IlsIwwW > f^ffAtr* A&*ri l fSS 5*.v» rf.v iiirtfei iitBiasfc € J>.. <2/ R1S7S2, A 1ST INTRODUCTION TO s. -«v. \ STRUCTURAL BOTANY V Digitized by the Internet Archive in 2015 https://archive.org/details/b21 91 5921 _0001 Fig. 1. — ( Frontispiece .) General view of a Wallflower plant ( Cheiranthus Cheiri), about £ natural size, (j — < j , level of ground. AN INTRODUCTION TO STRUCTURAL BOTANY (FLOWERING PLANTS) By DUKINFIELD HENRY SCOTT M.A., TH.D., F.L.S., F.G.S. HONORARY KEEPER OF THE JODRELL LABORATORY, ROYAL GARDENS, KEW FORMERLY ASSISTANT PROFESSOR IN BIOLOGY (BOTANY) AT THE ROYAL COLLEGE OF SCIENCE, LONDON LONDON ADAM AND CHARLES BLACK 1894 All Rights Reserved PREFACE This book is intended as a first guide to the study of the structure of plants. Botany is now taught in schools of all kinds, and wherever Botany is taught it has become customary to expect some knowledge of the construction of plants, and of the function of their organs. All that I have aimed at in this book is to secure that such knowledge, when first acquired, shall be correct as far as it goes. My purpose has been to write an Introduction to Structural Botany, not a manual of Botany in general. It is absolutely necessary that schoolboys and girls, if they are to learn this science at all, should also gain a knowledge of plants in the field. For this part of the work a guide is necessary, and some such book as Professor Oliver’s Lessons in Elementary Botany is indispensable. The type-system has been adopted, as far as practicable, because it seems better to gain as thorough a knowledge as possible of a few plants, rather than to acquire mere scraps of information about a larger number. The types have been specially in- vestigated for the purpose of this book, and many of vii V1U PREFACE the figures are original.1 For the rest, the authority is cited. An effort has been made to point out those structural characters which are of wide importance, as distinguished from those which are peculiar to the type, or its nearer allies. The subject-matter is not always easy, and some parts of the book will not be followed without close attention. There is really no reason why Natural Science should be regarded in schools as a specially easy subject. If Science is to be taken seriously, it rather seems desirable that those who study it should have to use their brains as much as in learning Euclid, Algebra, or Grammar. If any real knowledge of the subject is to be gained, practical work is essential. It is expected that the teacher should have sufficient training to be able to demonstrate to his class most of the structural features described in the book. We are now well supplied with laboratory guides, such as the works on Practical Botany by Professors Bower and Strasburger. If the present volume is found useful, it is proposed to add a second, treating in like manner of Crypto- gamic types. I much hope that not only pupils in schools, but also private students, may derive some help from this book, and that it may awaken in some readers a genuine interest in the study of living things. D. H. SCOTT. February 27, 1894. 1 Those signed “ R. S.” have been drawn by Mrs. D. H. Scott. CONTENTS — ♦ — PAGE Preface ..•••••• vu Introduction CHAPTER I Type I The Wallflower (Cheiranthus Chciri, L.) — I. External Characters — A. Vegetative Organs — a. The Shoot . . . • .12 b. The Root ..... 17 B. Reproductive Organs — a. The Flowers . . . . .18 b. The Fruit ..... 26 II. Internal Structure — A. The Cells ...... 26 a. General Structure of the Cell . . 28 b. Special Contents of Cells . . .33 a. Chlorophyll Granules . . .33 j3. Starch Granules . . .35 y. Oil . . . .37 2. Proteid Granules . . .38 c. Cell-Formation . . . .38 d. Continuity of the Protoplasm . . 40 e. Protoplasmic Movements . . .41 ix X CONTENTS PAGE B. The Tissues .... 45 a. Structure of the Stem . . .46 a. The Yascular Bundles . . .48 /3. Other Tissues of the Central Cylinder . 61 y. The Cortex . . . .62 S. The Epidermis . . . .62 b. Structure of the Leaf . . .64 c. Sti'ucture of the Root . . .72 cl. Transition from Stem to Root . . 78 Apical Development . . .82 Secondary Growth in Thickness . . 90 Periderm ..... 96 Secondary Thickening in the Root . 98 III. Internal Structure and Development ;of the Repro- ductive Organs ..... 103 a. Development of the Flower . . . 103 b. Structure of the Floral Envelopes . . 106 c. Stamens ..... 109 d. The Pistil . . . . .116 e. Fertilisation ..... 122 /. Development of the Embryo . . 130 The Ripe Seed .... 135 The Fruit 137 Germination . . . .138 CHAPTER II Type II The White Lily ( Lilium candidum, L.) — I. External Characters — A. Vegetative Organs . . . .142 B. The Flowers . . . . .149 CONTENTS xi PAGE II. Internal Structure of the Vegetative Organs . . 152 a. Stem .... . 152 b. Leaf 160 c. The Root ..... 164 d. Growing Points and Mode of Branching . 168 Summary of Monocotyledonous Anatomy . . 171 III. Internal Structure and Development of the Repro- ductive Organs . . . . .173 a. Development of the Flower . . .174 b. Structure of the Floral Organs . .176 The Pistil ..... 179 c. Ovules ..... 180 d. Pollination and Fertilisation . . 183 e. Development of the Embryo . . . 187 Endosperm . . . .190 The Ripe Seed and Fruit . . . 193 Germination . . . .194 Summary . . . . . .195 CHAPTER III Outlines of the Physiology of Nutrition . . . 198 The Food of Plants ...... 199 Assimilation of Carbon ..... 205 Absorption ....... 215 The Ascent of the Sap ..... 217 Respiration ....... 226 CHAPTER IV Type III The Spruce Fir (Picca excclsa, Link.) . . . .231 I. Anatomy of the Vegetative Organs — a. Stem . 233 b. The Leaves .... 246 xii CONTENTS PAGE c. The Root ...... 252 d. Growing Points and Lateral Appendages — Growing Point of the Stem . . . 258 Growing Point of the Root . . . 259 Summary of the Vegetative Structure of Conifers . 262 II. Structure of the Reproductive Organs — a. The Male Flower ..... 263 b. The Female Flower .... 268 c. Pollination and Fertilisation . . .274 d. Development of the Embryo . . .276 Summary ...... 280 Index ........ 283 STRUCTURAL BOTANY ♦ INTRODUCTION In beginning the study of plants we cannot perhaps do better than fix our attention on any common kind of plant with which we may happen to be familiar, and consider what are the most striking points in its construction. In this way we may hope, starting from the general knowledge which we all possess as the result of everyday observation, to pass on to that more accurate and systematic knowledge which is called Science. It matters little with what plant we begin. The Wallflower is well known to everybody, so we will take that as our example. The chief parts of which such a plant consists are easily distinguished (see Fig. 1). There is a branched colourless root, which is fixed firmly in the ground ; from this rises the upright stem bearing the fiat leaves. The stem is branched, each branch arising from the angle between a leaf and the main stem; the branches also bear leaves, and resemble in all 2 STRUCTURAL BOTANY respects the stem on which they grow. Later on the flowers appear, borne on the upper part of the stem and its branches, above the leaves. These flowers, when they wither, give place to the fruits or seed-vessels, and in these the seeds themselves are ripened, which, when sown, will produce a new genera- tion of wallflowers for next year. Now a plant, like all living things, is made up of organs ; thus a leaf, a stem, a root, or a flower is not merely a part of the plant, but it is a part which does some definite work for the good of the whole. The highest or most perfect plants are those in which the division of labour is most complete, in which the principle of setting apart a distinct organ for each distinct kind of work is most thoroughly carried out. The Wallflower is an example of a very highly organised plant. Later on we shall make the ac- quaintance of plants in which there is little or no division of labour — in which, that is, the organs are not distinct, or at least not evidently so. The work done by an organ is called its function. In the Wallflower, as in the higher plants generally, the root has two manifest functions : it has to hold the plant firmly in the ground, and also to take up food from the ground. The leaves have for then- chief office the absorption of food from the air; we shall soon find that a green plant obtains quite half its food from this source. The stem has to conduct the food, which the root and the leaves absorb, to other parts of the plant, and it also has to support the leaves in such a position that they may best be INTRODUCTION 3 able to do their work;1 and thirdly, the stem has also to bear the flowers. The flowers themselves have quite a different duty to perform. Their business is to produce the seeds, and so to provide a supply of new wallflowers for the future. We see, then, that the organs which we have mentioned are of two kinds. On the one hand, the root, stem, and leaves do work for the benefit of the particular plant to which they belong; these we call the vegetative organs. On the other hand, the flowers are concerned in the produc- tion of fresh plants ; these are called the reproductive organs. One important division of Botany, then, is concerned with such questions as those which we have just roughly indicated. We aim at finding out how a plant lives, what different kinds of work it has to do in order to live and to produce others of its kind, and by means of what organs the work is done ; also what is the structure of these organs by which they are enabled to perform their office. All these questions belong to Physiology in its widest sense. Physiology is concerned with the question what a plant docs, and what its various organs do. The answer to such questions must be obtained by experiment. The study of structure, or Anatomy, is from this point of view a necessary auxiliary to physiology. But we can also look at plants from a different point of view. Suppose, for example, that instead of considering the Wallflower alone, we compare it 1 It is often convenient to speak of the stem and leaves together as the shoot. 4 STRUCTURAL BOTANY with some other plant, such as a Potato. A potato plant differs in many points from the Wallflower, but Fig. 2. — Young Potato plant (reduced), raised from seed. ct, the first leaves, or cotyledons ; l, the foliage-leaves ; b, b' branches, bearing the tubers ; sc, scale-leaves ; r, r', roots ; tb, tubers ; ec, their scale-leaves ; bd, their buds. (From Sachs, after Duchartre.) we will confine ourselves to one of these differences. INTRODUCTION 5 It not only consists of the stem bearing its green leaves and ultimately producing the mauve flowers, and of the root with its fibrous branches, but it also forms the potatoes themselves (or tubers, as they are called), which we eat. Now these potatoes, as every one knows, grow, like the root, underground, borne on subterranean branches. Both they and the branches which bear them are colourless, and from their general appearance and position every one who has not learnt Botany at once classes them as roots. But is this their true nature ? If we closely examine the branches on which the potatoes grow, we shall find that they bear small scaly leaves (Fig. 2, sc). Similar little leaves ( ec ) will be found on the potato itself, adjoining the “ eyes,” and the “ eyes ” themselves are buds (bcl), which grow out into new plants when the potatoes are sown. Leaves and buds, however, are characteristics of stems ; the former are never, the latter rarely, found on undoubted roots. Besides this, a more thorough examination would show that the internal structure and the mode of growth of the potatoes and of the under- ground branches which produce them are those of stems and not of roots. Further, we may sometimes find on the parts of the potato plant above the ground bodies which are intermediate in form and structure between potato tubers and ordinary green branches. For these reasons, all botanists are agreed in regarding potatoes, hi spite of their appearance and underground position, as forming part of the shoot and not of the root. In other words, we come to the conclusion that 6 STRUCTURAL BOTANY certain shoots of the potato plant have become com- pletely modified, so as to lose all the more obvious characters of shoots, and to bear a superficial resem- blance to root - branches. These changes are the external manifestation of a complete alteration of function. The potato tubers have given up the usual functions of shoots, and become adapted to serve as storehouses of food (chiefly starch) for the young plants which will grow from the “ eyes ” next year. If, then, we compare the potato plant with the Wallflower, we find that, while in the latter the shoot consists entirely of above-ground branches bearing green leaves, in the potato plant certain parts of the shoot are changed into underground colourless organs, bearing scale-leaves, and ending in tuberous swellings, which are storehouses of food. But we must not suppose that all underground organs which serve this purpose are shoots. Turnips, radishes, and dahlia tubers are also underground parts of plants, and in function are similar to potato tubers, but here the structure and mode of growth show that we have to do with modified parts of the root. The comparison of various plants thus shows us that quite different parts may be changed, so as to serve the same function, and that the same part may be modified to perform quite different functions. We shall meet with plenty of examples of this later on, but the instances just given will suffice to introduce us to the second great division of the science of Botany, namely, the Morphology of plants. This is INTRODUCTION 7 based upon the comparative method of study as applied to plants and their members.1 To put the distinction shortly, while, as we have seen, physiology asks the question what each part of a plant does, morphology inquires what it is. The word “ morphology ” means the “ study of form,” but this literal meaning, as so often happens with scientific terms, gives a very imperfect idea of what is really implied. External form, at any rate, is quite an insignificant part of the subject of morphology, which is much more concerned with internal structure, relative position of parts, and the changes which they undergo during development. By the accurate com- parison of plants in all their members, morphology aims at finding out the relationships between them — 1 The word “members” rather than “organs” is used here, because morphology, strictly speaking, is not concerned with function. Fig. 3. — Conical root of Bryony. Reduced. (After Balfour.) Fig. 4. — Fleshy root of Turnip. Reduced. (After Balfour.) 8 STRUCTURAL BOTANY that is, it endeavours to construct a natural classifica- tion of the vegetable kingdom. Classification is often treated as a separate part of the science, called Systematic Botany, and this is a convenient division, but Systematic Botany can only be satisfactorily based on the comparative study of plants — that is, on morphology. While it is easy to distinguish sharply between the physiology and morphology of plants, it is more important to remember that neither can be pursued to any advantage without the other. Physiology without morphology would teach us much about the life of individual plants, but could give us no idea of the vegetable kingdom as a whole, or of the relation- ship between the innumerable species of which it is composed. Morphology without physiology, on the other hand, would be just as barren ; for the complex modifications of the organs of plants would be wholly unintelligible without reference to the functions to which they are adapted. Only by examining plants from both points of view can we attain to any knowledge of them which deserves the name of science. The method pursued in this book is to take in succession a series of types representing important groups of plants, to examine each of them as fully as our space permits, with reference to both structure and function, and to compare them together. It will often be necessary to supplement the study of our main types by that of other plants which resemble them on the whole, but better illustrate some particular INTRODUCTION 9 point, for Nature does not provide us with perfect types ready made. In this way we may hope to gain some real knowledge of a few plants, which may serve as a firm foundation for more extended study afterwards. In choosing the order of our types, we will begin with the more highly organised plants, in which the division of labour among the organs is most complete. We do this partly because the higher plants are most familiar to us in everyday life, and partly because we shall gain from them the clearest conceptions of organs and their functions. Besides morphology and physiology, there are two more divisions of Botany which we must mention. One of these is the study of fossil plants — that is, of the vegetable remains which have come down to us preserved in the earth from ages long past. This is called the Palaeontology of plants, or sometimes Palceo-botany — that is, the science of ancient plants. From one point of view this is simply a part of morphology ; we compare the fossil plants among themselves and with recent ones, and draw conclusions, so far as we are able, as to their relationships. We cannot study their physiology, for they are dead ; we may, however, draw some inferences as to their mode of life from their structure. So far, palaeontology is concerned with the same questions as recent Botany, but we also have to study the succession of the strata in which the fossils are found, and so to trace their relative geological age, or, in other words, their distri- bution in time. Inquiries of this kind are peculiar to 10 STRUCTURAL BOTANY palaeontology, and are of the greatest possible import- ance, because they enable us to a certain extent to trace the past history of the different kinds of plants. To take only one example, the enormously rich vege- tation which is so well preserved in the coal-measures consisted of plant-forms which have long since become extinct. The classes to which many of the great trees Fig. 5.— Stump and roots of a Fossil Tree ( Sligmaria fcoidcs) from the carboniferous sandstone. The diameter of the stump is 4 ft. 4 in., and the spread of the roots 28-30 ft. (After Williamson.) of that period belonged (such as the Stigrmria figured) are now only represented by a few small herbaceous plants (such as Selaginella). On the other hand, most of the groups which are now of the greatest importance had not then appeared, and their first representatives are found in much later rocks.1 This 1 Sec Jukes’s School Manual of Geology, p. 280. INTRODUCTION 11 part of the science, however, cannot be further touched on here. We shall only be able to concern ourselves with plants now living. Lastly, we have Geographical Botany, from which we learn the parts of the world in which the different kinds of plants grow wild, or, in other words, their distribution in space. This teaches us, for example, the very uniform character of alpine vegetation in all parts of the world ; the peculiarities of the Flora 1 of oceanic islands, deserts, and so on. This also leads to very important conclusions, especially when we study the distribution of a plant in relation to its structure and mode of life. We shall call attention to the distribution of our various types, but this part of Botany cannot be pursued far without a much wider knowledge of plants than is aimed at in this book. After these introductory considerations, we will now go on at once to our first type. 1 By the Flora of a country we mean all the plants which grow wild there, collectively. CHAPTER I Type I THE WALLFLOWER ( Cheiranthus Gheiri , L.1) I. EXTERNAL CHARACTERS A. Vegetative Okgans a. The Shoot The Wallflower, a native of Southern and Central Europe, is occasionally found naturalised in England, and derives its name from growing commonly on old walls. It is a perennial — that is to say, it lives for a number of years. It does not die down in winter, nor does it lose its leaves in autumn. The leaves, however, gradually drop off from the older parts of the stem. The stem is erect and branched (see Fig. 1), and its lower part is covered by scales of pale brown bark. This part is hard and woody. The upper portions, both of the main stem and branches, are softer and coloured green, and are rather hairy. The surface of the stem is ridged lengthways, the ridges being more conspicuous on the younger parts. The ridges are 1 The letter L is the initial of the author (Linneus) from whom the plant received its present name. 12 THE WALLFLOWER 13 generally five in number ; each of them runs vertically along the whole length of the stem or branch, and passes through the places where the leaves are inserted. The ridges are most prominent just below the insertion of the leaves. Both in the main stem and its branches it is evident that the lower parts are the older. For some distance above the ground there are no leaves ; they have dropped off, and we only find the scars which they have left. Then we come to leaves which are beginning to wither ; a little higher up they are green and fresh, and of full size. Higher up still we find them smaller and more delicate ; while at the top of each branch the leaves are only just forming, and we can trace them up to the extreme tip, until they are so small that we can no longer distinguish them. We see, then, that the fonnation of new organs goes on from below upwards, or, in other words, the growth takes place at the apex of the branch. The extreme end of the branch where the young leaves are just making their appearance is called the growing point. The growing point with the young leaves which surround it is called the leaf -hud. The branches do not arise indiscriminately on any part of the stem, but every branch grows from the angle between leaf and stem, just above the insertion of the leaf. This angle is called the axil, and the branches are said to be axillary. The main stem continues its growth indefinitely in the same direction, giving off its branches laterally. The arrangement of the leaves on the stem requires 14 STRUCTURAL BOTANY rather close attention to make out. We see at once that no two leaves arise at the same level. Every leaf is placed a little above or below its next neigh- bours. The leaves, therefore, are said to be alternate. We find further that they are arranged in a spiral. If we imagine a line connecting the bases of the successive leaves together, it would wind upwards round the stem, just like the thread of a screw, only here the spiral ascends from the right up to the left, and so is like a left-handed screw, and is the reverse of an ordinary right-handed one. Now if we count the leaves carefully, beginning from below, and not counting the leaf taken as our starting-point, we shall find that the fifth leaf comes exactly above the one we started with, and in order to reach this fifth leaf we have had to follow the spiral exactly twice round the stem. The ridges make it much easier to see which leaves come in the same straight line ; for, as we saw just now, each ridge runs straight from one leaf to that vertically above or below it. This arrangement of the leaves may be expressed by the fraction §, in which the denominator indicates the number of the leaf which comes directly above the starting-point, while the numerator tells us how many times the spiral line travels round the stem before the fifth leaf is reached. This fraction | also expresses the divergence or angle between two success- ive leaves, which is equal to f of the circumference of the stem. The way in which leaves are arranged on the stem is called pliyllotaxis, and varies very much in different kinds of plants. THE AVALLFLOAVER 15 The whole length of the stem is divided into nodes and internodes. The part where a leaf is borne is called a node, the spaces between the leaves are the internodes. Each leaf is attached to the stem by a narrow base. It has no distinct stalk, but the blade of the Fig. 6. — Diagram to illustrate § Phyllotaxis. The dotted line represents the leaf-spiral, which is seen in horizontal plan, the centre corresponding to the apex. The con- centric circles represent the successive nodes. (After Van Tieghem.) leaf is very narrow near the base, and broadens out quite gradually for about two-thirds of the whole length, and then narrows again rather more rapidly to the pointed end. The leaf is technically called sessile, because the blade is seated on the stem directly, without the intervention of a leaf-stalk or petiole, such 16 STRUCTURAL BOTANY as we see in the leaf of a Vine or Geranium. The narrow part of the blade, however, only differs from a leaf-stalk in being slightly winged. The part of the leaf where it joins the stem is called the leaf -base ; this remains in connection with the stem after the rest of the leaf has dropped off. The shape of this leaf is ex- pressed in botanical language by the words lanceolate, acute, and entire. The word lanceolate refers to the general outline, which is like that of a lance-head ; the word acute indicates the pointed apex ; while the term entire means that the edge is not toothed or divided in any way. The upper surface of the leaf is dark green and very slightly hairy. The under surface is of a lighter green, and has many more hairs. The hairs adhere closely to the surface, and cannot be seen well without a lens, though they can be Fig. 7. — LeafofWall- pi, flower, natural size, showing the chief The leaf is traversed from end veins. (R. S.) gild by a stout midrib, or principal vein, which is more prominent on the lower surface, and gradually tapers towards the apex. From the midrib branch - veins are given off at irregular intervals, which turn upwards so as to run nearly parallel to the midrib. These main branches THE WALLFLOWER 17 join on to each other, and themselves give off in- numerable lesser branches, which permeate every part of the leaf, and are united among themselves into a fine network. The finest branches of all, however, end blindly within the meshes of the network. The smaller veins can only be traced with the help of a lens. We have now described the chief external characters of the shoot-system. To complete our sketch of the vegetative organs, we will next consider the root. b. The Root If we pull up a Wallflower and wash away the earth which clings to the root, we find that this, like the stem, has a main axis, which grows straight downwards in the same line with the upward growing main stem. A main root of this kind is called a tap- root. Of course the root meets with more resistance to its growth than a stem does, and sometimes it is forced to diverge more or less from its vertically down- ward direction (see Fig. 1). From the tap-root a number of branches are given off. These branches do not grow straight down, but take an obliquely descending course, forming an angle a little less than a right angle with the tap-root. From these principal branches other much smaller ones arise, and these also ramify repeatedly. The finer branches of the root do not take a fixed course, but penetrate the soil in all directions equally. If the plant has been very care- fully taken up, we shall find that particles of earth stick firmly to the very youngest branches of the root 2 18 STRUCTURAL BOTANY a little above their extreme tips. This is because these parts of the root are clothed with very fine root-hairs (see Fig. 29, p. 77), which have withered away from the older parts. These root-liairs are barely visible to the naked eye, and must by no means be confused with the root branches or fibres already described, to which they bear no resemblance in structure, as we shall find when we come to the microscopic work. Growth goes on at the tips {apices) of the root and its branches, just as we found in the stem. We notice as con- spicuous differences between the root and the shoot, that the former bears no leaves and is not green in any part. We shall find a young seedling more convenient for studying the root than an older plant. On such a seedling it is possible to make out that the branches of the tap-root are not arranged irregularly, but form four vertical rows. B. Reproductive Organs a. The Flowers The branches which bear the flowers do not at first differ in any way from ordinary vegetative branches. They arise in the axils of leaves, and for some tune bear leaves themselves in the usual way. But soon the production of leaves ceases, and from that time onwards the branch bears flowers only. Each flower may itself be regarded as a modified branch (borne on the main branch), the leaves of which differ greatly in arrangement, form, structure, and colour from the THE WALLFLOWER 19 vegetative leaves. The flowers, unlike the ordinary branches, do not arise in the axils of leaves, for there are no leaves at all on the part of the plant which bears them. This is characteristic of the Wallflower and its relations. In most plants the flowers, like other branches, are axillary. The way in which flowers are borne upon the stem is called the inflorescence. In the Wallflower the flowers are borne laterally on a main axis, and are Fig. 8. — Flower of Wallflower. a, flower-stalk; c, calyx; p, corolla ; s, stamens. (After Balfour.) s, stamens ; b, pistil ; r, receptacle ; g, glands at base of sliort stamens. (After Balfour.) stalked. The main axis terminates in a bud which is continually forming new flowers as long as growth continues. Thus the oldest flowers are at the bottom and the youngest at the top. An inflorescence of this kind is said to be indefinite. An indefinite inflorescence with a single elongated main axis and stalked flowers is a raceme. 20 STRUCTURAL BOTANY Now we will consider the parts of a single flower in detail. The first point that strikes us is that, while the stalk ( pedicel ) is quite bare, all the leaves are crowded together at the top. The floral leaves are not Fig. 10. — Dingrammatic view of the flower of Wallflower (enlarged) in longitudinal section, cut in median plane of outer sepals, s, sepal ; p, petal ; st1, two of the long stamens ; si2, one of the two short stamens ; o, ovary ; ov, ovules ; stig, stigma. (R. S. ) arranged spirally, hut two or more grow at the same level. The end of the stalk on which the flower grows is the receptacle, or thalamus. Beginning at the outside of the flower, we first come THE WALLFLOWER 21 to four small narrow leaves of a purplish colour (see Fig. 8, c). These leaves are uot all inserted at the same level. They are in two pairs, placed at right angles to Fig. 11. — Diagrammatic view of tlie flower of Wallflower (enlarged) in longitudinal section, passing through median plane of inner sepals, s, sepal ; p, petal ; st1, long stamens ; st2, short stamens ; o, ovary ; ov, ovules ; dig, stigma ; n, nectaries at base of short stamens ; R, septum between the two carpels. (R. S.) each other, and one pair is inserted rather farther in than the other pair (see Figs. 1 2 and 13). This is seen very clearly in the bud (Fig. 12), though less easily 22 STRUCTURAL BOTANY in the open flower. Each of these four leaves is called a sepal, and collectively they form the calyx ; as the sepals are not joined together, the calyx is said to be polysepalous. ®A Fig. 12. — Transverse section through a flower-bud. A, posi- tion of axis of inflorescence ; S, sepals (four in number) ; p, petals (shaded) (four in number) ; si, stamens (six in number, each with four pollen-sacs) ; 0, ovary (of two carpels). Compare the diagram, Fig. 13. Magnified about 12 times. (R. S.) The diagram, Fig. 13, represents as it were the ground plan of a flower, and shows the relative position of all its parts. In order to fix this, it is necessary to know how the flower is placed with reference to the axis of the inflorescence. This axis is indicated by a dot in the diagram. It will be seen that the two outer THE WALLFLOWER 23 sepals are so placed that one lies next the axis ( posterior ), and the other remote from it {anterior). The two inner sepals are placed at the sides {laterally), alternating with the outer two. The two inner sepals are bulged out at the base ; we shall see the reason for this directly. In the bud before it opens, the calyx completely encloses all the other parts of the flower. Inside the calyx we find four floral leaves forming a single ring, or whorl. These are the petals, the most con- spicuous part of the flower. In the wild Wallflower they are, yellow; in cultivated specimens they are generally more or less brown. Each petal is broad and rounded above, and tapers into a narrow stalk below. The four petals form collectively the corolla, which is here said to be polypetalous, because the petals are not connected together. The petals are so placed as to alternate with the four sepals — that is to say, the middle of each petal comes exactly opposite the space between two sepals (see Figs. 12 and 13). Both the sepals and the petals are veined, but not in such a complicated way as the vegetative leaves. a i s.a. Fig. 13. — Floral diagram, a ground plan of the flower of Cheiranthus. a, axis of inflor- escence ; s.p, posterior sepal ; s.a, anterior sepal ; s.l, lateral sepals ; p, the four petals ; st, the six stamens (the four long ones are shown connected in pairs) ; c, the two united carpels, forming the pistil. 24 STRUCTURAL BOTANY So far the organs of the flower have borne an obvious resemblance to ordinary leaves. The next whorl is very different, and consists of bodies which, so far as their outward form is concerned, are not at all leaf-like. Each consists of a longish stalk, bearing at the top a little boat-shaped case of a yellowish colour. These are the stamens (see Figs. 8, 9, 10, and 11). There are six of them, four of which are longer than the other two. The two shorter stamens stand on either side of the flower, and each of them is placed just between two of the petals, and just opposite the two inner lateral sepals. The four longer stamens are placed rather farther in, so that all six stamens might be regarded as forming two whorls. The longer ones are in two pairs, each pair standing between two petals, and just opposite the two outer (posterior and anterior) sepals. (See diagram, Figs. 12 and 13.) The stalk of the stamen is called the filament ; the body at the top of the filament is the anther. The filament is continued up through the middle of the anther, and is here called the connective, because it connects together the two halves {lobes) of the anther. When ripe, the anther bursts to discharge a yellow dusty substance called 'pollen. Just inside the bases of the two shorter stamens are two large green honey-glands {nectaries). The filaments are bent outwards to make room for them, and this causes the bulging of the lateral sepals outside them which we noticed in examining the calyx. THE WALLFLOWER 25 The middle of the flower is occupied by the pistil. This is a nearly cylindrical hollow body, green in colour, tapering a little towards the top, and sur- mounted by a forked outgrowth. The lower thicker part, which occupies more than three-quarters of the whole length, is the ovary , and the forked outgrowth at the top of all is the stigma, while the short tapering portion which connects the two is the style (see Figs. 9, 10, and 11). The hollow ovary is divided lengthways into two compartments by a membrane called the septum. This membrane marks the boundary between the two carpels, the name given to the floral leaves of which the pistil is composed. We see, then, that the two carpels are so placed as to lie opposite the two inner lateral sepals. Within the ovary are the young seeds, or ovules, as they are called at this early stage. They grow on the walls of the ovary adjoining the septum. As, however, we are only dealing with external characters at present, we will postpone any further examination of the interior of the ovary and of the ovules. The ovules only develop into the seeds after fertilisation, which is brought about by the pollen. In order that the ovules may become ripe seeds, which can grow into young plants, it is necessary that the pollen should come into contact with the stigma of the pistil. The whole process of fertilisa- tion will be described later on, when we have made ourselves acquainted with the internal structure of the floral organs (see p. 122). 26 STRUCTURAL BOTANY b. The Fruit The fruit is the ripened pistil, and contains the seeds, which are the ripened ovules. It con- tinues to show the same general structure as the pistil, but grows much larger, and undergoes some important changes, a description of which will be found on p. 137. During the development of the pistil into the fruit all the other floral organs — calyx, corolla, and stamens — drop off. II. INTERNAL STRUCTURE A. The Cells If we examine under the microscope a thin slice, or section as it is called, from any part of an ordinary plant, say, for example, a section cut straight across the stem of a Wallflower, we find that its substance is not uniform, but is divided up by a network of walls into innumerable minute chambers of varying size and shape (see Fig. 20, p. 48). If we make another section at right angles to the first, say a section lengthways through the stem, we find the same structure, except that in this view the chambers appear to have somewhat different shapes (see Fig. 22, p. 52). Combining the two views together, we come to the conclusion that the whole substance of the plant is composed of a system of closed compartments. This is the reason why if a juicy fruit like the melon is cut into pieces, all the juice THE WALLFLOWER 27 does not run out, but only a little is lost. The juice is contained in these little closed chambers, which hold it fast, and only those which are actually opened by the knife are emptied. Now these chambers, of which the whole substance of almost all plants consists, are called cells. They are almost always so small that they cannot be distinguished at all except under the microscope. Few reach a diameter of yjyy of an inch, while many measure only yoVo of an inch, or less. The fact that plants have a cellular structure is far from being a modern discovery. Cells were discovered in plants by Eobert Hooke1 in 1667, and within the succeeding fifteen years the internal structure of many plants was worked out by Nehemiah Grew in England, and Marcello Malpighi in Italy. The early anatomists were struck by the similarity of many vegetable tissues to the honeycomb of a bee, and they applied the word “ cell ” to the chambers in both cases in the same sense. From the time of Grew and Malpighi onwards until well within our own century — for a period, that is, of quite a century and a half — those botanists who troubled themselves at all about internal structure attended chiefly to the walls of the cells. These are much the most conspicuous features in the section See his Micrographia ; or, Some. Physiological Descriptions of Minute Bodies made by Magnifying Glasses, with Observations and Enquiries thereupon (London, 1667), especially chapter xviii., “Of the Schematismc or Texture of Cork, and of the Cells and Pores of some other such frothy Bodies.” 28 STRUCTURAL BOTANY through any fully formed organ (when seen under the microscope). The network of walls enclosing the cavities strikes the eye at once. We do not at first particularly notice the contents of the cavities, but the research of the last half century has shown that it is the cell-contents, after all, which are the most important part of the cell ; it is the contents which form the wall, not vice versd, and in many plants the cell-contents are quite able to grow and increase without any wall at all, while the cell-wall without its contents is a dead shell, incapable of growth or activity of any kind. a. General Structure of the Cell We will now shortly consider the structure of a single cell, and we will begin by describing one from quite a young part of a plant, such as we should find in the bud at the growing end of a branch. A young cell (see Fig. 14, A), such as those which are found at a growing point, is surrounded on the outside by a delicate but firm and elastic membrane, the cell-wall, by which it is separated from its neigh- bours. The wall consists chiefly of a body called cellulose, which is chemically very similar to starch, and is composed of the same chemical elements (carbon, hydrogen, and oxygen) in the same proportions.1 The strength and elasticity of all parts of the plant are ultimately due to the cell-walls, which serve as a firm supporting framework for the whole structure. In such young parts as we are now considering, how- 1 The formula is CflH10O5. THE WALLFLOWER 29 ever, the cell-walls are still very thin, and for this reason growing points are very delicate, and require external protection, which in the case of stems is afforded by the surrounding leaves. The interior of the cell is almost completely filled by a soft, viscid, finely granular substance, the protoplasm. The outer part of the protoplasm, where it borders on the cell-wall, is clearer — that is, more free from granules — than the rest. The protoplasm consists of a mixture of various substances, called by chemists proteids, which are more or less similar in composition to albumen or white of egg. These are very complicated compounds, about which little is known chemically, and always contain the five elements, carbon, hydrogen, oxygen, nitrogen, and sulphur. Besides the proteids, active protoplasm always contains a large proportion of water. Physically it resembles a very thick fluid, but it has powers of spontaneous movement and continuous changes of form such as no fluid possesses.1 Proto- plasm is found in all cells without exception in which growth is going on, or in which food is assimilated, or any new structure formed, or any kind of spontaneous movement carried out. It is, in fact, the seat of all those processes (whether in plants or animals) which distinguish living organisms from lifeless matter. The word “ protoplasm,” meaning that which is first formed , was invented by the German botanist von Mohl in 1846, to express the fact that this substance is the No pait ol tlio W allflower plant is convenient for observing those movements of protoplasm, so some special illustrations of them from other plants will be given below (see p. 41). 30 STRUCTURAL BOTANY primary part of the cell, while the cell-wall is a secondary product derived from it. Embedded in the protoplasm is a roundish denser body, which has a more coarsely granular appearance than the protoplasm itself. This body is the nucleus. A nucleus is certainly present in all living cells of the higher plants, and possibly in all cells whatsoever. In B Fig. 14. — Young cells at various stages of growth. A, Very early stage ; the cell - walls are quite thin, and the protoplasm almost fills the cell. B, Rather older cell, cell-wall thicker, and vacuoles more numerous. G, Older still ; the greater part of the cavity now forms a large vacuole, n, nucleus ; v, vacuoles. (After Van Tieghem.) Magnified several hundred times. very young cells it is of relatively large size, and its diameter may be more than half that of the entire cell. Its complicated structure cannot he fully de- scribed here, but it may be mentioned that in many cases the apparent granules are found, under a THE WALLFLOWER 31 sufficiently high power of the microscope, to represent extremely minute threads or fibrils. The nucleus also contains one or more comparatively large rounded masses distinct from the fibrils, and called the nucleoli (see Fig. 14). The nucleus has a definite outline, which probably represents a denser layer of the sur- rounding protoplasm. The substance of the nucleus consists of various chemical bodies which are similar to, but not altogether identical with, those of which the protoplasm is built up ; a compound containing phosphorus (nuclein) appears to be constantly present in the nucleus. Most cells contain only one nucleus, but in very long cells a number of nuclei are often found. The protoplasm, nucleus, and cell-wall are the most constant constituents of a vegetable cell. The proto- plasm, as we have seen, is absolutely constant in all living cells without exception ; the nucleus is equally general in the higher plants, though in some of the lower ones its presence has not been proved ; the cell- wall is formed sooner or later in all cells of the higher plants, but is very frequently absent, at any rate for long periods, from certain cells of the simpler plants. If we examine rather older cells, such as those which we find at a little distance below the growing point, we shall see that the whole cell is larger, and that its cavity is no longer so nearly filled by the protoplasm (Fig. 14, B ). Clear spaces, which had already appeared in the protoplasm at an earlier stage (v in the figures), have now increased in number; they are filled with water containing various organic 32 STRUCTURAL BOTANY and inorganic substances (salts, sugars, acids, etc.) in solution, and called the ccll-sap. The spaces filled with cell-sap are known as the vacuoles, because they look as if they were empty. The nucleus increases little hi size, and now bears a much smaller proportion to the whole cell than it did at first. The cell-wall will have become some- what thicker (see Fig. 14, B and 0). A full-grown but still living cell, such as we might find in the older parts of the Wallflower, has its proto- plasm reduced to a thin layer, lining the inside of the wall. The whole interior of the cell appears empty — that is, it is occupied by one large vacuole containing cell-sap. This has been formed by the smaller vacuoles running together. The layer of protoplasm lining the cell-wall is often called the primordial utricle. The nucleus has undergone little change, but is now placed close against the wall, embedded in the protoplasmic layer which lines it. The cell-wall itself has been further thickened by the deposit of new cellulose from the protoplasm. This is the structure of most mature living cells of plants. The protoplasm does not keep pace with the growth of the whole cell ; it may even actually diminish in amount, owing to its being used up to make new cell-wall and for other purposes, quicker than it is renewed by taking in food. Some cells lose their protoplasm altogether ; then they are dead, and incapable of any further growth, though they may still be of use to the plant as passages for sap or as a mechanical support. We shall find plenty of examples THE WALLFLOWER 33 of altered cells of this kind in various organs of such plants as the Wallflower, more especially in the wood. b. Special Contents of Cells Many other structures are found in cells besides protoplasm, nucleus, and cell-wall. Some of these we will not trouble about until we come to them in de- scribing the internal structure of our types. One or two, however, are so important that they must be mentioned at once. a. Chlorophyll Granules If we examine, under the microscope, a living cell from the green succulent parts of the leaf or stem, we shall find, embedded in the protoplasm, a number of round bodies of a bright green colour (see Figs. 19, p. 42, and 26, p. 69). If we soak the leaf or stem in alcohol, and then examine its cells, we shall find that the green colour has gone, the colouring matter having been dissolved by the alcohol, which becomes of a dark green colour in consequence. The round bodies themselves remain, however, unaltered in size and shape, though now colourless. The green colour- ing matter is chlorophyll, the round granules which contain it are the chlorophyll granules or corpuscles. Protoplasmic granules of this kind are called j^astids. The chlorophyll granules consist of protoplasm, rather denser and firmer than the ordinary protoplasm of the cell, and saturated by the green colouring matter. All green parts of plants owe their colour 3 34 STRUCTURAL BOTANY to the presence of chlorophyll granules in their cells. We have said above that one of the two main functions of the shoot is to take up food from the ah’. The great food-substance which green plants obtain from the air is carbon. About half the solid substance of plants consists of this element, which enters into all the chemical compounds of which living things are built up. The whole of this carbon is obtained by ordinary green plants from the carbonic acid, or more properly carbon dioxide (C02), which is contained in small quantities in the air. In ordinary air the amount of this gas is equal to less than four parts in ten thousand. The carbon dioxide is decomposed, the oxygen given off again, and the carbon retained to help to form the various organic substances of the plant.1 Now this decomposition of carbon dioxide is entirely performed by the chlorophyll granules, and they can only do this work under the influence of light. We see, then, that the chlorophyll granules are of the greatest possible importance to the plant, for they are the organs by which alone, with the help of the sun’s rays, it can obtain its carbonaceous food from the air. Both parts of the granule are necessary for the process ; the chlorophyll itself is useless without the protoplasm of the granule, and the protoplasm is quite unable to do the work unless it contains chlorophyll. The process of nutri- 1 We sliall see presently that in all probability only half the oxygen of the C02 is given off, and that the other half of the evolved oxygen is derived from the simoltaneous decomposition of ILO. THE WALLFLOWER 35 tion just described is known as assimilation, though this term is sometimes used in a wider sense. To this process the formation of all organic substances whatso- ever is ultimately due, and assimilation by green plants may thus be fairly regarded as the most fundamentally important of all physiological processes. Assimilation is not really so simple a matter as might appear from the rough sketch just given ; we shall return to the subject later on, when we come to the special con- sideration of physiological questions (see Chap. III.). (3. Starch Granules Another important body very generally, though uot always, found in the cell-contents is starch. Starch contains the three elements, carbon, hydrogen, and oxygen, in the proportion C0H10O5. It occurs in the form of granules, which are easily identi- fied under the micro- scope on treating with iodine, which gives them a deep blue colour. The larger granules show stratifi- cation (see Fig. 15, A and C) ; that is to say, the substance of the granule is made up of a number of successive layers, which have been deposited one after the other, so that the Fig. 15. — Starch granules, and leuco- plastids. A, From an orchid (Phajus), large granule, showing stratification : l, the lcucoplastid ; c, proteid crystal- loid ; the hilum is at the opposite side of the granule, near the letter A. B, Very young granule, still much smaller than the leucoplastid. C, Intermediate stage. (After Stras- burger.) Magnified 540 times. 