\NICAL MEMOIRS. No. 4 
 
 ELEMENTARY NOTES ON 
 STRUCTURAL BOTANY 
 
 By 
 A. H. pHURCH, M.A. 
 
 HUMPHREY MILFORD 
 OXFORD UNIVERSITY PRESS 
 
 LONDON EDINBURGH GLASGOW NEW YORK 
 TORONTO MELBOURNE CAPE TOWN BOMBAY 
 
 1919 
 
 Price Two Shillings net 
 
BOTANICAL MEMOIRS. No. 4 
 
 ELEMENTARY NOTES ON 
 STRUCTURAL BOTANY 
 
 By 
 
 A. H. CHURCH, M.A. 
 
 .it 
 
 HUMPHREY MILFORD 
 OXFORD UNIVERSITY PRESS 
 
 LONDON EDINBURGH GLASGOW NEW YORK 
 TORONTO MELBOURNE CAPE TOWN BOMBAY 
 
 1919 
 
C5 
 
 CONTENTS 
 
 PAGE 
 
 I. INTRODUCTORY . . 3 
 
 II. GROWTH OF THE SHOOT 5 
 
 III. THE LEAF 7 
 
 IV. PHOTOSYNTHESIS 9 
 
 t V. r ^TRANSPIRATION ii 
 
 [ VlV ORGANIZATION OF THE HERBACEOUS STEM . . 13 
 
 'Vill-BT'Eiyr MECHANISM 15 
 
 VIII. THE WOODY STEM 17 
 
 IX. THE ROOT 19 
 
 X. ROOT MECHANISM 21 
 
 XI. THE TRANSPIRATION CURRENT 23 
 
 XII. PERENNATION .25 
 
 GENERAL LITERATURE . . .... 27 
 
Structural Botany : Introductory. ^ ^ , j|, 
 
 BOTANY, the study of Plant-life, as the autotrophic organisms of the world. 
 Autotrophic as implying life which works solely in terms of physical energy and 
 chemical material in the form of gases, water, and inorganic salts ; hence distinguished 
 from heterotrophic life (mainly animal) ; the latter being dependent on carbohydrate 
 and proteid material previously synthesized by plant-life. The plant normally utilizes 
 the simplest materials, as H 2 O, CO 2 , and a few salts dissolved in water, working in 
 terms of direct solar energy. Heterotrophic plants as Bacteria, Fungi, and a few 
 parasites among higher plants, are equally secondary in origin and mechanism. 
 
 A vast subject dealing with the structure, mode of life, methods of reproduction 
 and improvement of the race, as also its progressive evolution in past time ; ranging 
 from minute forms as Bacteria (I/A) to the vegetation of the sea_and fresh-water, 
 forms of prairie and forest-land, as also cultivated plants including organisms ex- 
 ploited by the human race, as plants grown for food (cereals and fruit-trees), decorative 
 effect (florists' flowers), timber, textile, and other economic products. 
 
 Botany in the original sense means fodder or herbage, and the science is 
 historically based on Herbs, as used in magic and medicine (Herbalism), cf. Herbalists 
 from Dioscorides (A. D. 77) to Fuchsius (1542): scientific botany as the scheduling 
 and identification of Herbs, evolved by necessity from Herbalism to Bauhin (1623) 
 and Morison of Oxford (1680); from the latter passing on to Systematy, or classifi- 
 cation by academic 'systems', culminating in Linnaeus (1735-53) and the Genera 
 Plantarum of Hooker and Bentham (1862-83). 
 
 Other aspects as anatomy and physiology, the former dealing with minute structure, 
 the latter with the mechanism of living processes, metabolism as including anabolism 
 and katabolism, the mechanism of irritability, as response to stimulus of changing 
 environment, reproduction, asexual and sexual, heredity, variations, and the relations 
 of the individual to the race. 
 
 Anatomy, dependent on the development of the microscope, dates to Nehemiah 
 Grew (1670-80), cf. his large volume with the first plant-sections and drawings of 
 tissues. All conceptions of plant-structure necessarily based on Land-Plants, in more 
 immediate association, taking common objects first; e.g. a piece of stick, seed, as 
 a bean, fruit, as an apple ; all show phenomena of cellular organization. ' Cell ' 
 from Robert Hooke (1667), from resemblance of a section of cork (cf. bottle-cork, 
 cut dry, low power, reflected light, tangential section) to honey-comb. Hence Cell 
 Theory of nineteenth century ; all higher organisms being constructed in terms of 
 cell-units, as living protoplasts building the tissues ; many die and give derivative 
 tissues of secondary significance, e. g. skeletal and mechanical in the aggregated 
 soma. 
 
 Cell-Mechanism : In the history of the word ' cell ' was restricted to the con- 
 spicuous wall or chamber, characteristic of plant-tissues, rather than animal : only 
 later were the contents recognized as essentially living, as by Von Mohl (1846), and 
 termed Protoplasm : a colloidal albuminous fluid state (sol), coagulated to gel-state 
 on death, containing over 90 % water (more than milk), the actually living substance 
 of all plants and animals ; apparently evolved originally in sea-water, and auto- 
 synthetic, from ions of H', O", N'", C iv , P v , S vi , together with other elements found 
 associated, as Ca", Mg", K', Na', Cl', Fe'". Mainly of proteid-organization, as 
 investigated dead, giving compounds involving CHOH and CHNH 2 groups, &c., in 
 indefinitely complex series. ' Killed ' by heat-coagulation at about 50 C., and by 
 strong chemical solutions (' fixing reagents ') : essentially a non-molecular complex 
 of indefinite chemical action and reaction. 
 
 A central tract of individualized nature, suspended in the general cytoplasm (in 
 both plant and animal), for metabolism and reproductive processes of more complex 
 organization, distinguished as the Nucleus; identified by Robert Brown (1833), 
 and discussed in detail in cytology. 
 
 Plant Tissues, so named by Grew from the similarity of sections to lace-work, 
 are based on typical cells of average dimensions and details, about 50 //, diam., 
 (commonly ranging from 10-100 /t or more): spherical under action of surface- 
 tension when left to grow freely ; but usually aggregated in close contact, with or 
 without intercellular spaces; hence appearing in sections as circles (when free), 
 
or more ^or less hexagonal if 'packed'. A general term for such ordinary units 
 of construction is retained in < parenchyma', given by Grew from the fancied resem- 
 blance to air-sacs of lung-tissue. 
 
 A Parenchymatous Cell, taken as a starting point, is a fairly isodiametric 
 living unit, with cytoplasm and one nucleus as the central control. All plant- tissues 
 are based on such a structure, or have been formed from them, however much they 
 may be modified for special functions : the parenchymatous cell is thus generalized in 
 metabolism and function, and aggregated units constitute a tissue as parenchyma. 
 In addition to the nucleus, the cytoplasm may present vacuoles with cell-sap, often 
 greatly distended until the plasma is left as a peripheral film, and the nucleus 
 is lateral : it is bounded by a wall of waste debris of metabolism, mainly 
 polysaccharide cellulose, so called from plant-origin, as a carbohydrate with 
 suggested complex construction (6 CHOH H 2 O) = C 6 H 10 O 5 of the type : 
 
 OTOH^CHOH . CO - CHOH - CHOH - CH 2 (ring-formula, Cross and Bevan, 1918).. 
 The wall is deposited on the confines of the cytoplasm, in thinner or thicker series 
 of lamellae ; i. e. when thick, stratification may be noticed, while thin areas (pits) are 
 left for conduction of soluble substances between adjacent units, as also perforations 
 (more minute) for protoplasmic continuity. 
 
 Living protoplasm in such cells may show movements of circulation in slender 
 strands (bridles), or a foam-like arrangement ; the peripheral tract (plasm a tic film) 
 is concerned more particularly with the regulation of osmotic mechanism. The 
 outer surface of the film may constitute a semi-permeable membrane, to which the 
 freely permeable cellulose-wall may act as a mechanical support : such a cell in water, 
 in virtue of solutes in the cell-sap, presents phenomena of differential diffusion- 
 pressure ; water can enter the cell, but the solutes do not pass out : an osmotic 
 pressure of 5-10 atmospheres, or more, is commonly established in ordinary plant- 
 cells, maintaining the protoplast in a state of turgidity. The mechanism is utilized for 
 the absorption of water, distension of the cell-wall, and growth ; such turgidity being 
 so far essential for plant-cells growing in air. On placing similar cells in a solution 
 of higher osmotic value, the action reverses, water leaves the cell-cavity, and the 
 protoplast retracted from the wall is said to be plasmolyzed. 
 
 Nob t all cell-mechanism was originally evolved in the sea for metabolism in 
 a circumambient food-solution of 3| % total salts, and later transferred to air with 
 only atmospheric gases and aqueous vapour outside. Hence complications and 
 readaptations. 
 
 Other cell-contents include plastids, as tracts of cytoplasm for special functions, 
 with a certain individuality of their own : the most general case that of the Chloroplast, 
 coloured green with chlorophyll, and of essential significance in photosynthesis : also 
 vacuoles with oil drops, crystals as calcium oxalate, tannin, pigments, &c., as the 
 commonest products of metabolism. 
 
 As good example of cell-detail, cf. blue hairs on stamens of the flowers of 
 Tradescantia (available till first frosts) : threads (4 mm.) of 20 cells, or so, as oval 
 units, in single series, up to 300 ^ long, to globular at 75 /A diam. distally : optimum 
 200 p, with thin cellulose walls, and violet-blue cell-sap ; the cytoplasm colourless, as 
 a granular parietal layer, with slender bridles across the cell-cavity ; the coloured cell- 
 sap filling vacuoles ; nucleus (20 /x) central or parietal. Healthy cells show circulation 
 of protoplasm in the bridles and around the wall, as active streaming movements ; 
 large granules at 3 /x per sec., smaller at 6-7 ^ per sec. Plasmolysis, by 5% salt- 
 solution, causes the plasma to retract from cell-wall in places ; ultimately to a wholly 
 free spheroidal mass, with sap-concentration to deep violet colour. Washing with 
 water will restore full turgidity, and the circulation of living plasma will be seen to be 
 unaffected. Addition of spirit kills the cell instantaneously; the plasma being 
 coagulated irregularly, and the nucleus taking up the pigment that was in the cell-sap 
 after death. 
 
 Less satisfactory phenomena of circulation may be noted in the epidermis 
 skinned from the inner scales of Onion-bulb ; but special interest centres in the fact 
 that these cells are units of a definite parenchymatous tissue-system. 
 
 Elodea : In elongated cells of leaf-midrib, cytoplasm streaming along the walls 
 takes chloroplasts with it at 10 /x per sec. 
 
Structural Botany : Growth of the Shoot. II. 
 
 A typical Land Plant, as derived from submarine vegetation, is a more complex 
 organism in that the first problem is how to obtain sufficient Nitrates, Phosphates, 
 and other compounds as ' food-salts ' to carry out proteid-synthesis, when these are 
 only present in the soil. This involves the necessary differentiation of an absorbing 
 root in the ground, with subaerial shoot', the function of the latter being to produce 
 and display as much body-surface as possible to air and sunlight. Since the 
 absorbing cells are in a solution of soil-water (often little better than tap-water), 
 a current has to be maintained from the roots to the leaves (Transpiration-current), 
 and enormous quantities of water require to be evaporated at the surface of the 
 leaves (Transpiration). 
 
 Hence the Primary distinction iii land-plants of Leaves, Stem^and Root', the 
 first being the essential photosynthetic and proteid-synthetic regions of the organism. 
 The Leaf is the Laboratory of the Plant ; with conductive system leading to it, as 
 also away from it, in the form of definite strands of vascular tissue known as 
 Vascular Bundles (V.B.) of Xylem and Phloem. 
 
 The main axis normally grows erect, at right angles to the surface of the ground, 
 to give optimum space-distribution ; bearing leaves as laminate extensions of the 
 soma, and branches repeating similar organization ; thus giving a * tree habit ' of 
 diffused growth spread over considerable space, as opposed to the concentrated and 
 condensed body of a locomotile animal. The Plant-axis also exhibits continuous 
 growth, by definite 'growing-points', which may go on producing new leaves and new 
 axes so long as nutrition is satisfactory ; i.e. the plant continues to grow at the ends. 
 
 Growing Point : essentially a mass of undifferentiated growing and dividing 
 cells (meristem); exhibiting phases of (i) centric growth, i.e. around a point; 
 (2) unilateral distribution, i.e. on one side only; maintaining the growing apex at the 
 end of a longitudinal axis ; (3) retarded, since the stem attains a definite ' adult ' size. 
 Increase is effected by the growth and sub-division of each individual cell in irregular 
 sequence : any cell may be dividing, but the general effect and dome-shaped 
 outline are retained ; i. e. the mass grows as a whole, and all the protoplasts are in 
 living communication, as typical parenchymatous cells, or in this case, embryonic 
 tissue, practically immortal, and including generalized germ-plasm. 
 
 Mechanism of Cell -Division : The nucleus divides first, presenting phenomena 
 of Mitosis : In higher plants and animals the nucleus presents complex organization 
 of plasmic ground-substance, with fluid in vacuolations, and framework of linin, 
 with chromatin distributed in a reticulum of chromosomes. The latter are of constant 
 number for a given species, and display a certain degree of individuality, being 
 capable of independent division by longitudinal fission. The mass of the nucleus is 
 fluid, and is held in spherical form among the cell-cytoplasm by surface-tension, and 
 is covered by a plasmatic film (nuclear membrane), as its ' resting-stage ' in the 
 metabolizing cell. In Mitosis the membrane disappears ; the linin arranges a ' spindle- 
 mechanism ', with ' spindle-fibres ' from two ' poles ', and the chromosomes separate 
 (prophase), arrange in the ' equatorial plate ', and each divides : spindle-fibres attach 
 to the halves, and the ' daughter-chromosomes travel to the poles (anaphase), there 
 uniting to build the new reticulum of the 'daughter nuclei' (telophase). In the 
 equatorial plane, across the central spindle-fibres, a zone of demarcation of the fields 
 of the two new centres is marked by a deposit of polysaccharide waste, as the first 
 indication of the dividing wall : this extends to the older walls, and constitutes the 
 primary septum. Later deposits of cellulose on this wall commonly present cleavage- 
 effects, with the production of intercellular spaces at the angles (more or less) ; the 
 original septum is distinguished as the ' intermediate lamella ', and the intercellular 
 spaces are required for the aeration of the tissues. 
 
