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ANATOMY OF

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Originally appearing in Volume V21, Page 736 of the 1911 Encyclopedia Britannica.
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ANATOMY OF PLANTS The term " Anatomy," originally employed in biological science to denote a description of the facts of structure revealed on cutting up an organism, whether with or without the aid of lenses for the purposes of magnification, is restricted in the present article, in accordance with a common modern use, to those facts of internal structure not concerned with the constitution of the individual cell, the structural unit of which the plant is composed. An account of the structure of plants naturally begins with the cell which is the proximate unit of organic structure. The cell is essentially an individualized mass of protoplasm containing a differentiated protoplasmic body, called a nucleus. But all cells which are permanent tissue-elements of the plant-body possess, in addition, a more or less rigid limiting membrane or cell-wall, consisting primarily of cellulose or some allied substance. It is the cell-walls which connect the different cells of a tissue (see below), and it is upon their characters (thickness, sculpture and constitution) that the qualities of the tissue largely depend. In many cases, indeed, after the completion A, Cell (individual) of the unicellular Green Alga Pleurococcus, as an example of an undifferentiated autonomous assimilating cell. pr., Cell protoplasm; n., nucleus; chi., chloroplast; c.w., cell-wall. B, Plant of the primitive Siphoneous Green Alga Protosiphon botryoides. The primitive cell sends colourless tubelets (rhizoids, rh.) into the mud on which it grows. The subaerial part is tubular or ovoid, and contains the chloroplast (clrl.). There are several nuclei. C, Base of the multicellular filamentous Green Alga Chaetomorpha aerea. The basal cell has less chlorophyll than the others, and is expanded and fixed firmly to the rock on which the plant grows by the basal surface, rh, thus forming a rudimentary rhizoid. D, Part of branched filamentous thallus of the multicellular Green Alga Oedocladium. cr. ax., Green axis creeping on the surface of damp soil; rh., colourless rhizoids penetrating the soil; asc. ax., ascending axes of green cells. E, Vertical section of frond of the complicated Siphoneous Green Alga Halimeda. The substance of the frond is made up by a single much-branched tube, with interwoven branches. cond. med., Longitudinally running comparatively colourless central (medullary) branches, which conduct food substances and support the (ass. cor.) green assimilating cortical branches, which are the ends of branches from the medulla and fit tightly together, forming the continuous surface of the plant. F, Section through the surface tissue of the Brown Alga Cutleria multifida, showing the surface layer of assimilating cells densely packed with phaeoplasts. The layers below have progressively fewer of these, the central cells being quite colourless. G, Section showing thick-walled cells of the cortex in a Brown Alga (seaweed). Simple pits (p.) enable conduction to take place readily from one to another. H, Two adjacent cells (leptoids) of a food-conducting strand in Fucus (a Brown seaweed). The wall between them is perforated, giving passage to coarse strands of protoplasm. I, End of hydroid of the thalloid Liverwort Blyttia, showing the thick lignified wall penetrated by simple pits. of the cell-wall (which is secreted by the living cell-body) the protoplasm dies, and a tissue in which this has occurred consists solely of the dead framework of cell-walls, enclosing in the cavities, originally occupied by the protoplasm, simply water or air. In such cases the characters of the adult tissue clearly depend solely upon the characters of the cell-walls, and it is usual in plant-anatomy to speak of the wall with its enclosed cavity as " the cell," and the contained protoplasm or other substances, if present, as cell-contents. This is in accordance with the original use of the term " cell," which was applied in the 17th century to the cavities of plant-tissues on the analogy of the cells of honeycomb. The use of the term to mean the individualized nucleated mass of living protoplasm, which, whether with or without a limiting membrane, primitively forms the proximate histological element of the body of every organism, dates from the second quarter of the 19th century. For a more detailed description of the cell see CYTOLOGY and the section on Cytology of Plants below). In all but the very simplest forms the plant-body is built up of a number of these cells, associated in more or less definite ways. In the higher (more complicated) plants the cells differ very much among themselves, and the body