36 STRUCTURAL BOTANY innermost layers are the oldest and the outermost the youngest. The oldest part of the granule round which all the layers have been formed is called the hilum. It does not necessarily lie at the centre, for some- times the layers are deposited very unequally, so that in an old granule the hilum is in a very eccentric position. This is always the case in potato starch. In a green plant starch is formed in two quite different ways. First it is formed in the chlorophyll granules under the influence of sunlight. It is easy to demonstrate the presence of small starch granules in the chlorophyll-corpuscles of the Wallflower leaf. These starch granules are the product of assimilation — that is to say, they are formed by the chlorophyll- corpuscles in sunlight as the result, though not the direct result, of the decomposition of carbon dioxide and water, and the rearrangement of their elements. But starch is also formed in the deep-seated tissues of the stem and the root, i.e. in parts which receive little or no light, and in which there is no chlorophyll. This starch cannot be the product of assimilation, for it is formed under conditions which render assimilation impossible. What happens is this : the starch formed in the chlorophyll granules is converted into sugar, and hi this soluble form passes down into the stem and root. When it has reached the cells in which starch has been deposited, the sugar is taken up by certain protoplasmic bodies, which are essentially similar to * THE WALLFLOWER 37 chlorophyll-corpuscles without the chlorophyll. These protoplasmic corpuscles (called leucoplastids) 1 use up the sugar to re-form starch granules (see Fig. 15, l). It is evident that the function of the leucoplastids is dependent on that of the chloroplastids, for unless the latter carried on the work of assimilation there would be no supplies of sugar available for the use of the leucoplastids. Starch is a very important reserve food-substance, and is very commonly found stored up in seeds, tubers, and fleshy roots, and, hi fact, in organs of almost every kind. It is especially important inas- much as it affords the material for new cell-walls, but it is used in many other ways also. Starch belongs to the great group of organic bodies called carbo- hydrates, which all consist of the three elements, carbon, hydrogen, and oxygen, the two latter elements being present in the same proportion as in water, H20. Sometimes we find carbohydrates stored up in the form of some kind of sugar, as in many fruits and in Beetroot. These and some other forms of reserved carbohydrates are always dissolved in the cell-sap. 7. Oil In many plants we find that the place of carbo- hydrates is taken by various kinds of oil. Oils are especially common as reserve food-substances in seeds, 1 The word plastul is a general term, including leucoplastids, chlorophyll granules, or chloroplastids, and also protoplasmic bodies which contain other colouring matters ( chromoplastids ). The three kinds of plastids are convertible into one another ; they always arise by the division of pre-existing plastids. 38 STRUCTURAL BOTANY e.g. Castor-oil seed, Hempseed, Linseed, etc. Oil con- tains the same elements (C, H, 0) as carbohydrates, but in very differ- ent proportions, the proportion of oxygen being very much less. The oil occurs in the form of small drops embedded in the protoplasm. Fig. 16. — A, Cell from the endosperm of the Castor-oil seed, examined in water ; note the large proteid grannies, each of which contains one or more crystalloids (c), also of proteid, and a globoid (g), of calcium - magnesium phosphate. B, Isolated granules, examined in olive-oil. (After Strasburger. ) Magnified 540 times. 8. Proteid Granules These form a very important part of the cell - contents, especially in seeds and other organs in which food is stored up for the use of the plant. They are chemically similar to the substances of which the protoplasm is composed, but are themselves inactive, and serve simply as food. An example is shown in Fig. 16, which represents the complex proteid granules of the Castor-oil seed. c. Cell-Formation Cells increase in number entirely by division. Every cell which exists owes its origin to the division into two or more parts of some cell which existed before it. At present we will only describe the kind of cell-formation which goes on in the vegetative parts THE WALLFLOWER 39 of plants such as the Wallflower, and indeed in all plants except the lowest. Wherever new organs are in course of development we find cell-division going on, as, for example, at all growing points whether of stem or root. We will suppose, as is generally the case, that the cell that is about to divide has a single nucleus to start with. The first obvious changes which take place are in the nucleus. The changes gone through Fig. 17. — Dividing cells from the young seed of Fritillaria. A, The two daughter-nuclei have been formed, but the new cell-wall between them does not yet reach across the cell. B, Later stage ; the new cell-wall now com- pletely divides the mother-cell into two parts. (After Strasburger. ) Magnified 240 times. are of a very complicated character, but their ultimate effect is that the nucleus becomes divided into two exactly equal and similar parts, the daughter -nuclei. So precise is the accuracy of the division, that every single minute fibril of which the framework of the parent nucleus consists is split lengthways into two identical halves, one of which goes to each of the daughter-nuclei. The latter remain for a time con- nected by delicate threads of protoplasm. The new 40 STRUCTURAL BOTANY cell- wall is formed at right angles to these threads and midway between the two daughter-nuclei. It is formed by the protoplasm itself. Generally the new cell-wall does not stretch all the way across the cell, but has to be completed at the sides later on (see Fig. 17). The nucleoli disappear during the division, and perhaps take part in the formation of the new wall. New nucleoli make their appearance in each of the daughter-nuclei, which soon assume the size and structure of the nucleus before division. We see, then, that the essential points in vegetative cell-forma- tion are the division of the nucleus into two similar halves, and the formation of a cell -wall between them, separating the protoplasm of the cell into two distinct parts. d. Continuity of the Protoplasm We have hitherto spoken as if the cells of which a plant is built up were perfectly distinct one from another ; the research of the last few years, however, has shown that this is not the case, but that often, if not always, the protoplasm of each cell is in com- munication with that of its neighbours by means of excessively fine protoplasmic fibrils, which extend through the cell-wall from one cell to another. The perforations through which the connecting protoplasmic fibrils pass are extremely minute, but are often very numerous. The importance of the discovery lies in the fact that we now know that the living matter in the plant is continuous, and not absolutely severed into isolated portions by dead cell-wall (see Fig. 18). THE WALLFLOWER 41 e. Protoplasmic Movements A good example of protoplasmic movement is afforded by the cells of the leaf of the American Water- weed ( Elodea cctnaclcnsis), a transatlantic alien, which is a great deal too common in our rivers and canals. Fig. 18. — Cells from the endosperm of the seed of a Palm (Bcntinckia), showing the fine strands of protoplasm con- necting the cells. Some ( w ) pass through the full thickness of the wall, others (p) through the thin places or pits. (After Gardiner.) Magnified 550. In order to see the movements of the protoplasm, a young leaf is removed from the stem and examined in water under the microscope. The movements may not begin at once ; in cold weather they are especially sluggish, but can be hurried on by gently warming the slide with the leaf upon it over hot water. 42 STRUCTURAL BOTANY The blade of the leaf is only two cells thick, except at the midrib, where it is thicker.' The cells of the midrib are long and narrow, those of the rest of the blade are shorter and broader. The movement generally begins first in the long narrow cells. Fig. 19. — A few cells from the leaf of the American Water- weed. a, b, c, d, e, nuclei of the various cells ;/, a strand of protoplasm crossing the cell-cavity ; g, li, i, chloro- phyll granules, some of which are dividing. The small bodies inside the chlorophyll granules are starch. (After Kny.) Magnified several hundred times. The narrow cell, to the left, would show rotation ; the broader cells, to the right, circulation. Each cell has a fairly thick layer of protoplasm (primordial utricle ) lining its cell-wall, and in this protoplasm numerous large chlorophyll granules are THE WALLFLOWER 43 embedded (see Fig. 19). The nucleus is very trans- parent, and hard to see. (It is shown dark in Fig. 19.) The protoplasm flows in a continuous stream round and round the cell, carrying chlorophyll granules with it. Owing to the presence of these granules, the movement is very easy to follow. If we fix our eye on one of the granules which is being swept along by the protoplasmic stream, we shall see it travel steadily along one of the side-walls until it reaches a corner of the cell. Here there may be a momentary hesitation, for at these points the current is often choked by the accumulation of chlorophyll granules. Soon, however, the movement goes on again ; the granule *is carried along the short end-wall of the cell, the next corner is reached and turned, and the journey con- tinued along the other side-wall, and so on until the circuit is complete. The revolving protoplasm here forms one single stream, which flows round the whole cell. The current, as we have seen, follows the lateral and terminal walls of the cell. What, then, is the behaviour of that part of the protoplasm which is in contact with the other two walls, those, namely, which form as it were the roof and floor of the cell ? This protoplasm also takes part in the general movement of rotation, and revolves in the same direction as the rest. But it has a shorter distance to cover, and the nearer we approach to the middle of the top and bottom walls, the shorter is the circuit, and con- sequently the slower is the speed. Now, just in the middle of each of these two walls there is a band of protoplasm which does not move at all, but forms a 44 STRUCTURAL BOTANY quiet eddy in the stream. This still part is called the indifferent band, and it is always present where rotation of the protoplasm goes on. But even in the other parts of the cell it is not really the whole protoplasm which rotates. The ectoplasm — that is, the thin outer layer in immediate contact with the cell-wall — remains stationary, while the revolving endoplasm (containing the chlorophyll granules) travels over its inner surface. This will serve to represent an important type of protoplasmic movement — that of rotation. Another form of movement may often be observed in the broader cells of the lamina of the Elodea leaf (see Fig. 19). In this the vacuole of the cell is traversed by strands of protoplasm, most of which are attached to the nucleus, which is here suspended in the cavity of the cell. In these cells the protoplasm is in active movement in all directions. The strands crossing the cavity show active currents by which chlorophyll granules are often swept along. In the thicker strands there may be two opposite currents flowing simultaneously. The protoplasm lining the cell-wall is also in movement, but not as in the first case all in one direction. Here also there are vary- ing streams running hi different directions in different parts of the layer. Neither the arrangement nor the direction of movement of the protoplasm is constant. Sometimes a protoplasmic strand is snapped in two, and both halves drawn back into the main mass. Sometimes a new strand is put out, which extends itself across the vacuole and joins on to the proto- THE WALLFLOWER 45 plasm on the opposite side. Moreover, in any part of the protoplasm the movement may stop altogether for a time, and then start again, perhaps hi the opposite direction to that which it pursued at first. Protoplasmic movement such as this, in which numerous distinct and varying currents are flowing at the same time in the same cell, is called circulation. In Elodca circulation tends to pass over into rotation ; all the protoplasmic strands become withdrawn into the primordial utricle, which then settles down into a simple movement of revolution. In circulation as in rotation the ectoplasm remains at rest. In one form or other it is probable that the proto- plasm in every active cell is in movement, and that power of spontaneous motion is the constant character- istic of all living matter. But it is often difficult to examine a cell under sufficiently natural conditions for its protoplasmic movements to be observed. When we come to consider the simpler plants, we shall meet with several other kinds of movement executed by the protoplasm. B. The Tissues We have now been able to form some idea of the essential points in the structure of a living vegetable cell. Of such cells and their modifications all plants consist, if we except some of the lower plants with which we are not concerned at present. We shall find, however, that the modifications undergone by many of the cells, of which plants are built up, involve very considerable changes of structure. 46 STRUCTURAL BOTANY The different kinds of cells are not distributed uniformly all through the substance of a plant. They have a definite arrangement, and the various kinds of cells are associated together so as to form more or less sharply defined layers or strands. Such an association of similar cells is termed a tissue. Certain kinds of tissue, again, are generally associated together, and such a combination of tissues is called a tissue system. We are now in a position to undertake the study of the internal structure of the Wallflower. We will begin with the stem, and will make ourselves acquainted with the tissues of which it is composed. a. Structure of the Stem The stem of the Wallflower is traversed lengthways by a number of strands of rather tough, stringy tissue. It is possible to dissect away the soft substance in which they are embedded, and so lay bare the strands themselves. If a stem be allowed to rot, it is this stringy part which will longest resist decay. Most people will have noticed at some time or other an old rotten cabbage stalk, in which all the soft parts have perished and only a network of hard, woody, inter- lacing strings is left behind. These strands are the vctscidar bundles. They constitute one of the systems of tissue of which the higher plants are composed. Their great function is to serve as conducting channels through which the various food-materials pass. In most cases they are also of importance as forming a supporting skeleton for the plant. THE WALLFLOWER 47 In the Wallflower the tissue of the stem, with the exception of the vascular bundles, is soft and succulent. A part of the soft tissue lies to the inside of the vascular bundles, and between them ; the remainder is situated to the outside of the bundles. The vascular bundles, together with all the tissue that lies between and within them, form the central cylinder ; the tissue lying to the outside of the vascular bundles is called the cortex. The surface of the stem is covered by a definite skin, which we can easily strip off with the aid of a pair of forceps. We find that it is a very thin, colourless membrane, which, in spite of its delicacy, is fairly tough and elastic. This skin, the epidermis, is only found intact on the younger parts of the stem. The older portions are covered by a much thicker and harder skin, the lark ; all the external “ pro- tective” tissues are called by the general name of dermal tissue, which thus includes both epidermis and bark. We will now examine, with the help of the micro- scope, a transverse section cut through the stem of the Wallflower (see Fig. 20). This shows us the cut ends of the vascular bundles, and we see that they are arranged in a ring. The space inside the ring of bundles is occupied by a large-celled tissue, the pith or medulla. A similar tissue extends between the bundles as far as the cortex which surrounds them. These bands of interfascicular tissue (if in Fig. 20) are called medullary rays. Vascular bundles, medullary rays, and pith collectively form the centred cylinder. This is surrounded on the out- 48 STRUCTURAL BOTANY side by the large-celled cortex, many cells in width (Co), and this again by the single layer of epidermis (Ep). Fig. 20. — Transverse section through the second internode of the stem of a seedling Wallflower, with seven vascular bundles. Ep, epidermis ; h, hair ; s, stoma ; Go, cortex ; en, endodermis, or innermost layer of cortex ; pc, peri- cycle, or outermost layer of cylinder ; ph, phloem of a vascular bundle ; x, xylem of vascular bundle ; c, cam- bium ; if, medullary ray, or interfascicular tissue ; p, pith. Magnified 55. (R. S. ) a. The Vascular Bundles We will now consider the vascular bundles more in detail, and first we will determine their course as traced longitudinally. Each bundle runs out into a leaf. This fact is most easily proved by tracing the course of the five largest bundles, which correspond in THE WALLFLOWER 49 position to the five projecting ridges seen on the surface of the stem. It is most convenient to follow the bundle from above downwards. It enters the stem from the midrib of the leaf, runs inwards for a short distance, and then turns straight downwards, continuing its descending course in the stem through five inter- nodes. It then reaches the leaf vertically below that from which it started, and joins on to its bundle. Besides the principal bundle, two smaller lateral bundles enter the stem from each leaf, and these behave in the same way. Hence in any transverse section of the stem we cut through about fifteen bundles, five principal ones, each representing the middle bundle of a leaf, and ten smaller ones, forming the continuation of the lateral leaf - bundles. The number is not always exact, for the smaller bundles may branch or unite together on their downward course. It is quite easy to understand the arrange- ment if we bear in mind that each ridge on the outside of the stem corresponds to a principal bundle, which is accompanied by its two smaller side bundles. At the insertion of each leaf the various bundles are connected together by cross branches. The description given applies with slight variations in detail to the whole stem of the Wallflower, with the exception of the first few internodes of the seedling.- These have a simpler arrangement of the leaves, and consequently a smaller number of bundles. The section (Fig. 20) is taken from one of these first- formed and simpler internodes. Vascular bundles of this kind, which are continuous 4 50 STRUCTURAL BOTANY from the stem into the leaf, are called common bundles, because they are common to both organs. That part of a common bundle which passes through the stem is called a leaf-trace, because its position is dependent on that of the leaf with which it is connected. We see, then, that the whole bundle - system of the stem is built up of these leaf-trace bundles, the upper ends of which run out into the leaves themselves. As the various bundles are united to one another at the nodes, it is equally easy for the food-substances which they conduct to pass out into one of the leaves or to continue their course straight up the stem, while, on the other hand, those food-substances which are formed in the leaves have ready access through the stem-bundles to all parts of the plant. In fact, the arrangement of the bundles is adapted to facilitate communication in every direction between leaf and stem. The connection with the root will be described later on. Where an axillary branch is given off, its leaf-trace bundles are continuous with those of the main stem. Having described the longitudinal course of the vascular bundles, we have next to examine their internal structure. We will fix our attention on a single leaf-trace bundle, such as that which is shown in transverse section in Fig. 21. The bundle is made up of two quite different kinds of tissue. The inner half of the bundle — that, namely, which is turned towards the centre of the stem — consists chiefly of rather large cells, the walls of which are somewhat thickened, and are hard and woody. THE WALLFLOWER 51 Many of these cells are quite empty — that is to say, they have no living contents, ancl only contain air or water. This inner half of the bundle is the wood or xylem {x in Figs. 20 and 21). Fig. 21.— Transverse section through a vascular bundle of a young stem of Wallflower, en, eudodermis, containing starch granules; pc, pericycle; ph, phloem ; c, cambium ; x, xylem or wood ) px, protoxylem, or first-formed wood. Portions of pith, interfascicular tissue, and cortex are also shown. Magnified about 210. (R. S.) The outer part of the bundle, lying towards the cortex, consists of much smaller cells, with thin walls of pure cellulose, and with abundant protoplasmic 52 STRUCTURAL BOTANY contents. This part of the bundle is called the last ox phloem (ph in Figs. 20 and 21). Between xylem and phloem is a band of very regular cells, which appear oblong in transverse sec- tion, and have very thin walls, showing that active division is going on (see Fig. 21). This layer is the cambium , which in plants of the class to which the Fig. 22. — Radial longitudinal section passing through a vascular bundle from a young stem of Wallflower. e, endodermis, containing starch ; pc, pericycle ; ph, phloem ; c, cambium ; xy, xylem ; pxy, protoxylem ; p, pith. Somewhat diagrammatic. Magnified 220. (R. S.) Wallflower belongs is constantly undergoing cell- division, and adding new cells to the wood internally, and to the bast externally. THE WALLFLOWER 53 The wood in the Wallflower consists of three kinds of elements, which are best studied in longitudinal section (see Fig. 22). The Wood or Xylem 1. The Vessels These are long, continuous tubes, which extend along the plant for considerable distances without any interruption. When mature, they are quite empty, containing no protoplasm, but only water and air. The vessels have very peculiar markings on their walls, which are due to their being unequally thick- ened, some parts of the wall growing much in thickness, while other parts remain quite thin. In the vessels nearest the pith the thickening generally takes the form of a spiral thread, which winds along the inside of the wall. The parts of the wall between the coils of the spiral are quite thin. The structure of one of these spiral vessels is exactly like that of an indiarubber hose with a stiff spiral wire coiled inside it to prevent its collapsing. In the innermost vessels the spiral is always loosely coiled. This is because the innermost vessels are the first to be formed, and their walls become thickened before the tissues have stopped growing hi length, so that as growth goes on the spiral gets drawn out. Sometimes, instead of the continuous spiral thread, we find isolated transverse rings round the vessel, which is then called annidar. This form also admits 54 STRUCTURAL BOTANY of subsequent growth in length, for as the vessel becomes longer the rings become pulled farther and farther apart. This form is not so very common in the Wallflower. These innermost vessels constitute the protoxylem , so called because this part of the wood is formed first, and is therefore the oldest (see Tig. 22 ,pxy). The vessels a little farther towards the outside are still spirally thickened, but here the coils of the spiral are closer together, for the thickening has been developed a little later, and so has not got so much pulled out by subsequent growth. Next we come to vessels in which the thickening on the walls forms a network (- reticulated vessels), the meshes of which are occupied by a thin membrane. This form is not capable of longitudinal extension, and is only developed after growth of the stem is complete. Lastly, in the outer and later-formed parts of the xylem, we find vessels in which the wall generally is thickened, but a number of little oval spaces are left thin. These vessels are said to be pitted, the thin parts of the wall being the pits. The thin places or pits serve for the passage of liquids from one vessel to another ; the various kinds of thickening have the object of stiffening the vessel and enabling it to resist pressure. We have seen that the vessels are open, continuous tubes. They are therefore quite different from ordinary cells such as we described above. A vessel is not a cell, but it is formed from a number of cells. THE WALLFLOWER 55 Each vessel consisted at first of a whole row of cells, each with its own protoplasm and nucleus. Then the cross walls separating these cells from one another were dissolved away, and so the whole row came to form one open passage. The cross walls, however, do not disappear entirely. A ring is left at the edge, and in the Wallflower we can easily see these rings, even in the mature vessels, marking the limits between the original cells which have fused to form a vessel. These rings are found in all vessels, and must not be confused with the rings of thickening, which are found in the annular vessels of the protoxylem only. We have already seen that the cell-wall is formed by the protoplasm. Where, as in vessels, certain parts of the cell-wall are specially thickened, these thicken- ing masses also are deposited by the protoplasm, which at last becomes almost entirely used up in the process. What little protoplasm remains is absorbed by the neighbouring cells, and the vessel is left quite empty. Thus the vessels when fully formed are dead structures ; they contain no living matter, and are incapable of further growth. They serve simply as open channels through which the sap can pass. 2. The Fibrous Cells The wood of the Wallflower also contains fibrous cells, which are scattered about among the vessels, and are more common in the later-formed parts of the wood. They are very long, thick-walled cells, with sharply pointed ends. The wall is thickened almost all over, but there are a few small narrow pits where 56 STRUCTURAL BOTANY the wall is left thin. These cells, unlike the vessels, keep their living contents, and do not fuse with one another. The chief function of the fibrous cells is a mechanical one. They serve to give rigidity to the stem. At the same time their protoplasm retains its activity, and consequently these cells are able to form and store up starch for future use. Cells of the form just described, with long pointed ends overlapping one another, are said to bey> rosenchymatous (see Fig. 36). 3. The Xylem Parenchyma Besides the vessels and fibrous cells, the xylem also contains cells with square ends. They are rather long, though very much shorter than the fibrous cells ; they have rather thick walls with small pits. They always contain protoplasm and a nucleus, and often starch granules as well. They are the chief tissue in the wood in which starch is stored up. They do not undergo fusion. These square-ended cells constitute the woody or xylem parenchyma. Cells of this kind are always found in contact with the vessels (see Fig. 36). The woody character of the cell-walls of the xylem is due to the presence of a substance called lignine, which is closely united with the cellulose, and greatly alters its properties. Lignified cell-walls are much harder and stiffer than those of pure cellulose. Such walls are characteristic of the xylem, which is a sup- porting as well as a conducting tissue, and thus forms part of the skeleton of the plant. But we often find cells with lignified walls outside the vascular bundle THE WALLFLOWER 57 altogether, or in its phloem portion, though this is not the case in the Wallflower. The xylem, therefore, does not of necessity form the whole skeleton of the plant. It is especially the thick parts of the cell-wall which are lignified. In the vessels these hard, thick, stiffening bands have a special function, for the cavity of a vessel is often almost a vacuum, only containing air at a very low pressure, and a very small quantity of water. Vessels in this state would very readily yield to the pressure of the surrounding tissues, and so collapse, if their walls were not specially strengthened. The Bast or Phloem In the Wallflower the bast consists entirely of cells with soft walls of pure cellulose. The most important cells are of three kinds (see Fig. 23). 1. The Sieve-Tubes or Bast- Vessels These are made up of long cells placed end to end. Their great peculiarity is that their end-walls are perforated — that is, bored through in a number of fine holes, like a sieve. From this fact they obtain their name. They are called vessels because the cells of which they are built up are in open communication with one another, but while in many wood-vessels the cross walls are almost entirely absorbed, in all bast- vessels they are only perforated. The sieve-tubes retain their protoplasm, and also contain a slimy substance which is rich in nitrogenous compounds ; sometimes starch grains are also present. The sieve- 58 STRUCTURAL BOTANY tubes when mature have no nuclei. Each of their constituent cells has a nucleus to begin with, but during the development of the sieve-tube it breaks up into fragments, and ultimately disappears alto- gether. Where sieve-tubes are hi contact with each other side by side, their lateral walls are also perforated, not all over, but in certain parts. Through the per- forations, whether of the transverse or of the longi- tudinal walls, the contents of the sieve -tube are continuous. We must remember that this perforation of the cell-walls and open communication between the protoplasm and neighbouring cells is not peculiar to sieve-tubes. We have already learnt that living cells generally have then’ protoplasm continuous through minute perforations in their walls. The perforations in the sieve-tubes, however, are often much larger than those of ordinary cells, and continuity of the protoplasm was recognised in them long before it was even suspected to exist between other living cells. That part of the cell-wall of the sieve-tube which is perforated is called the sieve-plate. 2. The Companion-Cells The sieve-tubes are accompanied by narrow, longish cells, each of which is densely filled with protoplasm, and contains a large nucleus, which is permanently retained. These cells are called the companion-cells. In a transverse section the companion-cells look as if they had been cut off from the corners of the sieve- tubes, and that is how they really originate, for THE WALLFLOWER 59 sieve -tube and companion - cell are formed by the division of the same mother-cell (see Fig. 23, A, B, and 0, z). 3. The Phloem Parenchyma The rest of the phloem is made up of rather large parenchymatous cells, which have the typical structure of living cells, each containing a layer of protoplasm lining the wall and a single nucleus. This tissue undergoes no special modification, and is called simply the phloem parenchyma. The cells at the extreme outer edge of the phloem are the first to be developed, and are therefore called the protophloem. Unfortunately, the phloem of the Wallflower is not well adapted for showing the minute structure, as all the cells are very small in this part. It has therefore been necessary to introduce Fig. 23, taken from another plant (the Pumpkin). Between xylem and phloem there are some layers of cells which do not as yet show the characters of either tissue. On the inner side these layers pass over gradually into xylem, and on the outer side into phloem. The layer in the middle is itself constantly dividing, and thus contributing new elements to wood and bast respectively. The thinnest walls indicate the most recent divisions (see Fig. 21). The divisions take place chiefly in the tangential direction — that is to say, in a direction of the line which separates xylem from phloem, which is parallel to a line tangential 60 STRUCTURAL BOTANY to the surface of the stem at a point opposite the bundle. As already mentioned, it is characteristic of the class to which the Wallflower belongs to have this actively dividing layer of cambium between the xylem and phloem. Vascular bundles which have a cambium Fig. 23. — Sieve-tubes and companion-cells from the phloem of the Pumpkin ( Cucurbita ). A, Transverse section, showing sieve-plate and companion-cell, z. B, Longi- tudinal section, showing a sieve-plate in sectional view : pr, primordial utricle of sieve-tube ; u, its contracted contents; z, companion-cells. C, Longitudinal section, showing sieve-plates covered by callus c and cv D, Con- tents of sieve-tube after the cell-wall has been dissolved away with sulphuric acid. (After Strasburger. ) Magnified about 400. are known as open bundles, because there is no definite limit to their growth in thickness, the forma- tion of new xylem and phloem from the permanently active cambium going on as long as the plant lives. We shall, however, understand the nature of the THE WALLFLOWER 61 cambium better after we have considered the develop- ment of tissues in the growing points. j3. Other Tissues of the Central Cylinder The central cylinder consists of the vascular bundles and of parenchyma, which is called conjunctive tissue because it unites the bundles together. That part of the conjunctive tissue which lies inside the ring of bundles, and is called the pith, is of very simple structure. It is made up of large, rather elongated, square-ended cells, which retain their living contents, and usually form starch granules. The interfascicular tissue, constituting the medullary rays, is similar to the pith, but its cells are smaller. It is important, because its cells divide up to form the interfascicular cambium, which unites the cambial layers of the separate bundles into a continuous ring of actively multiplying cells (see Figs. 20 and 21). Owing to the presence of this layer of cambium, the stem is able constantly to form new tissues, and thus to grow indefinitely in thickness (see below, p. 90). Surrounding the whole ring of vascular bundles on the outside is a layer of thin-walled cells, the pericycle (pc in Figs. 20, 21, and 22). This layer forms the outer limit of the central cylinder. Beyond this we come to the cortex. Those cells of the conjunctive tissue which border on the vascular bundles are generally rich in grape- sugar, dissolved in their cell-sap. It is through these cells that the carbohydrates pass on their way from the leaves to other parts. 62 STRUCTURAL BOTANY 7. The Cortex Adjoining the pericycle on the outside is a single layer of cells, with slightly thickened walls, and with numerous starch granules in their contents. This layer is the endodermis (en in Figs. 20 and 21, e in Fig. 22), and forms the inner limit of the cortex. The rest of the cortex consists of ordinary parenchyma, like that of the pith. Its outer layers, however, have rather thick walls, though they consist of pure unlignified cellulose. The outer cells contain chloro- phyll granules, and can therefore take some part in assimilation, though this function belongs chiefly to the leaves. The thick -walled layers are of some mechanical importance in helping to give greater stiffness to the stem. Parenchymatous cells generally, whether belonging to the central cylinder or to the cortex, do not as a rule fit closely together, but separate slightly from one another at their corners so as to leave little spaces between them. These interccllidlar spaces, as they are called, contain air and aqueous vapour, but not water. 8. The Epidermis The epidermis of the stem is one layer in thick- ness. The epidermal cells are of three different kinds ; most of them are elongated, with square or sometimes pointed ends, and fit closely together without any intercellular spaces. These cells each contain protoplasm and nucleus, but no chlorophyll THE WALLFLOWER 63 granules. The outer layer of the superficial cell-wall of the epidermis forms the cuticle, and does not con- sist like the other walls of cellulose, but of a substance (cutiri) resembling cork, which is extremely impervious to water and watery vapour, and which is chemically a remarkably stable body, resisting the action even of strong sulphuric acid, and of other substances which easily dissolve the cellulose. The inner and lateral walls of the epidermal cells consist of ordinary cellulose, and the same is the case with the innermost layer of the superficial wall. The epidermis of the Wallflower bears very large and characteristic hairs. Each hair is a single cell. It has the form of a spindle, and is attached at the middle by a short stalk, which is inserted between the ordinary epidermal cells (see Figs. 26 and 27, which show similar hairs on the leaf). These hairs are always so placed that the spindle lies close to the surface of the epidermis, and parallel to the axis of the stem. These hairs have an external cuticle, like other epidermal cells. Their surface is rough, with projecting knobs, which contain calcium carbonate (chalk). Experiments have shown that these rough and sharp hairs are of great importance to the plant, as a protection against the attacks of slugs and snails. It has already been mentioned that the ordinary epidermal cells have no intercellular spaces between them. There are, however, intercellular spaces in the epidermis, but they only occur between specially modified cells. The intercellular space with the cells 64 STRUCTURAL BOTANY which enclose it is called a stoma. Each stoma consists of a pair of cells, much smaller than those of the epi- dermis generally, and of quite a different form (see Fig. 2 0 , s, also Figs. 2 6 and 2 7, s, from leaf). These two cells are called the guard-cells. Each guard-cell is sausage- shaped and curved, the ends of the cells being firmly joined together, while in the middle they are separated a little, leaving a narrow pore or crack between them, which communicates with the intercellular spaces of the ground tissue. The guard -cells of the stomata differ from other epidermal cells in containing chloro- phyll-corpuscles and starch granules. We will defer the more detailed consideration of the structure of the stomata until we come to the leaf, where they have more important functions to discharge. . b. Structure of the Leaf We have already learned so much of the structure of the leaf as can be made out with the help of a lens only (see Fig. 7). The leaf is built up of the same tissues as we have found hi the stem, but with many differ- ences in arrangement and in the details of structure. The vascular bundles of the leaf are the direct continuation of those in the stem. We have already traced the general course of the veins of the leaf, and this is identical with the course of the vascular bundles, each vein corresponding to a bundle. In the larger veins, however, such as the midrib, the bundles are accompanied by a thick, enveloping layer of parenchyma, differing from that of the rest of the leaf. The principal bundle traverses the midrib from THE WALLFLOWER 65 end to end, tapering towards the apex of the leaf, and giving off branches on either side. All these branches ramify repeatedly, but always in the same plane, the successive offshoots becoming finer and finer, until we come to the extremely slender ultimate branchlets, which end blindly within the meshes formed by the larger bundles (see Fig. 24). Mar. 4 Mid. Fig. 24. — Part of a section, parallel to the surface, of a leaf of the Wallflower, passing through the palisade paren- chyma, and showing the xylem of the bundles as seen from above. Observe the endings of the bundles within the meshes, p.p, palisade parenchyma ; v.b, vascular bundle ; sh, bundle-sheatli. The arrows point towards the margin and the midrib of the leaf respectively Magnified 50. (R. S.) The lateral bundles also branch immediately on entering the leaf, and form the bundle-system of the lower part of the lamina. The larger vascular bundles of the leaf agree in Ft 66 STRUCTURAL BOTANY structure with those of a young stem, but the two parts of the bundle are so placed that the xylem is directed upwards, and the phloem downwards. This is a very constant rule for vascular bundles in flat leaves, and holds good both for the main bundles and their branches. We know that in the stem the xylem faces inwards and the phloem outwards, and as the bundle passes into the leaf without any twisting, the xylem necessarily comes to lie towards the upper, and the phloem towards the lower surface of the leaf. The loosely coiled spiral vessels of the protoxylem are found at the extreme upper edge of the wood, while the protophloem lies at the extreme lower edge of the bast. The development of the parts of the bundle follows precisely the same order as in the stem. In the leaf, however, the bundles are closed. The leaf being of limited growth, there is no need for a permanently active cambium. In the finer bundles of the leaf the structure is greatly simplified, the xylem consisting exclusively of spirally thickened elements. In the ultimate branches these elements are of the kind called trachcides ; they resemble vessels in all respects except that the cells of which they are composed do not fuse with one another, so that they do not form continuous tubes. If we trace the finest bundles to their termination, we find that the phloem comes to an end before the xylem, so that the extreme end of the bundle consists of spiral tracheides only. All the bundles, including their finest branches, are enclosed in THE WALLFLOWER 67 sheaths of closely-fitting parenchymatous cells (see Fig. 24). We must now say something as to the tissue which surrounds the vascular bundles in the larger veins of the leaf, especially in the midrib (see Fig. 25) . The parenchymatous tissue of the midrib contains but little chlorophyll, and closely resembles the cortical tissue of the stem. The cells towards the upper and lower surfaces of the midrib are rather thicker-walled than the rest. The other main veins of the leaf show a somewhat similar structure on a much smaller scale. Together with the midrib they form a mechanical supporting system for the leaf, constituting a stiff framework, between the ribs of which the delicate tissue of the lamina is extended, and thus kept in position. The larger bundles also have a little parenchyma within their sheath. Now we will consider the structure of the chlorophyll - containing tissue (mesophyll), of which the thin deep-green lamina chiefly consists. On examining a transverse section (see Figs. 2 5 and 26) we see at once that the parts of this tissue towards the upper and lower surface respectively have a different structure. The upper part of the mesophyll is made up of three or more layers of closely-packed cells, which are elongated in the direction at right angles to the surface of the leaf. These cells contain an immense number of chlorophyll-corpuscles embedded in their protoplasm, and chiefly ranged on the side- walls of the cells (see Fig. 26). From the appear- ance of its upright and regularly arranged cells this 68 STRUCTURAL BOTANY THE WALLFLOWER 69 tissue is called the 'palisade parenchyma. Its cells are cylindrical in form, and have narrow intercellular spaces between them. The lower half of the mesophyll, on the other hand, U.S. Fra. 26.