 Apical Differentiation. As cell-division proceeds behind the growing point, 
 organization is shown in the layering of the tissues, following periclinal construction- 
 lines, and indications of tissue-systems appear : 
 
 (1) A peripheral layer controlling relations with the outer air, as epidermis. 
 
 (2) A generalized intermediate region (cortex). 
 
 (3) A central conducting and skeletal region, conveniently included as the Stele. 
 The early distinction of these regions close to the growing-point led to the 
 
enunciation of three 'embryonic layers', as dermaiogen, peribhm, and plerome, 
 reflecting academic abstractions on the general lines of the germinal layers of the 
 higher animal (middle xix century). Such layering is often quite definite, but is 
 to be referred to a precocious differentiation of functional regions of the adult, 
 and is not causal. The terms survive as useful conventions and topographical 
 designations. 
 
 Behind the growing-point, zones of differentiation may be distinguished as 
 (i) The apical zone of growth and division in all directions ; (2) A zone of elongation, 
 in which longitudinal extension is most active, giving a capacity for intercalary growth 
 and rapid increase in length; (3) A zone of differentiation, in which the tissues 
 assume their adult histological character and special physiological activities. 
 
 HELIANTHUS, |in. stem: In a small herbaceous stem cells and tissues are 
 readily distinguished in transverse section, and may be tested by ordinary reagents : 
 e.g. examine a transv. sect, mounted in Iodine solution. The general circular 
 outline of the section is filled with more or less circular areas indicating the cell-walls, 
 grading from TO /A to 100 /A diam., with intercellular spaces between. Differentiation 
 of more specialized tracts is conspicuously marked at 12-15 spots, very uniformly 
 distributed about 300 /A from the periphery, as sections of the Vascular Bundles (V.B.). 
 A little distance external to these a distinct line of cells with black dots (starch- 
 grains stained by Iodine sol.) indicates the Endodermis. The whole section is 
 bounded by a single layer of cells as Epidermis, and special tracts may be 
 defined as: 
 
 The tissue beneath the epidermis, as far as and including the endodermis 
 = Cortex: The tissue internal to the vascular bundles = Medulla (pith): tracts 
 between the V.B. are termed Primary (Medullary) Rays ; and a region between the 
 V.B. and the endodermis as pericycle. 
 
 In further detail note : 
 
 (1) The Epidermis of oval cells, 30 /A diam., with scanty granular contents, 
 thicker outer wall covered by a distinct cuticle ; and giving rise to outgrowths 
 occasionally as hairs of single series, or on multicellular bases. 
 
 (2) Cortex of outer 3-4 layers of Collenchyma, the walls markedly thickened at 
 the angles (mechanically increasing cohesion of periphery). 
 
 (3) Cortical Parenchyma of larger units, 40-100 /A diam., with occasional ducts. 
 
 (4) Endodermis of a single line of cells, 50 /A wide, with starch-grains (5 /A) ; 
 in this stem curiously the only layer with starch. 
 
 (5) The central Medulla, the bulk of the section, of rounded units, up to 120 /A 
 diam., with scanty contents and soon dying off. 
 
 (6) The Vascular Bundles, differentiated by Phloroglucin -25 % H 2 SO 4 , into 
 a Xylem-region (x) of lignified tracheides, in 3-5 radial rows, to 40 /A diam., 
 surrounded by smaller non-lig. cells of 15 /A. : Phloem (</>) of small cells (15-20 /A) 
 with more definite contents : between x an d < the suggestion of a cambium-zone 
 of smallest cells (10-15 /*) 
 
 (7) External to the phloem the beginning of a group of fibres, as a rule not yet 
 lignified, belonging to the pericycle, 30 /A diam. In older axes these have thick 
 lignified walls giving the Phloroglucin-reaction. 
 
 Approximate details of measurement, freely interpolated, are not necessarily in- 
 tended to be checked, although it is a useful habit always to use a micrometer eye- 
 piece; so much as to afford a basis for accuracy in drawings, and keeping the 
 figures in their proper proportions. The most useful standardized drawings are those 
 constructed on the ' mu-millimetre ' scale (High Power), = x 1000, or in some simple 
 relation. 
 
 Powerful reagents used for differentiating celluloses require great care. Alkalis 
 (KOH, NaCIO) spoil lenses, strong acids metal-work and clothes. Fuming acids 
 (HC1) should be avoided ; weak H 2 SO 4 with phloroglucin takes longer time, but does 
 not affect the intensity of the reaction. Preparations involving strong acid, or Chlor. 
 Zn. lod. should be put in a glass of water as soon as finished with, and not left lying 
 about. Small quantities only should be employed. 
 
Structural Botany : The Leaf. III. 
 
 A well-grown leaf of CHERRY LAUREL (Prunus Lauro-cerasus) from 
 a sunny position, consists, of a broad lamina, 8 in. by 3, on a short stout petiole ( in.). 
 Note conventional usage of borrowed terms as midrib, more conspicuous on lower 
 surface ; apex attenuated as a drip-tip ; veins (nerves) as paths of conduction ; teeth of 
 margin, and especially dorsiventrality as implying bilateral construction with two sides 
 unlike, the distinction of D.V. surfaces being purely arbitrary. 
 
 Orientation on plant in spiral phyllotaxis, with lamina presented to maximum 
 illumination, the upper surface at right angles to direction of incident light; by 
 secondary twisting of petiole giving 2 rows ; more or less wanting in shaded shoots. 
 
 Venation seen particularly well on lower side, pinnate, in alternating sequence ; 
 with expressions of 'triple-arch' system towards margin, as mechanical adaptation 
 against shearing action of wind. Obscure glands (hydathodes) paired at base of 
 lowest lateral veins. 
 
 Transverse section of sample of midrib and lamina of fresh leaf (half-way up), 
 cut in pith, mounted in water : note shape of midrib, central vascular strand and little 
 green tissue ; lamina of uniform thickness (-| mm.) chiefly of green tissue (mesophyll) 
 with a few obliquely cut portions of small V.B. as ' veins '. Taking the tissues in 
 order from upper surface to lower : 
 
 (1) Upper Epidermis of cells fairly isodiametric (30-60 /* broad and 30 high), 
 with stout cellulose walls, simple pits, no chloroplasts, slight aqueous contents ; with 
 distinct cuticle on outer surface, 3 p thick ; differentiated by Chlor. Zn. lod. as blue 
 cellulose, yellow cuticle, and brown intermediate cuticularized layers. 
 
 (2) Palisade Mesophyll as 2 tiers of columnar cells set at right angles to 
 surface, averaging 50 /z deep each, and 18 p, wide; occasional attempts at 3 tiers; 
 shade-leaves have broader more rounded units. Walls thin, of cellulose ; contents 
 include discoid chloroplasts (3-6 /*) with small starch-grains (tested by Iodine sol.). 
 Note the manner in which the units of the basal tier are fitted on the subjacent 
 parenchyma. 
 
 (3) Spongy Mesophyll, a lower zone, 200 /* deep, of loose isodiametric 
 parenchymatous cells, with chloroplasts and starch in smaller quantity ; up to 75 \j. diam., 
 with greatly exaggerated intercellular spaces, but often irregularly pulled out giving 
 stellate effects. Chloroplasts with included starch-granules, being fewer, are seen to be 
 more distinctly restricted to a parietal layer in each cell. Note the netted effect of 
 the sponge-like type of spacing for effective aeration. Both palisade and spongy 
 mesophyll may contain large crystal-aggregates of calcium oxalate (sol. in dil. HC1) ; 
 crystal-sacs contain one stellate cluster (35 p), more general in palisade of older leaves, 
 or large single rhombs (45 /x), especially in spongy mesophyll. 
 
 (4) Vascular Bundles, in smaller veins, cut more or less obliquely, with 
 a distinct sheath of parenchyma (endodermis), and poorly differentiated spiral tracheides 
 (giving phloroglucin -H 2 SO 4 reaction), constantly on the upper side of the strand. 
 
 (5) Lower Epidermis, similar to upper, but cells smaller and more rounded ; 
 similar aqueous contents and cuticle, with the additions of 
 
 (6) Stomata, as slits communicating with the exterior, bounded by 2 guard-cells. 
 These are not well shown in Cherry Laurel, being obliquely cut; more distinct on 
 sloping side of midrib. Guard-cells with comparatively thick inner and outer walls, 
 outer pore-chamber, and cuticle following the contour of the guard-cell its whole course. 
 Irregularities in the epidermis, due to stomata being cut obliquely are more evident in 
 Chlor. Zn. lod. preparations. Stomata are readily seen in surface- view by skinning 
 off a small piece of lower epidermis and mounting in Iodine sol. There may be 200 
 per sq. mm., each 40 /x by 35 across the guard-cells when widely open. 
 
 Note the mosiac effect of lower epidermal cells with simple pit-communications 
 (giving beaded effect to walls). The guard-cells contain small chloroplasts, small 
 included starch-grains, and a central nucleus. 
 
 (7) Midrib, appearing as a broad area of generalized structure, bilateral and 
 eccentric ; central vascular strand as conspicuous band of Xylem of pitted tracheides 
 and vessels (to 30 /* diam.), with radiating bands of parenchyma as ' medullary rays ', 
 differentiated by Chlor. Zn. lod.; tracheides give red reaction with phloroglucin -H 2 SO 4 . 
 On the lower side of the Xylem is a narrow band of phloem, with cellulose walls, and 
 
small cavities, through which the ray lines are continued. The rest of the area is 
 mainly parenchymatous, grading to collenchymatous tracts at upper and lower surfaces, 
 as cells with few chloroplasts and few starch-grains. Stomata are rare : the green 
 mesophyll is continued over the upper surface, below the collenchyma, but without the 
 palisade-effect. The lower parenchymatous tract of thin-walled large cells, up to 90 
 //, diam., is marked with radiating bands of thick-walled, but non-lignified, fibres, up 
 to 30 //, diam.: there is no special cambium; but a good effect of radial rows at the 
 junction of x an< ^ </>> tne smallest cells 10 /* by 5. A zone of large rounded cells 
 (especially in sp. mat., in which they go brown as a tannin-effect) may be partially 
 isolated as an endodermis on the flanks of the strand. 
 
 Shaded leaves differ in smaller size, less differentiated palisade, and little or no 
 starch-content. 
 
 Other leaves may be compared, taking more particularly stout forms of evergreen 
 habit, as available fresh at any time of the year, as : 
 
 Privet (Ligustruni) with good differentiation of palisade, and abundant stomata 
 on lower surface ; but the spongy mesophyll is less marked and the intercellular spaces 
 are scanty. 
 
 Box (Buxus) with a very dense palisade-region and almost colourless lower half; 
 thick cuticle, prominent V.B. and good stomata. 
 
 Ivy (Hedera) less differentiated mesophyll, palisade very poor in shade-leaves, 
 abundant stomata. 
 
 Aucuba (yellow-spotted variety), ill-differentiated mesophyll and midrib ; chloro- 
 phyll, as also starch, conspicuously wanting in the yellow areas : variegation implies 
 malnutrition. 
 
 Holly (Ilex) presents more marked differentiation, but along similar lines: 
 palisade in 3 tiers, covered by aqueous hypoderm, as well as aqueous epidermis. Arcs 
 of fibres over the phloem of V.B. Elaborate spongy mesophyll, and good stomata on 
 lower surface. Cuticularized layer of upper epidermis 10 //, thick. 
 
 Stomata in surface-view are readily examined by skinning the epidermis of 
 leaves, as : 
 
 Lilium candidum, large stomata 65 /* long and 50 broad, all orientated along 
 the long axis of the leaf, on lower surface only: guard-cells with conspicuously 
 abundant chloroplasts: slit to 20 ^ wide, as measured by air-bubble in water-preparation. 
 
 Arum italicum, fresh leaves afford admirable example of mosaic epidermis; 
 stomata numerous, 100 per sq. mm. ; guard-cells 50 p. long, with subsidiary cells on 
 either flank. Slit opens to 15 /x; closed stomata show guard-cells straightened out 
 and closely approximated. Iodine sol. gives purple-red or violet colour to amyloid 
 material soluble in cell-sap of ordinary epidermal cells only ; the cytoplasm and well- 
 defined nuclei (15 //,) colour yellow. The guard-cells have no amyloid, but small 
 chloroplasts and starch-grains. 
 
 Iris, stomata about 120 per sq. mm., on both sides of leaf, all orientated in 
 longitudinal direction with elongated epidermal cells : guard-cells 50 //, long, slit opens 
 10 /u.. Other leaves as available, cf. Tulipa, Hyacinlhus, Scmpervivum. 
 
 Leaf-arrangement: Leaves arise at the growing-points of the stem only, in 
 strict acropetal order, as outgrowths of the meristem, with definite arrangement, more 
 or less a constant for different plants ; in rhythmic sequence, so working out a definite 
 pattern (phyllotaxis) ; normally spiral, or whorled. Points of insertion termed nodes ; 
 spaces elongated between as inlernodes. Whorled systems give 2-3 or more leaves 
 spaced at same level, and alternating at successive nodes; e.g. (2 + 2) system = 
 decussate, a common case ; cf. Privet, Box, Ash, Sycamore, Aucuba, Fuchsia, &c. 
 
 In spiral constructions, successive members are spaced around the growing-point 
 at about 137^ (Fibonacci angle), which gives optimum balanced effect, as also 
 maximum exposure to light. A pattern is worked out in which contact-lines may be 
 traced (2:3:5:8:13:21, &c.) according to the size of the members produced, as seen 
 in transverse sect, of the young bud. Low ratio (2 : 3) in Ivy, Holly, Cherry Laurel, 
 &c. : for better examples, cf. Pine-cone (5:8), or Sunflower-head (34 : 55), and even 
 (89:144). 
 
