, is known as a spurious fruit, or pseudocarp. 
 The distinction is in many cases very difficult to make 
 and from our point of view is quite unnecessary. Such 
 fruits as the pine-apple and fig, which are the product of 
 whole inflorescences and not of single flowers, may be 
 distinguished as aggregated fruits. 
 
 Among the succulent fruits the most prominent is 
 the berry, which exists in several varieties. It is seen 
 in its simplest form in the grape, while varieties of it are 
 illustrated by the gooseberry and the orange. The 
 hard parts of this mechanism are the walls of the seeds 
 the berries contain. Another succulent fruit is the 
 drupe, in which the middle layer of the fruit wall be- 
 comes pulpy while an inner layer becomes hard and 
 constitutes the stone. A collection of very small 
 drupes upon a dry receptacle is met with in the different 
 kinds of raspberry and blackberry. 
 
 Succulent fruits in which the growing axis of the 
 flower is concerned are met with in two conspicuous 
 forms. The strawberry has a very succulent convex 
 axis on which the fruits appear as small hard bodies, 
 containing, however, the seeds inside their hard coats; 
 in the apple and its allies the succulent axis has become 
 concave and has grown up round the carpels and enclosed 
 them. This fruit is called a pome. The carpels them- 
 selves are cartilaginous in texture, or in some forms 
 bony as in the hawthorn. 
 
THE FRUIT 127 
 
 Dry fruits show greater variety. In some cases they 
 consist of one carpel only, many such fruits arising from 
 a single flower, as in the buttercup. This single carpel 
 may remain permanently closed, the seed being set free 
 only by its decay, or it may open along its front or both 
 front and back margins. In the latter case many seeds 
 are generally found inside. In other cases the carpels 
 while in the flower are joined together in their develop- 
 ment. When such a pistil is cut across it very generally 
 shows as many cavities as there are constituent carpels. 
 Sometimes the walls of the latter do not meet in the 
 centre, so that there is but one cavity. The seeds are 
 attached with hardly any exceptions to an outgrowth 
 of the edge of the carpellary leaf which constitutes a 
 placenta. There is a good deal of variety in this form 
 of fruit, as it varies with the number of carpels and 
 the ways in which they are united. Such fruits are 
 commonly called capsules. A singular variety of the 
 capsule is seen in the fruit of the wallflower and its 
 allies. Two carpels are joined together by their edges 
 and at the lines of union where a placenta can be seen 
 a membrane is developed quite across the cavity. 
 
 Besides the dry fruits which open and let the seeds 
 out and those which retain the latter permanently, 
 another form is met with, consisting of several united 
 carpels, which separate from each other at maturity, 
 but each carpel continues to hold its seed. These 
 fruits are called schizocarps. 
 
 A modification of the polycarpillary fruit is seen in the 
 nut. It has two or three constituent carpels and the 
 young fruit shows as many cavities as carpels. In 
 development, however, some or all of the partition walls 
 become dried up and disappear, so that there is only one 
 cavity in the adult fruit and this seldom contains more 
 than one seed. It is associated with a very hard 
 woody wall. All these modifications of structure show 
 
128 BOTANY 
 
 particular adaptations to the mode of dispersal which the 
 plant has adopted and should be studied mainly from 
 this point of view. 
 
 Small fruits and seeds are blown about by the wind 
 or carried by birds. Some are furnished with buoyant 
 accessories, which enable them to remain in the air 
 for considerable periods. In some cases the fruit splits 
 open with explosive violence and the seeds are jerked 
 to some distance. Often small fruits or seeds are 
 carried long distances embedded in mud, into which 
 they have fallen and which has subsequently become 
 attached to the feet of birds or other animals. Larger 
 fruits become attached by means of hook-like append- 
 ages to the coats of similar wanderers. Many fruits are 
 capable of floating long distances in water are indeed 
 often furnished with special mechanisms to enable 
 them to float. Indeed, the means of dispersal are so 
 numerous and in many cases so intricate that it is im- 
 possible here to do more than indicate the merest out- 
 lines of the subject. The mechanisms are easy to study 
 and every plant one meets affords an example which 
 will well repay investigation. 
 
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