— Pcart of ,a transverse section of a leaf of tire Wall- flower. e, epidermis ; cu, cuticle ; p.p, palisade paren- chyma ; s.p, spongy parenchyma ; s, stoma ; h, hair ; U ,$' upper, L.S., lower, surface of leaf. Magnified 165. (R. 8.) consists of very loosely arranged irregular cells, with very large air spaces between them. These cells, like those of the palisade parenchyma, contain chlorophyll- 70 STRUCTURAL BOTANY corpuscles, but they are less numerous here, and are distributed uniformly in the protoplasm lining the cell-walls. This tissue is called the spongy parenchyma. The lower ends of the palisade cells never end blindly in an intercellular space, but always join on to cells of the spongy parenchyma. The finer vascular bundles pass through the upper part of the spongy tissue in such a position that their xylem almost reaches the lower ends of the palisade cells. The differentiation of the mesophyll into palisade and spongy tissue is characteristic of those flat leaves which are horizontal, and which therefore have an upper and a lower surface. When we come to deal with the physiology of the leaf, we shall see how these tissues are adapted to their special functions. The epidermis of the upper surface of the leaf consists entirely of closely-packed cells, without inter- cellular spaces. It bears a few long; spindle-shaped hairs, like those of the stem. There are no stomata on this upper surface. The epidermis of the lower surface has more numerous hairs, and possesses an immense number of stomata (see Fig. 27). The general structure of a stoma has already been described. Each stoma opens into a large intercellular space in the spongy tissue immediately within it. This space is called the air-chamber (see Fig. 26). The air-chambers, again, are in communication with all the intercellular spaces of the leaf, and through them, with those of the whole plant. The stomata, then, are the pores by THE WALLFLOWER 71 which the intercellular passages of the whole plant, con- taining air and watery vapour, open into the external atmosphere. A section across a stoma shows the two guard-cells with the pore between them (Fig. 26). The transverse section of each guard-cell is roughly a square ; the walls are very thick, especially at the two corners Fig. 27. — Portion of epidermis from the under side of a Wall- flower leaf, in surface view, h, h, hairs ; S, S, stomata. Magnified about 210. (R. S.) towards the pore, which are provided with projecting ridges. We have already learnt that the guard-cells contain chlorophyll-corpuscles and starch granules, and tli us differ from the ordinary epidermal cells. The stomata have the power of opening and closing. As a general rule, they open under the influence of light and warmth, and close when it is dark or cold. 72 STRUCTURAL BOTANY The pore opens when the guard-cells become more curved, so that the space between them is widened. It closes when they straighten, so that their sides are brought into contact. We shall see later on how this opening and closing of the stomata works in with the mode of action of the leaf as a whole (see p. 221). c. Structure of the Root We will now examine the structure of the main root or tap-root , which forms the direct downward continuation of the stem. It does not differ essentially from its branches, but has the advantage of being rather larger, and more convenient for investigation. In order to find the primary structure of the root unaltered, we must take a very young one ; for hi the Wallflower, and most members of its class, the cambium begins its activity early, and soon brings about a com- plete change in the distribution of the tissues. We will start as before with the vascular system, which at first sight appears very different from that of the stem. The middle of the root is traversed by a slender cylindrical strand of vascular tissue, which runs in a straight course from base to apex, and is directly continuous with the bundle-system of the stem. In a root which has reached the stage shown in transverse section in Fig. 28, the xylem forms a plate of tissue passing through the centre of the vascular cylinder ; the phloem is grouped in two distinct bundles, one on each side of the xylem-plate. If we had examined a still younger root, we should have found that only the cells at the two ends of the THE WALLFLOWER 73 xylem-plate were mature ancl lignified, the middle part of the plate consisting of thin-walled cells still in course of development. In other words, the X Fig. 28. — Transverse section of a very young root of the Wallflower, r.h, root-hairs ; p.l, piliferous layer ; c, cortex ; e, endodermis ; pc, pericycle ; x, xylem-plate ; the protoxylem is at its two ends ; ph, pli, the two phloem groups. Magnified 110. (R. S.) protoxylem, or oldest wood, of the root is formed at the outside, and development advances from without inwards, just in the reverse order to that which is followed in the formation of xylem in the stem. 74 STRUCTURAL BOTANY Consequently we should have to look for the first- formed spiral vessels at the extreme ends, as seen in a transverse section, of the xylem-plate of the root. The phloem develops from without inwards, just as in the stem, so we see that, in the root, wood and bast are both formed in the same direction, and not in opposite directions, as in the stem. We find that in this root there is no pith, for the wood extends quite to the centre. This is characteristic of very many roots, though not of all. There are a few parenchy- matous cells between xylem and phloem ; these cells form the conjunctive tissue of the central cylinder. A root with the structure just described is called cliarch, because there are, to begin with, two groups of xylem and two of phloem, the two xylem groups soon uniting in the middle to form the continuous plate. When there are three groups of each, the root is called triarcli ; when four, tetrarch ; and so on. We see that the arrangement of the xylem and phloem is quite different in the root and in the stem. While in the latter the xylem always lies just inside the phloem, the two together forming a collateral bundle, in the former the xylem and phloem bundles are distinct, and alternate with each other all round the vascular cylinder. The arrangement in the root may be called radial, because the xylem and phloem groups are placed on alternate radii of the central cylinder, which, as seen in transverse section, is almost circular. The chief points, then, in which the vascular system of the root differs from that of the stem are — THE WALLFLOWER 75 (1) The alternating arrangement of the xylem and phloem groups. (2) The centripetal development of the xylem. (3) The greater concentration of the bundles, and consequent smaller diameter of the central cylinder, which contains but little parenchyma between the bundles. The vascular cylinder is surrounded by a layer of thin-walled cells, the pericyclc, which here, as in the stem, forms the ontermost layer of the cylinder. In the root this layer is of special importance, for from it all branches arise. The 'pericyclc, , again, is surrounded by another layer, the endodermis, which here has the peculiarity that its cell-walls, in so far as they border on other endodermal cells, are cuticularised. This is not the case with the inner and outer walls, which abut on the pericycle and cortex respectively. We can recognise the endodermis in the root in transverse sections by the dark appear- ance of its cuticularised radial walls (see Fig. 28, c) ; we know that cuticularised walls resist the passage of water much more than cellulose walls do, so the effect of this arrangement is that water has free passage through the endodermis from the cortex into the vascular cylinder, hut cannot turn aside to pass from one endodermal cell into another. The cuticularised walls of the endodermal cells fit closely together, so that the intercellular spaces of the cortex are completely shut off from those of the cylinder. This structure of the endodermis is sometimes found in the stem also, hut not so constantly as in roots. The endodermis is 76 STRUCTURAL BOTANY really the innermost layer of the cortex. The rest of the cortical tissue consists of living parenchymatous cells, with thin cellulose walls. They clo not, of course, contain any chlorophyll, because the roots in the earth receive no light, without which chlorophyll cannot be formed, though the plastids may be present. The outermost layer of the young root, corresponding in position to the epidermis of the stem, consists of thin-walled living cells, many of which grow out into long hairs (see Fig. 28). These root-hairs are very important organs ; they take up all the food which the plant obtains from the soil. Each root-hair is a single cell, with protoplasm and nucleus, and a thin cell-wall. The delicacy of the outer cell-walls, and the absence or slight development of cuticle, are points in which the external layer of the root differs from the epidermis of the shoot. The meaning of these differences is, that the epidermis of the stem is a protective layer, which among other duties has to hinder the passage of water out of the plant, while the external tissue of the root is absorptive (so loug, that is, as the root is quite young), and has to admit into the plant water and various food-substances which are dissolved in it. Partly because of this great difference in function, and partly on account of some differences in the development, it is better not to speak of the external layer of roots as an epidermis, but to call it the piliferous layer, i.c. the layer which produces the root-hairs. These root-hairs grow between the small particles of soil, and attach their ends to them quite firmly, so THE WALLFLOWER 77 that it is impossible to pull up a plant out of the ground without lifting with it innumerable fragments of soil, which remain sticking to the hairs on the young roots so firmly that they cannot be washed off. This adhesion to the soil is due to the conversion of the outer layer of cell-wall into mucilage. The student must be very careful never to confuse the Fig. 29. — A, Young root, with root-hairs penetrating the soil. Magnified about 7 times. B, Root-hairs more highly magnified, showing their close adhesion to the particles of soil. (After Kerner.) root-hairs with the finer branches or fibres of the root itself. The latter may appear thin to the naked eye, but they are very thick in comparison with the hairs, and always have the same structure as the main root on a smaller scale, while, as we have seen, the root- hair is a single long cell (see Fig. 29). The root hears no leaves : its only appendages are — (1) the branches, which repeat in essentials the struc- ture of the root which bears them ; and (2) the hairs. The growing point of the shoot is, as we know, 78 STRUCTURAL BOTANY protected by the leaves of the terminal bud, which close in over it. The growing point of the root requires protection even more, for it has to make its way between the hard particles of the soil, and here a special organ is present which performs this sheltering function. This organ is the root-cap, a sheath of tissue shaped rather like a thimble, which envelops the delicate growing apex. As the root elongates, the cap is pushed before it, and has to bear the brunt of the friction with the soil (see Figs. 33 and 34, c). The outer layers of the root-cap become gradually worn away, and as this happens the cap is renewed from within by the growing point itself, which thus has to provide new tissue for the cap in front of it as well as for the body of the root behind (see Fig. 33, c). The presence of the cap is another very constant character by which roots are distinguished from stems. d. Transition from Stem to Root It has already been said that the tissues of the root are continuous with those of the stem. We will now see how the one organ passes into the other. In order to understand this, it will be necessary to study a fairly young seedling, before there has been much growth in thickness. The first two leaves which the Wallflower seedling forms are placed opposite to one another, as is almost always the case in the class of which it is a member. These two first leaves are called the seed-leaves or cotyledons, and differ considerably from the other leaves of the plant. For example, they are nearly round in THE WALLFLOWER 79 shape, and have distinct leaf-stalks \ their venation is also different from that of ordinary leaves. The part of the stem which comes below the cotyledons, i.e. between them and the tap - root, is called the hypocotyledonary stem or hypocotyl {hyp in Fig. 30). It is in this region that the transition between stem and root structure takes place. From each cotyledon we can trace two vascular bundles into the hypocotyl. As soon as they enter it they are joined by the bundles of the next leaves above ; but with these we need not concern ourselves, for only the bundles of the coty- ledons extend down into the root. The hypocotyl, therefore, contains four vascular bundles only, two on each side. Their posi- tion is different from that of the ordinary vascular bundles of the stem. In each pair the two protoxylem groups are not directed straight inwards, but are turned towards each other, and this is the case even at the point where they first enter the hypocotyl. The phloem groups are not Fig. 30. — Wallflower seedling, s, stem ; c, c, cotyledons, or first leaves ; hyp, hypocotyle- donary stem ; G — G, level of ground ; r, root. Two-thirds of natural size. (R. S.) 80 STRUCTURAL BOTANY placed directly opposite the xylem, but lie rather farther to the right and left, so that in each pair of bundles the two groups of phloem are farther apart from each other than are those of the xylem. Now, the farther we trace the vascular bundles downwards, ph. PK Fig. 31. — Part of transverse section of hypocotyl of Wall- flower, showing cylinder, px, px, protoxylcm groups of the four bundles from the cotyledons ; x, x, their xylem, ph, ph, their phloem groups. From the middle of the transitional region between stem and root. Magnified 220. (R. S.) the nearer to each other do we find the xylem groups of each pair, and as they approach each other their protoxylem turns more and more towards the outside (see Fig. 3 1 , px). Lower down still, we find in place of each pair of xylem groups a single one, with its proto- THE WALLFLOWER 81 xylem directed straight outwards. From this point onwards, the hypocotyl contains two groups of xylem only, and these groups now converge, and ultimately unite to form the diarch xylem-plate of the root itself. We see, then, that this xylem-plate forms the downward prolongation of the two pairs of xylem strands from the cotyledons. Consequently the xylem-plate lies in the plane which passes through the insertion of the two cotyledons. In the meantime, we find a corresponding change in the position of the phloem strands, but in the reverse direction. The two phloem strands of each pair of vascular bundles diverge more and more from each other, until the right-hand group of one pair meets the left-hand group of the opposite pair, and vice versd (see Fig. 3 1 , ph). Tracing them still farther down, we find that these two groups become one, so that at last there are only two groups of phloem alto- gether, one on each side of the xylem-plate. Each phloem strand of the root, then, is continuous with two phloem strands of the hypocotyl, one belonging to each cotyledon, and thus we see that in spite of the great difference in the arrangement of the xylem and phloem strands, the vascular system of the main root is the direct downward continuation of that of the stem ; the parenchyma of the central cylinder undergoes a great reduction in the transitional region, the pith gradually disappearing as the bundles converge. The pericycle forms throughout the external layer of the cylinder whether in stem or root. The continuity of the endodermis and other cortical tissues between stem 6 82 STRUCTURAL BOTANY ancl root is also complete. The change from the epidermis of the stem to the piliferous layer of the root is, however, a sudden one. Apical Development We will now describe rather more fully how the development of the Wallflower plant takes place. that the growing points, i.e. the region where the formation of new organs and tissues goes on, lie at the extreme ends of the branches of both shoot and root. In every branch the base is the oldest, the apex the youngest part. It is at the apex, then, that we shall find development in progress. (a) The growing point of the shoot. — If we examine a longi- tudinal section passing exactly through the middle of a growing point, we find that the end of the stem has a nearly flat or slightly domed surface. If the section has passed through one or more of the very youngest leaves, we shall see slight projections on the sides of Lower down, we come to rather more advanced leaves, which at first simply show an increase in size ; while farther down still, their tissues begin to be differentiated. Now, the growing point and the very youngest We already know Fig. 32, — Growing point of a stem in longi- tudinal section. I, young leaves ; ep, young epidermis, or dermatogen ; i, periblem, or layer from which the cortex is developed ; pi, ple- rome, which develops into the central cylin- der. (After Douliot. ) Magnified. that they arise as the growing apex. THE WALLFLOWER 83 leaves consist entirely of thin-wallecl cells rich in protoplasm, with relatively large nuclei. Such cells have already been described (p. 28). All these cells are in course of active growth and division, and therefore the tissue of the growing point is called meristem or dividing tissue. From this meristem all other tissues are derived. In favourable sections we can distinguish three distinct meristematic strata at the apex ; the outermost of these is a single layer of cells, dividing exclusively by walls at right angles to the surface, and giving rise to the epidermis only. This is called the dermatogen (Fig. 32, ep). It ex- tends not only over the growing point of the stem, but also over the young leaves, for the dermatogen forms the epidermis of the leaves as well as that of the stem. Next below the dermatogen we find a stratum which may be only one cell thick at the extreme apex, but which lower down divides by cell-walls parallel to the surface as well as by walls at right angles to it, and thus becomes several layers in thick- ness. This is the periblem, or young cortex (Fig. 32, i). Lastly, underneath this again, we find a group of meristematic cells which divide in all directions, and form the central cylinder of the stem, including the pericycle, vascular bundles, pith, etc. This last group is called th q plcrome (Fig. 32, pi). It is probable, though hard to make out for certain, that each of these three layers may be traced to a single cell at the extreme apex, and if so, we should have only three initial cells altogether, giving rise by their divisions to all the tissues of the shoot (see Fig. 32). 84 STRUCTURAL BOTANY The vascular bundles begin to develop at some little distance from the apex. The outer part of the plerome divides up more actively than the rest, giving rise to a small-celled tissue. Within this zone certain strands of cells divide more frequently by longitudinal than by transverse walls, so that they come to consist of rather elongated cells. These constitute the pro- ccimbial strands, each of which develops into a vascular bundle, as already described. The inner- most elements of each strand thicken them walls and become the protoxylem, while the outermost are converted into the protophloem. The successive development of the elements then goes on from within outwards in the xylem, and from without inwards ill the phloem. But, as it is an open bundle with which we are concerned, there is always a layer of cells left between wood and bast which remains capable of dividing, and forms the cambium of the bundle, by means of which it can increase the number of its elements indefinitely. Only the dermatogen and periblem take part in the formation of the leaves. To form a leaf, the outer layei’s of the periblem grow out, and divide by walls parallel to the surface, while the dermatogen follows their growth (see Fig. 32, 1). The latter gives rise to the epidermis of the leaf only. All its other tissues — vascular bundles as well as mesophyll — are formed from the periblem. The plerome of the stem takes no part in the development of the leaf. The growth of the leaf itself, unlike that of the stem, goes on cliiefiy at the base, the lower part of THE WALLFLOWER 85 the leaf continuing to develop long after the apex is fully formed. - We see that in each leaf-trace bundle that part which belongs to the stem is formed from the plerome, while that which belongs to the leaf arises from the periblem. The part at the node where the vascular bundle bends out towards the leaf is generally the first to be formed. From this point the differentiation of the vascular tissue proceeds upwards into the leaf and downwards into the stem. When a branch arises in the axil of a leaf, it is formed, like the leaf itself, entirely from the more external tissues. The branch is developed a little later than the leaf, and rather more layers of the periblem take part in forming it. But here again the plerome is not concerned in the process. (b) The growing •point and branching of the root. — The root, like the shoot, forms its new tissues at the apex of each branch, but as the structures to be produced are different it is evident that the meristem which produces them must be different also. In roots there are no leaves to be formed, and, on the other hand, the root-cap has to be pro- vided for. Hence arise the chief differences between the growing points of shoots and roots. It is obvious that we must here seek the meristem, not at the extreme end of the root, but inside the root- cap. In a root like that of the Wallflower the meristem of the apex consists of three layers (see Fig. 33). The innermost forms the plerome, which here, as in 86 STRUCTURAL BOTANY the stem; gives rise to the vascular cylinder. The next outer layer is the periblem, which develops into the; cortex; the pericycle is formed from the outer- most layer of the plerome ; and the endodermis from the innermost layer of the periblem. So far there is no difference from the meristem of the shoot. The outermost part of the meristem gives rise at once to the piliferous layer and to the root-cap. We may call it the calyptrogen, or cap-form- ing layer. Its cells divide not only by walls at right angles to the surface, but also in the direction parallel to it. The cells which are thus cut off towards the outside of the calyptrogen go to form the root-cap, those cut off towards the inside form the piliferous layer, which always remains one cell thick. As regards the differentiation of the various tissues from the meristem, there are a few points to be noticed, though, on the whole, what has been said of the stem applies here also. The cells of the root-cap remain living for a time, and grow larger. The oldest of them, however, which lie farthest towards the outside, gradually die off, and are replaced by new cells formed from the calyptrogen. The other tissue formed from the calyptrogen, namely, Fig. 33. — Young branch-root of Sin apis (allied to Wallflower) in longitudinal section, c, calyptro- gen ; p, periblem ; pi, plerome. (After Van Tieghem.) Magnified about 250. THE WALLFLOWER 87 the piliferous layer, is remarkable for the great growth in length of those of its cells which form the root-hairs, organs of which we have already learned the high physiological importance. The development of the cortex calls for no remark. The plerome here, as in the stem, gives rise to procambial strands, which in the root occupy much the greater part of the young vascular cylinder, as there is only a little conjunctive parenchyma between them, and no pith is present. The external part of the conjunctive parenchyma forms a single-layered pericycle, which extends all round the cylinder. In the root each procambial strand gives rise to xylem or to phloem only. The branching of the root is very characteristic of that organ, and quite different from that of the stem. All the branches arise endogenously — that is to say, they are formed from deep-seated tissues, and have to make their way through the external structures in order to reach the surface. In the Wallflower, and indeed in all flowering plants, the pericycle is the layer from which the branch-roots are developed. The position in which the branch-roots or rootlets are formed has a definite relation to the xylem. In the Wallflower and its nearest allies the rootlets usually form four rows, which are so placed that one lies a little to the right, and another a little to the left, of each of the two protoxylem strands of the parent root. Sometimes, in place of the four rows, there are only two, which are then situated exactly opposite the two protoxylem strands. The develop- 88 STRUCTURAL BOTANY ment of the rootlets begins fairly early, but not so close to the apex as is the case with the appendages of the stem. It begins, however, before the central vessels of the xylem-plate have become lignified. A group of pericyclic cells, situated (in the more usual case) a little on one side of one of the protoxylem strands, begins to elongate radially, the middle cell elongating most. All these cells then divide by tangential walls. The inner cells thus cut off go to form the plerome of the rootlet (see Fig. 34, pi). The outer layer divides again (with the exception of the cells lying at the extreme edge of the group), and of the two layers thus formed the more internal gives rise to the periblem (p), while the outermost becomes the calyp- trogen (c), producing the piliferous layer and root-cap. We thus have the three meristematic layers marked off from the first. They continue to grow and divide in the usual manner. The young rootlet grows out through the cortex of the parent root, the cells of which are not mechanically pushed aside, but are actually digested by the rootlet, and their substance absorbed by it as food (see Fig. 34, d). Eventually the piliferous layer is reached and absorbed, and then the tip of the rootlet becomes free and penetrates into the ground. We see that the origin of the branches of the root is just the converse of that of the leaves and branches of the stem. In the latter all the tissues of the leaf or branch are formed from the dermatogen and periblem, the plerome taking no part in the process ; in the root it is the plerome alone which forms the 89 THE WALLFLOWER Fig. 34. — A, Diagrammatic transverse section of a diarcli root, to show the position of rootlets. B and C, Transverse section of root of Hespcris (allied to Wallflower), to show a rootlet at two stages of development: r.h, root-hairs; cc, cortex; d, cells in process of absorption ; en, endodcrmis ; pc, pericycle ; co, conjunctive tissue ; ph, phloem ; g, cambium ; x, xylem ; c, calyptrogen of rootlet ; p, its periblem ; pi, its plerome. (After Van Tiegliem.) B and C magnified about 250. 90 STRUCTURAL BOTANY branch. It is clear that the rootlet, owing to its endogenous origin, has the advantage of a more pro- tected position from the first — a protection which it needs, as its delicate tissues would otherwise be exposed to injury by contact with the soil. The endogenous mode of branching is one of the most constant characters by which roots may be dis- tinguished from stems. Secondary Growth in Thickness The stem and root of the Wallflower, like those of most members of the class to which it belongs, go on increasing in thickness as long as the plant lives. This is due to the activity of the cambium, the origin of which, in the vascular bundles themselves, we have already described. It is impossible to draw a sharp line between procambium and cambium ; for the latter, as we have seen, is simply the permanently active middle part of the procambial strand ; but it is to be noticed that while, in the earlier stages of develop- ment, the cell-divisions in the procambium take place indiscriminately in all directions, after a time they become very regular, nearly all the new walls being formed in the tangential direction, i.e. parallel to a tangent drawn to the surface of the stem, while radial walls are formed at long intervals only. This regular tangential division is characteristic of the cambium as distinguished from the procambium. The cambium of the bundle, then, goes on growing and dividing without limit ; on its inner side it cuts off cells, which, after one or two further divisions, THE WALLFLOWER 91 gradually become converted into vessels, parenchyma, or fibres of the wood. In the same way the cells cut off on the outer side of the cambium divide once or twice, and then become sieve-tubes, companion-cells, or parenchyma of the bast. The amount of wood formed on the inside is much greater than the amount of bast formed on the outside of the cambium. Now, this production of new xylem and phloem only adds to the size of each individual vascular bundle. In an old Wallflower stem, however, we find that the vascular bundles are no longer isolated, but there is a thick continuous zone of wood, surrounded by a narrower zone of bast. How is this brought about ? It depends on the fact that the cambium does not remain isolated within each bundle, but becomes united to form a complete ring all round the central cylinder of the stem. When the stem is quite young, we find the vascular bundles separated from one another by radial bands of conjunctive parenchyma — the primary medullary rays (see Fig. 20, if). There is already an active cambial layer between the xylem and phloem of each bundle, and soon the infection of division spreads to the neighbour- ing cells of the ray parenchyma, which also begin to divide up by tangential walls. Thus a layer of cambium is formed across each primary ray joining the cambiums of the bundles on either side, and so the whole central cylinder comes to be encircled by a zone of actively dividing cells, producing new wood internally and new bast externally. The cambium within the vascular bundles, derived from the original 92 STRUCTURAL BOTANY procambial strand, is called the fascicular cambium ; that between the bundles formed by division of the cells of a primary medullary ray is the interfascicular cambium. The interfascicular wood, i.e. that formed by the parts of the cambium between the bundles, consists at first chiefly of fibrous cells and parenchyma, but farther to the outside vessels are formed here also (see Fig. 35). The limits between the original bundles can, however, always be traced, for the cambium forms a radial band of parenchymatous tissue two or more cells thick, which extends right through from pith to pericycle ; this is called a principal medullary ray. It corresponds in position with a primary ray, and forms, as it were, the continuation of it, but is much narrower, for the cambium only forms the ray parenchyma just opposite the middle of the primary ray. The cells of the ray are short, with square ends ; their walls are thick and pitted (see Fig. 36, mr)\ they contain abundant starch, and are easily distinguished by their form from any other cells of the wood. The part of the ray which is formed on the outside of the cambium broadens out, its cells becoming both larger and more numerous. Between the principal medullary rays there are shorter secondary ones (Fig. 35, s.m.r ), which are started by the cambium after growth in thickness has gone on for some time, and which therefore do not extend either inwards to the pith or outwards to the pericycle. The secondary as well as the principal rays broaden out in the phloem. As the wood constantly increases in thickness, it is evident that all the tissue outside it THE WALLFLOWER 93 must become stretched. This stretching is partly pro- vided for by the tangential expansion followed by radial division of the cells forming the phloem rays, so that the phloem itself does not get distorted. The cells of the secondary wood and bast are essen- e Fig. 35. — Diagrammatic transverse section of part of an old stem of Wallflower, including three of the original bundles, e, epidermis ; co, cortex ; pd, periderm ; pr.ph, primary phloem ; pli, phloem ; c, cambium ; sec, secondary xylem ; pr.x, primary xylem ; p.m.r, a principal medullary ray ; s.m.r, a secondary medullary ray ; p, pith. Magnified about 20. (R. S.) tially like those of the primary tissues. The chief forms of xylem elements are shown in Fig. 36. The cambial cells are elongated prisms with inclined ends. Where a medullary ray is being formed, how- 94 STRUCTURAL BOTANY ever, the cambium has short, square-endec! cells ; and when a new secondary ray is to be started, the cambial cells first divide up by transverse walls ; so that in this case the form of the cambial cells is Fig. 36. — Part of a radial section through the secondary wood of Wallflower, p, xylem parenchyma; v, vessel; f, fibrous cell ; mr, medullary ray. The arrows M and C point respectively towards pith and cortex. Magnified 220. (R. S.) similar to that of the elements which they produce. This, however, does not always hold good, for the vessels and fibres of the wood undergo considerable THE WALLFLOWER 95 changes during their development from the cam- bium. The vessels have a much greater diameter than the cambial cells, while the fibres are more elongated. If we think the matter over carefully, we shall see that these changes of dimensions in particular cells cannot go on without some disturbance of the original arrangement of the cells. Take a vessel, for instance : as its diameter increases, it has to find room for itself among the surrounding parenchymatous and other cells which do not grow much in diameter. To do this it has to displace its neighbours, and event- ually it comes into contact with many more cells than it touched when first formed from the cambium. Next consider the excessive growth in length of the fibrous cells ; each of these when full grown is about twice as long as it was in the cambial condition. In attaining its full length it has to insert its pointed growing ends between its neighbours, and as they, if they are fibrous cells, are all doing the same thing, the ends of these cells have to grow past each other (see Fig. 36,/). All this may happen in a part of the stem which has long ceased to grow in length, so that all the room required must be found by the mutual displacements of the growing cells. To all changes of this kind in which developing cells grow past each other, whether in transverse or longitudinal direction, the name of sliding growth is applied. The meaning of the term is, that the walls of these cells slide upon each other as they grow. 96 STRUCTURAL BOTANY Periderm We know that the young stem is covered by the epidermis ; the old stem, however, is coated by a greyish bark, much thicker and harder than the epidermis ; this bark splits lengthways here and there ; as the stem grows thicker, the cracks widen and expose the softer tissues below, until these too become coated over by new bark. If we examine the bark of the Wallflower micro- scopically, we find that its outer layers consist of the epidermis and the whole of the cortex in a dried and withered condition (see Fig. 35, e and co ). The inner part of the bark is formed of closely-packed cells, which are arranged in regular radial rows, and which have their walls corky or suberised. These walls have undergone a similar change to the cuticularisa- tion of the outer epidermal cell-wall, and so the corky tissue has become almost impermeable to water or watery vapour, just as is the case with the ordinary corks used in bottles, which are made of this tissue, obtained from the Cork Oak ( Qucrcus suber). The cortex, which lies outside this corky zone, is thus completely cut off from all supplies of water and food from within, and consequently withers and dies. How we will see how the bark is formed. The pericycle, which encloses the whole vascular cylinder, is at first one cell thick. Soon after secondary thickening has begun, however, its cells divide, so that it becomes three or four cells in thick- ness. These divisions take place rather irregularly. THE WALLFLOWER 97 Then the outermost layer of the pericycle begins to divide with great regularity by tangential walls (see Fig. 37, pd), so that a cambial layer is formed, which, like the true cambium, extends all round the stem. This is called cork cambium or phellogen. It produces cells, both on its inner and outer side. Those cut off towards the inside retain their living contents, and have rather thick cellulose walls. There are several layers of these cells in the Wallflower; together they constitute the phelloclerm. The cells cut off on the outside of the phellogen have their walls suberised and their contents perish. These cells form the cork. The whole of this second- ary tissue, including phel- loderm, phellogen, and cork, is called the periderm. The word bark is applied to everything outside the phellogen, and so here it includes the corky layers, together with the endodermis and the rest of the cortex and the epidermis. The bark consists entirely of dead tissues, for the corky walls allow of no absorption of water or food-materials. It is, however, of great importance as a protection to the living tissues within. Almost all plants with secondary growth in thickness form periderm and bark through Fig. 37. — Part of a transverse section of the stem of Wall- flower, to show the outermost layer of the pericycle dividing up to form the periderm, c, cortical cells ; e, endodermis ; pd, phellogen, arising by divi- sion of pericyclic cells. Mag- nified 460. (R. S.) 98 STRUCTURAL BOTANY the activity of a phellogen, which, however, does not always arise from the pericycle. The bark of our trees and large shrubs arises essentially in the same way as that of the Wallflower, though there are many differences of detail. Secondary Thickening in the Root The root, like the stem, grows indefinitely in thick- ness, owing to the formation of new tissues from a cambium. This secondary growth begins very early in the root, just after the stage shown in transverse section in Fig. 28, so that in the older roots much the greater part of the tissue is of cambial origin. The first formation of the cambium in the root is rather different from that in the stem, owing to the different arrangement of the primary tissues. It has already been mentioned that the two phloem groups are separated from the xylem by parenchy- matous cells belonging to the conjunctive tissue. It is in these cells that the tangential divisions begin. The cambium, consequently, at first forms two bands of actively dividing cells, lying on either side of the xylem-plate and immediately within the two phloem groups (see Fig. 34, 0, g). These cells give rise to new xylem on the inside and new phloem on the outside in the usual way, but the cambium does not as yet form a complete ring. It is completed in this way. The pericycle, which was at first a single layer of cells, divides and becomes two or more cells in thick- ness, at least at the places opposite the two protoxylem groups. The innermost of these pericyclic cells, i.c. THE WALLFLOWER 99 those lying next the protoxylem, now undergo regular tangential divisions, and so form the continuation of the cambium round the ends of the xylem-plate. It is a constant rule in plants of the class to which the Wallflower belongs, as well as in plants of the Fir kind, which belong to a different class, that the forma- tion of the cambium in the root begins by division of the conjunctive cells inside the phloem groups, and is completed by division of the pericyclic cells outside the xylem groups. Thus the cambium (as seen in transverse section) cannot at first form a perfect circle, for it has a depression opposite each phloem bundle ; but just at these places the cambium starts its work earliest, and is for a time most active, so that the inequalities soon become equalised, and we have a circular zone of actively dividing cells just as in the stem. When once started, there is nothing peculiar about the cambium of the root ; it forms internal wood and external bast just as it does in the stem, but as in this root there is no pith, it is evident there can be no principal medullary rays. One point deserves special attention : the primary xylem of the root develops, as we have seen, from without inwards ; the secondary xylem is formed by the cambium from within outwards, so that in roots we have a sharp distinction between centripetal primary and centrifugal secondary wood, whereas in the stem all the wood arises centrifugally. However old the Wallflower root may be, we can always tell its transverse section from that of the stem, for in the middle, instead of the pith, we find the little diarch xylem-plate unaltered. In 100 STRUCTURAL BOTANY the roots of other plants of the same class there are often more than two groups of xylem, and a pith may even be present ; still, if we can once make out the position of the protoxylem elements, we can always be certain whether it is a root or a stem with which we have to do. Only very young parts of the root retain their root- hairs, and absorb food from the soil. The older parts soon lose their hairs, cease to be absorptive, and now serve only to conduct the food, which the younger part of the root has taken up, towards the stem. These older parts of the root acquire a covering of periderm. The pericycle divides up to form a phellogen, which produces phelloderm internally and corky layers externally, just as in the stem.1 It is from the outer layer of the pericycle (where it is double) that the phellogen arises. We now see that the pericycle in these roots is an extremely important layer, being the seat of three distinct new formations — (1) Lateral roots. (2) Cambium (in part). (3) Phellogen. In consequence of the formation of cork by the phellogen, all the external tissues, from the endodermis outwards, wither away, so that the older roots consist entirely of the central cylinder surrounded by peri- derm. A curious effect of this throwing off of the cortex, which takes place very early in roots, is that 1 In the case of roots the origin of the phellogen from the pericycle is the rule, while in stems it is exceptional. THE WALLFLOWER 101 they appear thinner, after secondary growth in thick- ness has begun, than they did before. This is becanse the root at first loses more by the death of its external tissues than it gains by secondary growth. The loss, however, is soon made good. Now let us consider what purpose all this secondary formation of new tissues serves. So long as a plant like the Wallflower lives, it keeps putting out new branches ; every new branch bears a number of leaves, and so the older the plant grows the greater is the total surface of its foliage, in spite of the loss of old leaves. Now the leaves perform two main functions : they give off watery vapour through their stomata, and thus keep up the ascending current of sap from the roots ; and also they take up food from the air, namely, the carbon which they obtain by the de- composition of carbon dioxide. Therefore the greater the leaf-surface, the greater will be the amount of water which has to pass up the stem from the roots, and the greater also will be the amount of assimilated food which has to be conveyed away from the leaves to the growing parts. Consequently, as the leaf -surface increases, it is necessary that the amount of conducting tissue should increase also, and this applies both to the wood, which conducts the sap from the roots to the leaves, and to the bast, which conducts the assimilated foods from the leaves to the seats of growth. This increase is provided by the activity of the cambium, and while this is going on the absorptive surface of the root is also increasing. The root branches again and again, and the number of young 102 STRUCTURAL BOTANY roots that are able to take up food from the soil is constantly becoming greater. Thus we see how the growth of all the vegetative organs of the plant works together. As the tran- spiring and assimilating surface of the leaves increases, the amount of conducting tissue in the stem and older roots increases at an equal rate, while at the same time the branching of the younger roots pro- vides for the greater demands on their absorptive capacity. Secondary growth in thickness is important in other ways also : as the branching of the shoot goes on, it is obvious that the stem has an ever-increasing load to bear, due not merely to the increased weight, but also to the greater leverage of the spreading branches. Hence the necessity for increased mechan- ical strength, which is provided for by the formation of additional wood from the cambium, and especially by its fibrous elements. Lastly, the constant formation of new tissues has the further advantage that old elements which have become past work are replaced by young and active ones. This applies especially to the phloem, in which it is generally the newer layers which are alone active. This point, however, is not so important in the Wall- flower as in some other plants, especially trees, in which the duration of life is longer. Secondary growth, then, has three main functions : to increase the conducting tissues, to increase the mechanical tissues, and to replace old elements by young ones. THE WALLFLOWER 103 III. INTERNAL STRUCTURE AND DEVELOPMENT OF THE REPRODUCTIVE ORGANS We use the term “ reproductive organs ” here in a wide sense, to include all the parts of the flower, although many of these are only of indirect service in reproducing the plant. We do so because the sole function of these organs is to form a sexually produced embryo or young plant, and to send it into the world as well equipped as possible for its future life. The floral organs are only of service in so far as they con- tribute to this end. We have already learnt the general arrangement and outward appearance of the parts of the flower (see p. 18) ; we will now consider the order and method of their development. a. Development of the Flower The very young flower-bud arises from the stem as a rounded outgrowth, which soon acquires a short stalk (see Fig. 38). The calyx is the first part to appear. The anterior sepal takes the lead, appearing as a crescent-shaped ridge of tissue (see Fig. 38, A, s“), next the two lateral sepals (s') appear together, and lastly the posterior one, which lies next the mam flower- stalk (B, sp ) (see also diagram, Fig. 13). When the calyx has started its growth, the corolla begins to appear. All the four petals show themselves simultaneously, arising as small papillae immediately within the calyx at the points corresponding to the spaces between the sepals (see Fig. 38, C , pa and pv). 104 STRUCTURAL BOTANY The petals have no sooner appeared than their growth ceases for a time. Hence we shall find that the next inner whorl, that of the stamens, soon overtakes them. The stamens appear in the same order as the sepals, opposite which they arise. First an outgrowth is formed opposite the anterior sepal (Fig. 38, D, si ®). It immediately forks into two, and gives rise to the anterior pair of long stamens (E, eta). Next two papillae are formed opposite the two lateral sepals. These develop into the two short lateral stamens (D and E, stl ). Finally a posterior papilla is formed, which like the anterior one immediately branches and produces the posterior pair of long stamens (D and E, stp and et). The pistil is the last part of the flower to be formed. The two carpels appear as two slightly elevated ridges opposite the lateral sepals ; they are only distinct at their very first origin. A zone of tissue underneath their insertion begins to grow up immediately, carrying the carpels up with it, and forming a tubular column in the centre of the flower, on the upper edge of which the original carpels can scarcely be distinguished (Fig. 38, E, cp). On the inside of the earpellary tube two longitudinal ridges arise ; the position of these ridges is posterior and anterior, i.e. they come midway between the centre-lines of the two carpels (F and G). These ridges are the placentas, on which the ovules or young seeds are borne. Two elevations soon appear on the edge of the earpellary tube just above the placentas. These elevations become the stigmas (F and H, sg), and ultimately close in over the top of the THE WALLFLOWER 105 o' 02 r-n co a PV* & «3 F cd ■73 02 P . O ?h *r! i S o CP CD •" EL s ...S*g S P+J CO ,D g p a O -p D S © 02 o qs t3£) ~ 3*Pf O -- p*> f-t _5 CD «■ ^ C3 p, ■ o|o L" o 02 v-< i— v_/ r ^ CD 02 ^swij 5 r •SPsJ «-2 . 1 45 P— ) 02. 2^° g'£=~ x - 2, cl, o e .A'- s ■5 d ^ jS fto g C) CD .. 2., a/ X 02 CD P-1 f-t P . 02 S'g-S .