 B 
 
Structural Botany : Photosynthesis. IV. 
 
 ANABOLISM expresses the building of living plasma; initial stages only are 
 open to experimental observation, as : 
 
 Photosynthesis, more usually restricted to the elaboration of carbohydrate 
 from CO 2 and H 2 O in the presence of solar radiation: experimental observations 
 show that on exposure to sunlight under suitable conditions starch-grains, which can 
 be tested by Iodine solution, appear in the chloroplasts : e. g. in two hours in Funaria 
 leaf, and 5 minutes in cells of Spirogyra. Such general facts first noticed by Sachs 
 (1862): The presence of chlorophyll is apparently as essential as that of light; 6 fac- 
 tors in all : 
 
 (a) General factors for healthy metabolism of the plasma : 
 
 (1) Water-supply for aqueous plasma. 
 
 (2) Temperature with max. and min. range ; opt. about 80 F. 
 
 (3) Oxygen-supply, generally conceded from conceptions of katabolism. 
 (ft) Special factors of the chlorophyll-mechanism : 
 
 Chlorophyll in special plastids (chlorophyll-corpuscles, chloroplasts). 
 CO 2 supply as free gas of the atmosphere ; 3 parts in ten thousand. 
 3) Light of the Sun, of optimum diluted intensity (white-cloud illumination). 
 
 Chlorophyll, a green pigment, soluble in oil on surface of chloroplast ; general 
 constitution fairly known (Willstatter, 1910), containing Mg, but no Iron ; extracted 
 by solvents as spirit, with other substances, as a more or less complex mixture, giving 
 a similar green solution, fluorescing blood-red, and presenting a characteristic absorp- 
 tion-spectrum, with several bands (7) ; the chief one in the red (B ^ C) ; a faint band, 
 C ^ D; one in the green (beyond D); a faint one before E, and all beyond F (violet end) : 
 i. e. certain light is absorbed (mainly red), much is reflected as green. Hence infer 
 red light is the optimum for photosynthesis : but all rays may be used, as under trees, 
 a soft green light being residual. 
 
 Bacterium-method of Engelmann (1882) gives a means of checking the evolu- 
 tion of Oxygen as part of the work done. Depends on the fact that certain Bacteria 
 are motile only in presence of free O 2 : on placing a green algal filament in the field 
 of a micro-spectrum, bacteria congregate in the region of maximum evolution, and 
 a curve may be drawn. This again gives most active decomposition in the red (B J C, 
 wave length 660-680 /x/x), with less action in the rest of the spectrum. Hence con- 
 clusion accepted that primarily energy of red light is converted into chemical work, 
 with evolution of O 2 and production of Carbohydrate from CO 2 and H 2 O. Experi- 
 mental gas-analysis shows that there is no change of volume involved ; i. e. for an 
 equal vol. of CO 2 absorbed, equal vol. of O 2 is set free ; 
 
 The Theoretical Result may be provisionally expressed in the form: 
 
 OfT 
 O : C : O + H-O-H = O 2 + ]C' ^ the suggestion being that O 2 comes jointly from CO 2 
 
 and H 2 O ; the Carbon atom comes into association with the ions of water, with 
 the possibility of attaching indefinitely similar (CHOH)" groups in chain-forma- 
 tion. A suggestive formula involving 6 such groups, stabilized in molecular form, is 
 given for glucose, as the simplest monosaccharide found free in the plant : 
 CH 2 OH-CHOH-CHOH-CHOH-CHOH-COH 
 
 One such group would stabilize as H-COH (Formaldehyde), invariably associated 
 with the chlorophyll-mechanism. Monosaccharide groups condense with separation 
 of H 2 O to form polysaccharides of (C 6 H 10 O 5 ) W class, to which starch is ascribed. 
 
 Hence the appearance of starch -granules in a chloroplast implies the end of 
 a long sequence of actions in which starch, itself insoluble, is off the main line of 
 synthesis, and so far a by-product, expressing excess of carbohydrate production, 
 rather than the actual amount formed. Sugars as monosaccharides are probably 
 nearer the main line of proteid-synthesis : disaccharides (as cane-sugar) are similarly 
 to be regarded as by-products. These have diminished osmotic value, and starch 
 has none at all, hence the meaning of their production may be the avoidance of 
 injurious osmotic action in the cell-sap of the working units. All further production 
 of starch, celluloses, ligno-celluloses, benzene-derivatives, and terpenes, may be 
 regarded as roughly following a sequence of progressive depletion and substitution 
 of OH groups in an anhydro-aggregate of such primary monosaccharides. 
 
Proteid- Synthesis involves further stages of formation of groups as (CHNH 2 ") 
 from CHOH groups and Nitrates, with subsequent introduction of S and Phosphoric 
 Acid, as also traces of Iron, to build more complex proteids, as in nuclei, before 
 plasma can be attained : little is known of such stages : substances found in plants 
 being molecular ends and de'bris of processes, rather than steps in actual metabolic 
 reactions. The elementary consideration of photosynthesis stops at carbohydrate 
 synthesis, though free energy of light is probably utilized all through ; but proteid- 
 synthesis can go on in the dark, as in growing roots or germinating seeds, other 
 energy being employed (katabolic). The entire sequence of events is termed assimi- 
 lation from the analogy of the animal, but may be conveniently distinguished as (i) 
 photosynthesis and (2) proteid-synthesis. 
 
 Photosynthesis (='00,2 assimilation') thus involves the utilization of CO 2 of 
 the air, with consequent evolution of free O 2 , as a gaseous exchange. Other gaseous 
 exchanges are concurrently effected. 
 
 Respiration : including gaseous exchanges as evidence of katabolism, the latter 
 involving oxidation-phenomena in the plasma, with evolution of CO 2 , in the normal 
 process of aerobic katabolism as utilized by animals and plants, in absence of light, 
 as the general method of obtaining energy for living processes. Hence in absence of 
 light, and in all parts not containing chlorophyll even in the light, normal respiratory 
 katabolism takes place. In this way living plants may be presenting a gaseous 
 exchange the reverse of that in photosynthesis, under certain conditions, as by night ; 
 and both actions may go on concurrently in green and non-green parts: the net 
 result of the exchange being the balance of two distinct processes. Active photosyn- 
 thesis is however normally several times (20-40) more intense than the respiratory 
 exchange, and may completely mask it by day. Hence respiratory activity is usually 
 studied in the case of seeds or non-green parts, and in the dark. Rarely does such 
 katabolism raise the temperature appreciably. (Antm-spadix, 105-112 F.) 
 
 The ratio of CO 2 : 2 of respiratory exchange is normally less than unity, as 
 more oxygen is used in other reactions, and CO 2 is not the only product of such 
 oxidation, though the most highly oxidized form of carbon; e.g. the next most 
 highly oxidized carbon-compound is COOH COOH (Oxalic Acid), soluble in water, 
 and so retained in the plant, and poisonous. Hence this if neutralized by Ca. may 
 explain the common presence of Calc. oxalate crystals in actively metabolizing parts 
 of the plant. In extreme cases (some succulents) oxalate is formed in preference to 
 CO 2 . Katabolic processes taking place in absence of free oxygen, and expressed 
 in terms of other sources of chemical energy, are termed anaerobic (cf. Bacteria). 
 
 Orientation : Elementary plasmatic organisms respond to changes in environ- 
 ment by form-alterations, originally the expression of modified surface-tension, giving 
 euglenoid and flagellar contractility. In more massive cellular organization form- 
 changes are effected by alterations in osmotic mechanism, as effecting turgidity and 
 growth of cell-units. Little movement is possible once the cellulose framework is 
 established. Such movements express ' Irritability ' in response to ' Stimulus ', i. e. 
 change ; or may be autonomous (spontaneous), i. e. without ascertainable cause. 
 
 The most fundamental response is to sunlight, Heliotropism ; plant-shoots are 
 normally ' positively ' heliotropic, and tend to lie pointed in the direction of optimum 
 light-effect (orthotropic) ; leaves are * plagiotropic ', or tend to lie at right angles to 
 incident light. The movement is originally a general growth-response ; but may be 
 differentiated, as effected by the petiole to display the lamina, implying a certain 
 amount of conduction of stimulus, and foliage-leaves normally attain a fixed light- 
 position. 
 
 In special cases movement is determined by a distinct ' pulvinus '-organ at the 
 base of the leaf or leaflet, and daily movements of presentation, or light-regulation, 
 are effected (cf. Robinia, Oxalis, and ' Sleep-movements '). In extreme cases the 
 mechanism is so delicate that it responds to other stimuli (electrical, or even a touch), 
 cf. Mimosa, leaves only displayed by excessive turgidity of pulvini, and hence ' sensi- 
 tive ' plant. 
 
 All such mechanism of response, by turgid protoplasts, is subject to general tonic 
 actors, as temperature &c., and is only feebly expressed in these latitudes. For further 
 expression in warmer climate, cf. Sensitive Plant, and 'Praying Palm' (Bose, 1918). 
 
 10 
 
Structural Botany : Transpiration. V. 
 
 The Gaseous Exchanges of the leaf may be expressed as : 
 
 (1) CO 2 in, and O 2 out, in Photosynthetic Assimilation : the oxygen diffusing out 
 
 passively when in excess. 
 
 (2) O 2 in, and CO 2 out, in Respiration; CO 2 passively diffusing out in absence of 
 
 anything to stop it. 
 
 (3) H 2 O lost to the tissues in evaporation and Transpiration : all gases diffuse out 
 
 if apertures are available, when in excess ; or may pass away from solution in 
 aqueous media : diffusion inwards continues as long as the gas is used chemi- 
 cally, and so creates a reduced internal partial pressure. 
 
 Aeration of the Tissues is provided for by intercellular spaces, always of 
 secondary origin, in free communication throughout the plant as an internal atmos- 
 phere ; e. g. the gas-content of a potato is the same as that of the external air. The 
 paths of exchange are indicated by the stomata as intercellular spaces in the epidermis, 
 formed as a slit between twin guard-cells: cf. Theory of Multiperforate diaphragm. 
 
 Stomatal Control : Penetration by gases, or outward diffusion, may be com- 
 pletely checked by an impermeable (gas-proof) surface-layer ; the normal function of 
 the cuticle, if only thick enough : but this does not give control. Stomata, with guard- 
 cells, opening and closing the slit, act as doors controlling the exchanges if the 
 mechanism is suitably adjusted. All three exchanges are thus more or less under the 
 control of the living guard-cells : the most important being the exit of water-vapour. 
 
 Transpiration expresses the giving off of water- vapour from living plasma, as 
 opposed to the physical evaporation from free fluid or dead material. Since the only 
 available source of food salts (N,P,S, &c.) is from solution in soil-water absorbed by 
 roots, these have to be carried to the photosynthetic cells for further proteid-elabora- 
 tion, and a constant supply is required, as the Transpiration Current. The tissue 
 in which this ascends is the Xylem of the V.B. ; as the plant can only work in terms 
 of very dilute solutions, considerable quantities have to be passed through the tissues 
 in order that a small amount of salts may be retained. Transpiration is thus of 
 primary significance to all land-plants as the means of removing excess water; but 
 since too rapid desiccation will soon kill aqueous plasma, an effective mechanism of 
 regulation and control is also essential. Closing the stomata altogether would tend 
 to starve the plant. 
 
 Stomatal Mechanism. The guard-cells absorb water from the adjacent 
 epidermal cells (or subsidiary cells) through their thin lateral wall, in virtue of their 
 greater osmotic activity. The fact that guard-cells contain chloroplasts when aqueous 
 epidermal cells do not, and still greater quantity if the latter do, suggests that they 
 retain the power of autosynthesis of osmotic material (sugars or organic acids) as 
 required. The guard-cells thus become turgid at the expense of adjacent units ; and 
 the thickening of the wall is so adjusted that they dilate most on the outer lateral wall, 
 and may be practically immovable in other regions. Hence on dilatation they become 
 convex outwards, and pull away from each other in the middle line, opening the slit 
 the wider the more turgid they become. Any stimulus to the plasmatic film, increas- 
 ing the permeability, tends to reverse the process; and water escapes back to the 
 adjacent tissues as the guard-cells become flaccid ; the pore being thus closed to the 
 original position. Excessive loss of water is an effective stimulus, and the motile 
 guard-cells as the most sensitive units feel it first. Stomata thus close on a wilting 
 plant. The effect may be imitated by plasmolysis of the guard-cells. 
 
 Food Conduction : The mesophyll of the leaf gives sugars, starch, and proteid 
 as excess manufactured food-material, which may be conducted away to feed other 
 parts of the plant (so far parasitic on the green tissues). Sugars travel in parenchyma, 
 largely as disaccharides ; Proteids travel in sieve-tubes of Phloems : these substances 
 are less readily checked owing to lack of good test-reagents. Starch-grains stored by 
 day are readily redissolved by night, being hydrolyzed back to sugars by enzymes of 
 proteid-constitution, classed as diastase. These hydrolyze starch to dextrins, maltose, 
 dextrose in turn ; and a leaf full of starch at the end of a summer day may be quite 
 empty by next morning, and so start afresh. All parts using food-material create 
 a drain on the photosynthetic units, and under such stimulus a food-current once set 
 up may be long-continued. 
 
 ii 
 
FICUS ELASTICA (India-Rubber Plant) a tropical forest-tree with leaf afford- 
 ing a good example of high specialization: the lamina 18 in. by 7, with stout petiole, 
 3-4 in.; midrib, pinnate venation, entire margin, and connecting marginal vein, 
 much on plan of Cherry Laurel. Section at right-angles to the small lateral veins 
 more satisfactory: lamina -7 mm. thick, and highly differentiated. In sect, of fresh 
 material note : 
 
 (1) Epidermis, a remarkable aqueous system of 3 rows of cells (originally i), 
 outermost about io/z, next 40 /a, inner ioo/i; total 1 50 p of aqueous tissue as pro- 
 tective screen. Cuticle and cuticularized layers uniformly thick, 7 p. ; at intervals oval 
 cells enlarge to 200 /* long, and show peculiar concretions (Cystoliths) as partly crystal- 
 line deposits of CaCO 3 on a cellulose basis ; sol. in HA, and test Chlor. Zn. lod. for 
 blue cellulose rest and peg : stratified and striated organization suggestive of that of 
 wall-deposits, and of starch-grains. Function unknown, except as * waste '. 
 