- § g O -, <« ^r-j ^ ~--2 a <12 co Q2 Co rP , _ Ph 02 l— t f. r*-H p O A 0 P-.S Ch 02 f-i O 4J 02 02 P. -P 03 s p p H o 02 lii! ■75 P +J M £ „ ° & CQ 02 " p "2 1 ** O P • Q2 CO rrt - co 72 « *02 c5 106 STRUCTURAL BOTANY tube, which now forms the cavity of the ovary (see Fig. 38, H). We see, then, how it is that the position of the stigmas is alternate with that of the carpels (see Figs. 10, 11, 12, and 13). The septum appears rather late after the development of the ovules has begun (Fig. 38, G-, pi). Each placenta grows out between the rows of ovules ; the two pro- jecting walls thus formed at last meet in the middle to form the septum dividing the cavity mto two. The honey -glands, which he at the base of the two shorter lateral stamens, arise very late in the development of the flower, when all the more important organs are well advanced. All floral leaves arise like other leaves, by the growth and division of the more external cells of the growing point. b. Structure of the Floral Envelopes We will now consider more fully the structure of the various organs of the flower, and the respective parts which they play in the process of reproduction. We will first consider the structure of a sepal. The sepals form the outer protective covering of a flower while it is still a bud. When the flower opens, the function of the calyx in the Wallflower is almost at an end. Of all the floral organs the sepals are the most obviously leaves. They are, hr fact, merely leaves of simplified structure, having less complex functions to perform than the vegetative leaves. The sepal has a similar bundle-system to that of THE WALLFLOWER 107 an ordinary foliage-leaf, but with simpler ramifications. In each bundle the xylem and phloem are in the usual position, the xylem being placed on the upper side, i.e. on the side turned towards the middle of the flower, and the phloem on the lower side. The mesophyll is of uniform parenchymatous structure throughout, showing no differentiation between palisade and spongy tissue. The cells contain numerous plastids corresponding to the chlorophyll granules, but little or no chlorophyll is present. The dark colour of the sepal is due to purple cell-sap, which is present in the more superficial layers on both the upper and lower sides of the sepal. It is most abundant towards the lower surface, and especially in the hypodermal layer, though it also occurs in the epidermis. Stomata are fairly numerous on both surfaces of the sepal. Hairs of the structure already described and figured (see Tigs. 26 and 27) are pro- duced on the lower or outer surface only. There is reason to believe that they serve to protect the young flower-bud from the attacks of slugs. The petals, which form the next floral whorl, are in many respects of very different structure. The petals are at first very slow in their development, and become mature rather suddenly, shortly before the flower is ready to open. They are transitory struc- tures ; they have no protective function, nor do they take any direct part in fertilisation or the production of the embryo, but they are none the less important. The Wallflower is in a great degree dependent on the visits of insects for its fertilisation. The function of 108 STRUCTURAL BOTANY the petals is to render the flower conspicuous, and thus to help in attracting insect visitors. Hence it is that it is generally the corolla which constitutes the chief beauty of the flower in our own eyes. Its business is to be showy. The petal has a vascular system ; its bundles, though slender, are of normal structure, with xylem above and phloem below. The mesophyll consists entirely of spongy parenchyma without chlorophyll. The epidermal cells of both surfaces contain very numerous yellow chromoplastids — that is, protoplasmic bodies like chlorophyll granules, but containing a yellow pigment instead of chloro- phyll. To these the yellow colour of the petals is due. In cultivated Wallflowers a purple cell-sap is also present, chiefly in the lower epidermis. To the combination of the colours of the yellow chromo- plastids and of the purple cell -sap the flower owes those beautiful red-brown shades which we know so well. The wild Wallflower generally has the chromo- plastids only, and so its petals are simply yellow. There are no stomata on either surface of the petal. The epidermis is not smooth, but its cells all grow out into short hairs or papilhe, which are narrower and more prominent on the upper surface of the petal. They give it a peculiar velvety gloss, owing to the varying way in which they reflect the light, and this effect is perhaps enhanced by the fact that the cell-walls of the papillae are delicately ridged. The well-known pleasant scent of the Wallflower comes from the corolla. This scent is probably of importance as helping to attract insect guests. THE WALLFLOWER 109 c. Stamens The six stamens together constitute the andrceceum , or fertilising apparatus of the flower. Each stamen, as we already know, consists of two parts — a stalk or filament, and an anther borne upon it. The stamens, like the other floral organs, are known by their development to be leaves, but their structure is much simplified in comparison with the foliage-leaf, or even with a sepal or petal. We regard them as much modified leaves, which have no vegetative functions to perform. The filament is traversed by two vascular bundles, having two small xylem groups turned towards the inner surface, and two larger united phloem groups on the outer side. The vascular tissue is surrounded by an endodermis. The meso- phyll is of uniform structure all through. The epidermis of the filament has neither hairs nor stomata, but the latter occur on the anther. The filament tapers at the top, and is attached to the anther by a narrow neck. The anther may be com- pared to the blade of an ordinary leaf, but it shows little trace of the structure of a vegetative lamina, for its functions are entirely reproductive. It is, however, traversed by a midrib (the connective), which forms a direct prolongation of the filament. The connective has a single vascular bundle, which is continuous with the two bundles of the filament (see Fig. 39, v.b). The anther, when nearly ripe, is divided lengthways into two halves or lobes, with the connective between them (see Fig. 39, also Figs. 10 and 11). 110 STRUCTURAL BOTANY Each lobe of the anther consists of two compartments, called the pollen-sacs, so that there are four of these pollen-sacs altogether, running lengthways of the anther parallel to the connective. Their arrangement, as seen in transverse section, is shown in Fig. 39. When ripe, each pollen-sac contains an immense number of isolated cells, the pollen - grains, which are set free by the Fig. 39. — A, Transverse section through a yonng anther of Wallflower, at the stage when the pollen mother-cells are dividing. Observe the four pollen-sacs and the connective. v. b, vascular bundle of connective ; f, hypodermal layer, which will become the fibrous layer ; t, tapetum of radially elongated cells ; p.m.c, mass of pollen mother- cells, derived from archesporium of the pollen - sac. Magnified 110. B, Three pollen mother-cells from a pollen-sac in A, showing three stages of division. Magnified about 450. (R. S.) (Cf. Fig. 10, st.) bursting or dehiscence of the anther. The anther bursts by two longitudinal cracks, coinciding with the partition between the two pollen-sacs of each lobe of the anther (see Fig. 40). The outer wall of the sacs is ruptured along this THE WALLFLOWER 111 line, and at the same time the partition separating the sacs is broken down (see big. 40, w), so that a single opening serves for the escape of the pollen from each pair of pollen-sacs. At the time of dehiscence, there- fore, the anther appears to have only two cavities, and we have to go back to unopened anthers to find the four pollen-sacs intact. We see, then, that the stamen is a modified leaf, Fig. 40. — Transverse section of ripe anther of Wallflower, after dehiscence. Observe that the two pollen-sacs on each side have now opened into each other by the break- ing down of the wall w between them, v.b, vascular bundle of connective ; p.s, the four original pollen-sacs ; /, fibrous layer. Magnified 44. (R. S.) bearing four pollen-sacs, which produce in their interior numerous pollen-grains. It will now be well to trace the development of the pollen-sacs, and of the pollen which they contain. The anther is the first part of the stamen to be developed, so that in a moderately young stamen we find the anther almost full-grown while the filament 112 STRUCTURAL BOTANY is still quite short. If we cut a very young anther across, we find that the transverse section is nearly oblong, with rounded corners (Fig. 41, A). B C Fig. 41. — A, Transverse section of very young anther, v, young vascular bundle ; pp, periblem cells which divide to give rise to arcliesporium and cells of wall, w. B and C, More advanced stages of the same. The cells with dark outlines belong to the four arcliesporia. The numbers 1, 2, and 3 refer to the order in which the cell-walls are formed, t, tape turn. (After Warming. ) Magnified about 200. These figures are reversed in position as compared with Fig. 39. At each of the four corners two or three of the periblem cells next below the epidermis divide by walls parallel to the surface (see Fig. 41, A, p). The inner THE WALLFLOWER, 113 cells produced by this division form what is called the archesporium, i.e. the layer from which the spores, here called the pollen-grains, ultimately arise. Each arche- sporium thus consists at first of a single strand or layer of cells running the whole length of the anther. The archesporial cells grow and divide up in all directions, and we soon have, in place of the original layer, a solid cylindrical column of tissue, consisting of large cells (see Fig. 41, B and C), which are densely filled with protoplasm, and have large nuclei. In the meantime the cells immediately outside the archesporium have undergone several divisions, so that the wall of the pollen-sac becomes about four cells thick (see Fig. 41, A, B, and C), including the epidermis, which does not itself divide. The cells which are actually in contact with the archesporium become elongated radially, and form a well-marked internal lining to each pollen-sac, called the tapUum (see Figs. 39 A, and 41 C, t). The tapetum extends all round the sac. Its cells are at first rich in protoplasm. The function of this layer is to feed the developing pollen-grains, by which it is entirely used up. To return to the archesporium : after its cells have divided a great many times, the divisions cease for a while, the pollen-sac continues growing, its cavity becomes more roomy, and so the cells derived from the archesporium become isolated from one another and round off their corners (see Fig. 39, A). These cells now bear the name of the pollen mother -cells, because each of them divides up into four pollen-grains. The division takes place as follows : at the time when 8 114 STRUCTURAL BOTANY the mother-cells begin to be isolated, the nucleus in each divides into two, and the two daughter-nuclei at once divide again, so that there are four nuclei hi the mother-cell (see Fig. 39, B). New cell-walls are formed between the nuclei, and the mother-cell is thus divided into four distinct cells, each with its own nucleus. These four daughter-cells are arranged like four cannon-balls in a heap, the whole group having roughly the form of a four-sided pyramid or a tetra- hedron.1 Each daughter-cell next forms from its protoplasm a new cell-wall of its own. The wall of the mother-cell is dissolved, and the four pollen-grains are set free. The cavity of each pollen-sac is now filled with a host of free pollen-grains, four times as numerous as the original mother-cells. The grains are at first immersed hi a half-fluid substance derived from the protoplasm of the tapetal cells, which break down during the formation of the pollen. This substance is soon used up by the pollen-grains to complete their growth. The ripe pollen-grain is somewhat oval in shape. Its wall is considerably thickened, but there are three thinner bands which run lengthways of the grain. The pollen-grain hi Fig. 42 is shown as if in cross-section, so that it appears round, with three depressions, and we see the three thin places in its wall at the depressed pohits. The rest of the wall is thick and cuticularised, except a delicate inner layer of cellulose. 1 Hence only three are seen in one plane. See the mother-cell at the top of Fig. 39, B. THE WALLFLOWER 115 The pollen-grain at first, like most other cells, has a single nucleus. This nucleus divides while the grain is ripening, and one of the two daughter -nuclei divides again. Thus in the mature pollen-grain there are three nuclei, one of which is larger than the other two ; the two smaller nuclei have risen from the second division. From other plants which are more favour- able for examination than the Wall- flower, it is probable that the protoplasm around each of the smaller nuclei forms a distinct cell, so that the mature pollen-grain thus consists of three cells. The two layers of cells next out- side the tapetum, like it, become absorbed. One of them often dis- appears even before the tapetum itself, as shown in Fig. 39, A. When the anther is ripe, the wall of each of the four pollen-sacs consists on its free outer side of one or two layers besides the epidermis. These persistent layers are distinguished by a strongly-marked spiral or netted thickening on then- membranes. These cells form the fibrous layer (see Fig. 40,/). It completely surrounds each pollen-sac, except at the point just opposite the partition which separates the two pollen-sacs in the same lobe. Here the fibrous layer is interrupted by one or two thin- walled cells. It is thinnest (one cell thick) next the observations on Fig. 42. — A single pollen-grain of the Wallflower, show- ing the three thin places in the wall, at any one of which th e pollen- tube may subsequently grow out. Observe the three nuclei, of which the largest is vegetative and the two smaller are generative. Magni- fied about 450. (R. S.) 116 STRUCTURAL BOTANY partition, and attains its maximum thickness (about five cells) on the side towards the connective (Fig. 40). This layer is of great importance in the de- hiscence of the anther. It is owing to the contraction of the fibrous layer, when the ripe anther is getting dry, that the wall is ruptured and dehiscence takes place. If the anther is kept from drying, as may happen in very wet weather, dehiscence may fail to take place. d. The Pistil The development of the pistil has already been shortly described. The tissue of the carpellary tube is traversed by several vascular bundles, the largest of which are placed opposite the septum. The external epidermis is thickly clothed with hairs of the usual structure ; there are a few stomata. The pistil terminates above in the stigma, the two lobes of which are rough on their inner side, with long papillae, which, as we shall see presently, serve to catch the pollen-grains. The ovary contains the ovules, or young seeds (see Figs. 10, 12, 38 G). They are numerous, and grow in four rows, two rows to each compartment, on the placentas. Each ovule is attached to the placenta by a stalk, the funicle. The body of the ovule consists of a central mass of tissue, the nucellus} continuous with the funicle, and 1 In tlic ovule of Wallflower, when ready for fertilisation, the nucellus is only represented by the embryo-sac (e) and a small mass of tissue at its base (n), Fig. 43. THE WALLFLOWER 117 of two coats, the integuments, which arise from the lower part of the nucellus and enclose it, leaving only a narrow passage, the micropyle, leading down to the free end of the nucellus. The funicle contains a vascular bundle, branching off from the bundle- system of the ovary. The bundle terminates at the Fig. 43. Longitudinal section through an ovule and part of the ovary wall of Wallflower, ov, ovary wall ; li, a hair ; /, funicle of ovule ; ch, chalaza ; n, nucellus (which is only persistent at the base) ; in-,, inner integument ; w2, outer integument; m, micropyle: e. e. embryo-sac • base of the nucellus (see Fig. 43, ch). The part of the ovule where the integuments spring from the nucellus is called the chalaza. All these parts, except the vascular bundle, consist of delicate parenchyma. In the nucellus is a long cell, the embryo-sac (see Fig. 43, c), which is much larger than its neighbours, tn 7 118 STRUCTURAL BOTANY and contains in its interior a number of smaller cells. One of these, situated at the end of the sac towards the micropyle, is the ovum (Fig. 44, 8 and 9, o), the most important part of the whole structure, because when fertilised it develops into the embryo, or young plant of the next generation. The Wallflower has the peculiarity that a number of embryo-sacs, sometimes five or six, are usually formed in the same ovule. This is quite an exceptional character, and it will be more instructive to describe in detail a typical case, in which only one embryo- sac is formed. The development of the ovule will therefore now be traced in the Shepherd’s Purse (Capsella Bursa -pasto ris) , a common weed allied to the Wallflower, which will show us all the essential points. Each ovule arises as an outgrowth from the placenta. Several cells belonging both to the epidermis and to the next lower layer, take part in its formation. This outgrowth is the nucellus of the future ovule. When it has attained the form shown in Fig. 44, 1, that cell of the middle row which lies next below the epidermis (e.m in the Fig.) begins to be distinguished from its neighbours by its larger size and more abundant protoplasm. This cell is termed the arclicspormm. It divides by two transverse walls into a row of three cells (see Fig. 44, 2 and 3). The lowest of the three becomes the embryo-sac (Fig 44, 3, c). The two sister- cells above it become completely destroyed (see Fig. 44, 4), and are, in fact, devoured by the growing embryo-sac. THE WALLFLOWER 119 In the meantime the integuments have begun to form ; the inner one arises first, growing out as a ring of tissue at the base of the nucellus, and forming a sheath around it, which soon projects beyond its free end and almost encloses it (see Fig. 43, in{). The outer integument next appears in like manner, just below the inner one, which it soon overtakes by its more rapid growth (see Fig. 43, in,,). Ultimately the outer integument closes in over the inner, leaving only the narrow micropyle as a passage leading through them both to the nucellus (Fig. 43, m). In the mean- time the funicle has been formed by the growth and division of the cells at the base of the ovule (Fig. 43,/). Returning to the embryo-sac, we find that it not only absorbs its sister-cells, but, as it continues to grow, soon attacks the epidermis of the nucellus. This also is quickly digested, so that only the basal part of the nucellus, lying below the embryo-sac, remains (Fig. 44, 4 and 5 ; Fig. 43, n ), while the sides and top of the sac come into direct contact with the inner integu- ment (see Fig. 43). While these changes are going on, the nucleus of the embryo-sac divides, and the two daughter- nuclei travel to each end (see Fig. 44, 5). Each divides again (see Fig. 44, 6), and the four nuclei thus formed divide once more, so that there are now eight nuclei altogether in the embryo-sac, four near each end. So far the nuclei only have divided, so that all the eight lie free in the protoplasm of the sac. Now, however, the protoplasm at each end divides into three 120 STRUCTURAL BOTANY distinct cells, each surrounding a nucleus. Thus we have six cells in the embryo-sac, three at each end, and also two free nuclei, which lie in the undivided protoplasm which occupies all the middle part of the sac (see Fig. 44, 7). The three cells at the end towards the micropyle form what is called the egg- apparatus (Fig. 44, 7, e.a). Of these three the one farthest from the micropyle is the ovum (Fig. 44, 8, o). The two others are termed the syncrgidce (s, s), because they co-operate, as we shall see, in bringing about the fertilisation of the ovum. The three cells at the opposite end of the sac are called from their position the antipodal cells (see Fig. 44, 7 and 8, an). In this particular case the antipodal cells disappear before fertilisation (Fig. 44, 9). Lastly, the two free nuclei are called the polar nuclei (see Fig. 44, 7, px and p2, and Fig. 44, 8, p), because one comes from each pole of the sac. Their fate is remarkable. They do not remain in their original positions, but travel through the protoplasm to meet each other (see Fig. 44, 7 and 8). When they meet they fuse together into a single large nucleus, which may be called the secondary nucleus of the embryo-sac (see Fig. 44, 9, pp). Meanwhile the whole embryo-sac has increased considerably in size, and so has the egg-apparatus, the two synergidse becoming long and pointed (see Fig. 44, 9, s, s). The body of the ovule during its development becomes considerably curved on itself, owing to the more rapid growth of its lower side, so that the micropyle ultimately comes to point almost backwards Fig. 44. — Development of ovule and embryo-sac of Shepherd’s Purse (Capsella Bursa-pastoris). The integuments are not shown. 1, Early stage : e.m, archesporium or embryo-sac mother-cell. 2, The mother- cell has divided once. 3, A second division has taken pilace. 4, The lowest of the three cells, e (the embryo-sac), is obliterating the two above it. 5, The nucellus disappears, except at the base ; the nucleus of the embryo-sac has divided. 6, Each daughter-nucleus divides again. 7, Four nuclei at each end of sac ; three at each end have cells around them : e.a, egg-apparatus ; pu p2, upper and lower polar nucleus ; an, antipodal cells. 8, The polar nuclei approach each other ; s, s, synergidte ; o, ovum. 9, Fertilisation stage ; pp, secondary nucleus, arising from fusion of polar nuclei. Magnified about 200. (After Guignard.) 121 122 STRUCTURAL BOTANY towards the placenta. The embryo-sac takes part in this curvature (see Fig. 44, 9, and Fig. 43). Ovules in which the nucellus and embryo-sac are bent in this way are called campylotropons. The structure of the ovule, then, when ready for fertilisation, is as follows : it is attached to the placenta by a stalk, the funicle ; the curved body of the ovule is enclosed by the two integuments, which leave a narrow opening at the free end, the micropyle ; the interior of the ovule is almost entirely occupied by the long curved embryo-sac, which is in immediate contact with the inner integument. The rest of the tissue of the nucellus has disappeared except at the base. Within the embryo-sac, at the micropylar end, are three cells, the egg-apparatus. The most deeply- seated of the three is the ovum. In the protoplasm of the sac is a large nucleus, which has been formed by the fusion of the two polar nuclei. W e shall prob- ably find no trace of the antipodal cells, which at an earlier stage occupied the chalazal end of the sac. The description just given, though taken specially from the Shepherd’s Purse, applies to the ovules of the Wallflower also, except that the latter usually has several embryo-sacs side by side. Only one of them, however, contains an ovum capable of being fertilised. Fig. 43 is drawn from an ovule in which only one embryo-sac is visible. e. Fertilisation We have next to learn how fertilisation is brought about. The ovum alone is quite incapable of any THE WALLFLOWER 123 further development. It can only become an embryo or young plant after union with another cell, namely, one of the generative cells formed from the pollen- grain. The contents of the pollen-grams, therefore, must in some way be conveyed to the ovules in the ovary, in order that fertilisation may take place. The first stage in this process is that the pollen- grains have to be deposited on the stigma. This is called pollination, and is a necessary preliminary to fertilisation. It is easy to prove experimentally that pollination is necessary for the production of ripe seeds, i.e. seeds which contain embryos. In order to prove this, the stamens of some of the flowers are cut off before the anthers have dehisced to shed their pollen. It is, of course, essential to do this in good time, and the flower-buds must be opened artificially in order to perform the operation, for as soon as the flower opens naturally the pollen is shed. The plant must be kept under a bell-glass or under very fine gauze, so that no insects may reach it which might bring pollen from other flowers. There is no danger of pollen being brought by the wind, as the Wallflower pollen is too sticky to be blown away. It is therefore better to use gauze than a bell-glass, as the plant might become unhealthy if kept from the air. It will be found that in all the flowers which have had their stamens removed, and which are not allowed to receive pollen from other wallflowers, the pistils wither away without setting their seeds, and if we examine their shrivelled ovules we find that they contain no embryos. We can prove that it is not the 124 STRUCTURAL BOTANY mere “ shock; ” of the operation which prevents the seeds from ripening, for if pollen from another flower be placed on the stigmas of the mutilated flowers their seeds will ripen in due course, and when they are sown the embryos will grow into young plants as usual. We learn, then, that the access of pollen to the stigma is a necessary condition for the formation of fertile seeds. It was by experiments of this kind, per- formed about two hundred years ago, that the existence of distinct sexes in plants was first proved. We now know that all flowers are organs, the fimction of which is sexual reproduction. We are now in a position to study in detail how the process goes on. In plants like the Wallflower, which have con- spicuous, brightly-coloured, and sweet-smelling flowers, the work of pollination is chiefly performed by insects. The Wallflower is specially attractive to bees, which visit it in search both of honey and of pollen, the bee- bread, which is collected as food for the young brood. The bees are quite as useful to the Wallflower as the Wallflower is to the bees, for in the course of their visits they do it the essential service of bringing the pollen on to the stigmas of its flowers, and so ensuring fertilisation. We have already seen that there are two large honey-glands or nectaries in the flower. The honey formed by these glands collects in the pouches of the inner sepals. As the bee thrusts her proboscis down between the stamens and the pistil in order to reach the honey, her head comes into contact with the inner side of the anthers, on which a mass THE WALLFLOWER 125 of loose pollen is hanging, for the anthers are introrse, i.c. they open on the inner side. When the bee flies to another flower, it is pretty sure that some of the pollen which has stuck to her head will be brushed on to its stigma. If this happens, as it generally will, cross-fertilisation will be effected, i.e. the stigma of one flower will receive the pollen of another, and this will often happen between the flowers of different plants. The chief way in which bees and other insects are of use to flowers is in bringing about cross-fertilisation, which, as Darwin’s experiments prove, produces more and better seed than fertilisation by pollen of the same flower. In the Wallflower, however, self-fertilisation is also provided for ; the anthers of the longer stamens rise up immediately above the stigma, and some of their pollen will be almost certain to fall upon it. It is also, of course, possible that the insect may happen to brush some of the pollen on to the stigma of the same flower. Probably this does not happen very often ; for generally, as the bee works round the flower, one side of its head will touch the anthers, while the opposite side brushes against the stigma. In any case the Wallflower is sure to be cross-fertilised very often ; and if this fails, owing to bad weather and the absence of insects, there is still self-fertilisation to he relied on, which is a great deal better for the plant than no fertilisation at all. Many other flowers have much more elaborate arrangements ensuring cross- fertilisation, and rendering self-fertilisation impossible, but the simple mechanism of the Wallflower is eff ective enough. 126 STRUCTURAL BOTANY All that an insect directly does for a flower is to perform the act of pollination, i.e. to bring pollen on to the stigma. If all goes well, this act has its proper effect, i.e. the fertilisation of the ovules. Hence it is usual to speak of bees and other insects as fertilising flowers, though they are really only concerned in the preliminary process. How we will assume that the pollen has reached the stigma, whether by the agency of some insect or directly from the anther, and will see what next happens. The surface of the stigma, as we have already learnt, is coated with long unicellular hairs or papilke, between which the pollen-grains are caught. These cells secrete a slightly sticky fluid, containing sugar in solution, which the pollen-grains absorb. In consequence of this they begin to swell, and it is soon noticeable that the thin parts of their walls become bulged out. Under the influence of the food which it obtains from the stigmatic fluid, the pollen-grain begins to germinate. At one of the places where its outer coat is thin the protoplasm grows, and forms a protrusion covered by the thin cellulose membrane ; this protrusion increases in length and becomes the pollen-tube. The greater part of the contents of the pollen-gram, including all the three nuclei, pass into the tube, which continues its growth, makes its way between the stigmatic cells, and penetrates into the tissue of the style below (cf. Fig. 81, p. 181, from the Lily). It grows on among the cells of the style, and at last reaches the cavity of the ovary. Here the pollen-tube follows the placenta. THE WALLFLOWER 127 When it reaches an ovule which is ready for fertilisa- tion, the tube curves away from the placenta, and grows along the fnnicle, until its end reaches the micropyle of the ovule (Fig. 43, pt). We are at present unable fully to explain the causes which direct the growth of the pollen-tube with unerring accuracy hito the micropyle of the ovule. Eecent researches, however, make it probable that the attrac- tion is due to some substance secreted by the ovule. The tube grows on through the narrow passage, and eventually its tip reaches the summit of the embryo- sac, and meets the pointed ends of the synergidse. And now the growth of the pollen-tube has reached its limit, and its destination is attained. The length of the tube is enormous compared to the size of the pollen -grain which produced it. In the Wallflower the distance from the stigmatic surface to the lowest ovule in the ovary is about 6 mm. (-£ in.) in a straight line. The diameter of the pollen - grain is about •02 mm. The tube does not really grow straight, so that we shall be within the mark in estimating its length on the average at about three hundred times the diameter of the pollen-gram before germination. There are other plants in which the growth is ten times as great, or even more. It is evident that this enormous growth is only possible if food is taken up on the way from the cells of the style and ovary. In fact, the germinating pollen-grain lives at the expense of the tissues of the pistil, just as a parasite lives at the expense of its host. We must not, however, suppose 128 STRUCTURAL BOTANY that the whole tube is filled with protoplasm. The protoplasm with the nuclei is at all stages of growth to be found in the growing end of the tube. All the older part of the tube is almost empty. It is quite easy to cultivate the pollen-grains and make them germinate, if we provide them with an artificial stigmatic fluid. All that is necessary is to put some of the grains into a drop of sugar-solution on a glass slide, so that we can observe them under Fig. 45.— Pollen-grains of Wallflower germinating in 6 per cent, sugar solution. Observe tlie three nuclei, two of which are already in the pollen-tube. Magnified about 200. (R. S.) the microscope. For the Wallflower pollen a 6 per cent, solution of common sugar (cane-sugar) is about right. In this the grains will put out tubes which reach a considerable length, and the passage of the nuclei down the tubes can be observed. The pollen-tubes figured (Fig. 45) were obtained in this way. The details of fertilisation have not been followed in the Wallflower, which is not very favourable for the investigation, as the structures in question are small. What follows is a general description based THE WALLFLOWER 129 on observations recorded on other plants. Reference may be made to figures given at pp. 185-187 (Figs. 85, 86, 87), of fertilisation in the Lily, a plant in which all the details have been fully worked out. All three nuclei travel down the pollen-tube with the protoplasm. The vegetative nucleus, however, i.e. that which was the product of the first division (see p. 115), becomes disorganised and disappears, having nothing further to do with the process of fertilisation. The two generative nuclei alone remain, and when the pollen-tube has penetrated the micropyle of the ovule and reached the embryo-sac, these nuclei are to be found in the protoplasm at the extreme end of the tube. The cellulose wall of the pollen-tube is very soft, and allows the passage of the generative nuclei. Sometimes one, sometimes both, leave the tube. One of the two generative nuclei passes on, usually between the synergidse, and reaches the protoplasm of the ovum itself. It must be remembered that these cells have no cell-walls, and so no resistance is offered to the passage of the nucleus. Ultimately the generat- ive nucleus reaches the nucleus of the ovum. The two nuclei then fuse together into one, and fertilisation is effected. It is probable that in all cases some protoplasm passes over from the pollen-tube with the nucleus, and unites with a certain part of the pro- toplasm of the ovum. What, however, we at present know to be constant in fertilisation is the union of the nuclei of the sexual cells. This statement holds good for plants generally, and not for flowering plants alone. 9 130 STRUCTURAL BOTANY f. Development of the Embryo The effects of fertilisation are, that the ovum develops into the embryo, the ovule into the seed, and the pistil into the fruit. We will first con- sider the development of the embryo or young plant, for this is the really important process, to which the changes in the ovule and pistil are subservient. The immediate result of fertilisation is that the ovum surrounds itself for the first time with a cell- wall. Its nucleus, which, as we have seen, has been formed by the fusion of a generative nucleus of a pollen-grain with the original nucleus of the ovum, next undergoes division. This division of the nucleus is followed by that of the cell, which divides by a transverse wall into two cells. Repeated transverse divisions then go on, until we have a row of several cells (see Fig. 46, A) attached at one end to the wall of the embryo-sac near the micropyle, while the other end of the row hangs freely in the interior of the sac. The last cell at the free end is nearly spherical, and larger than the rest. The whole structure at this stage is called the pro-embryo. The round cell at the end is called the embryonic cell ; the rest of the row is the suspcnsor. From the “ embryonic cell ” the greater part, but not the whole, of the young plant is formed. This cell first divides by three walls, two longitudinal and one transverse (see Fig. 46, A and B, c), cutting each other at right angles, into eight cells, the octants. Next, each octant divides by a wall parallel to the THE WALLFLOWER 131 Fig 46. — Development of the embryo of Shepherd's Purse. b.c, basal cell of suspensor ; e embryonic group ; d, dermatogen ; h, hypophysis ; g.p, growing point of stem ; ct, ct, cotyledons ; pc, periblem ; pr, pi, cells of plerome ; s„ cells derived from the hypophysis, (After Han- stein.) Magnified about 200. 132 STRUCTURAL BOTANY outer surface into an inner and an outer cell. The eight outer cells thus cut off form the dermatogen of the embryo (see Fig. 46, C, cl). Their further divisions take place entirely by walls at right angles to the surface, so that the layer remains one cell thick. It gives rise to the entire epidermis of the shoot, and to ct no other tissue. The eight inner cells then divide in their turn, forming a central group and an intermediate layer (see Fig. 46, D). The central group be- comes the plerome, from which the central cylinder of the main stem is developed, while the intermediate layer between plerome and dermatogen becomes the periblem, from which the cortical tissues arise. The last cell of the suspensor immediately adjoining the embryonic cell also contributes something to the formation of the embryo. It divides by a transverse wall, and the daughter-cell next the embryonic cell by its further divisions forms the root- cap and periblem of the root, while the plerome of Fig 47. — Older stage of same ; the two cotyledons now project beyond the growing point, r, periblem of root; r.c, root-cap. Other letters as before. (After Iianstein.) Mag- nified about 200. THE WALLFLOWER 133 the root is formed from the same central group as that of the stem (see Fig. 47, r and r.c ). We see, then, that the apex of the root is turned towards the suspensor and the micro pyle, as is always the case in the embryos of flowering plants. The apex of the stem is placed exactly at the opposite end of the embryo. As growth proceeds, the embryo ceases to be spherical. It first becomes flattened at the top and then heart-shaped (see Fig. 47), forming two lateral protrusions. These are the cotyledons (Fig. 46, 1), and 47, ct), the two first leaves of the embryo. The growing point of the stem lies between them (y.y>). The class to which the Wallflower belongs derives its name, Dicotyledons, from the fact that its embryo always has two seed-leaves or cotyledons. The grow- ing point of the embryonic stem in this class is constantly terminal (i.e. in the middle line of the embryo at the end farthest from the micropyle), and the two cotyledons lateral (i.e. one on each side of the growing point). As the embryo attains its full size it gradually occupies the whole of the curved embryo -sac, and therefore of necessity becomes curved itself, so that the cotyledons are bent back and lie parallel to the young root or radicle. In the Wallflower the coty- ledons are so placed that their edges are turned towards the radicle (see Fig. 49) ; in this case they are said to be accumbent. This, however, is a character of no great importance, as it varies in closely-allied plants. 134 STRUCTURAL BOTANY In the Wallflower and all its immediate allies, such as the Cabbage, Turnip, Shepherd’s Purse, Watercress, and plenty of other well-known plants, the embryo in the ripe seed fills the greater part of the embryo-sac. The nucleus of the embryo-sac undergoes repeated divisions, and around the daughter-nuclei cells are formed. In this way a little tissue is produced, called the endosperm, a name given to any tissue formed thus in the interior of the embryo-sac after fertilisation. But in the case we are considering the greater part Fig. 48. — Embryo-sac of Elder ( Sam, - ag food py the grow- bucus ), which lias just become failed . J ° up with endosperm by cell-division, mg embryo before the persistent. Seeds which when ripe contain nothing besides the embryo, are called ex albuminous, because they contain no albumen, an old - fashioned name applied to the endosperm and analogous tissues from a fancied resemblance to the white of an egg. The Wallflower seed is commonly described as exalbumin- ous ; but this is not strictly true, for we know from recent researches that one layer of endosperm at least is persistent. In many plants the embryo-sac becomes completely filled with endosperm, the embryo itself occupying only a small space. Such seeds are said to of the endosperm is only a temporary structure. It never attains any consider- able size, and is to a great extent used up en, endosperm ; em, embryo. (Afl Hegelmaier. ) Magnified about 250. CAfter geec| r[p6j oniy the outermost layer being THE WALLFLOWER 135 be albuminous. Fig. 48 shows an embryo-sac filled with endosperm, from the Elder. We have now traced the development of the embryo, which is the primary and essential result of fertilisation. It remains to consider certain secondary effects of the process. The Ripe Seed We will first complete the description of the seed, which is the ripened ovule. The nucellus, as we have seen, disappears in great part even before fertilisation. By the time the seed is ripe scarcely a trace of it remains except a little group of cells at the chalaza, in contact with the tips of the cotyledons of the embryo. The integuments, on the other hand, remain (though some of their layers are obliterated), and develop into the hard seed-coat or testa, which when the seed is ripe consists of several layers of cells (see Fig. 49, t ). On the extreme outside is a layer of clear cells with very thick walls, so thick that there is no cell-cavity left at all. These walls are mucilaginous, and swell up in water, rendering the seed-coat sticky, and causing it to adhere to the soil or to anything which it touches. Beneath this are two or three layers of brown cells, more or less flattened, and on the inside of the seed- coat next the embryo is a layer of thin-walled cells, with very abundant contents, consisting chiefly of nitro- genous food-materials in the form of proteid granules (see p. 38). This innermost layer belongs to the endo- sperm, of which it is the only remnant. The outermost mucilaginous layer is the epidermis of the outer in- 136 STRUCTURAL BOTANY tegument ; the brown cells in between belong partly to one integument and partly to the other. The funicle remains as before, and its vascular bundle can still be traced in the ripe seed. The micropyle is Fig. 49. — Seed of Wallflower in section passing between coty- ledons. h, t, testa ; m, micropyle ; f, funicle ; r, radicle of embryo ; above this is the liypocotyl ; g.p, growing point of stem ; c, one of the two cotyledons. Magnified about 20. (R. S.) closed while the seed is ripening, but it still has a part to play, as we shall see presently. The ripe seed, then, consists essentially of the funicle, the testa or modified integuments, and the embryo. The whole interior of the seed is occupied by the large curved THE WALLFLOWER 137 embryo. At the end next the micropyle is the radicle of the embryo, and above this is the hypocotyl. At the upper end of the latter the two broad cotyledons are borne, and between them is the plumule, or growing point of the stem (see Fig. 49). Many seeds contain a much larger amount of permanent food- tissue, in the form of endosperm within the embryo-sac, to be used during germination, e.g. Buttercup, Foxglove, Castor-oil seed ; while in a few the nucellus is per- sistent, forming the perisperm, e.g. Pepper, Water-lily. The Fruit The pistil ripens into the fruit. In doing so it in- creases greatly in bulk, growing from a quarter of an inch to about two inches in length. Its tissues show changes of some interest. Perhaps the most important consist in the development of a layer of very thick- walled, elongated cells from the inner parenchyma adjoining the epidermis which lines the cavity of the fruit. The parenchyma lying between the sclerenchy- matous zone and the outer epidermis contains chloro- phyll. The outer epidermis is densely clothed with overlapping hairs of the usual Wallflower type, and has numerous stomata. When the fruit is ripe, it opens or dehisces. The sides of the fruit split away from the replum or persistent placenta, the severance beginning below and extending upwards. The valves thus formed remain attached at the top. The replum is left intact, with the seeds attached to it, and the septum stretched across it. 138 STRUCTURAL BOTANY The lines along which dehiscence is to take place are marked out beforehand as thinner bands in the wall (see transverse section, Fig. 50, cl, cl). A special layer of separation is formed at these places, the cells of which split apart during dehiscence, and so release the valves. Fig. 50. — Transverse section of ripe fruit of Wallflower (seeds omitted), d, cl, tlnn portions of ovary wall, where de- hiscence takes place ; r, r, septum, which is in two parts. The thick masses of tissue at the two ends of the septum form the replum. Magnified about 8. (R. S.) (Cf. Figs. 9, 10, 11.) The seeds hang on to the replum for a time and then fall, or are blown off by the wind on to the ground. Germination In the spring, when the temperature has become sufficiently high, and the weather is moist enough for the seeds to take up a supply of water, the process of germination begins. By germination is meant the development of the seedling from the embryo contained in the seed. The mucilaginous layer of the testa takes up water THE WALLFLOWER 139 in large quantities, swells into a slimy coating, and thus helps to fix the seed to the soil. Part of the water which is thus taken up is passed on to the embryo inside, and stimulates it after its long rest to renewed growth. The hypocotyl and radicle begin to elongate rapidly, and the apex of the latter is forced out through the micropyle. In whatever position the germinating seed may lie, the radicle as soon as it becomes free curves, so that its tip is directed straight downwards, and at once penetrates the soil (see Fig. 51). As soon as this has happened, the young root immediately begins to develop root-hairs, which take a firm hold of the particles of soil, and serve both for nutrition and for the fixing of the plant during its subsequent growth. Meantime the hypocotyl grows very rapidly, and curves so that the apex of the stem comes to point directly upwards. The cotyledons for a time remain imprisoned in the seed-coats, which are lifted up with them above the level of the ground, but they soon spread them- selves out flat, and in doing so strip off the seed- coat. We see, then, that in germination the root grows vertically downwards and the stem vertically upwards, while the cotyledons take an approximately horizontal Fig. 51. — Germinating seed of Wallflower, t, t, burst testa ; c, c, cotyledons ; h, hypocotyl ; r, root. Mag- nilied about 5. (R. S.) 140 STRUCTURAL BOTANY position. The apex of the stem (the plumule) now begins to develop further and forms leaves. The root at the same time begins to put out lateral branches, and we may now regard germination as completed ; the embryo has become a seedling (see Fig. 30). The food - supply necessary for germination is obtained at first from the organic substances (starch, etc.) stored up in the cotyledons themselves, and in part also from the remaining layer of endosperm, which, as we saw above, is rich in proteid granules. As soon as the root-hairs have taken hold of the soil, the seedling can begin to supply itself with mineral food, but it is only when the cotyledons have expanded that assimilation can begm and new organic substances be formed. We see, then, that in this plant the cotyledons have a double function ; they serve at first as stores of ready-made food for the seedling, and afterwards they act as assimilating leaves, and can manufacture new food on their own account. In most of the points described the germinatioii of the Wallflower may be regarded as typical of that of the Dicotyledons generally. For example, the exit of the radicle through the micropyle is very constantly the first stage in germination. The behaviour of the cotyledons, however, differs much in different members of the class. In some plants, such as the Broad Bean ( Vida Faba), the cotyledons never rise above the ground at all, but serve simply as storehouses of food, the first assimilating leaves being those of the THE WALLFLOWER 141 plumule. Iu seeds, again, which when ripe contain abundant endosperm, the first duty of the cotyledons is to absorb the food which the endosperm contains, and they do not emerge from the seed until this has been accomplished. CHAPTER II Type II THE WHITE LILY ( Lilium candidum, L.) It matters little which of the common kinds of Lily we take for our type. The following description refers in the main to the White Lily, but the general Figs. 52 and 53 are of Lilium aurdtum, and some of the other figures refer to different kinds. The White Lily grows wild in Southern Europe, Persia, and the Caucasus, and is one of the commonest Lilies cultivated hi gardens. I. EXTERNAL CHARACTERS A. Vegetative Organs In cultivation, Lilies are usually raised from bulbs. A bulb is really a large underground bud. It consists of a short conical stem, covered with densely-crowded, spirally-arranged, thick scale-leaves, which completely coat the surface, leaving no internodes between them. The base of the bulb produces fibrous roots. The bulb is that part of the plant which persists at all times, in winter as well as in summer. During the winter the 142 THE WHITE LILY 143 scale-leaves contain stores of food-material, which were laid by during the previous summer, while the green foliage-leaves were assimilating. When growth begins in spring, the short conical axis of the bulb elongates and grows straight upwards, pro- ducing the foliage- leaves, and ultimately the flowers. The White Lily is peculiar, inasmuch as it produces two sets of green foliage - leaves. One set appears in autumn, arising at the base of the rudiment- ary flowering stem of the following year. The other set is borne in summer on the tall flowering stem itself. If we examine a plant of the White Lily in autumn, after the flowering stem of the past summer has died down, we see a group of broad lanceolate green leaves, the bases of which ar< Fig. 52. — General view of flowering stems of Lilium auratum. From a photograph. Much reduced ; the plant was about 6 ft. high. underground, the stem on which they grow being hidden in the earth, These leaves we will call the ground-leaves. 