 (2) Palisade Mesophyll, of 2 well-marked tiers, leading to a third, in cande- 
 labrum type ; cells 40-50 /A by 12 wide, thin-walled, with abundant discoid chloro- 
 plasts, grading into spongy mesophyll of loose tissue aggregated around V.B. of veins. 
 
 (3) Vascular Bundles, larger ones cut transversely, with x on upper surface ; 
 broad crescentic tract of scl. fibres on lower (</>) side ; small phloem units between ; 
 palisade tissue divergent to make way for them. 
 
 (4) Lower Surface with attempts at a lower palisade (one row); 3-layered 
 epidermis on reduced scale ; occasional cystoliths, and conspicuously fine sunk 
 stomata. 
 
 (5) Stomata, guard-cells sunk midway in aqueous epidermis, 40 \L below level 
 of surface, with thick outer and inner walls, mere slit-lumen, with chlorophyll and 
 starch ; slit hour-glass shaped. Cuticle follows entire contour, and ledges define 
 outer and inner pore-chambers. Large outer vestibule, 30 /x deep and wide. Stomata 
 are cut in all directions in the same section, as many as 4-5 in the field of high power 
 at same time. Well-differentiated in Chlor. Zn. lod. 
 
 (6) Midrib of generalized organization ; parenchyma and V.B. may show abun- 
 dant calc. oxalate rhombs. 
 
 NYMPH A EA, the leaves of the Water-Lily present a quite different type, as 
 associated with flotation on the surface of water. The lamina is soft, 10 in. by 8, 
 with petiole of several feet in length adjusted to the depth of the water, and is over 
 i mm. thick. In sect, note : 
 
 The upper epidermis consists of a single layer of small aqueous cells with thin 
 cuticle. Stomata occur only on the upper surface ; the guard-cells being well supplied 
 with starch-granules, and flush with the outer surface. 
 
 The palisade is very pronounced, 200 /* deep, of short slender, thin-walled cells, 
 in vertical rows, with large discoid chloroplasts, 5 /*. 
 
 The spongy mesophyll is arranged to form rectangular lacunae, bounded with 
 plates of simple cells, in single series, with chloroplasts and nuclei (io/i) in a peri- 
 pheral layer. V.B. rudimentary with spiral tracheides. Remarkable sckrites, as 
 branched sclerenchymatous fibres, pushing irregularly between other cells in all direc- 
 tions (instead of in one line only), are left more or less suspended in the walls of the 
 lacunae on final distension of the lamina : these are lignified, picked out very prettily 
 by the phloroglucin-reaction. The lower surface shows tannin-sacs in the place of 
 more usual stomata. 
 
 Lavandula (Lavender) affords a good example of hair-derivatives of the epider- 
 mis ; as (i) branched hairs, of shrubby growth of dead air-containing units, forming 
 a protective screen over surface, and covering (2) glandular hairs, with 2-4 celled head 
 on short pedicel. The head-cells secrete ethereal oil underneath the impervious cuticle, 
 which may become enormously distended before rupture (60 /*). 
 
 Pelargonium tomentosum, intense odour, softly hairy; (i) simple dead hairs, 
 i mm. long, as velvety pile ; (2) glandular hairs on long pedicels, globular head-cell 60 /* 
 diam. ; secretion distended 80 /*. 
 
 Hedera (Ivy) affords pretty stellate hairs, protective on young shoots ; esp. larger 
 forms with rusty tomentum : limiting case in peltate closely-set scales of Eleagnus, as 
 glistening skin over transpiring surface. 
 
 12 
 
Structural Botany : Organization of the Herbaceous Stem. VI 
 
 The function of the stem is to support, space out, and display the photo- 
 synthetic leaf-members, to supply them with food-salts, and act as the general path of 
 conduction from the absorbing root. Hence its tissues are differentiated in connexion 
 with (i) Conduction of water and solutes to the lamina ; (2) conduction of elaborated 
 food-material to other growing but non-photosynthetic regions ; (3) mechanical sup- 
 port and ramification over a considerable space- dimension ; (4) in lesser degree for 
 storage of reserves. 
 
 The tissues concerned with these functions are respectively : (i) Xylem, (2) 
 Phloem, (3) Mechanical tissues (Stereome), more particularly as sclerenchymatous 
 fibres, to a lesser extent Collenchyma in young axes, (4) all living parenchymatous 
 units. 
 
 Ramification of the shoot is normally provided for by the initiation of new grow- 
 ing-points in the axils of the leaf-members, hence termed axillary-buds ; the limiting 
 case of one such bud being normal for all foliage-leaves. 
 
 A good example is afforded by the stem of the great Sunflower : 
 
 HELIANTHUS ANNUUS, which may grow 12 ft. high, and 2-3 in. diam., 
 in the course of a few months. Small stems, J in. diam., from an actively growing 
 plant are most convenient. Hard stems and soft ends may be rejected. In transv. 
 sect, the bundles are visible to the eye, as an enlargement of the ^ in. stem : about 
 20-30 are evenly distributed around the periphery, about 2 mm. from the margin, and 
 the centre includes a large pith, 9 mm. diam. Section need not include more than 
 3 bundles. Iodine sol. gives immediate differentiation of the V.B. masses, endoder- 
 mis with starch grains as before, tracts of scl. fibres (yellow) in the pericycle external 
 to the V.B., as if associated with them : a definite cambium-zone passes from bundle 
 to bundle. The cortex shows an outer collenchymatous region and a few ducts. 
 The bundles vary in size, some very small. Select a medium well-differentiated one 
 for more detailed examination. Tissues from the periphery inwards include : 
 
 (1) Epidermis, of oval cells, 30 /x wide, with aqueous contents, slight pitted areas, 
 
 thicker outer wall, and distinct cuticle. 
 
 (2) Collenchyma, of a dozen rows or more; cells 15-20^, oval, with cellulose 
 
 thickening at angles and on tangential walls ; scanty contents with a few chloro- 
 plasts ; grading into : 
 
 (3) Cortical Parenchyma, of thin-walled cells, oval, up to 100 /u wide, with scanty 
 
 contents ; many in active division by radial walls. Occasional resin-ducts show 
 a cavity, 20 p. diam., surrounded by a series of a dozen or more small, 20 \*. 
 secreting cells, grading into ordinary parenchyma. 
 
 (4) Endodermis, as a wavy line of cells, in single series, pressed outwards opposite 
 
 the V.B., and dipping between them ; cells to 50 //, more or less with starch- 
 grains. Radial walls in close contact. 
 
 (5) Pericyclic Fibres ; opposite each primary V.B. an oval tract of very distinct 
 
 tissue, as closely packed units (up to 500 or so) with minute intercellular spaces 
 only ; walls thickened, with slit-pits, and deeply lignified (giving phloroglucin 
 react.). Average diam. 25 n, but many smaller attenuated ends cut as the effect 
 of sliding growth. Little living contents left at this stage. 
 
 (6) Pericyclic Parenchyma fills the regions between the fibrous tracts, cells of 
 
 which may be also dividing by radial walls to keep pace with the growth of the 
 stem. 
 
 (7) The Phloem, immediately within the fibrous tract, consists of units with cellu- 
 
 lose walls and protoplasmic contents, differentiated into : 
 (a) Sieve-Tubes, 20-25 /* diam., with contents as dense coagulated proteid masses 
 
 (or falling out in sect, of sp. mat.). 
 (6) Companion- Cells, as small units (6-8 /*) with dense granular contents, 
 
 associated with the sieve-tubes, as at their angles. 
 
 (c\ Phloem-parenchyma, as all cells not distinctly referable to the preceding. 
 (d) Protophloem, suggestions of small cells at the external limit of the tract. 
 
 (8) The Cambium as radial rows of small cells, 15 ^ wide, actively dividing by 
 
 tangential walls to build new tissues in both directions radially, with occasional 
 
 13 
 
radial divisions to add new rows. The term covers the initial cell of each row, 
 as also, more loosely, all units still small and not obviously differentiated. 
 
 (9) The Xylem-region internal to the cambium shows at first thin-walled units in 
 
 process of elaboration ; vessels dilating at the expense of adjacent units before 
 acquiring the special wall-thickening, and retaining p. contents and nucleus. 
 The fully differentiated region, picked out by the phloroglucin-reaction, is ligni- 
 fied, and consists of: 
 
 I. Metaxylem of 
 
 (a) Pitted Vessels, appearing as large oval or rounded empty spaces, to 100 ^ 
 or more diam., wall thick, with beaded effect due to closely set small bor- 
 dered pits (5 //), deeply lignified ; between these 
 
 (t>) Xylem Fibres, as units similar to the fibres of the Pericycle, lignified, with 
 thick walls and slit-pits, closely packed by mutual pressure. 
 
 (c) Xylem Parenchyma, lignified, with conspicuous pits on end walls. 
 
 II. Protoxylem of rounded tracheides, in more or less radial series, diminishing in- 
 
 wards, often in finger-like series, including 
 
 (a) Spiral Tracheides, to 70 /z diam., showing cut end of a spiral thread-thicken- 
 ing : 3-4 in series, to smaller ones of 40 /*. 
 
 () Annular Tracheides, the smallest, innermost, lignified units, with no cut 
 thread (i. e. ring-thickening). Surrounding these 
 
 (c) Xylem Parenchyma, as small-celled undifferentiated tissue, ultimately 
 sclerosed. 
 
 (10) Medulla of larger parenchymatous cells, as also in Primary Medullary Rays, 
 
 20-60 fj. diam. In the centre of the pith, which soon dies, and contains air 
 only, cells distend to 300 p diam. 
 
 Note. The connexion of Interfascicular Cambium across the rays by the inner 
 pericycle; differentiation of Metaxylem takes place centrifugally, that of Phloem 
 centripetally. New vascular strands have no ' proto '-regions. The Protoxylems 
 differentiate centrifugally (' endarch ') : the vascular tissues will ultimately complete 
 the circle as a ' vascular cylinder '. On the flanks of the bundles pitted-tracheides 
 occur, intermediate to vessels, as at the periphery of the protoxylem reticulate tracheides 
 are intermediate to simpler spiral units. 
 
 Longitudinal section, cut radially through one V.B., shows general extension 
 of all the tissues more or less : isodiametric cortical parenchyma may be twice as long 
 as wide ; endodermal cells 3-4 times. The fibres are apparently indefinitely long, 
 with attenuated ends, and the slit-pits show on the face-walls. Sieve-Tubes, 200 /z 
 long, with sieve-plates obscurely perforated and 20 \L diam. ; the coagulated proteid 
 contents (and slight starch) staining deeply with Iodine-sol. Cambium units as 
 delicate longitudinally extended cells (200^), with thin walls, granular contents and 
 elongated nuclei. Pitted Vessels, formed of segments 300 p long, or more, with 
 close-set oval, eye-like, bordered pits (6 /x). Xylem fibres as slender units with pointed 
 ends, small slit-pits, both ends not seen ; mingled irregularly in metaxylem. On the 
 flanks of the bundle, pitted tracheides come next to pitted parenchyma of the M. rays, 
 rectangular cells, 100 //, or more long, with pitted walls. 
 
 Protoxylem of tracheides, variously thickened ; as reticulate, with broad meshes, 
 grading to pitted forms, 300-400 //, long; spiral tracheides with spiral thread (i or 2), 
 running distinct, anastomosing, or branching, pulled out more or less according to 
 age ; the thread alone lignified. Annular tracheides as the innermost, with perfect ring- 
 thickening alone lignified, and pulled out to ioo/x, apart. Transitions to spiral also 
 occur. 
 
 Iodine-solution is the best differentiating reagent for cell-contents; Phloroglucin- 
 2 5 % H 2 SO 4 for differentiation of lignified walls ; Chlor. Zn. lod. is often feeble for 
 spirit-material, though admirable for fresh : In extreme cases Iodine sol. and 66 % 
 H 2 SO 4 gives a deep blue with celluloses, but is apt to decompose delicate tissues. 
 
 Drawings of such an indefinite complex, no two sections being alike, are 
 preferably built up by taking one unit of each kind (or two), in succession, as 
 a composite figure including all cell-details, rather than a fancy sketch of the field of 
 view : In the case of pitted tissues of different order, note the detail on either side of 
 the party-wall, as seen by focussing the cut edges. 
 
Structural Botany : Stem Mechanism. VII. 
 
 Differentiation of form of cells, texture of walls, and character of contents imply 
 differentiation of function. Names given to tissues are of value only in so far as they 
 express a specialization of parts in a complex mechanism. Details of microscopic 
 observation are the chief method of deducing the meaning of the organization; 
 e.g. What each unit does, how it is done, and on what evidence, are the essential 
 problems. 
 
 The Epidermis secretes the cuticle as a more or less impermeable film restrict- 
 ing loss of water from the entire external surface ; Collenchyma affords mechanical 
 cohesion under wind-strain : the ducts secrete terpene-derivatives, but are of little 
 significance in this type (cf. Ptnus). 
 