144 STRUCTURAL BOTANY If we clig up the whole plant and look at it closely, we find the following structure : on the outside of the bulb there are a number of broad, thick scales, each of which has a blunt, withered tip. These scales ( b , Fig. 54) are the persistent bases of the ground-leaves of the year before. They are spirally arranged, and if we Fig. 53. — Flowers of Lilium auratum. From a photograph. Reduced. remove them one by one, we find opposite the inner- most of them the decayed base of last summer’s flowering stem (a, Fig. 54). After these scales are removed, we come upon bud-scales of another kind ; these are thick and fleshy like the former, but differ from them in having pointed and uninjured tips (c, Fig. 54). These scales are complete leaves which have THE WHITE LILY 145 never developed any further ; they also are spirally arranged. If we remove these, we next come to the green ground-leaves of the present season (d, Fig- 54). Their bases are already somewhat swollen, but not nearly so much so as they become later. Kemoving Fig. 54. — Bulb of White Lily in autumn, r, roots ; b, blunt bulb- scales formed from bases of old ground - leaves ; a, remains of old flower- ing stem ; c, pointed bulb-scales of new stem ; d, ground-leaves of new stem. Fig. 55. — Young flowering stem re- moved from the middle of bulb in Fig. 5 4. a, surface of stem ; b, undevel- oped stem-leaves ; c, bud from which next year’s bulb will arise. (Both after Irmisch.) these again, we find that they surround a large bud (Fig. 55), which is the young flowering stem ready to develop next summer. The leaves of this bud (b, Fig. 55) are simply the future stem-leaves of this 10 146 STRUCTURAL BOTANY flowering stem. In the axil of the innermost of the ground-leaves, and therefore at the base of the young flowering stem, we find a tiny hud (c, Fig. 55), with only two or three very minute scales. This is destined to develop into the next year’s bulb and stem. We find, then, that the White Lily bulb consists of (1) the base of the old stem, bearing the remains of its ground-leaves, (2) of the new stem which arose in the axil of the innermost ground-leaf. On this new stem the first leaves are the pointed bulb-scales which never develop any further ; then the ground- leaves, which come up in autumn ; and lastly, the stem-leaves, which can only expand when the flower- ing stem develops next summer. The little bud (c, Fig. 55) will repeat this whole process a year hence. In vigorous plants more than one of these buds may be formed, each giving rise eventually to a new bulb. All Lily bulbs are not so complicated as this. In the Turk’s Head (L. martagon), for instance, there are no ground-leaves, and consequently no scales formed from their bases ; all the bulb-scales here are of the pointed kind, i.e. they are all complete but simplified leaves. The leaves of the flowering stem are arranged spirally. The phyllotaxis is not very regular, but a divergence of f is common, i.e. if we trace the spiral upwards, each leaf is in these cases separated from the one next above it by f of the circumference of the stem. The leaves are lanceolate in form, without petioles, THE WHITE LILY 147 and have a base, which embraces about f of the circumference of the stem. The autumnal ground- leaves chiefly differ from the stem-leaves in being stalked, and in having a swollen base, and also in their broader ovate-lanceolate form. The arrangement of the veins in the leaf is different from what we found in our dicotyledonous type. In the Lily all the principal veins run from end to end of the leaf, and are approximately parallel to each other. The veins near the middle of the leaf run almost straight, while those towards the edges are more and more curved, so as to follow the outline of the leaf. There are generally three veins more strongly marked than the rest, the midrib being most prominent of all. These larger veins form projecting ridges on the lower surface. The longitudinal veins are connected by oblique branches. The whole system, therefore, forms a network, but of a simpler form than that in the Wallflower. The longitudinal veins are, on the whole, stouter than the oblique connections. There are, as a rule, no blindly ending branches within the meshes, another point of difference from the dicotyledonous type. The position of the blade of the leaf is roughly horizontal in the stem-leaves, and more oblique in the ground-leaves. As a rule the flowering stem does not branch ; there is a bud in the axil of each leaf, but it generally remains undeveloped. If, however, the main stem be injured, it may be replaced by a branch developed from one of these buds. In some other species, such as L. bulbifcrum, these axillary buds grow into little 148 STRUCTURAL BOTANY bulbs, which become detached and give rise to new plants, thus affording a good example of vegetative pro- pagation. We speak of vegetative propagation when a plant possesses some means of producing descendants without the help of the regular reproductive organs, the stamens and pistil. In most Lilies such propagation takes place by means of the bulbs. We have already seen that a vigorous old bulb in the White Lily may give rise to several buds, which develop into new bulbs. While the leafy stem is assimilating, the young bulbs receive the food which it forms ; part of this they use for their own growth, the rest they store up, especially in the thickened scale-leaves, for future use. Every new bulb can form roots and a leafy stem of its own, and thus ultimately produce flowers, giving rise, in fact, to a complete new plant. In our own country, Lilies are usually propagated from bulbs, seedlings being more rarely met with. A striking peculiarity of the Lily as distinguished from the Wallflower is the absence of any main root. All the roots of a mature Lily plant are adventitious — that is to say, they are borne on the stem. Usually it is the base of the stem, at the bottom of the bulb, which bears them ; sometimes, however, roots also arise from the lower part of the flowering stem above the bulb. When a Lily plant is very young, it has a main root formed from the radicle of the embryo ; but this root soon dies away, and the root-system is henceforth entirely produced from the stem. Most members of the class to which the Lily belongs THE WHITE LILY 149 ( Monocotyledons ) lose their main root early, and replace it by adventitious roots. B. The Flowers The dowers are borne in a terminal raceme, i.e. they arise on the main axis — the older dowers below, the younger above. Each dower is stalked, and grows in the axil of a small leaf called a bract. The dowers are large, and very easy to examine. The drst great difference which we notice, from the Wallflower, is that the whole perianth, including both calyx and corolla, is coloured alike, and its leaves are all of about the same size. In the bud, when just about to open, the three outer perianth leaves or sepals enclose all the other organs, only the midribs of the petals showing between them. A good idea of the arrange- ment of the parts of the flower is obtained by examining a transverse section across a bud (see Fig. 56 ; also diagram, Fig. 78). The sepals are three in number, and are all inserted at one level, forming a single whorl. They are lanceolate in shape, and their venation is much like that of the green leaves, only less conspicuous. In the bud each sepal is somewhat hooded, so as to arch over the other organs. At the base of each sepal, on its inner side, is a shallow groove with raised sides, into which honey is secreted. The three petals also form a whorl, and stand opposite the spaces between the sepals, or, in other words, are alternate with them. The petals are narrower at the base than the sepals, but rather 150 STRUCTURAL BOTANY broader above. A petal differs from a sepal in having a well-marked midrib, which projects as a ridge on its outer or lower surface. On either side of this ridge is a deep furrow, into which in the bud the edge of the adjoining sepal fits (see Fig. 56). The side veins of the petal diverge from the midrib, running obliquely upwards and outwards to the edge, these Fig. 56. — Transverse section across a flower-bud of the White Lily, s, one of the three sepals (unshaded) ; p, one of the three petals (shaded) ; st, one of the six stamens in two whorls of three ; 0, pistil consisting of three carpels. Magnified about 8. (R. S.) veins being connected by oblique branches to form a network. At the base of each petal is a honey- bearing groove like that on the sepal. The stamens are six in number, in two whorls of three each. The outer whorl is alternate with the petals, and therefore opposite the sepals. The inner whorl is alternate with the outer, and therefore opposite the petals. In fact, all through the flower THE WHITE LILY 151 the successive whorls of three alternate regularly with each other (see Figs. 56 and 78). The stamens have long filaments and large versatile anthers, i.e. the filament in the mature state is attached to the anther near its middle, so that the anther swings freely upon the filament. In all essentials the structure of the stamen is like that of the Wallflower, but its large size in the Lily makes it much more favourable for study. The anther consists of a con- nective and four pollen-sacs. Here, as in our former type, the two pollen-sacs on the same side of the anther become fused when it is ripe, so that the mature anther has only two cavities. Here also the anther is introrse, i.e. it opens on its inner side. In the middle of the flower is the pistil . The ovary is cylindrical and ribbed, the style about twice as long as the ovary and surmounted by a large three-lobed stigma (see Fig. 82), the velvety surface of which is visible even without a lens. The pistil is syncarpous, the carpels three in number, and alternate with the inner whorl of stamens. The ovary is three-celled ; the ovules are numerous, and are inserted in two rows in each cell, arising from its inner angle ; in other words, the placentation is ccxile (see Figs. 56 and 78). If we suppose each of the carpels folded inwards until its edges meet, and then all three carpels closely joined laterally, the united edges of all three meeting at the middle, we shall have a good idea of the structure of the ovary, though not one which is strictly accurate according to the development. In this ovary, as in 152 STRUCTURAL BOTANY many others, the ovules are inserted on that part which corresponds to the margins of the carpels. The development and minute structure will be dealt with below. II. INTERNAL STRUCTURE OF THE VEGETATIVE ORGANS In the Lily we shall find the same great systems of tissue which we observed in the Wallflower, and their general arrangement is also similar. There are, how- ever, some broad differences between the two types, chiefly connected with the course and development of the vascular bundles. a. Stem We shall find it best to begin with the leafy flower- ing stem (see Fig. 57).' If we examine a transverse section through an internode, the first thing that strikes us is the very well defined central cylinder, limited on the outside by a zone of thick -walled cells (pc hi Fig. 57). In the central cylinder we see the transverse sections of a large number of vascular bundles ( b ). They do not form a single ring, as in our dicotyledonous type, but appear at first sight to be irregularly scattered. We can, however, make out an arrangement in con- centric rings, though they are not very regular. Towards the middle of the stem we find the larger vascular bundles forming an irregular ring (ij). Sur- rounding these is a zone of smaller bundles (&2) ; while quite at the outside of the cylinder, next the thick- walled tissue, we find the smallest bundles (&3), which THE WHITE LILY 153 do not, however, form a complete ring. Now, this arrangement in irregular concentric rings is character- istic of the vascular bundles of most Monocotyledons as distinguished from the Dicotyledons, in which we usually find a single and regular ring. This difference in the transverse section depends on the course of the Fig. 57. — Transverse section of tlie flowering stem of White Lily. Somewhat diagrammatic, pc, thick-walled peri- cycle, forming outer limit of central cylinder; bs, one of the small vascular bundles on its way from a leaf ; b2, bundle cut through on its inward course ; blt one of the principal bundles which form the inner ring. Magnified about 20 times. (R. S.) vascular bundles through the stem. Here, just as in Dicotyledons, all the bundles are common to leaf and stem that is to say, each bundle in the stem is the direct downward continuation of one which comes from a leaf. The diagram (Fig. 58) will help us to under- stand this, though it is much simpler than the reality. 154 STRUCTURAL BOTANY We see that each bundle on entering the stem from a leaf (l) passes very gradually inwards, until it is only a short distance from the middle of the stem. It then turns straight down, and at last joins on to a bundle which starts from a lower leaf. Now, this course is not essentially different from what we found in the Wallflower. The great difference in the transverse section depends on the fact that the bundles pass very gradually inwards, so that when we cut the stem across, our section passes through a number of them at points of their inward Thus the small outer (Fig. 57, b3) in the transverse section are those which are nearest the leaf and have only just entered the cylinder. (The outline of the cylinder is indicated by dotted lines in the diagram, Fig. 5 8.) The intermediate bundles (bo) are cut through at a point nearer the interior ; while the largest (b() are those which have approached nearest to the centre of the stem and have turned straight down. In the bulb the course is still more complicated, for here each bundle first bends in towards the middle and then out again, and only joins on to other bundles near the outside of the stem (see lower part of Fig. 5S). o various course. bundles Fig. 58. — Diagram to show course of the bundles in the flower- ing stem and bulb of the Lilies, as seen in longitudinal section. I, insertion of a foliage leaf ; s, insertion of a bulb- scale. (After Falkenberg. ) THE WHITE LILY 155 In the White Lily three bundles enter the stein from each leaf. If we now consider the structure of the central cylinder more in detail, we find that the vasculai Fig. 59. — Part of transverse section through flowering stem of White Lily. e, epidermis; s, stoma; c.p, cortical parenchyma containing chlorophyll ; sc, sclcrenchyma of pericycle; v.b, vascular bundle. Magnified about 70. (R. S.) bundles are embedded in thin-walled, large - celled ground-tissue {conjunctive parenchyma). We may, if we like, call the central part of this tissue within the 15G STRUCTURAL BOTANY innermost ring of bundles pith, but there is no dis- tinction in structure between this and the rest of the conjunctive tissue, all the cells of which have the form of vertical cylinders with horizontal ends. They are always living cells, and besides the protoplasm and nucleus generally contain starch granules. Fig. 60. — Single vascular bundle from tlie last section, x, xylem; p.x, protoxylem; ph, phloem; p.ph, proto- phloem ; ().■ s, selerenchymatous bundle-slieath. Magni- fied 210. (R. S.) The outer layers (pericycle) of the cylinder, however, are different from the rest. They have thick, lignified walls, and are very long, with pointed ends (pc in Fig. 57, sc in Fig. 59). As the pericycle here forms a rigid case round the cylinder, it is evident that no further growth in thickness is possible. The pericycle THE WHITE LILY 157 here represents the chief mechanical tissue of the stem, serving to give it the necessary stiffness. The vascular bundles themselves have on the whole a similar struc- ture to those of the dicotyledonous type. They are collateral, the inner half being the wood and the outer half the phloem (see Figs. 60 and 61). Each bundle fth Fig. 61. — Longitudinal radial section through a vascular bundle of the stem of the White Lily, sh, bundle-sheath ; xy, xylcm, or wood; px, protoxylem ; ph, phloem; s, sieve-tube; c, companion-cell. Magnified 220. (R. S.) is surrounded by a special sheath of long thick-walled lignified cells (b.s. in Fig. 60, sh in Fig. 61). There is often a group of thin-walled cells between the sheath and the inner edge ol the xylem, which itself consists chiefly of vessels with a few living cells among them. The vessels are of the same kinds as those of 158 STRUCTURAL BOTANY the Wallflower. Starting from the side nearest the middle of the stem, we find first annular or loosely- coiled spiral vessels (often with a double spiral band), then more closely-coiled spiral vessels of larger size, and finally, next the phloem, vessels with simple pits. We do not usually meet with bordered pits in the Lily. The phloem is of simple structure, and is entirely made up of sieve-tubes and companion-cells, without any hard bast or phloem parenchyma. Thus both wood and bast are composed of the same elements as those of the Wallflower, but with rather less variety. The development of the vascular bundle is also such as we are already familiar with. In the xylem the first elements to be formed are those on the extreme inner edge of the bundle (px in Figs. 60 and 61). As usual, this protoxylem may be known by its annular or loosely spiral vessels, adapted to undergo much stretching after their walls are thickened. Conversely, in the phloem, the first cells to become mature are those on the extreme outer edge of the bundle. From these two remote points the development advances towards the middle. But now we come to a great difference from our former type. In the Dicotyledon we found that the development of the young bundle into mature wood and bast was never complete ; there was always a band of actively dividing cells left between the two, so that the formation of new wood and bast could go on without limit. In other words, the bundle possessed a cambium, and was therefore said to be open. In the Lily and all Monocotyledons, on the other THE WHITE LILY 159 hand, the whole of the procambial strand is used up to form permanent tissue. The xylem and phloem meet ; no layer of cambium is left between them ; the bundle is a closed one, and when once formed is quite incapable of any further growth. This is one of the most constant distinctions between the two great classes of which we have taken the Wallflower and the Lily as types. The one has open bundles, capable of indefinite growth by means of cambium, while the other has closed bundles with no cambium, and therefore with limited growth. All Monocotyledons have closed vascular bundles, and the great majority have no cambium at all, so that the stem or root, when once developed from the growing point, increases no more in thickness. There are, however, a few members of the class, such as the Dragon-tree and Yucca, which have a kind of cambium, and in which secondary growth in thickness conse- quently takes place. In these plants the cambium arises altogether outside the vascular bundles, so that the growth is quite different from that in typical Dicotyledons. We need not, however, concern our- selves here with these exceptional cases. The cortex requires little description. Its cells have thin cellulose walls, and in their protoplasm chlorophyll granules are embedded, by means of winch the stem is able to do some assimilation. The leaves, however, are the chief organs for this function. The epidermis has elongated cells, and rather numerous large stomata, placed lengthways, so that a transverse section cuts across both guard-cells. 160 STRUCTURAL BOTANY The base of the stem, which bears the scale-leaves of the bulb, has a rather different structure. The distinction between central cylinder and cortex is less sharp ; there are no chlorophyll granules in the cortical cells, because this part of the stem is never exposed to light. The vascular bundles have a different course, as explained above. The ground - tissue contains quantities of starch and also oil. The reserve substances of the bulb stem are, however, used up in summer, when the flowering stem develops, and then we find its cells almost empty. The bulb - scales, which we shall describe later on, are more important than the stem as storehouses of food. b. Leaf It matters little whether we take an autumnal ground-leaf or a summer leaf from a flowering stem. They are different, as we have seen, in shape, but in internal structure they are very much alike. We have already learned the general course of the veins. In the midrib and principal veins of the larger leaves several vascular buudles run side by side ; the smaller veins correspond each to a single bundle. If we trace the bundles outward from the stem into' the leaf, we find that they immediately begin to branch, the main branches taking a parallel course, while they are connected by minor oblique strands (see Tig. 62, v.b). Thus the whole surface of the lamina is provided with an elaborate system of conducting channels, chiefly differing from that of our dicotyledonous type THE WHITE LILY 161 in the absence of blindly ending branches and in the strongly marked parallel course of the stouter bundles. In the broad ground - leaves, however, the parallel veins are less conspicuous, and the network more resembles that of a Dicotyledon. In these leaves, as in those of most plants, the rule Fig. 62.— Part of section parallel to surface of leaf of Lily. v.b, vascular bundles ; p.p, palisade parenchyma. Magni- fied about 35. (R. S.) ° holds good that the wood of the bundle is directed to- wards the upper and the bast towards the lower surface. The minute structure of the bundle in the leaf is like that in the stem. The finer bundles, however, are much reduced, and here we usually find tracheides li 162 STRUCTURAL BOTANY instead of vessels (cf. p. 66). The bundles are accompanied by thin - walled parenchyma, and the whole strand, bundle and parenchyma together, is enclosed by a well-marked sheath of cells containing abundant starch (see Fig. 63). The parenchyma accompanying the bundles is continuous with that of the central cylinder of the stem, and it is through this tissue that the sugar formed in the leaf passes down Fig. 63. — Part of transverse section of foliage-leaf of White Lily, e, epidermis ; s, stoma ; p.p, palisade paren- chyma; s.p, spongy parenchyma; v.b, vascular bundle, with xylem above and phloem below. Magnified about 70. (R. S.) towards the bulb, to be there formed into a store of starch. In this leaf, as in most horizontal leaves, we find that there is a marked difference between the upper and lower parts of the ground-tissue ; in fact, we have here, as in the Wallflower, to distinguish between palisade and spongy tissue. The palisade cells, however, are of a peculiar form (see Fig. 63, p.p) ; they are branched, and the branches (two or three to each cell) are placed at right angles to the surface of THE WHITE LILY 163 the leaf. This kind of tissue is called arm-palisade ; the effect of the branching is that extra vertical wall surface is gained for the disposal of chlorophyll granules. We must not suppose that this kind of tissue is peculiar to Monocotyledons. The lower part of the tissue of the leaf is spongy ; its cells are branched irregularly in all directions (see Fig. 63, sp), so as to leave large intercellular spaces between them. The epidermis consists on both surfaces of the leaf of large elongated cells, with a thick outer wall, limited by a well-marked cuticle. The stomata are mostly on the lower surface, though we may find a few scattered about in the upper epidermis as well. Each stoma consists of two guard-cells, with a pore between them, leading to an air-chamber below. In fact, their structure is precisely the same as in the Wallflower, but in the Lily they are much larger and easier to observe. The bulb-scales also are leaves or bases of leaves, as we have seen. They have, however, quite different functions to perform from the foliage-leaves, and accordingly have quite a different structure. They are not exposed to light, and therefore have no chlorophyll. The epidermis is without stomata. The ground-tissue is very abundant, and is all alike, there being no need for any special palisade or spongy tissue. The cells are crammed with reserved food- material, especially starch, which is gradually con- sumed as the flowering stem develops. The vascular bundles are much fewer than in the foliage-leaf, and 164 STRUCTURAL BOTANY their xylem is small, for there is no active water- current to be conducted. To sum up : while the foliage-leaves are adapted to the functions of assimilation and transpiration, the scale-leaves of the bulb have only the work of storing up the food which has been formed elsewhere. c. The Root The main root, as we have already seen, only lasts for a short time, though it grows to a considerable length. After the seedling stage is past, the plant has adventitious roots only. Indeed, as seedling Lilies are not common in our gardens, the only roots which we generally see on these plants are adventi- tious ones. They usually arise from near the base of the stem, below the bulb-scales (see Fig. 54, r). They are little, if at all, branched. The roots are of large size, and very favourable for examination. The general structure follows the same lines as that of dicotyledonous roots, but there is this difference : a root of the Lily, like that of nearly all Monocotyledons, has no secondary growth in thickness, hence it has to depend entirely on its primary tissues, which are accordingly much more highly developed than is necessary in a dicotyledonous root, where there is a cambium ready to make good the deficiencies of the original structure. The Lily root is traversed by a large central cylinder, which includes a great many distinct bundles of wood and bast, often as many as a dozen of each (see Fig. 64). They are arranged alternately all THE WHITE LILY 165 round the cylinder. We thus see at once that the structure is essentially the same as in the young root of a Wallflower, hut in the Lily there are many bundles of wood and hast, while in the Wallflower there were only two of each. In other words, the Fig. 64. — Transverse section across the central cylinder and inner layers of cortex of a root of the White Lily, e, endodermis ; pc, pericyele ; ph , a phloem group ; x, a xylem group ; protoxylem at the outer end of each xylem group. Magnified 330. (R. S.) root of the Lily is polyarch, that of the Wallflower diarch (cf. Fig. 28). We notice that here also the smallest vessels lie to the outside of each group of xylem. These are the spiral vessels, which were the first to be formed, so that here, as indeed in all roots, we have external protoxylem. The inner larger vessels, which 166 STRUCTURAL BOTANY are formed later, are pitted. Sometimes two of the xylem bundles converge inwards, and meet in a single vessel. The small phloem groups have the same structure as those in the stem. Xylem and phloem are separated from each other by a layer of thin- walled tissue. In the root figured there is also some parenchyma in the middle of the cylinder, forming a small pith, but sometimes the xylem groups meet in the middle, so as to leave no room for a pith. Fig. 65. — Transverse section of tissues at tlie extreme outside of the same root, r.h, root -liair ; ex, exodermis, form- ing outermost layer of cortex. Magnified 330. (R. S.) The outermost layer of the cylinder, surrounding the whole ring of wood and bast, is a very definite thin-walled pericycle (Fig. 64, pc). Outside this, again, we come to the innermost layer of the cortex, the very conspicuous endodermis (e in Fig. 64), the cells of which fit closely together, and have those walls cuticularised by which they are in contact with each other. In the older roots the endodermal cell- walls also become thickened, especially on the radial THE WHITE LILY 167 and external surfaces, as shown in Fig. 64. The endodermis serves especially to cut off all communica- tion between the intercellular spaces of the cortex and the interior of the central cylinder. These spaces contain air, and if this air could make its way into the vessels of the wood, the water-current through the latter would soon be stopped. Hence the import- ance of the isolating endodermis. The cortex is many layers of cells in thickness, and is persistent instead of being thrown off, as in Dicotyledons. Outside this, again, is the absorptive or piliferous layer, which forms the external surface of the root. Many of its cells grow out into uni- cellular root-hairs (see Fig. 65, r.h.), which are the special organs for absorption, taking up water and dissolved salts from the soil. It is only, however, the young parts of the roots which absorb food ; sooner or later the piliferous layer dies away, absorption ceases, and the root hence- forth becomes merely an organ of conduction which serves to pass on the sap towards the stem. This conducting part of the root requires some kind of protective layer on the outside to guard it against abrasion, or the more serious danger of attack by parasites, and also to prevent possible loss of water if the soil should become dry. This protective function is performed by the outermost layer of the cortex, which soon becomes distinguished from all the inner layers by its cuticularised walls. For a time it has the structure of an endodermis ; subse- quently the corky change extends to all its walls 168 STRUCTURAL BOTANY alike. Thus the older part of the root is clothed by a complete protective covering, similar to the epidermis of the stem, while the piliferous layer, which formed the original outer surface, disappears. This protective covering is called the exodermis (ex in Fig. 65). It is found in many dicotyledonous roots also, but in them its functions are very temporary, and it is usually soon replaced by secondary periderm. In most monocotyledonous roots there is no periderm, and here the exodermis continues to be the only protective layer throughout the life of the root. The tip of the root is enclosed in a root-cap, the development of which will be described below. d. Growing Points and Mode of Branching The growth of the stem follows the same general course as that of a Dicoty- ledon. Here also the growing point, where new tissues and organs are produced, lies at the apex of the stem. In this position we find a meri- stem, which at first is made up of quite uniform cells. The description given of the growing point of a Dicotyledon holds good here with only slight differences (see Fig. 66). The ex- Fig. 66. — Longitudinal median sec- tions of the growing point of the stem in a Monocotyledon, ep, epidermis ; p, periblem ; pi, ple- rome ; i, initial cell common to periblem and pleroine ; l, young leaves. Magnified about 200. (After Douliot.) THE WHITE LILY 169 ternal layer of the apex is the dermatogen, from which the epidermis is formed. The periblem and plerome probably have a common initial cell. From the plerome, which is the inner part of the meristem, the vascular cylinder, including all its bundles, is developed. The development of the vascular bundles begins at the interior of the cylinder, and advances outwards. When the outermost bundles have been formed, the peri- cycle thickens its cell-walls, and all further development ceases. The leaves here, as usual, arise from below upwards, so that the youngest are always nearest the apex. Only the outer layers of the growing point take any part in the formation of the leaves and branches of the stem (see Fig. 66, l ). The growth of each leaf after its first origin goes on chiefly at its base. The apex of the root is on the whole similar to that of a Dicotyledon, and we find the same three layers of meristem, giving rise to the root-cap, cortex, and vascular cylinder respectively. The superficial Fig. 67. — Longitudinal median section of apex of young root of a Monocotyledon ( Amaryllis ). s (shaded), “digestive sac,” formed from endodermis of parent- root ; c, calyptrogen, giving rise to root- cap only ; p, periblem (the outermost layer formed from this bears the root- hairs); pi, plerome. Magnified about 150. (After Van Tieghem and Douliot. ) 170 STRUCTURAL BOTANY Fjg. 68. — Part of tranverse sec- tion of root of a Liliaceous plant, showing the first origin of a rootlet, cn, endodermis ; pi, pericycle ; x, xylem group ; ph, phloem group ; co, con- junctive tissue. All the above belong to the parent-root, s (shaded), digestive sac, formed from endodermis ; c, calyptro- gen, or cells giving rise to the root-cap of the rootlet ; p, peri- blem, which will form its cortex and piliferous layer ; pi, ple- rome, from which the central cylinder will be developed. Fig. 70. — Part of longitudinal section of a monocotyledonous root passing through a de- veloping rootlet. Letters as above. (All three Figs, are magnified about 150, and are after Van Tieghem and Douliot.) Fig 69. — Similar section from the White Lily, showing a more advanced root- let. d, cortex of parent-root, through which the rootlet is boring its way ; e, cortex at base of rootlet. Other letters as above. layer of the root, which bears the root-hairs, arises here, however, from the same group as the cortex, instead of having a common origin with the root-cap, as in Dicotyledons (see ' Fig. 67, and compare it with Fig. 33, p. 86). The branching of the root can also be dismissed in a few words, as in all im- portant respects it takes place in the same way as in the WalMower. THE WHITE LILY 171 Here, just as in Dicotyledons, the branch-root arises from a deeply-seated layer of the parent-root, namely, from its pericycle. The details of the development are sufficiently shown by Figs. 68, 69, and 70. The endodermis of the parent-root here takes some part in the process, as it forms a sheath around the developing rootlet during its growth through the cortex. As this endodermal sheath serves to absorb the cortical tissues, through which the rootlet as it were eats its way, it bears the name of the “ digestive sac.” When this function has been per- formed, the sac is cast off, the whole permanent part of the rootlet being thus formed from the pericycle. The origin of an adventitious root is similar to that of a rootlet. Just as a rootlet arises endogenously from the parent-root, so does an adventitious root arise endogenously from the stem, and in both cases it is the pericycle from which the new organ is formed. Summary of Monocotyledonous Anatomy The general working of the vegetative organs in the Monocotyledons goes on in the same way as in our first type. In both, the green leaves, by means of their palisade and spongy tissue, are the great organs of assimilation and of transpiration. The leaves are in communication with the stem through the vascular bundles and the tissue immediately surrounding them. The conducting tissues divide up their work as follows : the wood serves chiefly to convey the water aud mineral salts taken up by the roots ; the phloem 172 STRUCTURAL BOTANY conducts to the growing organs assimilated food which has been formed in the leaves, and especially that part of the food which contains nitrogen. On the other hand, the carbohydrate foods produced by the leaf, such as sugar, are conveyed through the thin- walled parenchyma which accompanies the bundles. Both in Monocotyledons and in Dicotyledons all these tissues are continuous throughout the central cylinder of the stem and its branches, and ultimately with the corresponding tissues of the root. Neither as regards structure nor function can we pohit to any profound difference between the two classes, so far as the primary tissues are concerned. The real distinction lies in the fact that in typical Monocotyledons there are no secondary tissues ; the entire structure is developed from the growing points once for all, and henceforth no further additions are ever made, how- ever long the plant may live. Consequently, we generally find that the primary structure of stem and root is more complex in Monocotyledons than in the other class, because in the former the primary tissues have to do all the work, while in the Dico- tyledons they are supplemented or even entirely superseded by the secondary tissues formed from the cambium. The few Monocotyledons which have a kind of cambium are here left out of consideration. This difficulty, however, arises : many Monocoty- ledons, when mature, have a stem which is of great thickness compared with that of the same plant when a seedling. Take the Maize, for instance : a well-grown plant may have a stem two inches in diameter, while THE WHITE LILY 173 the stem of the seedling is perhaps an eighth of an inch thick. In the Palms the difference is much more striking ; the thin stem of the seedling, only a small fraction of an inch in thickness, may give rise to a great trunk, two feet or more in diameter, and yet there is no cambium and no formation of secondary tissues. How is this possible ? The explanation is that the thin stem of the seedling never does become any thicker, but for a time the vigour of the growing point constantly increases, so that each node and internode which is added to the stem is thicker than the last. Thus for a time the stem has the form of a cone standing on its apex. But sooner or later the growing point reaches its full strength. From this time onwards the portions of the stem which it successively produces are of equal diameter ; thus all the stem above this region is cylindrical. The older part of the stem, which alone has the form of an inverted cone, generally becomes buried in the ground, or like the tap-root it may die away altogether. In typical Monocotyledons, then, the increasing diameter of the stem is due to the addition of suc- cessively thicker nodes and internodes, and not to the growth in thickness of those already formed. III. INTERNAL STRUCTURE AND DEVELOPMENT OF TEE REPRODUCTIVE ORGANS The flower of a Lily is not only very favourable for study, from the large size and simple arrangement of all its parts, but it has the more important advantage 174 STRUCTURAL BOTANY that it is an excellent type of the flowers of Monocoty- ledons generally, most of which are built on the same ground-plan. We already know the external characters of the flower ; we will now shortly trace its development. a. Development of the Flower The flower-bud arises, like any other branch of the stem, as an outgrowth from the external layers of tissue of the growing point. It is formed in the axil of a bract, and in some species, as in that figured, the axis of the flower itself produces a small lateral vegetative leaf, termed a bracteole (see Fig. 71, B, b). In the White Lily, however, the latter is absent. The future flower has at first a broadly rounded shape. The earliest stage shown in the figures is that just after the sepals have appeared. All the three sepals do not arise at the same time, but one after the other (see Figs. 71 and 72, s). Next, the three petals appear, all at the same time, and exactly between the sepals (see Fig. 73, p). After this three fresh outgrowths show themselves just inside the petals, and alternating with them, so that they lie exactly opposite the sepals ; these are the three outer stamens (sta, Fig. 74). Very soon afterwards the three inner stamens appear (si0, Fig. 75) exactly between the first three, and therefore opposite the petals. Lastly, around the centre of the growing point, yet three other structures arise, each shaped like a horseshoe, open towards the inside of the flower. These are the young carpels, forming the pistil, which is the innermost whorl of the flower tb pH a s-d ,spg-s« g ”« S *3 1-2-3 g, «*h r^-+-3 03 c6 ° s g- fe o o r: ^ bo^-N V, | * 2.S s? g£ S £ ;* ^ ® e " I * 142 d-a o ® •rH S O ~ >■ ,9 22^_g CO r X". .bp ' E THE WHITE LILY 176 STRUCTURAL BOTANY (Figs. 76 ancl 77, cy>). The carpels are alternate with the inner whorl of stamens, and are therefore opposite the outer stamens. Diuing the development of the floral leaves the stalk on which the whole flower-bud is borne goes on growing. We see, then, the extreme regularity and simplicity of the Lily flower. All the whorls consist of three members each, they arise in regular succession, and the members of each whorl always alternate with those of the whorl next outside them. The diagram (Fig. 78), repre- senting a ground-plan of the flower, illustrates this regularity. b. Structure of the Floral Organs The calyx and corolla are much alike in structure. Both closely resemble foliage-leaves, but are some- what simpler. The same systems of tissue, vascular bundles, mesophyll, and epidermis, are present, and the epidermis possesses stomata. The mesophyll, how- ever, is destitute of chlorophyll granules, and in the White Lily has no colouring matter at all. In the yellow Lilies, such as the Tiger Lily, chloroplastids are present, and where we get a distinctly red tint, as Fig. 78. — Floral diagram of a Lily, a, axis of inflorescence ; b, bract ; s, sepals ; p, petals. Within this are seen the two whorls of stamens, and in the middle the three carpels united to form the ovary with axile placentation. (After Eichler.) THE WHITE LILY 177 in the spots on Lilium auratum, this is due to the presence of red cell-sap. The groove at the base of each sepal or petal is lined by a tissue of thin-walled cells, rich in protoplasm, by which the nectar or honey is secreted. The stamens require no detailed description, as they have essentially the same structure as those of Fig. 79. — Division of tlie pollen mother-cell in a Lily. A, First division : n, n, daughter-nuclei ; c.p, granular cell-plate, from which the new cell -wall will be formed. B, Second division : n, n, the four granddaughter -nuclei ; c.p, c.p, the cell-plates from which the second cell-walls will be formed. The four cells in B become the pollen- grains. (After Strasburger. ) Magnified 800. the Wallflower, though they are much larger. The development, both of the stamen as a whole, and of the anther with its four pollen-sacs, takes place in the same way as in Dicotyledons. In each of the pollen-sacs there is a layer of tissue ( arcliesporium ), which gives rise to a great mass of pollen mother-cells. The division of a pollen mother- cell to form its four pollen-grains, which takes place 12 178 STRUCTURAL BOTANY in a slightly different way from that in Dicotyledons, is shown in Fig. 79.1 In the pollen-grain itself further changes take place. The nucleus divides, and around one of the two nuclei thus produced a small cell is formed (see Fig. 80, g). This, as in the Wallflower, is the generative cell, so called because it is directly concerned in fertilisation. It is much more easily seen in the large pollen-grains of a Lily than in the small ones of a Wallflower. The membrane of the ripe pollen- grain is cuticularised, and its sur- face is rough, with raised ridges, which form a delicate network. Fig. 80. — Pollen-grain of Lily at the begin- ing of germination. . g, small generative Along one side of the gram there eeli with its nueleus ; js a f0p|. the membrane of the p.t, first outgrowth ’ Oil fold is thicker than the rest ; it of pollen-tube, drops are shown out- side the grain. Mag- nified several hundred times. (After Dodel- Port. ) The dehiscence is smooth, and consists of pure cellulose. In the contents of the grain there are many starch granules. of the anther takes place as described in the Wallflower. Here also the wall separating the two pollen-sacs on the same side of the anther breaks down at the moment of dehiscence (see Fig. 40, of Wallflower), so that the perfectly ripe anther appears to have only two compartments instead of four. The abundant pollen is set free, but most of 1 For the whole development of the stamen careful reference must be made to the description given in the case of the Wallflower (pp. 109-114). THE WHITE LILY 179 it remains hanging on the burst anthers, for the grains are made sticky by the presence among them of innumerable drops of oil, of an orange colour. The Pistil The pistil has a very different structure from that of the Wallflower, though it resembles that of many other Dicotyledons, so that the differences in question are not characteristic of the two classes. We have already seen that each of the three carpels arises as a ridge, shaped like a horseshoe, on the growing point of the flower; very soon the ridge is completed on the inner side of each carpel, which thus becomes a closed ring. Where the carpels join on to each other they are completely united from the first, the wall separating their cavities being a single, and not a double one ; the pistil is, in fact, completely syncarpous. Henceforward growth goes on at the base of the pistil, and so the whole structure grows up into a triple tube. At last the upper edges of each carpel join together at the top, so as to close the tubes. The upper closed part of the pistil develops into the style. At the extreme tip the three carpels remain partly distinct all through, forming the three lobes of the stigma (see Fig. 82, stig), which here correspond in position to the carpels, unlike the arrangement which we found in the Wallflower. The ovules develop from the inner angle of each of the united carpels, forming two rows in each carpel (see diagram, Fig. 78). This placcntation is known as axile. 180 STRUCTURAL BOTANY c. Ovules Passing on now to the ovules themselves, we find that the first stages of their growth are like those of the Wallflower, and here also the ovule soon becomes curved. The curvature, however, takes place in a different manner. In the Lily the nucellus and integuments remain straight ; the bending takes place at the chalaza, so that the whole body of the ovule turns through an angle of 180°, and so comes to lie parallel to the funiculus, but with the micropyle directed towards the placenta instead of away from it (see Fig. 83). Such an ovule is called anatropous. The difference between this form and the campylo- tropous ovule of the Wallflower, in which the nucellus and integuments are themselves bent, will be evident on comparing Fig. 83 with Fig. 43. The anatropous ovule is the commonest form both among Dicotyledons and Monocotyledons. In the Lily the second or outer integument does not become closed, but remains widely open, the narrow micropyle being thus formed by the inner integument alone (see Fig. 83). This is common among Monocotyledons. In other respects there is a general agreement between our two types ; in fact, the main points in the development of the ovule are very uniform throughout the great majority of the members of both classes. When we come to the origin of the embryo-sac itself, however, there are some differences to note, though they are not characteristic of the two classes, THE WHITE LILY 181 as similar variations are found within the limits of both Monocotyledons and Dicotyledons. In the Wallflower we found that the original archesporial Fig. 81. — Longitudinal section through part of the stigma of a Lily, showing germinating pollen-grains, h, papillse of stigma ; p. rj, pollen-grains ; t, pollen - tubes. Highly magnified. (After Dodel-Port.) cell underwent several divisions before the embryo-sac was finally marked off. In the Lily the matter is simpler : a cell of the middle row of the ovule, next under its epidermis, grows larger than the rest, and 182 STRUCTURAL BOTANY itself becomes the embryo-sac, without dividing any more at all. It grows to a comparatively large size, so much so that there are no plants more favourable for the study of the embryo-sac than the Lilies. The divisions of the nucleus follow the same general course as that described above (see p. 119). We find as the final result three nuclei at the end of the sac towards the micropyle, three at the opposite end and two near the middle. The three nuclei next the micropyle have cells round them, though without any cell- wall ; these three cells are the egcj -apparatus, the ovum being the deepest seated of the three, while the two others are the helpers or synergidse (see Fig. 84, s). in like manner the three nuclei at the other end become the Fig. 82.