 The general Cortex is photosynthetic as far as light can penetrate, and acts as 
 a ground-tissue of units apparently not required for any more special function : the 
 endodermis controls lateral access to and from the stele. Pericyclic strands of fibres 
 afford mechanical cohesion, strength, and elasticity, to the axis in response to wind- 
 action. The Phloem includes tissues distributing synthesized ' food ' to non-photo- 
 synthetic regions, as soluble carbohydrate sugars and colloidal proteid ; the latter 
 requiring perforated sieve-apertures. The sieve-tubes, being destitute of a nucleus in 
 the functional condition, can have only a partial activity : the companion-cells, with 
 distinct nuclei, as sister-cells of sieve-tubes, apparently play a more active part, as in 
 storage and regulation. The Cambium is the meristem continually adding new 
 tissues, and implying that the older units become effete. Xylem includes tissues 
 devoted to the conduction of the transpiration-current. Pitted-vessels, with continuous 
 tube-cavity, formed by the obliteration of the original transverse septa, represent the 
 most highly efficient water-pipes; the bordered pits acting as countless valves per- 
 mitting the passage of water but not of air (cf. mechanism of pit-chamber, pit-membrane, 
 and torus). Spiral tracheides, universal in all vascular land-plants, as the first-formed 
 conducting units, merely present a spiral supporting internal thread of lignified 
 material on a cellulose wall, originally close-set. Annular tracheides, as variants, are 
 open to compression between the rings. Living parenchymatous cells, with osmotic 
 pressure of several atmospheres, are stronger than any dead wall unless thickened 
 solid. Medullary-ray parenchyma and pith are of the nature of ground-tissue, storing 
 starch, or dilating instead of dividing, and soon dying off in reduced supply of light, 
 water, and free oxygen. 
 
 CUCURBITA (Vegetable Marrow) an interesting example of herbaceous stem ; 
 also annual in duration ; with more exact number of large V.B. in a 5-6 ridged shoot, 
 bearing enormous leaves (18 in. across), and huge fruits : the plant is a tendril-climber, 
 and the mechanical tissues are ill-differentiated ; but the conducting tissues (^ and <) 
 are as fine as may be found, and far more elaborate than those of Helianthus, in 
 correlation with the great photosynthetic and transpiring area, as also the amount of 
 food to be conducted to the massive fruits within a few summer months. But the 
 cambium-ring is never completed, the bundles remain isolated, and the medulla 
 breaks down leaving a cavity. 
 
 Transv. sect, of the ridged stem shows V.B. in double series, corresponding to 
 the ridges and spaces alternating : the pitted-vessels of the metaxylem, -5 mm. diam., 
 can be seen with the naked eye. From the periphery inwards, note : Epidermis 
 little differentiated, with thin cuticle ; beneath it tracts of collenchyma, partial only, 
 a dozen cells deep (20 //,), with conspicuous cellulose thickening at the angles, giving 
 star-effects. Parenchymatous cortex of half a dozen rows : endodermis without starch, 
 and no special character ; but recognizable by being immediately outside a continuous 
 zone of pericyclic fibres, little thickened, 4-6 rows, and lignified. Within this zone 
 pericyclic parenchyma as a tract i mm. broad, of cells similar to med. rays, and in 
 older stems storing starch in cluster-groups. The Phloem contains large sieve-tubes, 
 to 70-90 fji diam., with associated conspicuous companion-cells: transverse sieve- 
 plates are seen in face-view, with netted pores (5 //,) and conspicuous proteid-contents, 
 irregularly coagulated, and staining brown with Iodine sol. Cambial cells are 30 //, 
 wide, giving very regular radial rows ; the latter much disturbed in the young 
 metaxylem by dilatation of young vessels. The Metaxylem of Pitted Vessels and 
 Pitted Tracheides shows no mechanical fibres, and but little parenchyma. The 
 
protoxylem of ill-differentiated spiral tracheides extends to a broad tract of small 
 celled (20 fj) parenchyma. Internal to the latter, a semicircular arc of Medullary 
 Phloem repeats the organization of the outer phloem, but is not capable of extension 
 by a cambium. 
 
 In Longitudinal sect, note, the large sieve-tubes of the primary (outer) 
 phloem, 600-750 /x long, and 50-75 /x wide, with sieve-plates as transverse septa, 
 5-6 ^ thick, and clearly perforated. In sp. mat. the coagulum of proteid may be 
 squeezed through the pores, and take the impression of the sieve. Pitted areas on 
 the side-walls communicate with the companion-cells, as lateral ' sieves '. In older 
 parts these are all blocked with callus, as carbohydrate waste of proteid-conduction, 
 30 /x thick on the main sieve, and as thick masses on the laterals. Later sieve-tubes 
 from the cambium are shorter, 200 /x; companion-cells correspond in length to their 
 sister sieve-tube: young sieve-tubes show nuclei 30 /x diam., but these disappear later. 
 Cambium cells are 150-200 /X long: the Pitted Vessels are built up from segments 
 100-200 fji long, all stages being found ; the bordered pits are in the meshes of a fine 
 reticular thickening. 
 
 ZEA (Maize) as a strong Grass, affords a convenient example of the Monocot. 
 type of construction : the stem is of annual duration, with well-marked nodes and 
 internodes ; the latter with considerable capacity for intercalary growth at basal region. 
 Avoiding pieces too hard, with silicified epidermal wall, transv. sect, of a young 
 growing stem (10-15 mm - diam.) shows V.B. scattered somewhat irregularly over 
 the section, about 500, of which f are more peripheral ; each V.B. consists of x an d <, 
 but with no cambium, and no secondary increase. The general scheme of preceding 
 types appears at first sight wholly wanting, but fibres are more numerous in the 
 peripheral region with smaller bundle-ends, and each V.B. presents normal orientation ; 
 the best V.B. are those nearer the centre. Individual V.B. are small tracts 2oo/x 
 (100-300) diam., all on the same plan ; one may be taken in detail : 
 
 Two large pitted- vessels placed laterally (60 /x) are most conspicuous ; as also 1-2 
 smaller ones in the radial line, 50-30 p, as roughly a V-shaped group, the apex of 
 which is occupied by an intercellular lacuna, and the fork subtends a small oval 
 phloem tract (100 /x). The innermost tracheide is annular (often only as suspended 
 loose rings), the next is spiral, and between the large p. v. a series of pitted-tracheides, 
 56 wide, closes the gap ; while small cells of xylem parenchyma fill the space around 
 the spiral tracheide. The Phloem consists of sieve-tubes (15 /x), as the larger units 
 of the tract, more or less in a chess-board alternation with smaller cells (6 JM), 
 apparently functioning as companion cells, with marked contents. Crushed relics of 
 Protophloem appear at the limit of the V.B., pressed against an investing sheath of 
 sclerosed fibres (1-2 deep) which more or less surrounds each bundle, being greatly 
 exaggerated in the cortical region of the stem. Ground-tissue of thin-walled 
 parenchyma, to 150-200 /x, with marked aeration, fills the intermediate regions of 
 the stem. 
 
 In Longit. Sect, different units may be checked by their position in transverse 
 scheme. The pitted-vessels are made up of segments 600 /x long, with close set oval 
 bordered pits, 6 /x; the annular vessels with hoops 50 /x diam., spaced as much apart 
 or more, are the finest elements of this stem. The sieve-tubes are feebly differentiated, 
 with sieves hardly noticeable. Note general absence of starch, tannin, and crystals. 
 Cellulose tissues are differentiated by Iodine and 66 % H 2 SO 4 . 
 
 The biological organization of the stem is based on strength and flexibility rather 
 than rigidity, and this is secured by a very uniform distribution of fibrous tissues 
 associated with the vascular strands. 
 
 H IPPU R IS (Mare's Tail) ; Dicot. aquatic type with greatly deteriorated conduc- 
 tion system. In transv. sect, of internode, 5-6 mm. diam., Note central stele, circular, 
 i mm. diam. or more, and cortex with large lacunae ( to i mm.) separated by plates of 
 cells, i deep, giving great internal atmosphere. 
 
 Epidermis of small cells with thin cuticle ; Endodermis conspicuous, cells oval 
 (40 /x), with starch-grains, and ' radial dot '. No differentiation of V.B., no cambiumj 
 no M. Rays, no mechanical fibres, no distinct pericycle : a zone of ill-differentiated 
 phloem of small cells, largest (sieve-tubes) 20 /x diam., surrounding Xylem of reticulate 
 and spiral tracheides (30x1), mingled with small-celled parenchyma. Broad central 
 parenchymatous medulla. A trace of cambial division outside xylem in older stems. 
 
 16 
 
Structural Botany : The Woody 'Stem VIII. 
 
 The massive timber-tree which may commonly attain a height of 100-200 ft., 
 and live for hundreds of years, yet may take 20-30 years to attain reproductive 
 maturity, is in no sense as efficient an organism as the rapidly growing herbaceous 
 annual with quick returns. Indigenous trees are characteristically deciduous, and 
 work in terms of a short season (May-October), enduring the vicissitudes of the 
 winter. The amount of seasonal increment is relatively small; the tissues are 
 densely sclerosed, giving rigidity to the axis, and the cell-units remain typically small 
 and closely compacted. 
 
 The vascular cylinder from the beginning is preponderant in the stem ; the 
 xylem constituting a close cylinder around the pith ; individual V. B. are not noticed. 
 Growth continues by a circular cambium-ring, external to which narrow zones of 
 phloem and cortex are popularly but incorrectly considered as ' bark ', because they 
 can be peeled off at the cambium. 
 
 Special features require detailed notice : 
 
 I. Cambium cells have the shape of greatly elongated rectangular prisms, with 
 definite orientation, and may divide by transverse, radial, and tangential walls 
 respectively ; i. e. in 3 dimensions. 
 
 (a) Transverse walls, beyond the zone of primary extension of the axis, merely 
 produce septation into shorter segments in longitudinal series, as in medullary-ray 
 formation, and production of xylem parenchyma. 
 
 (/?) Radial walls increase the number of initial units, giving more radial rows of the 
 same size to fill the circle as the stem increases in circumference. 
 
 (y) Tangential walls give new tissue-units, as phloem on the external side (centri- 
 petal), and xylem on the inner side (centrifugal). Each division involves mitosis, and the 
 formation of a new cell-wall : in this last case in the position presenting greatest 
 difficulty. Initial cells (i. c.) in each radial row now control the mechanism, with 
 some sort of power of choice as to which segment shall continue as initial. 
 
 II. Annual Rings. Increase by cambium may continue indefinitely, and one 
 cambium may persist throughout the life of the tree as a permanent meristem of 
 embryonic activity, but restricted to special tissue-production : a cambium does not 
 originate any other part of the plant (as a branch or root). Where annual periods 
 are marked by climatic changes, tree-types respond by seasonal periodicity. T.he 
 cambium may be dormant, or ' resting ', at certain periods of the year, and start again 
 with renewed activity (as in Spring of the N. Temp.). Differences in the tissues 
 formed at different seasons may give the effect of zones in the wood; i.e. anew 
 growth each year as an annual ring. Spring wood is commonly indicated by wider 
 vessels ; summer wood by smaller vessels and tracheides, abundance of fibres and 
 increase in deposit of polysaccharide. Decreasing size to a minimum in late summer, 
 followed by sudden growth of larger units in the spring, with special function, 
 emphasizes the ring-effect. Such rings vary in width from 1-20 mm., as annual 
 increments, and afford a clue to the age of the tree, giving the ' grain ' of timber as 
 more or less concentric circles ; though again naturally wanting in trees growing with 
 little change of season. 
 
 III. Medullary Rays : The spaces between the primary bundles of a young 
 stem are conveniently known as primary rays. These tend to be obliterated by 
 interfascicular extensions. Secondary rays are more important, as formed at and by 
 the cambium for a special purpose. Any initial cell may be set apart to produce 
 a ray. These rays are distributed at fairly constant intervals (varying in different 
 types) ; and as the circle of cambium increases, new rays are initiated to maintain equal 
 distribution, midway between the older ones. Being initiated in the cambium, the rays 
 differentiate both ways in ^ and < for ever after, following radial lines at right-angles 
 to the curve of the cambium ' circle ' ; and so with the concentric rings constituting an 
 orthogonal geometrical construction, as seen in cross-section, centric or eccentric, 
 according to the growth of the stem. 
 
 Typically of living parenchymatous units, radially elongated, more or less 
 sclerosed in the xylem, and storing starch, tannin, or crystals, the primary function is 
 to act as living units controlling the mechanism of radial and transverse conduction to 
 other living cells of the vascular cylinder. Commonly only i cell wide, and several 
 
 17 
 
deep ; but often very wide and very deep (broad rays of Quercus). In wood split 
 down radial planes the rays show as a ' silver-grain ' effect. 
 
 IV. Cork: The epidermal cells constitute a specialized integumentary system 
 producing the cuticle, but are not capable of indefinite growth and division. Typically 
 in woody stems with rapid secondary growth, the epidermis is replaced by a new 
 formation (Periderm). A special meristem arising from the cells below the epidermis 
 (in the general case hypodermal, but often deeper) presents the dividing mechanism of 
 a cambium, so far as ' initial cells ' and ' radial rows ' may be concerned. This 
 constitutes the Phellogen ; and the tissues formed from it centripetally are cork, those 
 centrifugally phelloderm : the latter is rarely well- developed, and merely adds new units 
 to the cortex, only distinguished by being in radial rows. 
 
 The cork-cells retain close lateral contact, but soon die ; the wall is ' suberized ', 
 and becomes very impermeable to water and gases. The cork-formation may be 
 continued indefinitely from the same phellogen (cf. Bottle-cork), or new phellogens 
 may be initiated internal to the first one. All parts external to the last phellogen are 
 cut off from communication with the cortex, and soon die, decay, or may be exfoliated. 
 
 V. Bark : The term bark botanically covers all such dead layers cut off by 
 a Phellogen, whatever their origin, as opposed to the popular and commercial concep- 
 tion of the word. Such bark may be cut off in flakes or rings, sheets or ' scales ', the 
 last by oblique sectorial phellogens, and may contain ' stone-cells ' (Pintts) and layers 
 of different texture. Where the phellogens are initiated deep in secondary phloem, 
 the bulk of the bark of an old tree may consist largely of crushed phloem. The 
 products of one phellogen are traced in terms of radial rows to each initial. 
 