— Young pistil centres of the antipodal cells, of a Lily, o, ovary; . sty, style ; stig, stigma, which, unlike their opposite Natural size. (After neighbours, have cell-walls. The two polar nuclei are of exception- ally large size, especially the one from the antipodal end (see Fig. 84, p.2). A more interesting fact is, that these two nuclei do not fuse till very late — in fact, only just before fertilisation. On the whole, however, the processes inside the embryo-sac are closely similar to those in the dicotyledonous type, the description of which should be carefully compared (pp. 119-122). THE WHITE LILY 183 The nueellus of the Lily does not disappear, hut encloses the embryo-sac until after fertilisation (see Fig. 83). Fig. 83. — Longitudinal section of ovule of Lily. /, fuuicle ; r, raphe ; ch, chalaza ; inu inner integument ; in„, outer integument ; m, micropyle ; n, nueellus. Within the nueellus (n) is seen the long pear - shaped embryo - sac containing egg - apparatus, polar nuclei with vacuole between them, and antipodal cells. Magnified 82 diameters. (R. S.) d. Pollination and Fertilisation The pollination of Lilies is carried on by moths and butterflies, which come in search of the honey secreted hy the nectaries on the perianth. The flowers are made 184 STRUCTURAL BOTANY very conspicuous to the visitors by their great size and bright colours, and in some species the sweet smell is an additional attraction. Generally the style projects so far that an insect comes into contact with the stigma before touching the stamens. Hence it will usually bring pollen from the flower which it has last visited, and will thus effect cross-fertilisa- tion. But sometimes it happens that the anthers and stigmas of the same flower brush against each other, and then self-fertilisation may result. As the visits of the insects are rather uncertain and very dependent on weather, this power of fertilisation by its own pollen is very useful to the Lily as an extra chance of setting its seeds. When the pollen-grains, by one means or the other, are brought into contact with the stigma, they stick to its viscid surface and soon begin to send out their tubes. The grain takes up water from the stigma, and begins to swell. The fold of pure cellulose (see p. 178) bulges out, and, following the growth of the protoplasm within, forms the membrane of the pollen-tube (see Fig. 80). Fig. 84. — Embryo-sac of a Lily, s, s, syner- gidse ; o, ovum ; v, v, vacuoles ; p1 and p„, the polar nuclei ; an, antipodal cells. Magnified about 250 diameters. (After Guignard. ) Note that this figure is reversed as compared with Fig. 83. THE WHITE LILY 185 The tube at first creeps along among the papillae of the stigma, and then grows down between the lobes and penetrates the middle of the style, until its growing end reaches the ovary (see Fig. 81). Here it grows along the surface of the placenta, and at last turns aside into the micropyle of an ovule. Its growth still con- tinues, until it makes its way between the cells of the nucellus to the em- bryo-sac, which it enters, usually pene- trating for a short distance between the synergidse to the ovum (see Fig. 86). Here the growth of the tube ceases. The length which it has attained is enormous compared to the size of the pollen-grain, for in a large Lily the nearest ovule may be as much as six inches from the surface of the stigma. The large size of the pollen grains and tubes has rendered it possible to trace exactly what is going on inside, and these changes have been followed more minutely hi the Lilies than in any other plants. When the pollen begins to germinate, the vegetative nucleus belonging to the large cell of the pollen-grain is the first to enter the tube. It takes Fig. 85. — Pollen-tube of a Lily on its way down the style. g, g, the two gen- erative cells, each nearly idled by its large nucleus ; nv, vegetative nucleus, already becoming disorganised. Mag- nilied about 450 diameters. (After Guignard. ) the lead all through so long as it exists, but it 186 STRUCTURAL BOTANY becomes gradually disorganised, and before the ovule is reached is altogether lost in the protoplasm. The small generative cell follows the vegetative nucleus into the tube and immediately divides, so that there are now two such cells, one behind the other, each with a large nucleus (see Fig. 85). When the embryo-sac is reached, the nucleus of the leading generative cell, accom- panied by part of its pro- toplasm, passes out through the softened membrane at the end of the tube, and makes its way into the protoplasm of the ovum (see Fig. 86, g.n). It travels on until it reaches the nucleus of the ovum, and then the two nuclei apply themselves closely together (see Fig. 8 7, g.n, o.n ). The protoplasm which had accompanied the generative nucleus unites with the protoplasm of the ovum. The ovule now acquires a cell-wall of its own, and fertilisation is complete. The nucleus of the fertilised ovum, consisting of the united male and female nuclei, now divides, its division being followed by that of the ovum itself, and thus the development of the embryo or new plant is started. The synergkke, having done their work, which Lily, showing fertilisation, s, s, synergidse ; o, protoplasm of ovum ; o.n, nucleus of ovum ; g.n, generative nucleus from pollen-tube, about to fuse with nucleus of ovum ; g. n.,, second generative nucleus ; p.t, pollen- tube. Magnified about 350 diameters. (After Guignard.) THE WHITE LILY 187 probably consists in guiding the end of the pollen- tube to the ovum, now disappear. The antipodal cells also undergo no further development. The embryo - sac is in future shared between the embryo and the endosperm, which latter structure is much more import- ant here than in the Wallflower. e. Development of the Embryo We need only de- scribe fully those points in the forma- tion of the embryo in which the monocoty- ledonous and dicoty- ledonous classes differ. These differences are important, and among them we shall find one of the most constant points of distinction between the two e.n. < i o. n. \:V'l . A-V n- ^V;wl iWPif Fig. 87. — Part of the embryo-sac of a Lily, just after fertilisation, s, s, re- mains of the synergidse ; g.n, gener- ative uucleus; o.n, nucleus of ovum (these are now closely united) ; v.v, vacuoles ; e.n, e.n, endosperm nuclei in course of division. Magnified about 250 diameters. (After Guig- nard. ) divisions. The fertilised ovum divides by two transverse walls. Of the three cells thus formed, the one next the micropyle (which we will speak of as the loivcst ) under- goes no further divisions, but often grows to a great 188 STRUCTURAL BOTANY size, as in the embryos shown in Figs. 89 and 90, and then forms a temporary store of food for the embryo. The uppermost cell, instead of forming nearly the whole of the embryo, as in our dicotyledonous type, here only gives rise to one part, namely, the single seed-leaf or cotyledon, which in this case forms the free end of the embryo. The middle cell of the three divides by further transverse walls, forming, after some longitudinal divisions have taken place, the groups marked st and r in Fig. 90. The group of cells marked st is a very important one, for from the cells on one side of this group arises the growing point of the stem, which in Monocotyledons is a lateral structure. The group marked Fig. 88 — Longitudinal section of ovule of Scilla (a plant allied to the Lily) after fertilisation, p.t, pollen -tube; cm, embryo ; c, c, endosperm nuclei still free in the protoplasm of the embryo-sac. Other parts as in Fig. 83. Magnified 55 diameters. (R. S.) r forms the hypocotyl and root, and also contributes something to the suspensor. The form of the more advanced embryo, after a great many cell-divisions have taken place, is shown in Fig. 91. Towards the top of this figure we see the radicle (r), with its growing point directed towards THE WHITE LILY 189 e.s. the micropyle (vi). On one side of the embryo is a de- pression, in which the lateral growing point of the stem (g.p) is situated. All the part beyond this is the cotyledon (c), which at this stage occupies more than half the entire length of the embryo. The essential dif- ferences, then, be- tween the embryos of a Monocotyledon and a Dicotyledon are these. In the Monocotyledon there is a single terminal cotyledon, and the apex of the stem is lateral ; while in the Dicotyledon there are two lateral cotyle- i • , , , , , n Fig. 89. — Very young embryo of Alisma Cions, witn the ter- Plantago , a Monocotyledon, n, cells minal apex of the of nucellus ; e.s, membrane of embryo- . ‘ . sac. The embryo with its suspensor Stem between them. consists of three cells, the basal cell These distinctions are beins veiT lal'Se- Magnified about 500 diameters. on the whole con- stant, in spite of great variations in the embryology of both classes. The dimensions of the embryo, as compared with the whole seed when ripe, are shown in Fig. 91. At the stage figured the chief systems of tissue in the young plant are already marked out. The embryo now enters on a period of rest. Its further develop- ment is postponed until the germination of the seed, and 190 STRUCTURAL BOTANY will be described below. In the meantime we must shortly consider the changes which take place in other parts of the ovule during its conversion into a seed. Endosperm In the Lilies the union of the two polar nuclei of the embryo-sac takes place comparatively late — in fact, only just before the moment when the ovum is ready for fertil- isation. The polar nuclei unite, whether fertilisa- tion takes place or not, but the changes about to be described only take place if fertilisation has been accomplished. The united nucleus of the embryo-sac divides, but without any forma- tion of cell-wall between the daughter-nuclei. The division is repeated again and again in the same manner (see Fig. 87). Soon the nuclei thus formed arrange them- selves in the layer of pi’otoplasm lining the wall of the sac, and there they still go on dividing. All this time the embryo-sac is increasing enormously Fig. 90. — Older embryo of same, seen in section. The basal cell is still larger than before. Its enormous nucleus has two nu- cleoli. cot, cells which will form the cotyledon ; st, disc of cells from which the stem arises later ; r, radicle, diameters Magnified about 500 THE WHITE LILY 191 in size, and the increase in the number of the nuclei keeps pace with its growth (see Fig. 88). For a long time no cell-walls are formed between them. They lie scattered in the undivided protoplasm. Only after hundreds of free nuclei have been produced does tissue formation begin. Then for the first time cell-walls make their appearance between the nuclei, so that the Fig. 91. — Longitudinal section of the ripe seed of a Lily. t, testa ; m, micropyle ; e, endosperm, in which the embryo is embedded ; r, radicle of embryo ; g.p, growing point of stem ; c, cotyledon. Magnified diameters. protoplasm is partitioned up into as many distinct cells as there were free nuclei before. The endosperm tissue, as we may now call it, at first forms a single layer lining the sac. Growth and cell-division, however, go on continuously, until by the time the seed is ripe the whole cavity of the sac, except the small part 192 STRUCTURAL BOTANY occupied by the embryo, is filled by a mass of endosperm (see Fig. 91). Its cells contain reserve starch and proteids, and have very thick cell-walls. The whole tissue forms a great storehouse of food, from which the young plant can draw supplies during germination, before it is ready to shift for itself. This is the most striking difference between the seeds of the Lily and of the Wallflower : in the latter the greater part of the endosperm is a transitory and unimportant structure, which is used up long before the seed is ripe ; in the Lily it forms a massive tissue constituting the bulk of the ripe seed, and is only consumed during germination. The Lily thus has a typically albuminous seed, while the Wallflower is nearly, though not quite, exalbuminous. \ As regards the presence or absence of endosperm, there is no con- stant difference between the two classes. While, as we have seen,, a large number of Dicotyledons have albuminous seeds, many Monocotyledons are ex- albuminous, as, for example, the Orchids and the Water-plantains. From one of the latter ( Alisma Plantago ) our Figs. 89 and 90 were taken. In both classes a majority of the members have albuminous seeds. The albuminous seeds of some of the Monocotyledons have great practical interest. In Wheat, Rye, and the other grains it is the endosperm of the seed from which flour is prepared. In fact, when we eat bread we are converting to our own use the stores of food which the Wheat or Rye plant had provided for the benefit of its seedlings. THE WHITE LILY 193 The Ripe Seed and Fruit The seeds of the Lilies are arranged in six rows in the fruit, two rows in each compartment ; they are packed very close together, and are flattened, so that each row has somewhat the appearance of a pile of coins. Fig. 9 1 shows the section parallel to the broad side of the seed. Each seed has a hard, thick testa, formed from the two integuments of the ovule, the outer of which constitutes the thicker and harder layer. The ripe seed, therefore, consists of three essentially different structures : (1) the embryo, developed from the fertilised ovum ; (2) the endosperm, or tissue formed inside the embryo-sac; (3) the testa, formed from the integuments of the ovule. The nucellus has by this time been absorbed. The point where the seed is attached to the funicle is called the hilum, and as the ovule from which the seed was developed was anatropous, it follows that the hilum and the micropyle are close together. The fruit is called a capsule, i.e. it is syncarpous, dry, and dehiscent. The structure shows very little differ- ence from that of the ovary, but there is an enormous increase in bulk, and some change in shape, the fruit being thicker hi proportion to its length than the ovary. The style withers away during the ripening of the fruit. The opening or dehiscence of the capsule to set free the seeds takes place by three longitudinal splits, running along the middle of the outer wall of each compartment throughout its whole length. The fruit 13 194 STRUCTURAL BOTANY is thus cleft into three divisions, each representing two half carpels. Germination The organ which grows most actively in the early stages of germination is the cotyledon, which elongates to an enormous extent, attaining a length of several inches. The radicle is first pushed out through the micropyle and then grows down into the earth. The tip of the cotyledon remains immersed in the endo- sperm, and acts at first as an absorbing organ, by means of which the food-substances in the seed are taken up by the young plant. The cellulose, starch, and proteids of the endosperm have to be rendered soluble before they can be absorbed. This is brought about by means of various ferments, which are probably secreted by the cotyledon (see p. 214). In its lower sheathing part the cotyledon encloses the bud of the stem, and below this again are the rudimentary hypocotyl and the main root, which is developed from the radicle and grows straight down- wards. Henceforth the seedling is able to provide its own mineral food. The seed-coat is lifted high above the ground on the tip of the cotyledon, which becomes a long narrow green leaf, and thus constitutes the first assimilating organ of the seedling. The young stem develops into the bulb, the first bulb-scale being formed by the sheath of the cotyledon. Other scale-leaves are soon formed from the growing point. The bulb appears at first as a lateral growth at one side of the base of the cotyledon. The main THE WHITE LILY 195 root may attain a length of a couple of inches, hut it sends out few, if any, branches. Its function is only temporary, for it is soon replaced by adventitious roots arising from the base of the bulb. After a time the bulb sends up green foliage-leaves; the cotyledon decays ; the seed, the endosperm of which is now exhausted, is reduced to an empty shell ; and the embryonic stage is now ended. Summary If we now shortly sum up those essential points in the life-history of the Lily in which it serves as a type of the Monocotyledons generally, we find that the following are the chief : — (1) In the external morphology the most striking point is the predominance of adventitious roots, the mam root serving a merely temporary purpose, namely, the nutrition of the seedling. The explanation of this peculiarity is to be found in the mode of growth of the plant as a whole. The main root is formed simul- taneously with the basal part of the stem, which is also the thinnest part, and is in proportion to its small dimensions. Hence, as the stem becomes for a time larger and larger in each successive node and internode, the root becomes more and more inadequate to supply its needs. The root has no secondary growth in thickness, and therefore cannot make good its deficiencies. Thus the requirements of the plant can only be satisfied if the later-formed bulkier portions of the stem produce new and larger roots proportioned to the organs which they supply. 196 STRUCTURAL BOTANY (2) The primary bundle-system in the stem and root is on the whole more complex than in Dicoty- ledons, the individual bundles being more numerous, and their course, in the stem at any rate, less simple. In the leaf we have, as a rule, what is called “ parallel venation,” which means that there are a number of large longitudinal bundles with more delicate trans- verse connections. The presence of numerous main bundles in the leaf goes together with the complex conducting system of the stem. The fact that the leaf generally has a broad base also helps to ex- plain the number of the vascular bundles which enter it. (3) The closed structure of the vascular bundles, and the general, though not universal, absence of secondary thickening in Monocotyledons, have been sufficiently dwelt upon above. These characters render necessary the greater complexity of the primary tissues, and also explain the absence of a persistent tap-root, which without cambium cannot satisfy the needs of the plant as it grows in bulk. (4) Perhaps the most constant character of all is found in the structure of the embryo, with its single terminal cotyledon and lateral growing point. It is this peculiarity, more than any other single character, which leads us to regard the Monocotyledons as a class distinct in origin from the Dicotyledons. We have now considered the distinction between the two classes ; their points of agreement scarcely need to be emphasised. The general morphology is alike in both ; stem, THE WHITE LILY 197 leaf, and root show the same distinctive characters, both internal and external. The structure of the tissues, as distinguished from their arrangement, is similar in the two classes. The most striking agreement, however, is in the floral organs. Speaking broadly, we may say that there are no characteristic differences between the two classes here. Their similarity is equally marked, whether we consider the general plan of the flower, the structure of its several organs, or the minute details of their development. Apart from one or two trivial and not very constant differences, we may say that every detail in the development of the perianth, of the stamens, of their pollen-sacs and pollen, of the pistil, the ovules, the embryo-sac and the endosperm, as well as the process of fertilisation, shows the most minute correspondence in both classes. In fact, so close is the agreement in all important characters, that if we want to find a ready means of distinguishing these classes by their flowers, we have to fall back on arithmetic, and make what use we can of the rule that in Monocotyledons the parts of the flower are usually in threes, and in Dicotyledons in fours or fives ! On the whole, we may say that the points of agreement between our first two types much outweigh the points of difference. In the classes which remain to be considered this will no longer be the case. The classes Dicotyledons and Monocotyledons together constitute the sub-kingdom Angiosperms, characterised by the fact that the ovules are contained within a closed ovary. CHAPTER III Outlines of the Physiology of Nutrition In studying our first two types we have obtained a general idea of the form and structure of the root and shoot in green plants. We have also learned some- thing as to their functions. It will be well for us, however, before proceeding further, to endeavour to gain a more connected view of the mode of working of the vegetative organs. We have to learn what food the plant needs ; from what sources its various kinds of food are obtained ; what changes they have to undergo in order to be made use of by the plant ; we have also to inquire how plants breathe ; and gener- ally, what are the actions which a plant carries on as a living organism. All these are physiological questions, for we have already learned that physiology asks what is the work done by plants and their organs. We shall only attempt here to give a very short and simple answer to these inquiries ; anything like a full answer would require an advanced knowledge of physics and chemistry, which we must not take for granted. A little elementary knowledge of these two subjects is indeed absolutely necessary in order to understand even the simplest facts about the life of plants. 198 OUTLINES OF THE PHYSIOLOGY OF NUTRITION 199 The Food of Plants. — The first step necessary in order to understand anything about the nutrition of plants is to learn what chemical elements a plant contains. Probably most of the elements which exist have been detected at some time or other in plants ; those, however, which are constantly present, are only thirteen in number, namely, carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, calcium, magnesium, iron, sodium, chlorine, and silicon. But although these elements are very con- stantly found, on chemically analysing green plants, it does not follow that they" are' all necessary to the life of the plant. Many of them we know to be so, because they enter into -the chemical composition of the substances of which the tissues of the plant consist. For example, the three elements, carbon, hydrogen, and oxygen, constitute cellulose and starch ; the proteids, of which the protoplasm is composed, contain, in addition, nitrogen and sulphur ; while phosphorus is regularly present hi the substance of the nucleus. As regards the other seven elements, analysis does not tell us whether they are necessary to the life of the plant, or whether they may not be taken up simply because they are present in the soil. This question can only be answered by experimental cultures, so arranged that we know exactly which chemical bodies are supplied to the plant. Ihe simplest method is by means of cultivation in water. "We must take care that the water used is pure to begin with, and therefore distilled water must 200 STRUCTURAL BOTANY always be chosen. To this water we add small quantities of the chemical compounds with which we wish to experiment. Plants never take up their food directly in the form of chemical elements; the elements which the plant absorbs are always already combined to form compounds. It will be well to choose such compounds as are known to be present in a natural soil. We may begin with a food-solution of the following composition : — Water (H20) . . . . 1000 cubic centimetres Potassium nitrate (KN03) 1 ‘0 gram Sodium chloride (NaCl) = common salt . . ’5 gram Calcium sulphate (CaS04) .... '5 gram Magnesium sulphate (MgS04) . ... -5 gram Calcium phosphate (Ca3(P04)2) .... '5 gram Iron chloride (Fe 2 Cl 0) a trace It will be seen that the amount of the various salts is very small in comparison with the bulk of water. Plants in nature take up their food, in so far as it is supplied to them by their roots, in very weak solutions. A seed (say, of the Maize, or Bean, or Pea, or Buck- wheat) is allowed to germinate in damp sawdust. The seedling is washed in distilled water, and is then fixed in a split cork fitted into a wide-mouthed bottle, with its root dipping into the food-solution with which the bottle is filled. We have spoken of a food solution, but one of the salts, calcium phosphate, is very slightly soluble in water, and most of it will remain as a powder at the bottom of the bottle. OUTLINES OF THE PHYSIOLOGY OF NUTRITION 201 Now, in a water culture of this kind, the young plant will remain healthy and continue to grow; it will even in time flower and ripen its seed. In fact, under these artificial conditions, the plant may be brought successfully through its entire life-history, while the amount of its organic substance is increased, it may be several hundredfold, as compared with the quantity contained in the seed to start with. The success of this experiment proves that the compounds presented to the plant contain all that is necessary for its nutrition. Yet among the elements contained in the water and salts of the food-solution one is con- spicuously absent ; we have given the plant no carbon whatever. We know, however, that the dry-weight of a plant — that is to say, its weight after evaporating the water which it contains — is half made up of carbon. We know also that every organic substance — cellulose, starch, sugar, proteid, or anything else which goes to build up an organism — is a compound of carbon ; we know, then, that as the organic substance of a plant has increased some hundredfold, so also has its carbon increased in like proportion. Our specimen, then, has obtained abundant carbon, but not from the food- solution, and therefore not by means of its roots. The only remaining source is the atmosphere, and the carbon can only have been obtained by means of those organs which are exposed to the air, namely, the leaves and stem. We will return to the question how the plant obtains its carbon ; our immediate object is to determine which of the other elements are essential. 202 STRUCTURAL BOTANY It will be noticed that no compound of silicon was added to the water, and this element certainly cannot be obtained from the air. Some trifling quantity might possibly be derived from the glass of the bottle or from chance dust, but such amounts are too small to be of any importance. We infer, then, that silicon, in spite of its general presence in plants, is not essential to their nutrition. This does not prove that it is of no use. To many plants, such as grasses, silicon is of great importance in hardening the outer surface of the epidermis, and thus rendering it a better protection against parasites, but as food silicon is not necessary to the plant. Of the salts offered to the plant, sodium chloride can easily be proved not to be essential. If it is left out altogether, the plant will still flourish and come to maturity. The sodium, then, can certainly be dispensed with. It is said that for some plants chlorine in some form or other is needed, but certainly this is not the case with most ; yet these two elements are constantly present in plants, simply because they are constantly present in the soil. If, however, we omit any of the other elements from our food-solution, the plant suffers. Suppose, for example, that no compound of iron be added to the water ; for a time the seedling grows normally ; the new leaves which are formed, however, are not green, but white — in fact, they contain no chlorophyll. Hence no assimilation of carbon can take place, and no further increase in the organic substance of the plant can be produced. The reason why the first few OUTLINES OF THE PHYSIOLOGY OF NUTRITION 203 leaves are green is that a certain amount of iron is contained in the seed. The disease caused by the want of iron is called chlorosis, which means pallor, and is very easily cured. If a few drops of a solution of an iron salt be added to the water, the pale leaves soon begin to turn green, and the same result is produced if the surface of the leaf be painted with the dilute iron compound. It is very important to add no more iron than is necessary. In quantities sufficient to colour the water it is poisonous to the plant. The way in which iron is necessary is thus made quite clear. Without iron no chlorophyll can be formed. The necessity for the six elements which are known to enter into the composition of the tissues is self-evident, and is easily proved experimentally. Suppose, for example, that we wish to determine whether phosphorus is essential. All we have to do is to leave out the calcium phosphate and substitute for it the same quantity of calcium sulphate. The growth of the seedling soon ceases — as soon, in fact, as the phosphorus in the seed has been used up. In like manner we can prove the necessity for the other tissue-forming elements. The presence of potassium, calcium, and magnesium is also essential. If we substitute sodium nitrate for potassium nitrate, the seedling stops growing ; and the same is the case if we replace the calcium and magnesium compounds by those of any other metals. These elements are in some way or other necessary for the process of assimilation ; in their absence the 204 STRUCTURAL BOTANY formation of new organic substance is checked. Their exact mode of action, however, is unknown. We learn, then, by this method of water culture that the essential elements for the nutrition of a green plant are only ten in number, namely, carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, calcium, magnesium, and iron. Something must be said about the source of nitrogen. Nitrates, such as potassium nitrate, are, as we have seen, quite successful as a source of nitrogen to the plant. If we use ammonium com- pounds, such as ammonium phosphate ((NH4)3P04), instead of nitrates, the plant will also flourish, but it is possible that in this case the ammonium compound is converted into a nitrate before being made use of. If, however, we leave out nitrogen compounds from the food-solution altogether, most plants will starve. Yet we know that the atmosphere consists of 4 of nitrogen to \ of oxygen, approximately. In most cases, then, the free nitrogen of the ah' cannot be assimilated by plants. It appears, however, that there are important exceptions to this rule, and that plants of the Pea and Bean kind are able, under certain conditions, to supply their demand for nitrogen entirely from the air, while no nitrogenous compounds are offered to their roots. The conditions under which this takes place, however, can only be under- stood after we have made ourselves acquainted with some of the simplest vegetable organisms, for only by the help of certain fungus-like companions can the Bean plant and its allies make use of atmospheric nitrogen. OUTLINES OF THE PHYSIOLOGY OF NUTRITION 205 Assimilation of Carbon. — We will now consider the question how a plant containing chlorophyll obtains its supply of carbon, the most important of all the elements for nutrition. We have already learnt that the plant certainly does not obtain this element by its roots, and that the air is the only source of carbon available. Now, the atmosphere contains carbon in the form of carbon dioxide, more commonly, though inaccurately, called carbonic acid.1 The quantity of this gas contained in the ah- is very small, amounting only to 3 or 4 volumes in 10,000 volumes of ah. Yet from this relatively small amount the entire supply of carbon for most green plants is obtained. Carbon dioxide is formed whenever carbon is com- pletely burnt, or, in other words, completely oxidised. It contains the maximum amount of oxygen with which carbon can combine. Now we know that the substance of plants is combustible ; in fact, all our fuels are directly or indirectly of vegetable origin. It follows, then, that the tissues of plants must be com- paratively poor in oxygen, or they would not burn. Hence we see that the carbon dioxide of the air must lose the whole or part of its oxygen when it is assimilated by a plant. That green plants in sunlight give off oxygen is easily proved. Some pieces of any green water - 1 Carbonic acid is, properly speaking, the compound of carbon dioxide with water, and has the formula H2C03. Hence carbon dioxide is sometimes called carbonic anhydride , the latter word implying that the acid is deprived of its water. 206 STRUCTURAL BOTANY plant (such as the Elodca, mentioned on p. 41) are put into a glass of ordinary water, which always contains some carbon dioxide in solution. The glass with the Water- weed in it is allowed to stand in a sunny place, and we soon see that bubbles are given off from the plant in a constant stream. If we catch these bubbles, as they rise to the surface, in an inverted test-tube filled with water, we find that they consist chiefly of oxygen, as can be easily shown by plunging a glowing match into the gas thus collected ; the match will at once catch light again. If, however, the plant be placed in water which has been previously deprived of its carbon dioxide by boiling, no oxygen will be given off. If a plant, or simply some green leaves, be placed in a bell-glass, dipping into mercury below, and con- taining a measured quantity of carbon dioxide, and left for some time exposed to light, the volume of the gas in the jar remains unaltered. If the gas be analysed after some hours, the carbon dioxide will be found to have diminished, and the oxygen to have increased, and in fact the volume of carbon dioxide lost will be exactly equal to the volume of oxygen gained. In other words, the plant has given out exactly as much oxygen as it has absorbed carbon dioxide. This seems at first to prove that all the oxygen of the carbon dioxide is given off, and only the carbon retained by the plant. We shall see presently, however, that another explanation is more probable. The decomposition of so stable a compound as OUTLINES OF THE PHYSIOLOGY OF NUTRITION 207 carbon dioxide is a very difficult process to carry out iu the laboratory. Thus, in order to reduce carbon dioxide (C02) to carbon monoxide (CO) by heat, a temperature of no less than 1300° C. is required. Yet this same work is carried on by the plant at the ordinary temperature of the air — say 10° to 30° C. The energy which enables a green leaf to do its work is derived from the rays of the sun ; plants will only assimilate in the light. This is easily proved ; if we repeat the experiment just described in a dark place, no carbon dioxide is absorbed, and no oxygen is set free. Other sources of light may be substituted for the sun. Thus, plants will assimilate actively and con- tinuously when exposed to the electric light. As in nature sunlight alone is in question, we may say that all the organic material of the world, all the animal and vegetable substance which it contains, has been directly or indirectly produced through the assimila- tion of carbon from carbon dioxide by green plants in sunlight. There are not many rules in science without an exception : it has lately been shown that certain very simple organisms are able to obtain their carbon from inorganic sources without the help of light ; the amount of organic substance thus formed, however, is too small to affect our statement appreci- ably. It is only green plants, and parts of plants, which can decompose carbon dioxide in sunlight. If we had used for our experiments coloured or white flowers, or colourless roots, or any part of a plant containing no chlorophyll, we should have found that no absoi ption of carbon dioxide or evolution of oxygen 208 STRUCTURAL BOTANY took place. It is not necessary that the plants should appear green externally, it is enough that they con- tain chlorophyll. For instance, the leaves of the Copper Beech are of a brownish-red colour, yet they assimilate like ordinary green leaves, because they contain chlorophyll, which, however, is disguised by the presence of a red colouring matter in addition. We already know that chlorophyll is contained in the protoplasmic chlorophyll-corpuscles or plastids. The whole apparatus is necessary for the work to be done ; the chlorophyll is useless for the purpose of assimilation without the protoplasmic plastid, and the plastid cannot assimilate without the chlorophyll. We are not at all clear at present as to the special duties performed by the two partners. Most probably the chlorophyll serves to catch certain rays of light, by the energy of which the plastid is enabled to do the work of decomposing carbon dioxide and water. It has been proved that the red rays of light are the most active in promoting assimilation. The absorp- tion spectrum of chlorophyll shows a conspicuous dark band in the red. Hence we know that these red rays are absorbed by the chlorophyll, and do not pass through it as light. It is very probable that it is the energy of these absorbed rays which is employed on the work of assimilation. We have now seen that assimilation is dependent on light ; it is also dependent on temperature. Although all other conditions may be favourable, yet if the temperature be too low no assimilation takes place. Thus, in our experiment with the Elodca , we OUTLINES OF THE PHYSIOLOGY OF NUTRITION 209 should find little or no oxygen given off unless the temperature of the water was at least 6° C. The minimum temperature varies very much for different plants, but for every plant there is a certain temperature below which no carbon dioxide can be decomposed. Light is not only necessary in order that the chlorophyll-containing organs may perform the work of assimilation, but also for the formation of chloro- phyll in the first instance. Although the protoplasmic plastids are always present, the pigment chlorophyll is not formed in them in the absence of light. If a Potato plant be grown in the dark, it remains of a sickly yellowish colour. This colour is due to a substance called etiolin, which is distinct from chloro- phyll, and may become converted into it when exposed to the light. We must be careful to distinguish between these two actions of light, which are quite distinct : a plant will become green in the light even though it be kept in air containing no carbon dioxide, so we see that assimilation is not necessary for the production of chlorophyll, though the presence of chlorophyll is one of the necessary conditions of assimilation. A green plant kept in the dark cannot decompose carbon dioxide, and therefore under these circumstances it forms no new organic substance. A potato kept in a warm and damp but dark cellar will often grow. It produces a pale, unhealthy plant, with long stems and small leaves. If such a plant be dried and weighed, we find that its solid substance is not H 210 STRUCTURAL BOTANY greater, but rather less, than that of the tuber from which it has developed. It has grown entirely at the expense of the food-substances ready stored in the tuber ; a great deal of water has been absorbed, but no new organic substance has been formed ; on the contrary, some has been lost, for a reason which we shall explain presently. The plant, in fact, has earned nothing, but has been living on its capital, and has naturally become poorer by doing so. The same thing would happen if a plant were allowed to grow in the light, but in an atmosphere destitute of carbon dioxide. For the purpose of this experiment the specimen is grown under a bell-glass, into which air is admitted through a tube, containing pieces of pumice-stone soaked in a solution of caustic potash (KOH). The potash absorbs the carbon dioxide, so that none reaches the plant. Here also the result is that no fresh organic material is formed ; if the plant grows at all, it does so at the expense of the food - substances which it had previously stored up. We may sum up our conclusions up to this point as follows : — Green plants, as a rule, obtain all their carbon by the decomposition of the carbon dioxide of the atmosphere. Consequently, in the absence of carbon dioxide, no fresh organic substance is formed. The decomposition of carbon dioxide can only take place under the action of light (the red rays being the most effective) and at a sufficient temperature. OUTLINES OF THE PHYSIOLOGY OF NUTRITION 211 It can only be performed by those parts of plants which contain chlorophyll in their plastids. This process of the assimilation of carbon from carbon dioxide may fairly be considered the most important of all physiological phenomena, for on it the whole existence of plants and animals is dependent. Herbivorous animals eat plants, and are in their turn eaten by carnivorous animals and by man. Our own life, and that of the whole animal kingdom, would thus be absolutely impossible without the work done by the chlorophyll-corpuscles of plants. We will now inquire what becomes of the carbon after it is taken up by the plant. We know in a general way that it is employed with other elements to build up the protoplasm, starch, cellulose, and other bodies of which a plant consists. Can we trace the stages of the process ? Our knowledge here is still very imperfect, but yet something has already been made out. It is improbable that the carbon dioxide which is absorbed is completely deoxydised. We know that the simplest organic bodies found in plants contain at least the elements of water, oxygen, and hydrogen in addition to carbon. Now it is not likely that carbon and water should enter into combination in the plant, for both are chemically inactive bodies at ordinary temperatures. If, however, water (IT,0) as well as carbon dioxide (C02) is decomposed, and the oxygen given off is thus derived half from the carbon dioxide and half from the water, the bodies remaining in the plant will be carbon monoxide (CO) and nascent 212 STRUCTURAL BOTANY hydrogen. Both of these are bodies which readily form new combinations. Now, as we learnt some time back (see p. 36), we find in most plants, though not in all, that starch granules soon make their appearance in the chlorophyll corpuscles when assimi- lation is going on. In some of the simpler plants an exposure to sunlight of five minutes only is sufficient to cause the formation of starch in the chlorophyll bodies, when none had been present before. In darkness, or in the absence of carbon dioxide, the starch disappears, and is not replaced. The chemical elements of starch are present in the proportions C6H10O5 ; we must not suppose, however, that starch is ever the direct product of assimilation. In all cases a sugar of some kind makes its appearance before starch. In some eases the sugar formed appears to be grape-sugar or glucose, which has the formula C6H1206 ; while in other cases cane-sugar (C12H22Ou) is said to be the first demon- strable product of assimilation. How the sugar is formed we cannot say : it is easy to give a chemical equation for the process ; thus, supposing grape-sugar to be formed, the equation would be — 6 (CO) + 6 (Ho) 4 C0H12O6 ; it is quite certain, however, that nothing so simple as this takes place in nature. There is a body called Formic Aldehyde (CH20) which has been supposed to represent an intermediate stage in the process — co + h2 = ch2o, 6(CH20) = C6H1206. Many kinds of sugar have recently been made from inorganic compounds in the laboratory, and this is OUTLINES OF THE PHYSIOLOGY OF NUTRITION 213 perhaps the most important advance which organic chemistry has made in our time in a physiological direction. It is an interesting fact that bodies of the nature of aldehydes are important links in the chain of reactions by which sugars have been artificially produced. It is, however, perfectly possible that the carbon monoxide and nascent hydrogen may first be taken up by the proteids which constitute the protoplasm, and that the sugars may be formed by a subsequent breaking down of the highly complex proteid sub- stances. Our actual knowledge of what takes place in a plant does not go beyond this : that the first demonstrable product of assimilation is some form of sugar, which is produced, however indirectly, as the result of the decomposition of carbon dioxide and water. A certain part of the sugar, which is formed as an ultimate result of assimilation, is in most plants employed by the protoplasm for the manufacture of starch ; the remainder is probably conveyed away at once to other parts of the plant as some form of sugar, without necessarily passing through the intermediate form of starch. Starch, in fact, is nothing more than an insoluble form of carbohydrate, which is convenient for storing purposes. Both in the chloroplastid and in the leucoplastid starch is produced at the expense of sugar. The difference between them consists in the origin of the sugar, which in the former case is the result of the assimila- tion of inorganic compounds, while in the latter it is derived from previously existing carbohydrates. 