 VI. Lenticels : As the cork in its special impermeability replaces cuticle in 
 function, so aeration of the tissues must be provided for, and the stomatal control is 
 replaced by lenticel-formation. These appear on corky surfaces as small wart-like 
 growths, often increasing with age to considerable dimensions on the bark, but are of 
 essentially similar origin from the phellogen : certain cells over localized areas are set 
 apart to give new tissue-units centripetally, differing from cork- cells in that the walls 
 readily separate, and the tissue becomes hygroscopic and powdery, as ' lenticel tissue ' : 
 on decay, this leaves free communication to the phellogen and subjacent phelloderm, 
 which is usually well-developed beneath the lenticel. Such lenticels may persist for 
 several years, and show seasonal periodicity ; closing in winter by a production of 
 cork, and opening in spring as the cork-layers are in turn ruptured, and thrown back 
 like the pages of a book, by new loose-tissue formation. Clogging or closing of these 
 pores by dust or smoke, by cutting off the free oxygen-supply from the internal 
 cambium, where there is little photosynthesis in the cortex, may be distinctly injurious. 
 
 Elder (Sambucus) shoots cut at the end of the first summer show a well-defined 
 production of large cork-cells (50 /x, wide), in radial rows of half a dozen or so from 
 hypodermal phellogen ; the dead epidermis still persisting, and not conformable with 
 the subjacent radial rows. Phelloderm represented by 1-2 cells per radial row; the 
 initial cell of the Phellogen as the last cell out with protoplasmic contents. Under- 
 neath the Periderm collenchymatous cortex. The woody ring is extremely hard, and 
 may be avoided ; slips of outer cortex, stripped at the cambium, being cut in pith. 
 Lenticels show as lenticular areas, about 2-3 mm. long, and i mm. broad ; in section, 
 note great extension of dead layers, ruptured peripherally to a broad gap; loose 
 lenticel-tissue, and great development of phelloderm, a dozen cells or more in the 
 radial rows. Larger growths on second year's shoots, to 2-3 mm. diam., show 
 characteristic reflexed tissue-zones. 
 
 Bottle Cork (Quercus Suber), as a secondary growth of cork, induced after 
 stripping primary mass ('Virgin Cork*'), of very uniform texture, with lenticels as 
 conspicuous pore-canals. These pass radially and transversely, giving the orientation 
 of the tissue. 
 
 Note annual zones of growth (2 mm. wide) ; in transv. sect, radial rows of fairly 
 isodiametric rectangular units, about 40 /x, wide ; radial sect, similar radial rows ; but 
 in tangent, sect, no radial rows are seen ; the cell being isodiametric and polygonal. 
 
 Tilia (Lime-tree) is a convenient type of woody stem, owing to softness of wood. 
 (Separate detailed Schedule omitted from elementary course) 
 
 18 
 
Structural Botany : The Root. IX- 
 
 The Primary Hoot of a seedling (e. g. Bean) appears as the direct continuation 
 of the primary axis of the shoot, passing down into the soil, as a main Tap-root, 
 with definite growing Apex, and lateral ramification ; the laterals spread more 
 horizontally, and again branch to a highly ramified system penetrating the soil in all 
 directions. 
 
 Ordinary Soil is a very special material, of inorganic particles and colloidal 
 organic de*bris, with spaces containing air and water, as a complex organization 
 physically and chemically ; and also biologically as containing races of micro-organisms 
 (Bacteria and Protozoa). The essential part of soil may be preferably regarded as 
 a living complex of bacteria engaged in breaking down decomposing plant-residues, 
 largely of cellulose origin, as humus, to which inorganic particles are largely accessory, 
 One gram of good soil may contain anything from 10-50 millions of Bacteria. For 
 successful penetration by roots the material requires to be loose in texture, well-aerated, 
 with considerable water-content, as facts of general horticultural experience. Land- 
 plants are normally furnished with ' soil-roots ' : ' water-roots ' and ' air-roots ' involve 
 subsidiary problems. 
 
 The essential function of the root is Absorption of solutions in the soil ; the 
 mechanism of the process, being complex, involves a stationary habit ; fixation of the 
 plant to the ground, as adaptation to resist uprooting effect of wind, &c., is quite 
 secondary. Normally the connexion has become so intimate that adult plants with 
 less power of regenerating new root-systems are with difficulty ' transplanted'. The 
 solutions in the soil vary enormously, but are normally extremely dilute ; inorganic 
 solutes being largely ionized. Essential substances are determined by 
 
 Culture Solutions, experimentally. A typical laboratory medium may contain 
 Potas. Nitrate, i g., Sod. Chlor., Calc. Sulph., Mg. Sulph., Calc. Phosphate, each -| g., 
 and a trace of Iron salt; or about 3 g. of salts to a litre (1000 g.) Sea-water, 35 g. 
 per litre, is over ten times as strong; lake- and river-water about -^fa the latter. 
 The osmotic value of sea- water is 23 atmospheres, that of rain-water may be nil; 
 soil waters range between. Absorption can only take place at the peripheral layer of 
 cells in direct contact with the medium ; and in typical soil-roots the units of this layer 
 are produced into special filamentous growths presenting greatly increased surface, as 
 1 Root-Hairs' ; these may attain a length of 1-5 mm., with a diam. of 6-30 /*. The 
 layer producing them is known as the ' Piliferous Layer ', as quite distinct from the 
 conception of the epidermis of the aerial shoot-portion. 
 
 A typical Boot-Hair extends as a dilatation from a localized area of the original 
 cell-wall, with protoplasm, aqueous contents, and nucleus controlling its extension ; 
 the plastic apex pressed between particles of soil may become irregular in shape, and 
 contains little cellulose. Such units have usually but a short life, and die away as new 
 ones are continually produced at the growing apex ; e. g. on pulling up a plant the 
 root-hairs are usually broken off, and remain in the soil (cf. Groundsel, root-hairs 
 10 /A diam.). 
 
 Boot Apex : The root increases by a terminal growing-point, essentially similar 
 to that of the stem, and presenting similar differentiation into distinct layers, con- 
 ventionally distinguished as calyptrogen, periblem, and plerome. The calyptrogen is 
 many-layered, and constitutes a definite ' Root-cap ' region, giving the apex a pointed 
 conical appearance, the older layers of which are gradually worn away by friction in 
 passing through soil. The initial layers are internal, marking a dome-shaped curve 
 as the true growing point. The periblem (or cortical region) in some cases (Zea) 
 reduces to a single row of cells at the actual apex, giving very distinct layering ; in 
 others (Bean) it is ill-marked. The central region (plerome) gives rise to the con- 
 ducting stelar tract. The calyptrogen is either entirely worn away as root-cap (Zea), 
 or leaves one layer only (Dicots.) ; in the latter case this becomes the piliferous layer ; 
 in the former, root-hairs are produced from ' periblem '. Such discrepancies show that 
 apical layering is of no strict morphological value, but may characterize minor groups 
 as interesting variations on a general physiological mechanism. 
 
 Zones of Growth are more condensed than is usual in the case of typical 
 stems. The second ' zone of elongation ' is but short, giving increased power to the 
 thrust behind the dividing meristem of the apex ; and this region is again responsible 
 
 19 
 
for all correcting curvatures. Adult differentiation of primary tissues is rapid ; the 
 first expression of tissue-differentiation being seen in the initiation of protoxylem 
 tracheides at usually only a few points. Hence roots are described in terms of 
 protoxylems as ' diarch ', triarch, tetrarch, &c., and ' polyarch ' when the number is not 
 obvious at a glance. Maximum number in large Monocot. types may be 100 or more. 
 The limiting case of centric symmetry is diarch ; and this becomes the commonest 
 case for all small roots. Root-structure also shows a characteristic arrangement of 
 the xylem, possibly as retention of an older mode of organization; the xylems 
 alternate with the phloems on different radii (' Radial ' arrangement, as opposed to 
 ' collateral ' in the stem), and the xylem units differentiate centripetally ; the first-formed 
 tracheide being external in the stele, abutting on the pericycle, and hence termed 
 exarch, as opposed to endarch and centrifugal in the stem. 
 
 RANUNCULUS repens (Buttercup): The general scheme of primary tissue 
 organization may be followed in stout roots, 2 mm. diam., in which no secondary 
 complications obtain. More normal roots present secondary thickening by a cambium 
 at an early stage, and ultimately become woody axes, like old stems, in which 
 conduction alone remains ; absorption being restricted to the growing tips penetrating 
 new ground. In transv. sect, note : 
 
 Cortex: wholly parenchymatous, of rounded cells (50-100 ^ diam.), with 
 intercellular spaces, and storing abundant starch-grains (12 //,); bounded externally by 
 Piliferous layer, of simple papillose cells, 25/1,, without root-hairs, and Exodermis of 
 thin walls, no special contents, and closely approximated radial walls ; internally by 
 Endodermis of oval cells (25 /x), radial walls with 'dot '-effect, owing to chemical 
 alteration of walls at point of contact. 
 
 In older roots the cortex becomes ragged and lacunar, the endodermal cells 
 lignify at the dot, add a thickening layer, and this in turn is lignified. 
 
 Stele, circular, (300 //, diam.) conveniently fills field of high power. 
 
 Type tetrarch (varying 3-5), with 4 protoxylems symmetrically spaced, alternating 
 with 4 phloem tracts. The former continue differentiation to meet in the centre, thus 
 leaving no medulla. In young roots only the first spiral tracheides are in evidence, 
 and the others as thin-walled dilatations ; in older roots fully differentiated pitted- 
 vessels extend to the centre, and sclerosis may extend to conjunctive tissue on the 
 flanks of the xylem. 
 
 The Pericycle (as the region between the protoxylem and the endodermis) is 
 reduced to the limit of one row of small units (12-25 //,). 
 
 The Protoxylem units as spiral tracheides are small (12 /A), and a large pitted 
 vessel (75 /A) may occupy the centre of the stele : the entire xylem-tract is limited to 
 about half a dozen tracheides. 
 
 The Phloem tracts are vaguely oval regions between the xylems, of a few small 
 parenchymatous units (12 /x), with few (1-2) sieve-tubes (20 //,) with companion cells. 
 A zone of small-celled conjunctive parenchyma separates the $ from the x> and follows 
 round internally. 
 
 In older roots definite indications of tangential division in some of the cells of 
 the innermost phloem express the initiation of cambtal mechanism, but get no further. 
 
 Deposits of starch in the cortex express the utilization of these roots for storage 
 of food-reserves over the winter ; these are conveniently cleared by the use of potash ; 
 the tissues are well differentiated by Phloroglucin-H 2 SO 4 , or wholly cleared in Eau 
 de Javelle. 
 
 Special Cases : The roots of water-plants, attached in mud, show extremely 
 deteriorated organization ; root-hairs are wanting, and vascular tissues are reduced to 
 a minimum. Many epiphytic orchids, of tropical rain-forests, and in greenhouse- 
 cultivation, send out aerial-roots, in which a peripheral tissue, delimited by an exodermis, 
 and of varying thickness, is specialized as dead units with tracheidal thickenings, which 
 store rain-water, as a spongy system, Velamen : conduction is effected by living 
 ' passage-cells ' distributed in the exodermis, and storage-tracheides occur in the cortex. 
 The stele is of the polyarch Monocot. type. 
 
 Primary structure is also well shown in the primary roots of seedlings of 
 Phaseolus (Kidney Bean), about 2 in. from the tip. The tissues are soft, but the 
 radial dot of the endodermis is particularly well-defined. 
 
 20 
 
Structural Botany ; Root Mechanism. X. 
 
 Secondary Thickening is adjusted to the primary construction ; no residual 
 meristem (procambium) being left in the stele as first outlined. Division by tangential 
 walls is first observed in the conjunctive parenchyma immediately internal to the primary 
 phloems ; and this continues to give each a definite cambium tract, from which normal 
 differentiation ensues, as xylem products on the inside (centrifugal), and phloem units 
 on the outside: such extension naturally pushes the protophloems farther out, and 
 these are soon crushed out of existence. Similar tangential divisions follow in the 
 pericycle units external to the protoxylems ; and the preceding partial cambium arcs 
 are linked up in a continuous system, more or less wavy at first, but soon adjusting to 
 a definitely circular cambial zone, similar to that of the stem, but leaving the proto- 
 xylems on the inside more or less unaffected. Quite old roots may be thus distinguished 
 from stems in section by tracing the original protoxylems. The new tissue opposite 
 the protoxylems commonly remains parenchymatous as 'primary ray', rendering the 
 pattern more conspicuous. With a normal cambium in working order, growth follows 
 on as in the stem, and annual rings are produced in successive years ; but the first ring 
 is complete in the first year. The cortical region may fail to keep up with the new 
 growth, and is (in a tree-type) commonly exfoliated, including the endodermis. 
 
 Phellogen giving a cork-formation, as in stems, normally follows from the outer 
 layer of the pericycle, which thus in old roots looks very much like a cortex. Secondary 
 tissues with vessels, secondary medullary rays, sieve-tubes, fibres, &c., agree with the 
 details of the corresponding stem for different types. 
 
 Lateral Hoots : Ramification of the stem follows the lines of pre-existing 
 organization, and the laterals are thus associated with the very definite plan of leaf- 
 arrangement as repeating an ancestral mechanism of construction : ramification of the 
 root, in absence of leaves, follows pre-existing organization as that of the protoxylem 
 centres. Owing again to the special mechanism of the absorbing peripheral layers, 
 laterals are endogenous ; they are initiated in the pericycle, opposite the protoxylems, 
 and are hence put in immediate communication with the water-conducting tissues. 
 
 Tangential divisions are initiated in a few cells of the pericycle, thus delimited 
 in transv. sect., but irregularly spaced in the longitudinal dimension, as a rhizogenic 
 plate. Two sets of such divisions suggest the 3 tiers of apical meristem (as calyptrogen, 
 periblem, and plerome), and the new apex is fully constituted; the young root extending 
 radially to reach the surface and free medium across the cortex. The endodermis 
 normally takes part ; its cells divide radially to keep pace with the new growth, forming 
 a pocket over the young root; the cells of this region become glandular, secreting 
 enzymes which digest the tissues ahead, so that penetration is not merely mechanical. 
 On reaching the exterior this digestive pocket is worn away ; though in water-plants it 
 may persist as a conspicuous glove-finger over the apex (Lemna). The xylem of the 
 young root is linked by tracheides with that of the old, at the point of origin; the 
 phloems are connected laterally, and the gap in the endodermis is made good at 
 a * 3~dot ' cell, as seen in transv. sect. Tissues of unlike origin have to be fitted together 
 or the machine would not work, and a considerable range of variation may be noted. 
 
 Adventitious Roots arising on stem, or even leaf-structures, are similarly 
 endogenous and are initiated in the pericycle. From very embryonic tissues with im- 
 differentiated epidermis, ' bud-roots ' may take off exogenously (Water-cress). The 
 rooting of ' cuttings ' is effected by the initiation of root-centres in the meristem of 
 * wound-callus '. In many trees the factors required to initiate a stem-apex are sub- 
 stituted for those of a root, and stem-buds may thus break out endogenously from the 
 pericycle, to corne to the surface as ' suckers ' (Elm, Poplar). Adventitious roots, 
 emitted from a stem under stimulus of contact with a damp surface, may be utilized 
 for climbing, Ivy (Hedera) and Poison Ivy (Rhus toxtcodendron) ; in the event of 
 efficient water-supply these may attain a considerable size. 
 
 The origin of lateral roots may be traced in Bean seedlings, from the first visible 
 protrusion of the young laterals to near the apex ; but owing to the vague apical 
 differentiation of the Bean root, seedlings of Zea give more detailed results. 
 
 The development of secondary tissues may be followed on the Bean, but more 
 conveniently in the roots of : 
 
 21 
 
Vitis pterophora, a greenhouse climber, which drops adventitious air-roots for 
 several feet, in lengths with clean surface and very uniform structure. 
 
 (1) Softer young roots, 2 mm. diam., show well-marked protoxylems, 4 or more 
 (7), and conspicuous phloem-tracts of small cells, with distinct medulla and broad 
 cortex. The endodermis is thin-walled, and only distinguished by being the first row 
 of cells without intercellular spaces. Conjunctive parenchyma on the flanks of the 
 xylems may become sclerosed; but is distinguished by the different tint with 
 Phloroglucin-25% H 2 SO 4 . Root-hairs are wanting; the 2 layers subjacent to the 
 peripheral layer are distinct, and contain tannin. Protoxylem units grade up to 75 //, 
 diam., the first formed small and soon crushed. 
 
 (2) A slightly older (firmer) stage shows cambial divisions freely produced along 
 the inner margin of the phloem tracts, giving distinct radial rows : large metaxylem 
 units are soon differentiated on the inner side of the cambium (90 /x) ; the phloem may 
 show tannin sacs ; a few cells of the pericycle become fibres, outside the phloems. An 
 active phellogen on the outside of the cortex gives cork-cells cutting off the peripheral 
 layers. Tangential divisions extend to the pericycle external to the protoxylems, and 
 the cambium is linked up, though distinctly 4-angled in outline in tetrarch roots. 
 
 (3) Roots 2-| mm. diam. show a well-marked vascular cylinder with masses of 
 metaxylem alternating with the protoxylems, and large pitted vessels (ioo/x); but the 
 xylem is mainly parenchymatous: the tissue external 10 the protoxylem remains a broad 
 primary ray, and a secondary ray (often even broader) is initiated in the new metaxylem 
 tracts. The cambium zone is fully established, with particularly good radial rows. 
 Pericyclic fibres are followed up by fibres in the phloems ; tannin-sacs, raphides, and 
 cluster- crystals of calc. oxalate, add details of interest ; all parenchymatous units store 
 abundant starch. Thyloses are frequent. 
 
 (4) In older roots, 4 mm. diam., with further increase, the large xylem vessels 
 (150/1) have fibres grouped around them : the protoxylems remain clearly defined, and 
 the tissues are clearly differentiated by Iodine, Potash, or Phloroglucin-H 2 SO 4 . 
 
 Quercus (Oak) affords a good example with annual rings, in roots 3-15 mm. 
 diam. ; the protoxylems (6 or so) persist as a stellate group in the centre, and the 
 primary rays (broad rays) are clearly seen by the naked eye. The details of the 
 tissues follow those of the stem. 
 
 Monocotyledonous type of root-construction typically differs in the complete 
 absence of secondary changes, and in polyarch organization. Succulent roots, 4 mm. 
 diam., of Asparagus present a wide cortex of rounded parenchymatous units (50-90 /x 
 diam.) with intercellular spaces. Stele, i mm. diam., the endodermis with U-thickening, 
 of small cells (12 /x). Pericycle reduced to one row (15 /x). Protoxylems about 20, 
 of very small tracheides (6 p) peripherally, but extending as a complete ring of vascular 
 tissue, with large p. v. up to 80 /x diam. Central medulla of 50 /x parenchyma. Phloem 
 tracts small, between the protoxylems, each with 2-3 sieve-tubes, and well-marked 
 cambiform units, apparently functional as companion-cells. The section contains no 
 starch, and clears admirably in Potash, or Phloroglucin-H 2 SO 4 . The peripheral layer 
 may have conspicuous root-hairs, but the subjacent tissue (6 rows or so) is thickened 
 and suberized. 
 
 Orientation of the Boot is normally effected in terms of positive Geotropism 
 and positive Hydrotropism ; i. e. roots tend to grow downwards into the soil and to 
 ' seek water ', as part of their essential biological equipment. In lateral roots 
 hydrotropism is more pronounced ; the strongly-marked positive geotropism of 
 a primary sinking root is an adaptation to the same end of water-supply. Rarely, 
 in water-logged soils, roots may grow upwards to obtain free oxygen. Sensitivity is 
 more concentrated in the apical meristem (Zone I), and the mechanism of response, 
 by initiating a growth-curvature, is restricted to the zone of elongation, usually only 
 a few millimetres behind the apex. The very general occurrence of starch-grains in 
 the cells of the root-cap (not in the growing layers of the apex) has given support 
 (since 1900) to hypotheses of j/tf/tory/^-rnechanism, interpreted in terms of 'Falling 
 starch ', still largely speculative. 
 
 Experimental observations are followed on the Clinostat, in which rotation on 
 a horizontal axis neutralizes gravity by giving equal presentation to the stimulus on 
 all sides. The effect of a continually acting force as 'g' may be imitated, or 
 replaced in intensified degree, by the utilization of ' centrifugal force '. 
 
 22 
 
Structural Botany : The Transpiration Current. XI. 
 
 Absorption of Water by the root-hair is regarded as mainly a matter of 
 osmotic activity, dependent on the solutes of the cell-sap and the control of the 
 semipermeable plasmatic film ; the osmotic pressure of the cell being normally much 
 greater than that of the external dilute solution. Capacity for absorption implies 
 a similar capacity for getting rid of excess fluid ; and this when passed to subjacent 
 parenchyma implies Conduction. The physical mechanism of plasmatic conduction 
 is wholly obscure ; speculations in terms of osmotic mechanism remain unsatisfactory, 
 as implying that each cell is an isolated mechanism. Colloidal plasma is undoubtedly 
 freely permeable for water as the original medium of its existence, and continuity of 
 the medium through colloidal membranes implies molecular continuity of the water 
 all the way. The fact of absorption by the root-hair implies turgidity oFthe entire 
 range of living cells; i.e. living units freely conduct until all are turgid; cells losing 
 water draw freely on the others ; younger units drain older ones ; as in the familiar 
 process of putting flagging cut-flowers in water, by stalks or tops. 
 
 A new departure takes place at the first dead tracheide : here a mechanism of 
 discharge or active secretion must be postulated. Plasma of land-plants necessarily 
 secretes water as a necessity of existence ; there being no other way of obtaining 
 food-salts in quantity from a dilute solution (cf. Myxomycete plasmodia secrete drops 
 of fluid, Dry Rot Fungus, Merulius lachrymans, Water-glands of leaves, and water- 
 pitchers of Nepenthes). In connexion with such secretion of water into a dead 
 * conducting' tracheide, it may be noted (i) the tracheide was full to begin with, and 
 (2) there is no evidence that it was dead or empty when it began to conduct. Such 
 secretion into the tracheidal system marks the beginning of the 
 
 Transpiration Current, and further rise of water in the dead xylem is mainly 
 physical ; the system is full to begin with, and remains filled ; fluid being drawn off 
 above by the transpiring cells, much as the root-hairs below draw on the soil-solution ; 
 as loss is made good, water moves in the stem in the dead tracheides. Such a tracheidal 
 system, more perfectly expressed in terms of vessels, is in full communication throughout 
 the plant or even a tall tree. In a tree 300 ft. high, the water-column with a pressure 
 of 10 atmospheres would only just neutralize the osmotic pressure of the turgid cells 
 at the base of the system with 10 atmospheres capacity. The height to which water 
 can rise is thus probably connected with the osmotic possibilities of the parenchymatous 
 conducting cells ; while owing to the wet colloidal walls there is practically no waste of 
 energy in friction. Lignified walls cannot be imitated by glass tubes. 
 
 Root Pressure : A special case of intensely active absorption, conduction, and 
 secretion is seen in the rise of fluid in the tracheal system of many trees, more 
 particularly in the spring ; this being noticed before the new leaves appear as a ' rise 
 of sap ' (cf. Sugar-Maple, Birch), and is measured as a positive pressure by water- 
 column, mercury manometer, or pressure gauge (e.g. Vine 1,000 mm. Hg; Birch 
 2^ atmospheres): when the leaves are expanded, a negative pressure is generally 
 noticed. Such activity is not confined to the root ; and ' bleeding ' may be induced 
 in other parts with active parenchyma; cf. Arenga (Toddy Palm) * bleeding* induced 
 by blows, long-continued, and the fluid contains sugars. 
 
 Absorption of Salts is independent of osmosis, the latter being solely a pheno- 
 menon of water-pressure. In a dilute solution the inorganic solutes are practically 
 completely ionized ; diffusion is assisted by the colloidal plasma adsorbing ions and 
 complex molecules with little selective capacity ; e. g. poisonous substances are taken 
 freely, as ions of CuSo 4 , and dyes as Methylene Blue. 
 
 Selective Absorption follows in the long run, as only the substances removed 
 from the solution in metabolism create a drain lor more of the same kind, to give an 
 absorption equilibrium. Hence from the same solution different plants give a net 
 absorption differently for the same salts. There is no excretion of osmotic material 
 from an undamaged root-hair ; but an acid effect (as seen by corrosion of marble 
 slabs) may be the effect of (i) contents of dead hairs, (2) excretion of CO 2 in respira- 
 tion, (3) excessive removal of basic ions (Ca", Mg", K', Na') in metabolism. 
 
 Culture Solutions : by the omission of one or more constituents at a time, 
 it is possible to trace the possible functions of different materials : e. g., omission of 
 Iron renders green plants chlorotic, owing to failure of chlorophyll-production, although 
 chlorophyll contains no iron. Lack of Potassium stops the photosynthetic mechanism, 
 
 23 
 
and implies starvation. Addition of Nitrates stimulates proteid-synthesis, and so reduces 
 polysaccharide deposits, giving rank but often mechanically weak growth. Phosphorus 
 stimulates nuclear activities. A certain relation between Ca, K, Mg suggests that these 
 may be individually toxic, but that they neutralize each other, as possibly inherited 
 from sea-water. 
 
 Water Problems are obviously insistent for all subaerial vegetation, but the 
 water-problem is but a part of the wider problem of the source of food-salts and 
 combined nitrogen. Special cases attract attention, as evaporation becomes more 
 acute. Although all land-plants require to transpire excessively to obtain food-salts, 
 regulation is required. Plants in danger of losing water faster than they can replace 
 it are termed Xerophytes ; and commonly present striking Xeromorphic adaptations 
 in form, anatomy, and general habit. 
 
 The leaves as the essential transpiring organs are the first to be affected ; followed 
 by the entire shoot-system. As available examples, cf. : (i) Leaves reduced in area 
 (Box, Heath); (2) Reduction of intercellular spaces (Conifers); (3) Diminution in 
 immber of stomata (Aloe, 10-20 per sq. mm.); (4) Thick cuticle (Holly, Yucca); 
 (5) Deposits of wax ('bloom' of Cabbage); (6) Clothing of hairs ( Verbasctim) ; 
 (7) Rosette-habit, with overlapping leaves (Sempervivuni) ; (8) Leaves edge to light 
 (Eucalyptus), or vertically orientated (Iris] ; (9) Extreme succulence by osmotic effect 
 (Sedum} ; (10) Sclerosis of covering-sheets of lignified tissue (Pinus) ; (i i) Entire loss 
 of leaves (Cereus); (12) Replacement of leaves by flattened cladodes (ppuntia\ 
 phyllodes (Acacia sp.), or phylloclades (Ruscus, Asparagus}; etc. 
 
 Also any combination of such factors, producing characteristic xerophytic vegeta- 
 tion, where such conditions are predominant throughout the year. But a plant may 
 be xerophytic (i) at one period of ils life and not another; (2) at some time of the 
 day; or (3) at some particular season: examples as (i) Seedlings with feeble root- 
 system; (2) Herbaceous plants wilting in hot afternoon; (3) The evergreen tree 
 (Holly) with feeble root-absorption in winier, and the deciduous tree, as the common 
 example of the N. Temp, region, shedding leaves when the transpiration-system fails. 
 
 Special cases of Aquatics and Hygrophytes present variants on the water- 
 problem, due to insufficiency in the supply. Aquatics, with all subaqueous parts 
 rooted in mud, impoverished for free oxygen-supply (cf. Hippuris\ utilize a voluminous 
 internal atmosphere in lacunar spaces, with free communication to subaerial portions 
 of the body. 
 
 Transpiration is wholly eliminated in plants growing in a saturated atmosphere, 
 or entirely submerged : in such case (cf. Zostera under the sea), salts can be obtained 
 with difficulty by direct adsorption; or by active secretion of excess water, as in 
 Nepenthes of tropical rain-forest. 
 