214 STRUCTURAL BOTANY If starch is formed, it has to be rendered soluble, in order to be transportable to the other parts of the plant. It is well known that starch can be converted into sugar by means of a ferment called diastase, which is derived from the protoplasm, and is found, for example, in large quantities in germinating seeds, such as those of Barley during the process of malting. It has been proved that diastase is present in the leaf, and serves to convert into sugar the starch which is formed in the chlorophyll-corpuscles. The soluble carbohydrates are transported through the parenchyma, and especially through that part of it which immediately accompanies the vascular bundles. They may either be conducted directly to the seats of growth, such as the apical growing points, or the cambium, to be employed chiefly in the formation of new cell-walls, or they may be conveyed to organs which serve as storehouses of reserve food, such as the endosperm or the cotyledons in a seed, the tuber of a Potato, or the root of a Turnip. In these regions the sugar is very often reconverted into starch by the leucoplastids ; it may, however, be stored up in a soluble form, as inuline in the Dandelion, or cane- sugar in the Beet. In some cases it is employed to form enormously thick cellulose walls, as in the endosperm of many Palms ; these cell-walls are dissolved again on germination, and so rendered available as food for the seedling. In other cases, again, as in oily seeds, the non-nitrogenous food- materials are stored, not as carbohydrates, but as fatty oils. OUTLINES OF THE PHYSIOLOGY OF NUTRITION 215 We know little or nothing at present about the first formation of proteids in the plant. It is most probable that, like the carbohydrates, they are first formed in the leaves. In addition to carbon, hydrogen, and oxygen, nitrogen and sulphur also enter into their composition, while the nuclein or chromatine of the nucleus further contains phosphorus. The three elements last named are, as we have seen, taken up in the form of various salts with the water absorbed by the roots. It appears probable that the proteids formed in the assimilating tissue are taken up by the phloem-cells at the ends of the vascular bundles, and it may be regarded as certain that they are conveyed to other parts of the plant through the sieve-tubes. We often find the proteids stored up in the form of definite granules, such as the aleurone-grains, which occur very generally in seeds. Absorption. — We have now obtained some idea of the chief nutritive processes of a chlorophyll-containing plant, but we have still to inquire how the various food-substances are absorbed. As regards the carbon dioxide, it has been proved that this gas can be absorbed through the cuticle of the leaves. This was shown by taking two leaves, with stomata on the under surface only, and joining them firmly together back to back, so that only the upper surfaces, destitute of stomata, remained exposed to the air. Under these circumstances assimilation went on as usual, showing that the stomata are not necessary for the absorption of carbon dioxide, though they are no doubt in some cases utilised for this 216 STRUCTURAL BOTANY purpose.1 The carbon dioxide absorbed is at once dissolved in the cell -sap, and it is the dissolved gas which is decomposed by the chlorophyll-corpuscles. The mineral substances taken up by the root are absorbed by the root-hairs, where these are present, and must be taken up in solution. Yet we know that many plants flourish well in a comparatively dry soil. In such a soil, however, each particle of earth is surrounded by a film of water, which adheres to it firmly. The root -hairs attach themselves to such particles, and are thus enabled to absorb a part of the watery film which surrounds them (see Fig. 29, p. 77). The root-hairs, however, are not dependent entirely on salts which are ready dissolved in the water of the soil. They are able to dissolve other compounds for themselves. Calcium phosphate, for example, is only soluble to a very small extent hi water. The root- hairs, however, excrete an acid by which minute quantities of such salts are brought into solution, and can then be absorbed. This solvent action of the root-hairs can be proved by growing plants in a thin layer of earth covering a slab of polished marble. After some time, an exact print of the course of the roots will be found on the smooth surface of the marble, which has become corroded wherever the hairs of the roots have been in contact with it. The absorption by the root-hairs is in part a merely physical process (osmosis), which can be imitated 1 Researches now in progress are said to show that the stomata are more important as organs for the absorption of C02 than has been hitherto supposed. OUTLINES OF THE PHYSIOLOGY OF NUTRITION 217 artificially. If we take a glass tube and close it at the lower end with a piece of ordinary bladder, and if we then pour into the tube a solution of salt, and dip its lower end into distilled water, the liquid in the tube will rise. The bladder is permeable to water, the dissolved salt exerts an attraction on the water outside, and a passage of water from without inwards goes on continuously through the membrane. At the same time a much smaller amount of the salt solution passes out through the membrane into the surrounding water. The primordial utricle and cellulose wall of a root- hair form together a permeable membrane. The cell-sap contains various substances in solution, among which organic acids are said to be the most active in osmosis. The cell-sap is thus denser than the water outside, in spite of the salts which the latter holds in solution. Consequently the external fluid passes inwards, through cell-wall and protoplasm, into the cavity of the cell. A similar process causes the sap of the root-hair to pass on into the next inner cell, and so on. When the root-hair absorbs water, a little acid passes out at the same time. Whether this happens or not, depends upon the protoplasmic lining of the cell, which controls the osmotic processes, and often lets in water readily, while it quite refuses to let out the substances dissolved in the cell-sap. It is this control exercised by the protoplasm which distin- guishes osmosis as it occurs in living cells from the process as it takes place through dead membranes. The Ascent of the Sap. — The ascent of the sap 218 STRUCTURAL BOTANY through the plant is a subject which is by no means thoroughly understood at present. There are two distinct processes which have to do with it : one is root-pressure, the other is transpiration. The water taken up by the root through its hairs is forcibly pumped upwards, as can he easily proved by cutting off the stem or branches of a plant, especially in spring, when the ascent of the sap has begun, and before there is any appreciable loss of water from the surface of the leaves. From the cut stumps of the stem or branches great quantities of water, amounting in some cases to several quarts, are exuded. This has long been observed in the case of the Vine in spring, when it is said to bleed. The same thing can be seen at any time during active vegetation, but in the summer the plant is losing water rapidly by its leaves, and consequently the water-conducting tissues are comparatively empty. Under these circumstances it is necessary to wait for a time after cutting off a branch, before the welling-out of water from the stump begins. This movement of the sap is undoubtedly due to the action of the living cells of the root. Through the root-hairs they take up water from the outside, in the manner explained above. The elastic cell-walls become distended, while the sap within is at a high pressure, which may amount to three or four times the pressure of the atmosphere. The cell-wall is easily permeable by water, and such pressure in the interior of the cell could not be maintained if it were not for the protoplasmic lining by which the entrance OUTLINES OF THE PHYSIOLOGY OF NUTRITION 219 and the exit of the sap is regulated. Cells or whole tissues which are thus distended by sap are said to be in a turgid condition. The turgid parenchyma cannot go on taking up water indefinitely ; as fresh supplies are taken in from the soil, a portion of the cell-sap already contained in the tissue is continuously forced out again. The sap thus expelled from the cells under pressure does not leave the plant ; it is pumped into the vessels and tracheides of the wood. These elements, as we already know, only contain air at a very low pressure, so that little or no resistance is offered to the sap which enters them. As the process goes on, the column of water in the vessels rises, and when a cut stem bleeds it is from the open ends of the vessels, or tracheides, that the sap escapes. Under natural conditions, if root-pressure acted alone, the result could only be that all the wood of the plant would soon become completely water-logged. This actually happens in certain cases ; after a damp, cool night, when little or no evaporation can take place, we find that the leaves of many plants are studded with drops of water, which we might very easily mistake for dew. If wiped off, however, the drops soon reappear in the same spots, and in fact they come from within, not from without, being excreted by the plant. When a plant is completely gorged with sap, and no transpiration is possible, it gets rid of its superfluous water in the liquid form, and in many plants there are special glands for the purpose. This state of things, however, is exceptional. In 220 STRUCTURAL BOTANY the day-time, under the influence of light and heat, the stomata are active, and through them water is given off in the form of vapour. That this is so is easily proved. If a plant be placed under a bell- glass, the soil in which it is growing being completely covered with tinfoil, so that no evaporation from the earth is possible, we find that dew is soon deposited on the inside of the glass, if there is a slight fall in the temperature of the surrounding air. This dew is produced by the condensation of the aqueous vapour given off by the plant. Under ordinary conditions the air in the bell-glass soon becomes saturated with vapour, and no further transpiration is possible. If, however, we place under the bell-glass some calcium chloride (CaCl2), or other substance which absorbs watery vapour, we can obtain a measure of the transpiration of the plant, for the increase in weight of the calcium chloride in a given time is due to the water which it has absorbed and which was previously transpired by the plant. The quantity of water given off by transpiration is very large, amounting, as has been calculated, to from 50 to 100 litres a day in the case of a large tree. Now, this emission of watery vapour goes on entirely through the stomata. In very young leaves, before the cuticle has been fully formed, the surface of the epidermis itself can give off vapour ; but in mature functional leaves the cuticle is quite impermeable to the vapour, and the stomata are the only means for its exit. The stomata, therefore, by their opening and closing, can regulate the transpiration of the plant. OUTLINES OF THE PHYSIOLOGY OF NUTRITION 221 Generally speaking, they open in light and close in darkness, i.e. they open when the conditions are favourable for assimilation. Thus transpiration and assimilation go on together, an arrangement which is evidently highly expedient, for the current set up by transpiration brings to the leaves the water and mineral matter without which assimilation cannot take place. A few words must be said as to the mechanism of the stomata, though the subject is too difficult to be discussed fully here. As a general rule, the stomata open when the guard-cells are turgid, and close when they are flaccid. This can be proved simply enough in the case of large stomata, such as we find in many Monocoty- ledons. A pieee of epidermis is removed Fig. 92. — Stoma of Amaryllis, p, pore. A, In 2 per cent, salt solu- tion ; the guard -cells are flaccid, and the pore closed. B, In distilled water ; the guard- cells are turgid and the pore wide open. Magnified about 150 diameters. (R. S.) from a living leaf and mounted under the microscope, first of all in distilled water. The guard-cells curve more and more, and the pore between them opens to its full extent (see Fig. 92, B)\ next we replace the water by a solution of salt — a strength of 2 per cent, is sufficient. The curvature of the guard-cells now diminishes, and as they straighten themselves the 222 STRUCTURAL BOTANY pore between them closes (see Fig. 92, A). Now, how are we to explain this ? The pure water is absorbed into the guard-cells by the denser cell-sap, and so their turgidity is increased ; conversely, the com- paratively dense salt solution withdraws water from the guard-cells, and so they tend to collapse. But why do the guard-cells become more curved as their turgidity increases ? This is mainly due to the ridges by which the wall of each cell is strengthened on the side towards the pore. This side of the cell is more rigid than the other, and so offers more resistance to stretching when water is taken up. Hence the more turgid the guard-cells become, the more do their outer walls stretch as compared with their inner walls, and hence the cells become more and more convex towards the exterior, and concave towards the pore. When water is withdrawn from the cells, just the reverse happens ; the outer walls shorten more than the inner, and so the guard-cells tend to become straight and to close the pore between them. The mechanism of the stomata is in reality much more complex than appears from the above account, but the changes described are among the most important of those which bring about the opening and closing of these organs. We have learnt, then, this much : the stomata open in light, and they open when the guard-cells are turgid. Can we, however, explain why the guard-cells become turgid in light? This question admits of an easy answer. We know that as a rule the guard-cells of the stomata are the only cells in the epidermis which contain chlorophyll granules ; in other words, they are the OUTLINES OF THE PHYSIOLOGY OF NUTRITION 223 only epidermal cells which can assimilate for them- selves. Now, when exposed to light, the chlorophyll- corpuscles of the guard-cells become active ; they form new organic substances, which pass into solution in the cell-sap. Hence the concentration of the cell-sap is increased, water is absorbed by osmosis from the neighbouring cells, and the turgidity of the guard- cells, which is necessary to effect their opening, is thus attained. The aqueous vapour transpired by the stomata comes immediately from the intercellular spaces, into which it has been given out by the mesophyll-cells, especially those of the spongy parenchyma. As transpiration goes on, the cells of the mesophyll con- stantly tend to become poorer in water, and they make good their loss by taking up fresh supplies from the water-conducting tracheides of the vascular bundles, by which every part of the leaf is traversed. We see now how appropriate is the comparison which has been drawn between the network of bundles in the leaf and the system of irrigation channels in a water meadow. When transpiration is active, the tracheae become comparatively empty, the water forming a series of short columns, the spaces between which only contain watery vapour and an extremely small quantity of air, so that the pressure is very much below that of the atmosphere, and often becomes so reduced that we may speak of an approximate vacuum. From this fact alone it is quite evident that the pressure from the root does not keep pace with the transpiration from the leaves. If an actively transpiring plant be 224 STRUCTURAL BOTANY cut off just above the ground and placed in water, the amount which its leaves give off as vapour may- be more than ten times as much as that which is pumped up from the stump by root-pressure during the same time. That the water ascends the stem through the wood can be well shown in any dicotyledonous tree in which the wood forms a continuous ring. Indeed, the ex- periment might be quite well carried out on our dicotyledonous type, the Wallflower, hut a larger woody plant is more convenient for the purpose. A ring of tissue is removed from the stem, the part stripped off reaching inwards as far as the cambium, so that the whole of the cortex and phloem are taken away. The wound is covered up with damp cotton- wool, or protected in some other way against evapora- tion. No withering of the leaves takes place, they remain indefinitely fresh and turgid as before ; and if we measure the transpiration, the amount of watery vapour given off is found to be undiminished. Hence it is evident that the current of water goes up just as well when the phloem and cortex are interrupted as it does when they are complete. Only the wood and pith remain for it to pass through. It makes no difference to the result of our experiment if, as is usually the case, the pith is completely dried up and quite incapable of conducting water. Hence we can only conclude that the whole ascending current of sap passes through the wood. In many trees, as for example the Beech, the whole thickness of wood continues to conduct water, even in OUTLINES OF THE PHYSIOLOGY OF NUTRITION 225 trees more than a hundred years old. In others, as in the Oak, it is only the outer layers, forming the sap-wood or alburnum, by which water is conducted. In the latter case, if we repeat our ringing experiment, but make a deeper incision than before, so as to remove the outer, more lightly-coloured wood, as well as the phloem, we get quite a different result. The leaves wither and dry up, and after a time all transpiration ceases ; the water supply has evidently been cut off from the part of the tree above the incision. Experiments in which plants are made to absorb coloured fluids have proved that it is through the vessels and tracheides of the wood that the water ascends ; the fibres take no part in the process. Whether the living cells of the woody parenchyma and of the medullary rays do or do not help in the conduction of water is still an open question. As to the causes of the ascent of water in the wood, we are still very much hr the dark. The pressure in the living cells of the root, due to osmosis, no doubt, has much to do with the forcing of water into the vessels in the first instance. The constant giving-off of watery vapour by the transpiring leaves is also of great importance, for room is thus made for fresh supplies of sap from below. The extremely low pressure in the vessels and tracheides must help the movement of the short columns of water which they contain, and this movement will tend to be in the right direction, for it lias been proved that the pressure becomes still less as we approach the leaves than it is in the lower parts of the stem. 15 226 STRUCTURAL BOTANY No known physical causes, however, will explain the whole phenomenon. Atmospheric pressure will only raise water to a height of about 33 feet. We know that some trees are 400 or more feet high, and yet their topmost branches suffer no lack of water. Capillary action, by which water rises in narrow tubes, will not account for the rise of sap in the vessels and tracheides, for these natural tubes are not nearly narrow enough to raise water to anything like the height required. Thus, ha a tube thW hich in diameter, which would be rather small for a wood-vessel, water will only rise by capillarity to a height of about two feet. A very tempting explanation, which many botanists have adopted, is to suppose that the living cells of the wood take part ha the work, absorbing water from the vessels by osmosis, and pumping it out again under pressure, just as we know takes place in the case of the living cells of the root. As, however, it has been proved that the sap will continue to rise in stems more than 33 feet high, which have been completely killed, we are not at liberty to accept this explanation. We are thus left for the present without an answer to one of the most obvious physiological questions, namely, How does the sap rise in the tree ? Physio- logists are hard at work on the question, and we can only hope that they will find a satisfactory answer before long. Respiration. — We will now leave the subject of nutrition, and say soiaiething about a chemical process OUTLINES OF THE PHYSIOLOGY OF NUTRITION 227 of quite another kind, by which the plant instead of gaming substance loses it, hut in doing so gains energy for the work which it has to perform. This process is the breathing or respiration of plants. So long as plants are in active life, they, like animals, are continually taking up oxygen from the ah', and giving off a corresponding amount of carbon dioxide. The latter, as we know, is a product of combustion. The oxygen which is absorbed is taken up by the protoplasm, and ultimately combines with a portion of its carbon. The product of oxidation, carbon dioxide, is given off again into the ah. The breathing of plants is, in fact, just the same thing as our own breathing, or that of other animals. We must be very careful not to confuse respiration with assimilation. In both processes an exchange of gases between the plant and the atmosphere goes on. In assimilation, however, the plant gains carbon at the expense of the carbon dioxide of the air, while in respiration it loses carbon at the expense of its own protoplasm. The former is a constructive process, by which more com- plex bodies are built up out of simpler ones ; the latter is a destructive process, by which complex sub- stances are broken down into simpler ones. In assimilation deoxidation takes place, while respiration is a process of combustion or oxidation. In the case of green organs exposed to light, the two processes go on together. In light of ordinary intensity assimilation is much more vigorous than respiration, so that carbon dioxide is decomposed much more quickly than it is formed. In darkness, however, 228 STRUCTURAL BOTANY assimilation ceases, and then it is easy to prove that carbon dioxide is given off. The same is the case, whether in light or darkness, with any organs which are not green, such as flower-buds or seeds just begin- ning to germinate. In order to prove that carbon dioxide is evolved, a number of seeds are allowed to germinate under a bell-glass, which is fitted with tubes through which a current of air can be drawn. The air which enters the apparatus is deprived of all its carbon dioxide by passing through a solution of caustic potash. The same air after it has passed over the seeds is drawn through lime water, in which it throws down a precipitate, owing to the formation of calcium carbonate (CaC03). By drying and weighing the precipitate, we can tell how much carbon dioxide has been formed by the seedlings in a given time. As a rule, subject, however, to many exceptions, the volume of carbon dioxide given off in respiration is about equal to that of the oxygen taken in. Organs which are altogether deprived of oxygen can, however, go on breathing for some time. This is called internal, as distinguished from normal, respiration. It is probable that in both kinds of respiration the oxygen as well as the carbon is derived from the proto- plasm itself. In normal respiration, however, the loss of oxygen is made good by fresh supplies from the atmosphere. In internal respiration, where this is not the case, the process can only go on for a limited time. It cannot be too strongly insisted upon that all parts of plants breathe at all times so long as they OUTLINES OF THE PHYSIOLOGY OF NUTRITION 229 are in active life. It is only when the protoplasm is completely at rest, as in a dry seed or bulb, that respiration ceases. If respiration is stopped, all the work of the plant is stopped also ; all the chemical changes in the plant come to an end ; growth ceases, and so do the protoplasmic movements, such as we studied in Eloclea. Just as the combustion of the coals in the fire-box of a steam-engine supplies the energy for the work of the engine, so does the com- bustion of carbon in the plant’s protoplasm supply the energy for the growth of the plant, and for such movements as it performs. The plant, in fact, like an animal, must breathe in order to live, but as in most cases the plant does not have to execute such active movements as an annual, the actual amount of oxygen which it has to consume is so much less. Respiration being a form of combustion, heat must of necessity be set free. As a rule, however, no con- sequent rise of temperature is observed in the case of plants ; for the plant in most cases loses heat more rapidly than it gains it. An ordinary leafy plant has an enormous surface in comparison with its bulk, and is thus exposed to very rapid cooling, both by radiation and conduction. The evaporation of water during transpiration also uses up more 1 1 eat than respiration can supply. It is only when actively breathing organs are crowded closely together in a confined space that a rise of temperature can be detected. If the bulb of a thermometer be thrust into the midst of germinating seeds, a temperature about 1-5° C. higher than that of the surrounding air may be observed, and in some 230 STRUCTURAL BOTANY plants with very crowded flower-buds, enclosed in a sheathing bract, a much greater rise of temperature takes place in consequence of respiration. We have now very rapidly sketched some of the chief phenomena connected with the nutrition and respiration of plants. These subjects, however, form only a part of the field of physiology. The whole question of the growth and movements of plants still remains. For this, however, more advanced text- books must be consulted. We have only aimed at giving so much physiological information as is necessary in order to understand the morphological facts which it is the chief object of this book to teach. CHAPTER IY Type III THE SPRUCE FIR ( Picea excelsa, Link.) The Spruce, a type which will be well known to all readers, is a forest tree, sometimes reaching as much as 150 feet in height. It is not a native of Britain, though frequently planted. Small specimens of this Fir are those generally used as our Christmas trees. In Northern Europe and in the mountainous districts of Central Europe this tree forms large woods, and extends almost to the extreme limits within which trees can grow. In the North it reaches Lapland and Arctic Russia, while on the Alps it is found up to a level of more than 6000 feet. The tree has a pyramidal form ; its main stem is straight and vertical. The principal branches grow out from it almost horizontally or slope slightly downwards ; the smaller twigs are nearly upright. The leaves are spirally arranged and crowded together. They are needle - shaped, and are bluntly four-cornered in transverse section. Each leaf is traversed from end to end by a single vein (see Fig. 93). The branches arise in the axils of the leaves, but 231 232 STRUCTURAL BOTANY only a very small proportion of the axillary buds are ever developed. The root-system consists entirely of the tap-root and its numerous ramifications. Both stem and root, like those of Dicotyledons, grow in thickness from year to year. Hence the main stem and its branches have a somewhat conical form, for they are thickest at the base and taper gradually towards the apex. The older parts of the stem are covered by a scaly bark, which peels off in thin flakes. The tree is evergreen — that is, it does not shed all its leaves in any one season ; the same leaves remain upon it for about three years, but they are changed gradually at all times of the year, so that the tree is never bare. The young shoots, which will expand in the follow- ing year, pass the winter in the form of buds, which are enclosed within the foul-scales ; these are modified leaves of a light brown colour, and thin membranous texture. The Spruce like most, though not all of its allies, is monoecious, i.e. the flowers are of distinct sexes but are both borne on the same plant. They are extremely different from any flowers which we have considered hitherto, and indeed bear little resemblance to the flowers of any of the Angiosperms. One peculiarity which is common to the flowers of both sexes is that the floral axis is elongated and bears spirally-arranged leaves, so far resembling an ordinary vegetative branch ; this form of flower is called a cone, whereas in typical angiospermous flowers the axis is shortened, THE SPRUCE FIR 233 and the floral leaves are therefore brought close together at nearly the same level. The male flowers of the Spruce are borne in the axils of the leaves of a shoot formed in the previous year (see Fig. 93, -£>). The flower or cone begins with a short stalk, on which are borne some bright green Fig. 93. — A, Twig of Picea cxcelsa, bearing a young female cone. B, Twig bearing several male cones. C, Semin- iferous scale from the female cone of A, showing two winged ovules on the upper surface. Enlarged. (After Beissner. ) bracteoles. There is no perianth ; the floral axis above the bracteoles only bears the stamens, which are numerous and arranged in a spiral. Each stamen is evidently a leaf. It has a distinct lamina, which is bent upwards at an angle with the stalk. Its colour is bright red. On its lower surface the stamen bears two pollen-sacs, lying side by side, parallel to the 234 STRUCTURAL BOTANY stalk (see Fig. 106). The details of the pollen-sacs and pollen will be described later. The female flowers or cones grow at the ends of last year’s twigs (see Fig. 93, A). At the end of May, when these flowers open, they are from 1\ to 2 inches in length and of a beautiful red colour. At this time they are erect ; later on they become pendulous. The cone consists of a central axis, bear- ing crowded spirally-arranged scales. Each scale is of an obovate form (see Fig. 93, C), often with a blunt point at the end. The scales are not themselves the leaves, though any one would take them for leaves at the first glance. More careful examination, however, shows that each scale stands immediately above a small narrow pointed leaf. Except in the youngest stages these little leaves are very inconspicuous com- pared with the scales, but they are developed before them, and in fact each scale arises at first as an out- growth from the upper surface of the little leaf close to its base. The scales soon outgrow the leaves which produce them, and by the time the flower opens they alone can be seen from outside, the leaves being quite buried between them. Each scale bears on its upper surface and near its base two ovules, with their micropyles directed inwards towards the axis of the cone. Each ovule is attached to a flat wing, much larger than itself, which is formed from the upper surface of the scale (see Fig. 93, G ). When the female flower becomes a ripe fruit, after its ovules have been fertilised, it undergoes but little change compared with an angiospermous flower. The THE SPRUCE FIR 235 whole cone becomes much larger, attaining a length of from four to six inches. The scales first turn green and then brown, growing hard and woody, and the ovules, on becoming seeds, acquire a hard testa. Their wings remain attached to them, and become dry and membranous. The scales of the cone close firmly together, shutting in the seeds. We thus see that all the parts which constituted the flower are still present in the fruit, which is not often the case in Angiosperms. The seeds are ripened in the autumn after flower- ing, but it is only in tire following spring that the scales of the cone open out and the seeds fall. The wing to which each seed is attached offers a broad surface to the air, and so enables the seeds to be scattered more widely by the wind. The most striking point in the whole history of the female flower is that there is no closed ovary at the time of pollination. When this process takes place, the scales are separated from one another sufficiently to leave an open passage down to the ovules, and it is upon the micropyle of the ovule itself that the pollen falls. There is thus no need for a stigma and a style. On this character, which is common to the whole class of cone-bearing trees and their allies, the name Gyvinosperms, or plants with naked seeds, is based. It is only after pollination that the scales close up so as to shelter the developing seeds, opening again when the latter are ripe, and so allowing them to escape. The gymnospermous structure is an obvious difference from other Flowering plants, and will prepare us for 236 STRUCTURAL BOTANY the still greater differences which we shall find when we come to consider the development in detail. Having now given a brief sketch of the chief points in the external morphology of one type, we will go on to examine its more minute structure. I. ANATOMY OF THE VEGETATIVE ORGANS a. Stem The structure of the young stem hi its main features is closely similar to that of a normal Dicoty- ledon (see Fig. 94). The central cylinder contains a single ring of vascular bundles enclosing a pith. The bundles themselves, just as in Dicotyledons, are collateral, with them xylem directed towards the centre, and their phloem towards the exterior of the cylinder. These bundles pass out one by one into the leaves, each of which receives a single bundle only (see Fig. 94, 1). The bundles in the cylinder are separated from one another by the primary rays, which are only two or three cells in thickness. The primary bundles are of simple structure. The xylem consists entirely of tracheides and parenchyma ; no vessels are developed at any time in the wood, whether primary or secondary, of the Fir or any other cone-bearing trees. The protoxylem, which lies at the extreme inner side of the bundle, has the usual spiral or annular tracheides ; the phloem consists of sieve- tubes and parenchyma. There are no companion-cells, but certain cells of the parenchyma bordering on the THE SPRUCE FIR 237 sieve-tubes are richer in proteicl materials than the rest, and may discharge the function of companion-cells. Between xylem and phloem we find a layer of actively dividing cambium, just as in the vascular bundle of a Dicotyledon. Later on, too, divisions take F IG. 94. — Semi-diagrammatic transverse section of a two-year- old stem of Picca cxcclsa. b, base of leaf ; pd, periderm ; c, cortex ; r, resin canal ; pli, phloem ; a:2, second year’s wood ; Xj, first year’s wood ; l, leaf- trace bundle. Magni- fied about 40. (R.. S.) place in the medullary rays, so as to form an inter- fascicular cambium which completes the ring. Fig. 94 shows a transverse section of a twig of the second year, when a good deal of secondary tissue has already been formed. The cambium acts precisely in the same 238 STRUCTURAL BOTANY way as in a Dicotyledon, and needs no special description. The cortical tissue is chiefly remarkable because it contains a ring of resin canals (r in Fig. 94). The resin canal is an intercellular space, surrounded by a circle of thin-walled cells rich in protoplasm. These cells secrete the resin which is poured into the canal. The canal itself arises by the splitting apart of a group of cells, so as to leave a space between them. The canals extend for long distances through the stem. The secreting cells lining the cavity are called the epithelium of the canal. The epithelium is surrounded by another ring of cells, which have rather thicker walls than those of the cortex generally. The resin is of use to the plant by rendering it distasteful to animals, which might other- wise attack the young twigs ; it probably has other functions also. The outline of the young stem, as shown in Fig. 94 in transverse section, is rendered irregular by the projecting bases of the leaves. The older branches, however, are cylindrical. This change is due to the formation of periderm, which arises, as in the Wall- flower, from a phellogen or cork cambium, but in the Fir the phellogen is formed by the division of a ring of cortical cells. Opposite the depressions between the leaf-bases the phellogen arises quite near the surface, but opposite the leaf-bases them- selves it is deeply seated, and thus the whole layer is nearly circular, as seen in transverse section (see pel in Fig. 94). Cork is formed by the phellogen towards the exterior, and everything out- THE SPRUCE FIR 239 side the layer of cork dries up, and is cast off. Thus the irregularities of the surface are removed, and the new surface formed by the cork is smooth and cylindrical. Later on, however, new layers of phellogen arise, by the division of deeper cortical layers, and so a scaly bark is formed, such as we find on the older stem or branches. The bark, as we know, consists of the cork, together with all the dried- up tissue outside it. We will now go on at once to describe the changes which take place in the twig when it becomes a thick branch or trunk. After what we have already learned of the development of the Wallflower, it is quite easy to understand this process, though the change from a little twig, yg of an inch thick, to the stem of a great Lir- tree a yard or more in diameter, is certainly a remarkable one. We have seen that there is always a layer of cambium between the wood and bast of each bundle, and that by the divisions of the interfascicular tissue the cambium becomes united into a complete ring. When the cambium divides, it cuts off cells on the inner side, which after one or more further divisions become elements of the wood. In like manner the cells cut off from the cambium on its outer side, after subdividing, become part of the phloem. Hence, just as in the Wallflower, the oldest part of the wood is always that nearest the pith, while the oldest part of the bast is that nearest the cortex. The most recently formed part of each is found near the middle of the secondary zone, next the cambium. 240 STRUCTURAL BOTANY The secondary wood is of simple structure compared with that of most Dicotyledons. It consists entirely of tracheides and medullary rays ; there are no vessels, and there is no woody parenchyma, but the wood is traversed by resin canals, which have their secretory epithelium. The tracheides are long, pointed elements, Fig. 95. — Radial section of the wood and bast of the Scotch Fir ( Pinus sylvestris). xy, xylem ; c, cambium ; yli, phloem; a, r.s, a to t, s, t, medullary ray ; t, t, tracheides of ray ; s, starch -containing cells ; a. a, albuminous cells ; r.s, starch-containing cells. Magnified. (After Strasburger.) c should be two cells farther to the right. reaching a length of from two to four millimetres ; they are as a rule pitted on their radial walls only. The pits are of the bordered kind, and afford the best examples of this structure. On each radial wall of the tracheide there is a single row of large bordered THE SPRUCE FIR 241 pits. Their construction is shown in Fig. 96. Seen in surface view, i.e. in a radial section through the wood, each pit appears as two concentric circles (Fig. 96, A). The meaning of the two circles only becomes clear when we examine a pit in sectional view, such as we see in either a tangential (B) or a transverse ( G ) section of the wood. We now find that the pit, like all .other pits, is cell-wall. The thin part of the wall or closing membrane is over-arched on both sides by a circular rim, which projects over it so as to leave only a narrow opening in the middle of the rim. The closing membrane itself is not of equal thickness all over, but has a thicker part in the middle, called a torus (t in Fig. 96, B and C). We now see that in the surface view (A) the outer circle corresponds to the external ed«-e of the projecting border, while the little circle in the middle is the opening in the border leading to the closing membrane. The structure of all bordered pits is similar, but they are most easily studied in plants of the Fir kind on account of their large size. We must remember that the tracheides are closed IG essentially a thin place in the Fig. 96. — Bordered pits of a Fir. A, Part of traeheide with pit in radial view. B, Pit in tangential section : t, torus. C, Transverse section of trach- eide, showing pits : t, torus ; m, middle lamella ; i, inner layer of cell-wall. Magnified about 400. (After Stras- burger. ) 242 STRUCTURAL BOTANY cells, so that no communication is possible between them except through their pits. The whole of the water which goes up a Fir-tree has to pass through the bordered pits thousands of times on its journey from the roots to the leaves. The thin membrane is protected from rupture by the torus. When the pressure is much higher in one tracheide than in its neighbour, the closing membrane is bulged out, be- coming convex on the side towards the lower pressure. Thus the torus is pressed closely against the opening of the pit border on that side (Fig. 96, B). The thin part of the membrane is thus supported by the border, and only the thickened torus is exposed to the full pressure. The border has another important function ; it serves to keep the closing membrane moist. In times when transpiration is very active, and the water- supply deficient, as in hot, dry summers, the tracheides become very nearly emptied of water. Within the minute cavities of the bordered pits, however, the water is held fast by capillary action, and so the closing membranes are prevented from drying up. Thus the entrance of air into the tracheides is guarded against. This is a very important matter, for if the tracheides once became filled with air, no more water would be able to pass through them. The wood is not uniform all through. As everybody has seen, the wood of the Fir, like that of other trees in temperate climates, shows annual rings, each ring representing a year’s growth, so that by counting them the age of the tree can be ascertained. THE SPRUCE FIR 243 The reason why we can detect the annual rings is that the wood formed in autumn has a different structure from that developed in spring. Thus there is a sharp boundary between the autumn wood of one year and the spring wood of the next. This difference in the case of the Fir-tree is due to the fact that in the autumn wood the tracheides are much narrower radially and have much thicker walls than those formed in spring (see Fig. 97). The last-formed Fig 97.— Portion of transverse section through the wood of Picea cxcclsa, showing spring and autumn wood. The anow c points towards the cambium, the arrow p to- wards the pith ; a, autumn wood ; sp, spring wood ; a1, beginning of autumn wood of second year. Magnified 330. (R. S.) ° layers of the autumn wood, unlike all others, have pits on their tangential as well as on their radial walls. Through these tangential pits the water passes to the cambium, and to the spring wood as soon as it is formed in the following year. The medullary rays vary very much in size. Many of the rays are only one cell thick, and some of these may be only one or two cells in height ; others are several cells in thickness, and proportionately higher. 244 STRUCTURAL BOTANY The elements of which the rays are composed are of two kinds. Some are living parenchymatous cells in which starch is formed (see Fig. 95, s ). Others, however, completely lose their living contents, and become tracheides (see Fig. 95, t, t). These have bordered pits like the long tracheides of the wood. Their walls are irregularly thickened. These trach- eides in the ray serve for the passage of water in the radial direction, so that by their help communi- cation is kept up between the different layers of the wood. The parenchymatous ray-cells have only simple pits, except where they are in contact with tracheides, in which case the pits are bordered on the side towards the tracheide. The parenchymatous cells serve for the storage of reserve food-substances, especially starch and oil. The two kinds of elements in the ray are arranged in rows. Often we find rows of tracheides at the top and bottom of the ray, and parenchymatous cells in the middle. The resin canals which traverse the wood resemble those already described ; their cavity is surrounded by a layer of thin-walled secreting cells. In each of the larger medullary rays a horizontal resin canal is usually found. These horizontal canals are in communication with the vertical ones. Summing up the peculiarities of this example of coniferous wood, we find that its great characteristic as compared with the wood of a Dicotyledon is its less perfect differentiation. Vessels and woody fibres are both absent. The tracheides have to do duty for both, those of the spring wood taking the chief THE SPRUCE FIR 245 part in conducting water, while the thick- walled autumnal traclieides serve mainly for strength. Ordinary wood-parenchyma is also absent, the only parenchyma apart from the rays being the secreting tissues of the resin canals. On the other hand, the rays themselves are more complex than those of most Dicotyledons. As the wood increases in thick- ness, new secondary rays are started by the cambium, just as we found in the Wallflower. The secondary phloem of the Dir is made up of sieve-tubes , variously modified parenchyma, and 'phloem rays of complex structure. The sieve-tubes are long, pointed elements, resembling in form the traclieides of the wood. They have sieve-plates on their radial walls only (see Fig. 95, ph). The minute pores of the sieve-plates do not completely perforate the wall. The sieve-tubes, as in other vascular plants, contain proteids, and no doubt serve to transport them. The phloem parenchyma forms tangential bands between the zones of sieve-tubes. Some of its cells contain starch or crystals of calcium-oxalate, while others become very thick- walled, forming the stone-cells. The medullary rays in the phloem consist of two kinds of cells: (1) ordinary parenchymatous cells which form starch (see Fig. 95, r.s) ; (2) cells which are more elongated in the vertical direction, contain no starch, and are very rich in proteids (see Fig. 95, a, a). These cells (which are called the albuminous cells) communicate by pits with the sieve-tubes, and are believed to fulfil the same functions as the companion-cells of angiospermous sieve-tubes. 246 STRUCTURAL BOTANY The phloem ray is simply the outer part of the same medullary ray which passes through the xylem, and the cambium serves for the growth of both (see Fig. 95). We see, then, that the portions of the medullary rays which pass through the phloem show a similar differentiation to that of the same rays in the wood. In both alike there are two kinds of elements, one kind serving for the storage of reserve materials, while the other forms part of the conducting system. b. The Leaves The leaves of the Fir and those of most Coniferse differ greatly from the typical leaves of Dicotyledons. On the whole they are much simpler in structure, and the bundle - system especially is rudimentary in character. As we have seen, a single vascular bundle enters each leaf from the stem. Some- times this bundle divides into two, which run side by side through the leaf. In other cases, as in that figured, the bundle is a single one (see Fig. 98). The bundle forms part of a central cylinder which traverses the leaf from end to end, and is con- tinuous with the tissues of the central cylinder of the stem. The cylinder of the leaf is surrounded by the mesophyll, which in this species shows no distinction between palisade and spongy parenchyma, though in some other Conifers this differentiation is present. We will now consider the structure of a leaf more in detail. The form of the leaf as seen in transverse section is quadrangular. Its natural position is such THE SPRUCE FIR 247 that the diagonals of the square are vertical and horizontal (see Fig. 98). The epidermis has a thick outer wall, which is cuticularised. The stomata are arranged in lines along each of the four flat surfaces of the leaf. The guard-cells are sunk below the surface, and are almost covered in above by two very thick-walled epidermal cells, so that scarcely anything Fig. 98.— Transverse section of a leaf of Picca excelsa. s, stomata ; r, resin canal. In the middle observe the central cylinder with a single vascular bundle, in which the xylem is directed upwards. Magnified 82. (R. S.) can be seen of the stoma itself hi a surface view of the leaf. In a transverse section (see Fig. 99) there appear to be two pairs of guard-cells, one above the other, but only the lower cells belong to the stoma, and they alone take part in the opening and closing of the pore. Beneath each stoma is a large inter- cellular air-cavity. 248 STRUCTURAL BOTANY Next below the epidermis comes the hypoderma, a layer of cells which are elongated parallel to the axis of the leaf. They have extremely thick walls, and help much in giving rigidity to the whole organ. The hypodermal layer is interrupted below the stomata, while at the corners of the leaf it may be more than one cell thick. Fig. 99. — External tissues of leaf from tlie same section as Fig. 98. Enlarged to show details, cj, guard-cells of stoma ; a.c, air-chamber; e, epidermis; h, hypoderma; p, assimilating parenchyma. Magnified 330. (R. S.) Within the hypoderma we find the most important part of the leaf-tissue, namely, the assimilating parenchyma, containing chlorophyll granules. This forms a thick layer, and indeed makes up the greater part of the substance of the leaf. Its cells are thin- walled and rather irregular in shape. The cell-walls are often somewhat folded. The cells contain in- THE SPRUCE FIR 249 numerable chlorophyll granules, which are embedded in the protoplasm lining the cell- wall (see Fig. 99). In the mesophyll we very often find two resin canals placed near the two lateral corners of the leaf (Fig. 98, r). The resin canals have the usual structure, namely, a layer of secreting epithelium next the cavity, and a strengthening ring of sclerenchyma surround- ing the epithelium. These resin canals are not very constant in their occur- rence. Sometimes there is only one, sometimes there are none at all. When present, the canals end blindly at the base of the leaf, so that they do not join on to those of the stem. This shows that the leaf forms its own resin and keeps it to itself. We now come to the central cylinder. This is marked off from the mesophyll by a regular endodermis, which has essentially the same structure as the endodermis of a root (see Fig. 100, en). Within the endodermis we find the conjunctive tissue of the cylinder, and in the middle of this is the vascular bundle. Fig. 100. — Part, of the central cylinder and adjoining tissue of the leaf of Picca excclsa in trans- verse section, corresponding to the left-hand part of the central cylinder in Fig. 98. p, cells of assimilating parenchyma ; cn, endodermis ; t, transfusion tissue of cylinder ; nl, albuminous cells ; ph, part of the phloem of bundle; x, part of the xylem of bundle. Magnified 330. (R. S.) 250 STRUCTURAL BOTANY We will describe the latter first. It is generally a double bundle in the lower part of the leaf, but nearer the apex only one bundle can be distin- guished, as in Fig. 98. The wood consists of tracheides, with plates of parenchyma, resembling medullary rays, between them. The first - formed tracheides, on the upper side of the xylem, are spiral, the rest pitted ; the phloem has much the same structure as in the young stem. Between wood and bast is a layer of cambium. As compared with the leaves of most Dicotyledons or Monocotyledons, we see that the Fir leaf is very scantily supplied with vascular bundles. It has, however, an arrangement of its own, which to some extent makes good the deficiency. Among the cells of the conjunctive parenchyma there are a number of short tracheides with bordered pits (see Fig. 100, t). These tracheides are in communica- tion both with one another and with the xylem of the bundle. They no doubt take up water from the bundle, and pass it on through the pitted endodermis to the assimilating tissue, where it is wanted. These tracheides outside the bundle form what is called the transfusion tissue, a very characteristic feature in the anatomy of coniferous leaves. The phloem as well as the xylem enjoys the advantage of an extension into the conjunctive tissue. On each side of the bundle we find a large group of cells with dense protoplasmic contents (see Fig. 100, al). They join on to the phloem of the bundle. These albuminous cells, as they are called, appear to answer the same purpose as the enlarged companion- THE SPRUCE FIR 251 cells at the ends of the finer bundles in dicotyledonous leaves. It is supposed that they collect the proteid substances formed in the mesophyll, and transfer them to the phloem for transport into the stem. The conjunctive tissue further contains a strand of thick-walled fibres placed just outside the phloem (see Fig. 98). We see, then, that the tissue of the cylinder is altogether rather complicated, for besides the vascular bundle and the ordinary parenchyma- tous cells, in which starch is formed, it con tarns the transfusion- tracheides, the albuminous cells, and the sclerenchymatous fibres. The leaves remain on the tree for three or four years. During this time a certain amount of new tissue, chiefly phloem, is formed by the cambium. Secondary growth in a leaf is not so very common, and is limited to leaves which last for a long time. The leaf is constricted at the base, so that the mesophyll here comes to an end. Only the cylinder extends through into the stem. Hence all the food- material which goes to and fro between leaf and stem must pass through the tissues of the cylinder, which thus forms part of the conducting system of the plant. The scale-leaves which protect the buds have a very simple structure compared with the foliage- leaves, for they have no assimilating work to do. They often contain no vascular bundle at all ; two resin canals, however, are present in the mesophyll. The epidermis on the outer surface of the scale has an extremely thick cuticularised external wall, and this is no doubt the most important part of the whole 252 STRUCTURAL BOTANY scale-leaf, as it protects the young organs of the hud from the risk of drying up. c. The Root The root-system of the Fir, and of Conifers in general, consists of the main tap-root and its Fig. 101. — Transverse section of tire root of Picea excelsa at the commencement of secondary thickening, px, px, the two protoxylem groups ; uniting them is the primary xylem -plate. Outside each protoxylem group is a resin canal. x.2, first layers of secondary xylem ; e, cambium ; pli, phloem; p.c, pericycle ; pd, periderm ; en, endo- dermis. Outside this is cortex. Magnified 33. (R. S.) branches. Adventitious roots hardly ever occur in these plants, unless it be on the base of the hypo- cotyl, where they scarcely differ from the ordinary rootlets. The general structure of the root is of the dicoty- ledonous type — that is to say, we find as a rule a THE SPRUCE FIR 253 simple primary structure with a small number of strands of wood and bast. This simple primary structure soon becomes completely transformed by the early occurrence of secondary growth on a great scale. The mode of development is like that of a Dicotyledon, and consequently the resulting structure \ r.c. Fig. 102. — Semi-diagrammatic transverse section of a root of Picea excelsa at a later stage, px, pa?,protoxylem groups ; r.c, resin canal; x-, secondary wood ; ph~, phloem ; pc, pericycle ; pd, periderm ; co, remains of cortex. Magni- fied 22. (R. S.) is also similar. We have found many such resem- blances in the vegetative organs, though as we shall see, the two classes are not at all closely related. The young root is clothed on the exterior by a piliferous layer of thin-walled cells. The root-hairs, however, remain short (Fig. 105, r.h). The Coniferte 254 STRUCTURAL BOTANY generally are badly provided with root-hairs, and many species do not form them at all. As a matter of fact they are not wanted. The use of root-hairs is to increase the absorbent surface by which water can be taken up. The greater the transpiration, the greater is the quantity of water needed, and the more does the absorbent surface require to be increased. Now the leaves of most Coniferse have a very thick cuticle, while the stomata are not very numerous and lie sheltered at the bottom of deep depressions. The water-conducting tissue of the leaf is also, as we have seen, but moderately developed. Hence the amount of watery vapour given off in transpiration is comparatively small, and the supply of water taken up is small in proportion. Thus we see why the absorbent surface of the young roots need not be specially large, and so the absence or slight develop- ment of the root-hairs is explained. Within the piliferous layer is a wide zone of parenchymatous cortex, limited on the inner side by the endodermis, which immediately surrounds the central cylinder (see Fig. 101). This endodermis has the structure with which we have already become familiar. Its radial and horizontal walls are corky and undulated, and show the characteristic dark line when seen in section (see Fig. 106, en). The cylinder of the root in the Fir may be either triarch or diarch. The main-root always has the former structure. We will describe the diarch type which our figures illustrate. The triarch roots do not differ from this except in the number of bundles. THE SPRUCE FIR 255 The first thing that strikes us as peculiar in the structure of the cylinder is the great width of the pericycle, which forms a broad zone from six to nine cells in thickness. It is thickest opposite the middle of the two phloem groups (see Fig. 101, p.c). Most of the pericyclic cells contain a quantity of starch, which is ready for use in the formation of the cell- walls of the new tissues as they develop. Within the pericycle comes the vascular tissue, which, as seen in transverse section, occupies a somewhat elliptical area (see Fig. 101). The two protoxylem groups lie at the ends of the long axis of the ellipse, and therefore at the points where the pericycle is narrowest (Fig. 101, px). This first-formed xylem consists of spiral tracheides. The tissue lying between the two protoxylem groups slowly becomes converted into wood. The differentiation advances from either end towards the middle of the cylinder, until ultimately a continuous plate of primary wood is formed, extend- ing across the middle of the cylinder (see Figs. 101 and 102). The primary wood is now complete. Its development goes on very gradually, so that the secondary growth has usually begun before the primary structure is finished. We will, however, consider the two stages separately. All the middle part of the primary plate of wood consists of pitted tracheides. Just outside each protoxylem group a resin canal is formed in the pericycle (see Figs. 101, 102, and 103). The primary phloem forms two bands, one on each side of the xylem-plate, from which they are separated 25G STRUCTURAL BOTANY by a few layers of conjunctive parenchyma (see Fig. 101, ])h). Even before the primary tissues are completely developed, cambium begins to arise by tangential Fig. 103. — Part of a transverse section of the root of Picea excelsa, showing the completion of the cambium outside the protoxylem by division of pericyclic cells, px, proto- xylern ; xh primary xylem -plate; x2, secondary xylem ; b, bordered pits ; c, cambium ; phlt primary phloem (crushed); ph2, secondary phloem; r.c, resin canal; pc, pericycle ; pd, periderm. Magnified 165. (R. S.) divisions of the conjunctive cells lying just inside the phloem groups. As usual, secondary wood is formed on the inner side of the cambium, and THE SPRUCE FIR 257 secondary bast on its exterior. As it is the layer of cells next the phloem which divides to form the cambium, a band of unaltered parenchyma is always left between the primary and secondary wood (see Figs. 101, 102, and especially 103). For some time the cambial divisions are limited to the part immediately within the phloem groups (see Fig. 101). Eventually, however, the division spreads to the pericyclic cells lying outside the two resin canals, which border on the protoxylem groups. In this way the cambium is completed round the ends of the xylem-plate, and henceforward it forms a continu- ous ring. Fig. 103 represents the stage when the cambial divisions have already extended to the cells beyond the resin canal. No secondary tissue has as yet been formed in this position, although at the sides of the xylem-plate a considerable amount of secondary wood and bast has already been developed, so that the primary phloem is crushed and obliterated. A later stage, in which the secondary tissues already form a complete ring, is shown in the diagrammatic figure 102. The structure of the secondary tissues needs no special description. It is the same as in the stem. We may, however, notice that in the first few layers of the secondary wood the pits are formed on all surfaces of the tracheides indiscriminately (see Fig. 103, b). In the later-formed layers they are limited to the radial walls (see, however, p. 243). As soon as the cambium has begun its activity, the most external layer of the pericycle also becomes the seat of new formations. The cells of this layer divide 17 258 STRUCTURAL BOTANY up by tangential walls, and thus give rise to a phellogen which forms corlc-cells on its outer side (see Figs. 101, 102, 103, pd). From this time onwards the root ceases to be an absorptive organ. The corky periderm completely isolates the cortex from all communication with the conducting cylinder, and consequently the whole external tissue, from the endodermis outwards, dies away. Henceforward the root is an organ of conduction only, consisting solely of the central cylinder, which by means of its cambium retains the power of unlimited growth. In all these respects the root of a Conifer agrees closely with that of a typical Dicotyledon. d. Growing Points and Lateral Appendages Growing Point of the Stem The apex of the shoot in a Fir has essentially the same structure as that of an angiospermous plant. It consists of meristem, the cells of which are small, and have the usual characteristics of embryonic tissue, large nuclei, abundant protoplasm, and thin cell-walls (see Fig. 104). The formative layers of the meristem are less distinct here than in Angiosperms. Whereas in the latter it is a constant rule that the external layer of the apical meristem gives rise to the epidermis only, this is not the case in Conifers. Here the superficial cells of the growing point may divide by walls parallel to the surface, and thus contribute to the formation of cortical as well as of epidermal tissue THE SPRUCE FIR 259 (see Fig. 104). The limit between plerome and periblem is often impossible to trace near the apex. Thus in our figure the sharp line of demarcation at p corresponds to the limits of the pith, not to those of the plerome. Without entering further into the question of the meristematic layers, which is a difficult and disputed one, we may safely say that tjjteir differ- Fig. 104. —Median longitudinal section of the growing point of the stem of Picea excelsa. a, apex ; l, l , very young leaves ; p, p, young pith ; c, c, cortex. Between c and p the vascular bundles arise. Magnified about 140. (After Sachs.) entiation at the apex of the stem is on the whole less sharp in Gymnosperms than in Angiosperms. The leaves and branches arise, just as in Angio- sperms, exogenously, i.e. they are formed entirely by the growth and division of cells which belong to the outer layers of the meristem (see Fig. 104, /, l). Growing Point of the Root The apex of the root in the F ir and its allies grows in much the same way as that of a Dicotyledon. 260 STRUCTURAL BOTANY There is an outer layer of meristem (the calyptrogen), forming both the root-cap and the piliferous layer (see Fig. 106, c), a middle r.h. Fig. 105. — Portion of a transverse section of a root of Pinus Pinea (allied to Picea ), showing an early stage of development of a rootlet, x, protoxylem group of main root ; c.s, resin canal; d, remains of cortex of main root ; e.p, its piliferous layer, in which root-hairs, r.h, are shown ; pi, plerome of rootlet ; p, periblem ; c, calyptrogen ; s (shaded), digestive sac, cover- ing the apex of rootlet, and formed from the outer layer of the pericycle of the main root. Magnified about 150. (After Van Tieghem and Douliot.) group or periblem, which gives rise to the cortex (p in Fig. 106), and within this is the plerome, from which the central cylinder is developed (pi in same figure). This, at least, is the arrangement at the apex of young roots, such as those shown in Figs. 105 and 106, soon after their first formation. It is doubtful whether the limit between the periblem and the calyptrogen can always be traced at the growing point of the older roots. The formation of the rootlets or branches on the parent-root also follows the dicotyledonous type, with slight modifications. The rootlets are arranged in as many vertical rows as there are protoxylem groups in the main root. Thus, in our diarch root of the Fir there would be two opposite rows of rootlets lying in the same THE SPRUCE FIR 261 plane as the xylem-plate. The rootlet here, as in all Flowering plants, arises from the pericycle of the main root. The layer of pericyclic cells which divides to form the rootlet is here the second layer from the outside. Tangential walls are first formed, which separate off the plerome, or future central cylinder of the rootlet, while the outer cells resulting from the first division again divide tangentially into .s Fig. 106. — Similar section of a root of Picea excelsa, showing origin of a rootlet, en, endodermis of main root ; pc, outer layers of pericycle ; other lettering as in Fig. 105. Magnified about 150. (After Van Tieghem and Douliot.) periblem and calyptrogen. The more internal layers of the pericycle merely form the base of the central cylinder of the rootlet, by which it is attached to the vascular tissue of the main root. The outermost layer of the pericycle also divides, and follows the growth of the young branch, round which it forms a temporary envelope (s in Figs. 105 and 106) during its growth through the cortex. This envelope is the digestive sac (see p. 171). 262 STRUCTURAL BOTANY The whole of the cortex, as well as the piliferous layer, has to be bored through by the rootlet before it reaches the exterior. It completely absorbs all the tissues which lie in its way. The only peculiarities in the mode of origin of the branches of the root, hi this instance, depend on the fact that the pericycle is of unusual thickness, and are not characteristic of the class generally, but only of the Fir and its nearer allies. In all the essential points the mode of branching of the root is identical in Gymnosperms and in Dicotyledons. Summary of the Vegetative Structure of Conifers In the general morphology and anatomy of both stem and root the Conifers agree closely with Dicotyledons, while they bear little or no resem- blance to the Monocotyledons. The distribution and longitudinal course of the vascular bundles and the mode of secondary growth are practically identical in Conifers and Dicotyledons. Yet in other respects we find very great differences. This is especially the case as regards the leaves. Some Conifers have leaves of more complex struc- ture than those of the Fir, but still it is characteristic of the order to have a comparatively simple bundle- system in the leaf. Its simplicity is constantly com- pensated for by the formation of transfusion tissue and albuminous cells outside the actual bundles. In the more minute structure there are also important differences ; the entire absence of vessels from the wood is a striking character. THE SPRUCE FIR 263 As regards the phloem, we find distinctive peculiarities in the structure of the sieve - tubes, and in the replacement of true companion- cells by elements of a different origin. These characters suggest to us that after all we are dealing with a group fundamentally distinct from Dicotyledons. It will be noticed that in the points in which Conifers differ most strikingly from Dicotyledons, they differ equally from Monocotyledons. We shall find later on that in some peculiarities of their vegetative structure the Conifers approach certain families of the Flowerless plants, which we have yet to consider. The study of the vegetative organs of these plants, however, throws comparatively little light on their real systematic position. We will now proceed at once to make ourselves acquainted with their reproductive phenomena. II. STRUCTURE OF THE REPRODUCTIVE ORGANS a. The Male Flowers The male flower of the Fir, as we have already learnt, is a short branch, bearing a few bracteoles on the lower, and numerous spirally-arranged stamens on the upper part of its axis. The stamens are more evidently leaves than those of the Angiosperms, for here they have unmistakable laminae. The pollen-sacs are only two in number on each stamen, and are inserted side by side on its under surface. In some other Conifei’S, however, they are much more numerous. While in Angiosperms 264 STRUCTURAL BOTANY the number of the pollen-sacs on the stamens is very constantly four, in the Gymnosperms their number is subject to very great variations. The dehiscence of each ripe pollen - sac takes place separately by a longitudinal slit. Fig. 107. — A, Male flower of the Binuspumilio in median lon- gitudinal section. Magnified about 7 times. B, Longi- tudinal section through a single stamen, showing one of the pollen-sacs (p) below : s, lamina. Magnified 14. O, Transverse section of a stamen : p, p, the two pollen-sacs. Magnified about 20 times. I), A ripe pollen-grain : g, small generative cell ; v, large vegetative cell ; w, wings formed by inflated cuticle. Magnified 290 times. (After Strasburger. ) For details of pollen-grain, see Fig. 10S. Each pollen-sac arises from a small group of cells on the under side of the stamen while it is still very young. One cell of this group, lying immediately below the epidermis, becomes the archesporium, and THE SPRUCE FIR 265 after dividing up a great many times, gives rise to a large mass of pollen mother - cells. Here, as in Angiosperms, the pollen - producing tissue grows at the expense of the surrounding layers ; the innermost of these layers, called the tapetum, is rich in proteids, aud is the first to be digested. The outer wall of the pollen-sac is at first about three cells in thickness, but before maturity all except the epidermis have dis- appeared (see Fig. 107, B, C). The division of the pollen mother-cells takes place in the early spi’ing. Each mother - cell divides into four daughter-cells, each of which becomes a pollen- grain. An enormous number of pollen - grains are produced altogether. If we shake a Fir-tree in May, when the male flowers are ripe, the pollen comes out in great clouds of yellow dust, which float for a long time in the air, and are carried to great distances if any wind is blowing. The Fir is a wind-fertilised tree — that is, the transport of the pollen-grains from the stamens to the ovules is entirely dependent on currents of air. Hence a very small proportion of the pollen-grains ever reach the ovules, and in order that there may be a fair chance of pollination taking place at all, a very large number must be produced, the vast majority of which are wasted. We will now see why the pollen-grains are so light and float so long in the air. If we examine the ripe pollen-grains under the microscope, we find that each grain has a curious winged form, owing to the development of two large air-cavities in the cell- wall (see Fig. 107, 1), w). The wall of the pollen-grain is covered on 266 STRUCTURAL BOTANY the outside by a layer of cuticle. At two places on opposite sides of the grain the cuticle becomes separated from the cellulose wall within. The spaces thus formed are at first filled with water, so that a kind of blister arises on each side of the pollen-grain. The water is absorbed and the spaces become filled with air when the pollen is ripe. This curious arrangement has the effect of reducing the weight of the pollen-grain in proportion to its surface, these lateral expansions acting as veritable wings. Thus the grain is more easily carried to a distance by the wind. Before the pollen-grain is ripe, important changes go on in its interior. Each pollen-grain, when first formed by division of its mother-cell, is merely a single cell with one nucleus. But before the de- hiscence of the pollen-sac takes place, a series of divisions goes on in each pollen-grain. First, a small cell is cut off on the side of the grain farthest from the insertion of the wings. This little cell is separated from the large one by a thin wall shaped like a watch- glass. Two more divisions take place in the large cell, cutting off two more small cells, so that there is now a row of three small cells placed one upon another, and projecting into the cavity of the large cell. The first two of the small cells collapse, the third grows bigger and divides into two, forming a little stalk-cell and a larger terminal cell, which is long enough to reach quite half way across the cavity of the gram. The pollen-grain is now ripe, and we see that THE SPRUCE FIR 267 within its winged membrane no less than five cells are enclosed : first, the large cell, which still forms the mam part of the whole ; second and third, the two little collapsed cells next the wall ; fourth, the stalk- Fig. 108. — A, Pollen grain of Picca excelsa shortly before it is ripe, w, w, the wings ; 1, the large vegetative cell ; its protoplasm is contracted ; n1, its nucleus ; 2, 3, the two collapsed cells ; 4 and 5, the stalk-cell and terminal or generative cell, not yet completely divided from one another. B, Pollen-grain beginning to germinate. The large cell, 1, is growing out to form the pollen-tube. Lettering as before. C, Germination more advanced ; only upper part of pollen-tube shown. The generative cell, 5, has divided into two, each with a very large nucleus. Magnified 240. (After Strasburger.) cell ; fifth, the terminal cell. We see, then, that the pollen-grain of a Fir is a much more complicated structure than that of an Augiosperm (see Fig. 108). 268 STRUCTURAL BOTANY b. The Female Flower The arrangement of the organs of the female cone has already been described. We have seen that each of the conspicuous scales of the cone is an outgrowth produced on the upper surface of a much smaller scale, which is the true leaf or carpel. Hence we call the little scales first formed, the carpellary scales. The much larger out- growths which they bear, and which in them turn bear the ovules, are called the ovuliferous or seminifer- ous scales. The latter may be regarded as forming a placenta, which in this case much exceeds in size the carpel which produces it. In the Fir two ovules are borne on the upper surface of each ovuliferous scale. Neither the number nor the position of the ovules is at all constant in the Conifer® generally. In the Araucarias, for example, to which the “ Puzzle Monkey ” belongs, there is only a single ovule to each carpel, while in some of the Cypress family a great number are produced on each. The ovuliferous scale is often completely absent, as in Araucaria, where the ovule is borne directly on the carpel. In the Yew, which is classed with Conifers, though it does not bear cones, the ovule is not formed on a leaf at all, but is seated on the end of a short branch. To return to the Fir : each ovule is straight ( ortho - tropous), and has a single integument. The micropyle points inwards towards the axis of the cone. It is not necessary to follow the early development THE SPRUCE FIR 269 of the ovule in detail, for up to a certain point it grows in very much the same way as the ovule of an Angiosperm. If the reader will refer to the develop- ment of the ovule (see pp. 118 and 180), as described in the Wallflower or the Lily, he will understand what goes on here. Here, as in Angiosperms, the important changes all go on in the nucellus or middle part of the ovule. The archesporium arises from a cell just below the epidermis of the nucellus. The arche- sporium is a single cell to begin with. It divides by a tangential wall ; from the upper part tapetcd cells are formed. The lower part divides up into a row of three cells, and it is usually the lowest of the three which becomes the embryo-sac. Both the epidermis and the tapetal cells undergo a great many divisions by tangential walls, so that the embryo-sac comes to be buried deep down in the nucellus, beneath a thick mass of tissue. The embryo-sac at first, like any other cell, has a single nucleus. This nucleus divides up repeatedly, until there are a large number of nuclei in the proto- plasm lining the cell-wall of the sac. These nuclei are at first free, but after a time cell-walls are formed between them, cutting up the parietal layer of proto- plasm into a layer of cellular tissue. The cells thus formed continue to grow and divide until the whole embryo-sac is filled. The growth of the sac still goes on, until by the time of fertilisation, in the latter part of June, it is about £ of an inch in length. The growth and cell-division of the endosperm keep pace with the enlargement of the embryo-sac. 270 STRUCTURAL BOTANY We see, then, that in the Fir the embryo-sac becomes filled with endosperm before fertilisation. This char- acteristic is common not only to all Conifers, but to all Gymnosperms, and is one of the most important distinctions between this class and all other flowering plants. For reasons which we shall understand when we come to the flowerless plants, the endosperm of the Gymnosperms also bears the name of a prothallus. After the embryo- sac is filled with tissue, but before it has reached its full size, certain cells of the endo- sperm, at the end towards the micropyle, begin to be distinguished from their neighbours by their larger size and denser protoplasm. These cells are from three to five in number, and are always superficial cells of the endosperm in contact with the embryo-sac wall at its upper end.1 While the neighbouring cells go on dividing as fast as they grow, these few special cells grow enormously without dividing. After a time, however, each of them divides near its upper end by a transverse wall. The upper and smaller daughter-cell thus formed under- goes several further divisions, both transverse and longitudinal. The lower cell remains for a long time undivided, but continues to increase greatly hi size. The stage which we have now reached is this : the embryo-sac has grown to a great size, and is filled with endosperm. At its upper end are from three to five groups of cells, each consisting of a single very large 1 We always use the word upper, in describing an ovule, for the end towards the micropyle, without reference to the position which the ovule may happen to occupy. THE SPRUCE FIR 271 oval cell below and a well-defined column of small cells above ; each group is called an arcliegonium. The large lower cell is the venter of the arcliegonium ; the little column of cells above is the neck. Fig. 109.— Longitudinal section of an ovule of Picea at tlie time of fertilisation, i, integument ; nc, nucellus ; s part of the wing ; p, pollen-grains j t, pollen-tubes’; c. neck of an arehegonium ; a, venter of arcliegonium con- taining the ovum, o ; n, nucleus of ovum ; e, endosperm. Magnified 9 times. (After Strasburger.) A general idea of the structure of the whole ovule at this period is given by Fig. 109. In this we see 272 STRUCTURAL BOTANY the pointed nueellus, nc. Most of its lower part is occupied by the large oval embryo-sac, which is shaded to indicate that it is filled with endosperm, e. Two of the archegonia are shown in section. Fig. 110. — A, Upper part of embryo-sac in longitudinal section, showing two archegonia : o, ovum of an arche- gonium ; c, neck ; cl, ventral canal cell ; n, n, nuclei of the two ova. B, Portion of upper surface of endosperm, showing the neck of an archegonium, c. C, Enlarged longitudinal section passing through the neck of an archegonium at the time of fertilisation. The pollen- tube, p.t, has penetrated the neck. Near the lower end of the tube its two generative nuclei are seen. Magnified — A, about 50 diameters ; B, about 125 diameters ; C, about 125 diameters. (After Strasburger. ) The archegonia are the true female organs, in which fertilisation takes place. Just before the archegonium THE SPRUCE FIR 273 is ready for fertilisation, a very small lens-shaped cell is cut off from the venter at its upper end. This is called the ventral canal cell (see Fig. 110, A, cl). The immensely larger lower cell which remains after this final division is the ovum itself, which will give rise to the embryo after fertilisation. The ova in the Fir and in most Gymnosperms are of relatively very large size, so as to be easily visible to the naked eye. The large scale of the whole reproductive apparatus at the time of fertilisation is convenient for observation, but is some- times puzzling when we have become accustomed to the minute size of the corresponding organs in Angiosperms. Fig 110, A, shows the upper part of the embryo-sac with two archegonia buried hi the endosperm. Each ovum has a large nucleus, and is densely filled with protoplasm, in which there are numerous vacuoles, often bearing a deceptive resemblance to nuclei. The neck of the archegonium as seen in longitudinal section consists of two to four tiers of cells (Fig. 110, A and C ). When seen in surface view (Fig. 110, B), it appears as a rosette of six or eight cells, so that the total number of cells in the neck may vary from twelve to thirty-two. We see, then, that the arrangements within the embryo-sac at the time of fertilisation are totally different in Gymnosperms from those in all other flowering plants. On the other hand, they agree closely with the corresponding structures in some of the higher Cryptogams. We must now consider the processes of pollination and fertilisation. 18 274 STRUCTURAL BOTANY c. Pollination and Fertilisation Pollination, as we have already learned, is brought about by the wind, which blows the light winged pollen-grains in all directions, so that some of them chance to alight on the female cones. This takes place in May. Just before this happens, an interesting change takes place in the female flowers, which pre- pares them to receive the pollen. The axis of the cone elongates a little throughout its whole length, so that the ovuliferous scales are all shifted a little farther apart. Thus an open space is left between each two successive scales, through which the pollen-grains can reach the ovules. At this time the ovule is still at an early stage of development. The embryo-sac is still small, and is not yet filled with endosperm. The integument opens widely, leaving an open passage through the micropyle, leading down to the apex of the nucellus. At the same time a small quantity of liquid is secreted within the micropyle on the surface of the nucellus. Some of the pollen- grains carried by the wind pass between the ovuliferous scales and come to rest on the edge of the integument of an ovule. Another change now takes place : the micropyle closes, the lips of the integument bending inwards, and bringing the pollen-grains into contact with the top of the nucellus, to which they adhere owing to the liquid which has been secreted. The germination of the pollen-grain now begins. Our Fig. 108, A, shows a pollen-grain, highly magnified, THE SPRUCE FIR 275 at the beginning of this process. The large vegetative cell sends out a tube, which penetrates the tissue of the nucellus. The vegetative nucleus passes into the tube, but becomes disorganised, and has no further part to play. So far the group of small cells within the grain has remained unchanged. Now, however, the end cell of the group, which is the generative cell, divides into two. The stalk-cell behind them breaks down ; the two generative daughter-cells are set free and pass hito the pollen-tube (see Fig. 1 0 8, G, 5). Their nuclei have now become very large and granular. Pollination takes place in May, fertilisation not until near the end of June. The interval in the Spruce Fir is about six weeks ; in some other Conifers, such as the Scotch Fir, Pimis sylvestris, and the Juniper, the interval is very much longer. The pollen falls on to the ovule in May of one year, but fertilisation is not effected until June of the next year. To return to Picea : the pollen-tubes eventually grow down through the nucellus as far as the embryo-sac. They contain numerous starch-grains, which serve as food during their growth. In the meantime the ovule has ripened. By the time that the pollen-tubes have reached the embryo- sac, it is completely filled with endosperm, and the archegonia have been formed (see Fig. 109). After the 20tli of June the actual fertilisation takes place. A pollen-tube penetrates the wall of the embryo-sac just above the neck of an archegonium. It continues to grow on, passes between the cells of the neck, absorbs the ventral canal cell below, and reaches the 276 STRUCTURAL BOTANY protoplasm of the ovum itself (see Fig. 110, c). The two generative cells are now at the growing end of the pollen-tube. Their nuclei are shown in the figure. The leading generative nucleus, accompanied by a small amount of protoplasm, now passes through the softened membrane at the end of the pollen-tube (Fig. Ill, A, sn). It passes on through the protoplasm, and eventually fuses with the nucleus of the ovum (Fig. Ill, B). Fertilisation is now effected. We see now that, different as many of the preliminary pro- cesses have been, yet the act of fertilisation in itself is essentially the same as in Angiosperms. Fertilisation is, in fact, an identical process in all plants in which it occurs at all. d. Development of the Embryo The fertilised nucleus, resulting from the fusion of the male and female nuclei, passes into the lower end of the ovum. It there undergoes two divisions, so that there are now four nuclei, all of which lie in the same horizontal plane (see Fig. Ill, C, where, of course, only two of them are shown). Successive divisions now take place in a direction at right angles to the long axis of the ovum, until there are sixteen nuclei altogether, lying in four tiers one above another. Each nucleus of the lower three tiers is enclosed with its surrounding protoplasm within a cell -wall; the uppermost four are free nuclei (see Fig. Ill, D, E ). It is to be noticed that only a part of the ovum is concerned in these divisions ; much the greater portion remains undivided, and has nothing to do directly with THE SPRUCE FIR 277 the development of the young plant. It only serves as a store of food for the embryo. Fig. 111. — A, Ovum at tlie time of fertilisation : p, end of pollen-tube ; sn, generative nucleus from the pollen- tube ; on, nucleus of ovum. B, Ovum a little later : the two nuclei are in the act of fusing. C, Base of ovum showing first divisions after fertilisation : two of the four cells are shown. D, More advanced stage : four of the eight cells are shown. E, Still more advanced stage : a, cells from which suspensor will be formed ; e, cells of the embryo. F, Still later stage : the suspensor s has grown to a great length (it is composed of four cells). The rudimentary embryo, e, has been pushed downward by the growth of the suspensor. Magnified about 70 diameters. (After Strasburger. ) 278 STRUCTURAL BOTANY cl The four free nuclei now become disorganised ; the tier of four cells next below them undergoes little change, and remains in its original position. The third layer, however, s in Fig. Ill, E and F, elongates enormously, forming the suspensor, which here consists of four parallel cells. The lowest group of four cells is pushed forward by the suspensor deep into the middle of the endo- sperm. It is this lowest group, e in Fig. Ill, E and F, which alone forms the actual embryo. In the Spruce Fir only one embryo can be formed from each archegonium. All the arch ego nia, however, may be fertilised ; so, for a time, from three to five develop- ing embryos may be present side by side in the endosperm of the same ovule. One, however, always gains the upper hand, and in the ripe seed only this one remains. In some other Conifers, such as the Scotch Fir, the four cells of the suspensor separate from each other, and each bears a separate embryo at its end. In this case, therefore, each archegonium gives rise to four embryos, and altogether there may be from twelve to twenty in the young seed. Here also, however, only one out of the whole number comes to perfection. Fig. 112. — Longitud- inal section of the embryo of Picea taken from a ripe seed, cp, root-cap ; pi, growing point of root ; m, pith ; cy, vascular tissue of cylinder ; h, liy- pocotyl ; c, cotyle- dons, of which only two are shown. Magnified 10 dia- meters. (After Strasburger.) THE SPRUCE FIR 279 We will not follow the further development of the embryo in Picea in detail. Its growth, with which the cell-division keeps pace, goes on from June imtil October, by which time the seed is ripe. The sus- pensor at first serves to supply the young embryo with food, but by the time the seed is ripe its func- tion is at an end. The embryo then extends through the whole length of the embryo- sac, but is still enveloped in a mantle of endosperm ; the middle part of the endosperm has been absorbed by the embryo for its own nutrition. The structure of the embryo in the ripe seed is shown hi Fig. 112, which represents it in longitudinal section. The root is directed towards the micropyle. The apex of the root is protected by an enormously thick root-cap. The root itself is still but little developed, while the hypocotyl occupies nearly half the length of the embryo. The cotyledons are numerous, generally numbering about eight. This is not, however, a character common to Conifers generally, for many of them, such as the Yew, have only two. The cone opens in the spring, and the winged seeds are scattered by the wind, as already described. When the seed germinates, the root grows out through the micropyle, and turns downward into the ct, cotyledons ; h, hypocotyl ; ?•, roots. Natural size. (After Kemer.) 280 STRUCTURAL BOTANY ground. The cotyledons during germination absorb the remainder of the endosperm. The apex of the stem directs itself upwards ; the seed-coat is at first lifted up upon the cotyledons, but is soon cast off as they expand. The seedling has now reached the stage which is shown in Fig. 113. Summary We will now pass rapidly in review those facts in the reproduction and development of the Fir which are typical of gymnospermous plants generally, and which indicate their relations, on the one hand to the Angiosperms, and on the other hand to the fiowerless plants. Apart from the peculiarities in the general morphology of the male flower, which have beeu sufficiently dealt with already, we find a characteristic feature in the considerable number of cells which are formed within each pollen-grain. This, as we shall see later on, is a very significant fact. The produc- tion of all these intermediate cells, before the actual generative cells are set apart, is really a cryptogamic character, and is one indication, among many, that the Gymnosperms are on a different level from all other Flowering plants. The gymnospermy, or direct pollination of the ovule without the intervention of a stigma and style, is itself a point of great importance. Fertilisation by means of a pollen-tube is one of the chief characters of Phanerogams. In Gymnosperms we still have the pollen-tube, but its work is more limited than in THE SPRUCE FIR 281 Angiosperms, for its function only consists in con- veying the generative cells through the tissues of the nucellus. On the side of the female organs, the differences as compared with Angiosperms decidedly outweigh the resemblances. The filling of the embryo-sac with an extensive endosperm or prothallus before fertilisation is exactly what we shall find occurring in Cryptogams, hut is quite different from anything in Angiosperms, where only a few cell-divisions intervene between the formation of the embryo-sac and that of the ovum. The archegonia, with their multicellular necks, are quite foreign to the typical Phanerogams, while they agree in every detail with the female organs of the higher Cryptogams. In fact, we may say that from the formation of the embryo-sac up to the act of fertilisation the whole development is cryptogamic. The embryology, on the other hand, though highly peculiar, is not of such far-reaching significance, for the mode of development varies greatly among the Gymnosperms themselves. The seed is still a marked phanerogamic character ; in fact, after fertilisation, the differences from Angiosperms become much less marked. The Gymnosperms are known, from geological evidence, to be enormously more ancient than any other Flowering Plants ; they still retain many of the characters of their yet more primitive cryptogamic progenitors. INDEX Absorption, 215 Accumbent, 133 Acute, 16 Adventitious roots, 148, 164 Air-chamber, 70, 163, 248 Albuminous, 135, 192 — — cells, 245, 250 Alburnum, 225 Aleurone-grains, 215 Alisma Plantago, 189, 190 Alternate, 14 Anatomy, 3 of Monocotyledons, 171 Anatropous, 180 Andrceceum, 109 Angiospenns, 197 Annual rings, 242 Annular vessels, 53 Anterior, 23 Anther, 24, 151 dehiscence of, 111, 178 development of, 112 Anther-lobes, 24, 109, 110 Antipodal cells, 120, 182 Apex of root, 85, 169, 259 of stem, 82, 168, 258 Apical development, 82 Araucaria, 268 Archegonium, 271 Archesporium, 110, 113, 118, 264, 269 Assimilation, 35, 36, 205 Autumn wood, 243 Axil, 13 Axile placentation, 151, 179 Axillary, 13 Bark, 12, 47, 96, 239 Bast, 52, 57 Bast-vessels, 57 Beetroot, 37 Bentinckia, 41 Bordered pits, 240 Bract, 149, 174 Bracteole, 174 Branch, 13, 85, 147, 231 of root, 87, 170, 266 Broad Bean, 140 Bryony, root of, 7 Buds, 5 Bud-scales, 232, 251 Bulb, 142 Bulb-scales, 163, 194 Bundles, closed, 66, 159 common, 50, 153 leaf- trace, 50 open, 60 vascular, 50, 65, 153, 236 Bundle-sheath, 65, 162 Bundle-system, 50 Buttercup, 137 Callus, 60 Calyptrogen, 86, 169, 170, 260 177, Calyx, 19, 22, 106, 149 Cambium, 48, 51, 52, 60, 99, 237, 256 cork, 97, 238 fascicular, 92 interfascicular, 61, 84, 90, 92 237 283 284 INDEX Campylotropous, 122 Capsella Bursa-pastoris, 118, 121, 131 Capsule, 193 Carbohydrates, 37, 61, 213 Carbon, 34, 201, 205 dioxide, 34, 36, 205, 227 monoxide, 207 Carbonic acid, 34, 205 Carpel, 21, 25, 174 Carpellary scale, 268 Castor-oil seed, 38, 137 Cell, 26 albuminous, 245, 250 antipodal, 120, 182 companion, 58, 60, 157 embryonic, 130 fibrous, 55 generative, 178, 185, 267 guard, 64, 71, 163, 221, 247 initial, 83 pollen mother, 110, 113, 177, 265 stone, 245 vegetative, 267 Cell-division, 39 Cell-formation, 38 Cell-sap, 32 Cell-wall, 28, 40 Cellulose, 28 Central cylinder, 47, 61, 74, 80, 81, 152, 164, 236, 246, 254 Centrifugal, 99 Centripetal, 99 Chalaza, 117, 183 Gheiranthus Clieiri, L., 12-141 Chemical elements, 199 Chlorophyll, 33, 203, 208 corpuscles or granules, 33, 67, 208 Chloroplastid, 37 Chlorosis, 203 Chromoplastid, 37, 108 Circulation of protoplasm, 42, 44 Classification, 8 Closed bundles, 66, 159 Common bundles, 50, 163 Companion-cells, 58, 60, 157 Cone, 233, 234 Conifer, 252, 262 Conjunctive tissue, 61, 74, 155, 249 Connective, 24, 109, 151 Continuity of protoplasm, 40, 41 Cork, 96 Cork-cambium, 97, 238 Corolla, 19, 108, 149 Cortex, 47, 48, 62, 73, 159, 167, 237, 254 Cotyledons, 78, 132, 137, 139, 189, '278 Crystalloid, 38 Cucurbita, 60 Cuticle, 63 Cutin, 63 Cylinder, central, 47, 61, 74, 80, SI, 152, 164, 236, 246, 254 Dahlia, 6 _ Dehiscence of anther, 111, 178 of fruit, 137, 193 of pollen-sac, 264 Dermal tissue, 47 Dermatogen, 82, 169 Diarch, 74, 254 Diastase, 214 Dicotyledons, 133, 140 Digestive sac, 171, 261 Ectoplasm, 44 Egg-apparatus, 120, 182 Elder, 134 Elodea canadensis, 41 Embryo, 118, 187, 276 Embryo-sac, 117, 119, 130, 182, 269 Embryonic cell, 130 Endodermis, 48, 51, 52, 62, 73, 75, 165, 249, 254 Endogenous, 87 Endoplasm, 44 Endosperm, 134, 187, 190, 270 Entire, 16 Epidermis, 47, 48, 62, 68, 69, 70, 71, 159, 163, 247 Epithelium, 238 Evergreen, 232 Exalbuminous, 134, 192 INDEX 285 Exodermis, 166, 168 Eyes, 5 Fascicular cambium, 92 Fertilisation, 25, 122, 129, 183, 184, 274 cross, 125, 184 self, 125, 184 Fibrous cells, 55 Filament, 24, 109, 151 Fir, Spruce, 231-281 Flora, 11 Floral diagram, 23, 176 envelopes, 106 Flower, 2, 3, 18, 103, 149, 174 female, 234, 268 male, 233, 263 Food of plants, 199 Food-solution, 200 Fossil plants, 9 Foxglove, 137 Fritillaria, 39 Fruit, 2, 26, 137, 193, 234 dehiscence of, 137, 193 Function, 2 Funicle, 116, 136, 183 Generative cell, 178, 185, 267 nucleus, 115, 129, 186, 276 Geographical Botany, 11 Germination of pollen, 126, 181, 267 of seed, 138, 194, 279 Glands, 19, 106 Globoid, 38 Glucose, 212 Grape-sugar, 61, 212 Grew, Nehemiali, 27 Growing point of stem, 13, 82, 168, 258 of root, 85, 168, 169, 259 Guard-cells, 64, 71, 163, 221, 247 Gymnosperms, 235, 280 Hairs, 16, 48, 63, 68, 69, 70, 71 root, 18, 73, 76, 77. 167, 216, 253 Ilemp-seed, 38 Hesperis, 89 Hilum of starch grain, 36 of seed, 193 Hooke, Robert, 27 Hypocotyl, 79, 80, 137, 188, 278 Hypoderma, 248 Hypophysis, 131 Indifferent band, 44 Inflorescence, 19 Initial cell, 83 Integuments, 117, 119, 183, 271 Intercellular spaces, 62, 70 Interfascicular cambium, 61, 84, 90, 92, 237 tissue, 47, 48, 61 Internode, 15 Introrse, 125, 151 Lamina, 65, 160 Lanceolate, 16 Leaf, 1, 15, 146, 160, 231, 246 function of, 2 ground, 143 origin of, 84, 169, 259 scale, 4, 143 structure of, 64, 68, 69, 160, 246 Leaf- base, 16 Leaf-bud, 13 Leaf-stalk, 15 Leaf-trace, 50 Leucoplastid, 35, 37 Lignin e, 56 Lilium auraium, 142, 144 bidbifcrum, 147 candidxcm, L., 142-197 martagon, 146 Lily, 142-197 Linseed, 38 Lobes of anther, 24 Malpighi, Marcello, 27 Medulla, 47 Medullary rays, 47, 48, 61, 91, 237, 243 Members, 7 286 INDEX Meristem, 83, 168, 258 Mesopliyll, 67, 70, 246 Micropyle, 117, 119, 183, 274 Midrib, 16, 64, 67, 68, 147, 160 Mohl, Hugo von, 29 Monocotyledons, 149, 195 Monoecious, 232 Morphology, 6 Nectary, 21, 24, 124, 177 Nitrogen, source of, 204 Node, 15 Nucellus, 117, 119, 183, 269 Nucleolus, 31, 40 Nucleus, 30, 39, 42 generative, 115, 129, 186, 276 polar, 120, 182 vegetative, 115, 129, 185 Nutrition, 198-230 Oil, 37 Open bundles, 60 Organs, 2 reproductive, 3, 18, 103, 173, 263 vegetative, 3, 12, 142, 152, 236 Orthotropous, 268 Osmosis, 216 Ovary, 20, 25, 151 Ovule, 20, 25, 116, 151, 180, 234, 269 Ovuliferous scale, 268 Ovum, 118, 120, 182, 273 Paleobotany, 9 Palieontology, 9 Palisade parenchyma, 65, 68, 69, 162 arm, 163 Parenchyma, palisade, 65, 68, 69, 162, 163 phloem, 59, 245 — — spongy, 68, 69, 70, 162 xylem, 56, 68 Pedicel, 20 Pepper, 137 Perennial, 12 Perianth, 149 Periblem, 82, 86, 168, 169, 170, 260 Pericycle, 48, 51, 52, 61, 73, 75, 153, 155, 156, 255, 261 Periderm, 93, 96, 100, 238, 256 Perisperm, 137 Petal, 20, 23, 107, 149, 174 Petiole, 15 Phajus, 35 Phelloderm, 97 Phellogen, 97, 238 Phloem, 48, 51, 52, 57, 60, 68, 73, 156, 165, 236, 245, 255 parenchyma, 59, 245 Phyllotaxis, 14, 146 Physiology, 3, 198-230 Picca cxcclsa, Link., 231-281 Piliferous layer, 73, 76, 86, 167, 253 Pistil, 19, 25, 116, 151, 174, 179 Pith, 47, 48, 61, 236 Pits, 54 bordered, 240 Pitted vessels, 54, 158 Placenta, 104, 268 Placentation, axile, 151, 179 Plastids, 33, 37 Plerome, 82, 85, 168, 169, 170, 260 Plumule, 137, 140 Polar nuclei, 120, 182 Pollen, 24, 110, 114, 178, 265 Pollen mother-cells, 113, 116, 177, 265 Pollen-sac, 22, 110, 151, 177, 263 Pollen-tube, 117, 126, 178, 181, 267, 271 Pollination, 123, 183, 274 Poly arch, 165 Polypetalous, 23 Polysepalous, 22 Posterior, 23 Potato, 4 Primordial utricle, 32, 42 Procambial strand, 84, S7 Pro-embryo, 130 Prosencliymatous, 56 Proteid-granules, 38, 215 INDEX 287 Proteids, 29, 215 Prothallus, 270 Protophloem 59, 66, 156 Protoplasm, 29 circulation of, 42, 44 continuity of, 40, 41 rotation of, 42, 43 Protoplasmic movements, 41 Protoxylem, 51, 52, 54, 66, 79, 156, 165, 236, 255 Pumpkin, 59, 60 Quercus Suber, 96 Raceme, 149 Radicle, 133, 137, 18S Radisli, 6 Raphe, 183 Receptacle, 19, 20 Replum, 137 Reproductive organs, 3, 18, 103, 173, 263 Resin canal, 238, 244, 249, 251 Respiration, 226 Reticulated vessels, 54 Root, 1, 7, 17, 72, 148, 232 adventitious, 148, 164 branching of, 87, 170, 260 function of, 2, 216 growing point of, 85, 169, 259 lateral, 100 secondary thickening of, 98, 256 tap, 17, 72, 252 Root-cap, 78, 85, 170, 260 Root-hair, 18, 73, 76, 77, 167, 216, 253 Root- pressure, 21 S Rootlet, 87, 170, 260 Rotation of protoplasm, 42, 43 Rye, 192 Sambucus, 134 Scilla, 188 Secondary growth, 90 bast, or phloem, 93, 245 wood, or xylem, 94, 240 Seed, 2, 25, 38, 135,191,193, 235, 279 Seed, germination of, 138, 194, 279 Seed-leaves, 78 Seed-vessel, 2 Seedling, 78, 79, 140, 194, 2S0 Seminiferous scale, 233, 268 Sepal, 20, 22, 106, 149, 174 Septum, 21, 25, 137 Sessile 1 5 Shepherd’s Purse, 118, 121, 131 Shoot, 3, 12 Sieve-plate, 5S, 60 Sieve-tube, 57, 60, 157, 236, 245 S inapis, S6 Sliding growth, 95 Spiral vessels, 53, 158 Spongy parenchyma, 68, 69, 70, 162 Spores, 113 Spring wood, 243 Stamen, 19, 24, 109, 150, 174, 233, 263 Starch granules, 6, 35, 212 Stem, 1, 12, 46, 142, 152, 231, 236 function of, 2 growing point of, 13, 82, 168, 258 Stigma, 20, 25, 116, 126, 181 St igmaria ficoid.es, 1 0 Stoma, 48, 64, 68, 69, 70, 71, 163, 221, 247 Style, 25, 179, 182 Sugar, 36 cane, 212 grape, 61, 212 Suspensor, 130, 188, 278 Syncarpous, 151, 179 Synergidse, 120, 182 Systematic Botany, 8 Tap-koot, 17, 72, 252 Tapetiun, 110, 113, 265 Testa, 135, 193, 235 Tetrarch, 74 Thalamus, 20 Tiger-Lily, 176 Tissue, 45 Tissue-system, 46 Torus, 241 288 INDEX Tracheide, 66, 161, 230 Transition from stem to root, 78 Transpiration, 218 Triarch, 74 Tuber, 5 Turk’s Head Lily, 146 Turnip, 6 Vacuole, 30, 32, 44 Vascular bundle, 46, 48, 65, 152, 236 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