 Orientation of the main stem is effected largely in terms of positive Helio- 
 tropism ; but also in terms of response to stimulus of gravity, as negative Geotropism. 
 The plasma of stationary land-vegetation becomes sensitive to the direction of the fall 
 of its own material particles; and the response to this sense of direction may be 
 positive, negative, or transverse (plagiotropic) ; or even vary from time to time in the 
 same organ. Interpretations of geotropism in terms of ' falling starch', popular since 
 1 900, as hypotheses of ' statocyte ' nature, remain purely speculative. The cells of 
 a growing apex, with maximum sensitivity, have neither extensive vacuoles nor starch- 
 grains. The mechanism of response, by a growth-curvature, can be only effective in 
 regions in which the cells present a capacity for dilatation and extension. 
 
 Note, that changes in environment can never be postulated as Causal : the 
 individual plant is not an isolated mechanism, but the result of ages of inherited 
 response to similar stimuli, fixed by natural selection as part of the present 
 equipment of the race. Just as it is quite unjustifiable to read into the plant 
 ideas of thought, prevision, and design, based on our own more elaborated percep- 
 tions and actions ; so it is equally wrong to regard the plant as a mere physical 
 mechanism, and the passive victim of environment. The object of physiology is to 
 hold the balance between these two standpoints, with a view to understanding what is 
 the nature of living response. An inherited capacity for adaptability, within a certain 
 range, with every change of condition, may be termed the Vitality of the organism, 
 ultimately translated as self-determination. The mechanism of such inheritance, and 
 racial progression, is included under the heading of Keproduction. 
 
 24 
 
Structural Botany : Perennation. XII. 
 
 Special adaptations to enable organisms to exist in a more or less dormant 
 condition over periods of unfavourable environment are included as phenomena of 
 Perennation; since such vicissitudes are most commonly presented in terms of 
 seasonal periodicity. Conspicuous examples are ( i ) the case of the Cold Northern 
 Winter, (2) that of the hot and dry Southern Summer. Both trace back to problems 
 of water-supply, rather than temperature which only affects the rate of metabolism : 
 e. g. in cold soil, mechanism of root-absorption fails, and plants perish for want of 
 water in cold wind, even if rooted in water. In hot deserts lack of water is more 
 insistent than extreme insolation as leading to xerophytic habit. In cold winter of 
 British Isles trees perennate from mid-Oct.-Nov. to April-May, vegetating in short 
 summer season of 6 months ; and leaf-fall becomes a conspicuous feature of the zone 
 of deciduous trees (Central and N. Europe), as opposed to highly specialized xerophytic 
 foliage of evergreens of the Mediterranean region. 
 
 Mechanism of Leaf-fall, a special case of the general method of cutting off 
 all useless parts by the formation of an absciss -layer ; a tract of cells whose walls 
 readily exaggerate intercellular spaces, to loosen entirely along a line of cleavage ; all 
 parts intentionally separated follow this method, whether leaf, flower, fruit, or seed. 
 Autumnal leaf-fall may be more elaborate, as commonly preceded by a formation of 
 phellogen across the leaf-base (except through V.B.), giving a cork-layer in connexion 
 with the cork of the stem, and thus ' healing the wound ' before it is made. The 
 cork-layer becomes uniformly continuous, except across the axillary bud. 
 
 Populus (Poplar), leafy twigs cut in early October show the cork-zone with 
 phellogen on inner side, in usual radial rows ; V.B. pass through it : all tissues on 
 the leaf side are dead, and full of cluster-crystals; but a trace of starch at the 
 extreme base. On the stem side all living units are gorged with starch (as also calc. 
 ox., and patches of stone-cells, as conspicuous details). The absciss-layer is formed 
 externally to the first-formed cork (often giving phloroglucin-reaction) ; when formed, 
 the leaf loosens, hence it is not seen fully developed in sp. mat. Mechanical action 
 of wind or frost separates the dead leaf, rupturing the vessels and fibres of the V.B, 
 The V.B. are subsequently plugged by outgrowth of bundle-parenchyma, as a wart- 
 like scar on each end. The surface of the entire plant is then sealed with a uniform 
 zone of cork. 
 
 The Winter Bud : Such deciduous trees protect the shoot-apex and young 
 growth by special formation of" bud-scales (leaf-base, or stipules) ; and in order to start 
 rapid growth in spring, the leaves of the succeeding season are commonly all laid 
 down in the previous summer, remaining in a resting-stage enclosed within the ' bud ', 
 with its investment of scales (hibernaculum). The finest winter bud is that of 
 
 Aesculus (Horse-chestnut), not indigenous ; buds cut in October show Bark 
 with Lenticels, Leaf-scars with smaller scars of V.B., 5-7 in an arc; large T-bud, and 
 smaller axillary laterals following the decussate arrangement of the leaves. The 
 series of bud-scales similarly decussate imbricately in 4 rows, longer distally to the 
 pointed apex, and sticky with resinous exudation (sol. in spirit) from special glands 
 (colleters) on the scales. 
 
 In longit. med. sect, note the length of successive scales ; the enclosed foliage- 
 members with cottony packing-hairs; and in finer shoots the central terminal 
 inflorescence of young buds. Section of bud-scales shows colleters as small wart- 
 like sessile glands, with secretion poured into chinks between the scales. The latter 
 develop cork on outer surface from hypodermal phellogen. 
 
 Similar buds in April show rapid extension of shoot-internodes, divergence of the 
 bud-scales ; and often interesting transitions in form from protective leaf-base scale to 
 distal palmately lobed lamina. 
 
 Winter-buds are wholly wanting in trees of warm climates ; just as leaves remain 
 on more than one season as ' evergreen' habit. But leaf-fall may be again induced by hot 
 dry season (desert vegetation), resting buds similarly perennating over heat-period. 
 
 Herbaceous Perennials commonly vegetate over winter by a ' root-stock ' 
 portion, at the soil-level or below, of stem parts with reduced internodal extension ; but 
 sending up rapidly elongating axes each season, as foliage and flowering-shoots ; cf. 
 Polygonum, Hop. 
 
 25 
 
Special cases may be distinguished, as : 
 
 Rhizome of many forms, as in Iris ; the main axis remains prostrate at soil 
 level, or even below, out of the way of desiccation, sending up erect foliage-leaves 
 and inflorescences under optimum Spring conditions : essentially an adaptation to hot 
 desert with sandy soil. Note short seasonal growths, leaf-scars, advt. roots, and 
 utilization of stem-tissues for storage of reserves, as starch. Perennation over 
 summer heat may be continued over winter cold as well. By further concentration of 
 shoot-system and deeper soil-penetration the case of the ' bulbous ' Iris, with starch- 
 storage in leaf-members. 
 
 The Tuber (cf. Potato, Artichoke) as a special case of subterranean shoot- 
 formation, i or more internodes of a rhizome-system being swollen, filled with food- 
 reserves in parenchymatous tissues, and readily separated from parent plant. More 
 or less rounded form ; succulence due to water-storage ; with corky covering ; 
 enduring desiccation, and also utilized for dispersal. 
 
 Potato (Solarium tuberosuni), a plant of Central America adapted for perennation 
 over hot dry season in sand, to germinate in wet season ; tubers originally small 
 (i in.) and round. Utilized in this country for perennation over cold winter (under 
 cover), and often grown in clay soil : cf. stem-structure, stalk-end and apical crown 
 with T-system of leaf-scales and buds (' eyes '), or scars of subtending leaves ; 
 section near stalk shows V.B. distributed; delimiting cortex and broad central 
 medulla. Spiral vessels in V.B. (test phloroglucin -H 2 SO 4 ), and large starch-grains 
 (to 90 //,) with conspicuous stratification and eccentric hilum. 
 
 The Corm : cf. Crocus as a more extreme case ; the entire plant dried off and 
 reduced to a solid short main axis, with dense starch-storage ; enclosed in dead scale- 
 leaves, with lateral buds (i or more); these break out as flowering leafy shoots in 
 spring : cf. Yellow Crocus with white scale-leaves, green foliage-leaves, and 1-3 
 flowers in each such shoot. Plants dug up in March show new corm forming at the 
 base of each new shoot. Cf. Montbretia, similar enlargements of successive seasons 
 remain attached in linear series. 
 
 The Bulb (Lilium, Narcissus, Hyacinthus), a limiting case in which the entire 
 plant is reduced to one perennating bud-construction : cf. Narcissus ; reduced axis at 
 base of bulb giving off adventitious roots ; bulk of structure as a mass of leaves, 
 functional in successive seasons as (i) dead scales remaining as protective wrapping; 
 (2) storage-leaves, with starch and mucilage ; (3) young leaves of next season, pale 
 green, with no storage ; (4) in section of apex only, rudiments of a few primordia of 
 another season. (Differentiate by Iodine sol.) 
 
 In fine bulbs note central flower (Narcissus), or inflorescence with many buds 
 (Hyacinthus). In Narcissus the flower is lateral, and the apex, at its base, follows on : 
 in Lilium and Hyacinthus the flower spike is terminal, and a lower axillary bud takes 
 on the new growth. The adaptation for summer rest in sandy soil is, in this country, 
 continued over the cold winter, and such plants vegetate in spring only, on a short 
 season (April-June), fruiting and seeding before midsummer; though exceptional 
 species of Narcissus and Crocus may send up flower-shoots in Autumn months 
 (cf. Saffron Crocus). 
 
 Cf. also Onion bulb, with reserves as glucose and mucilage, no flower. Tulip 
 (bulb), Anemone (Rhizome), Polygonatum (Rhizome) all storing starch. 
 
 Perennation structures of root-origin as Tuberous Hoots : cf. Ficaria, swollen 
 segments, J in. long, storing starch, as extension of Buttercup root : Dahlia, large 
 * tuber '-growths, 6 in., store inulin. Orchis sp. swollen roots ; but greenhouse Orchids 
 commonly perennate as ' Pseudo-bulbs ', of swollen stem-internodes with remains of 
 foliage-leaves or scales. 
 
 The limiting case of perennation is that in which all somatic structures die and 
 vanish at the approach of winter, leaving the race only in the form of seeds, as the 
 Annual plant ; such perennation may be made effective in these latitudes as the 
 result of frost (Reseda, Phaseolus) ; or may be part of the specialized organization of 
 the plant, as a consequence of exhaustion in seed-production (Helianthus, Zea). 
 
 Special interest attaches to the case of the Biennial herbaceous plant which 
 takes a second season to attain reproductive maturity, storing reserves in the first 
 season, in foliage-leaves (Cabbage), bulb-scales (Onion), root (Beet, Carrot, Parsnip), 
 or hypocotyl (Turnip), to be exploited as food-crops for animal organism. 
 
 26 
 
Concluding Note. The preceding pages are arranged to cover a course of 
 12 lectures with associated practical work, as an Introduction to the Somatic 
 Organization of higher Herbaceous Land-plants; the general anatomical and 
 microscopical observations being strung together by elementary ideas on physiology 
 sufficient to give cohesion to the story. 
 
 The notes have been written as schedules to accompany, and not to replace 
 lectures ; it being assumed that the lecturer can add explanatory emendations and 
 enlargements on special points within the time as required. Their defects and 
 omissions are sufficiently obvious to the botanical teacher ; but so long as botanical 
 courses are designed largely by non-botanists, on the law of the minimum, it is 
 interesting to see how much can be done within such limitations; and though much 
 ground may be covered in theoretical lectures, the time (24 hours) allotted for obtain- 
 ing a practical acquaintance with the material is quite inadequate. 
 
 One of the results of a mediaeval University System, originally based on 
 conditions of agriculture, as a phase of Plant-life, in the North Temperate Region, 
 is that the introduction to such elementary knowledge of vegetation, as the response 
 of autotrophic life to the sunshine and temperature of the Northern Summer, has to 
 be taught, in this part of the world, during the darker winter months, when all active 
 vegetation is perennating ; and so the University term begins with the ' Fall of the 
 Leaf. Only in the Long Vacation can plant-life be studied at its optimum ; hence 
 outdoor observation, and experimental work under normal conditions, are necessarily 
 curtailed ; while laboratory investigation of the dead plant, or its pickled parts, takes 
 the place of direct contact with living organism of the type of Helianthus, Zea, and 
 Cucurbita^ then non-existent. So long as the present educational system prevails this 
 sort of botanical work appears as the unavoidable method of introducing the subject ; 
 Botany being naturally the only science affected in this manner, and it being so far 
 nobody's business to change the system. 
 
 General Literature. 
 
 SCOTT, Structural Botany ', Part I. 
 
 BOWER (1919), Botany of the Living Plant. 
 
 STRASBURGER (1912), Text-book of Botany (Eng. Trans.). 
 
 HABERLANDT (1914), Physiological Plant Anatomy: Stomata, p. 447; Vascular 
 
 Bundles, p. 346; Secondary Xylem, p. 659; Mechanical System, p. 150; 
 
 Statocysts, p. 595. 
 PFEFFER (1899), Physiology of Plants (Eng. Trans.), Vol. I; Spectrum, p. 342; 
 
 Aerobic Respiration, p. 518. 
 Josx (1907), Lectures on Plant Physiology (Eng. Trans.); Assimilation, p. 103; 
 
 Transpiration, p. 35; Geotropism, p. 429; Heliotropism, p. 460. 
 RUSSELL (1915), Soil Conditions and Plant Growth. (Monog. Biochem.) 
 DIXON (1914), Transpiration and Ascent of Sap, p. 27. 
 PHILIP (1910), Physical Chemistry ; Osmosis, p. 33 ; Adsorption, p. 219. 
 BAYLISS (1911), The Nature of Enzyme Action. (Monog. Biochem.) 
 ARMSTRONG (1919), The Simple Carbohydrates and the Glucosides. (Monog. Biochem.) 
 
 BOTANIC GARDEN, OXFORD, Nov. 1919. 
 
 27 
 
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