is composed of definite systems of these units, each system with its own characteristic structure, depending partly on the characters of the component cells and partly 1 Q st. J, End of hydroid of the Moss Mnium, showing particularly thin oblique end-wall. No pits. K, Optical section of two adjacent leptoids of the Moss Polytrichum juniperinum. The leptoids are living and nucleated. They bulge in the neighbourhood of the very thin cross-wall. Note resemblance to H and R. L, Optical section of cell of parenchyma in the same moss. Embedded in the protoplasm are a number of starch grains. M, Part of elongated stereid of a Moss. Note thick walls and oblique slit-like pits with opposite inclination on the two sides of the cell seen in surface view. N, One side of the end of hydroid (tracheid) of a Pteridophyte (fern), with scalariform pits. 0, Optical section of two adjacent leptoids (sieve-tube segments) of Pteridophyte, with sieve plates (s. pl.) on oblique end-wall and side-walls. P, Part of spiral hydroid (tracheid) of Phanerogam (Flowering Plant). Q, Three segments of a " pitted " vessel of Phanerogam. R, Optical section of leptoid (sieve-tube segment) of Phanerogam, with two proteid (companion) cells. s. pl., sieve-plate. S, Optical section of part of thick-walled stereid of Phanerogam, with almost obliterated cavity and narrow slit-like oblique pits. T, Part of vertical section through blade of typical leaf of Phanerogam. u.e., Upper epidermal cells, with (c) cuticle. (p) Assimilating (palisade) cells. sp., Assimilating (spongy) cells with large lacunae. l.e., Lower epidermis, with st., stoma. U, Absorbing cell, with process (root-hair) from piliferous layer of root of Phanerogam. V. Endodermal cell of Phanerogam, with suberized central band on radial and transverse walls. on the method of association. Such a system is called a tissue-system, the word tissue being employed for any collection of cells with common structural, developmental, or functional characters to which it may be conveniently applied. The word is derived from the general resemblance of the texture of plant substance to that of a textile fabric, and dates from a period when the fundamental constitution of plant substance from individual cells was not yet discovered. It is convenient here to define the two chief types of cell-form which characterize tissues of the higher plants. The term parenchyma is applied to tissues whose cells are isodiametric or cylindrical in shape, prosenchyma tissues consisting of long narrow cells, with pointed ends. We may now proceed to a systematic account of the anatomy of the different groups of plants, beginning with the simplest, and passing to the more complicated forms. Thallophyta.—The simplest members of both the Algae and the Fungi (q.v.) (the two divisions of the Thallophyta, which is the lowest of the four great groups into which the plant-kingdom is divided) have their bodies each composed of a single cell. In the Algae such a cell consists essentially of: (r) a mass of protoplasm provided with (2) a nucleus and (3) an assimilating apparatus consisting of a coloured protoplasmic body, called a chromatophore, the pigment of which in the pure green forms is chlorophyll, and which may then be called a chloroplast. The whole of these living structures are covered externally by the dead cell-membrane (fig. 1 A). It is from such a living and assimilating cell, performing as it does all the vital functions of a green plant, that, according to current theory, all the different cell-forms of a higher plant have been differentiated in the course of descent. Among the Green Algae the differentiation of cells is comparatively slight. Many forms, even when multicellular, have all their cells identical in structure and function, and are often spoken of as " physiologically unicellular." The cells cell and are commonly joined end to end in simple or branched Tissue filaments. Such differentiation as exists in the higher Differeniiatypes mainly takes two directions. In the fixed forms think, Alg~. the cell or cells which attach the plant to the substratum often have a peculiar form, containing chlorophyll and constituting a rudimentary fixing organ or rhizoid (fig. r C). In certain types living on clamp soil, the rhizoids penetrate the substratum, and in addition to fixing the plant absorb food substances (dissolved salts) from the substratum (fig. r B and D). The second type of differentiation is that between supporting axis and assimilating appendages. The cells of the axis are commonly stouter and have much less chlorophyll than those of the appendages (Draparnaldia). This differentiation is parallel with that between stem and leaf of the higher plants. In the group of the Siphoneae both these types of differentiation may exist in the single, long, branched, tube-like and multinucleate " cell " (coenocyte) which here forms the plant-body. Protosiphon (fig. r B) is an example parallel with Oedocladium; Bryopsis, with Draparnaldia. In Caulerpa the imitation of a higher plant by the differentiation of fixing, supporting and assimilating organs (root, stem and leaf) from different branches of the single cell is strikingly complete. In the Siphoneous family of Codiaceae the branches of the primitive cell become considerably interwoven one with another, so that a dense tissue-like structure is often produced. In this we get a further differentiation between the central tubes (branches of the primitive cell), which run in a longitudinal direction through the body, possess little or no chlorophyll, and no doubt serve to conduct food substances from one region to another, and the peripheral ones, Which are directed perpendicularly to the surface of the body, ending blindly there, contain abundant chlorophyll, and are the assimilating organs (fig. i E). None of the existing Red Seaweeds (Rhodophyceae) has a unicellular body. The thallus in all cases consists of a branched filament of cells placed end to end, as in many of the Green Algae. Each branch grows simply by the transverse division of its apical cell. The branches may be quite free or they may be united laterally to form a solid body of more or less firm and compact consistency. This may have a radial stem-like organization, a central cell-thread giving off from every side a number of short sometimes unicellular branches, which together form a cortex round the central thread, the whole structure having a cylindrical form which only branches when one of the short cell-branches from the central thread grows out beyond the general surface and forms in its turn a new central thread, from whose cells arise new short branches. Or the thallus may have a leaf-like form, the branches from the central threads which form the midrib growing out mainly in one plane and forming a lamina, extended right and left of the midrib. Numerous variations and modifications of these forms exist. In all cases, while the internal threads which bear the cortical branches consist of elongated cells with few chromatophores, and no doubt serve mainly for conduction of food substances, the superficial cells of the branches themselves are packed with chromatophores and form the chief assimilating tissue of the plant. In the bulky forms colourless branches frequently grow out from some of the cortical cells, and, pushing among the already-formed threads in a longitudinal direction, serve to strengthen the thallus by weaving its original threads together. The cells belonging to any given thread may be recognized at an early stage of growth, because each cell L s' ~ ~ u s.". is connected with its neighbours belonging to the same thread by two depressions or pits, one at each end. The common wall separating the pits of the two adjoining cells is pierced by strands of protoplasm. The whole structure, consisting of the two pits and the wall between is known as a genetic pit. Other pits, connecting cells not belonging to the same branch, are, however, formed at a later stage. Many of the lower forms of Brown Seaweeds (Phoeophyceae) have a thallus consisting of simple or branched cell threads, as in the green and red forms. The lateral union of the branches to form a solid thallus is not, however, so common, nor is it carried to so high a pitch of elaboration as in the Rhodophyceae. In a few of the lower forms (Sphacelariaceae), and in the higher forms which possess a solid thallus, often of very large size, the plant-body is no longer formed entirely of branched cell-threads, but consists of what is called a true parenchymatous tissue, i.e. a solid mass of cells, formed by cell division in all directions of space. In the Laminariaceae this tissue is formed by cell division at what is called an intercalary growing point, i.e. a meristematic (cell-dividing) region occupying the whole of a certain transverse zone of the thallus, and cutting off new cells to add to the permanent tissue on both sides. In the Fucaceae, on the other hand, there is a single prismatic apical cell situated at the bottom of a groove at the growing apex of the thallus, which cuts off cells from its sides to add to the peripheral, and from its base to add to the central permanent cells. The whole of the tissue of the plant is formed by the division of this apical cell. In whatever way the tissues are originally formed, however, the main features of their differentiation are the same. According to a law which, as we have seen, applies also to the green and red forms, the superficial cells are packed with chromatophores and form the assimilating tissue (fig. i, F). In these brown types with bodies of considerable thickness (Laminariaceae and Fucaceae), there is, however, a further differentiation of the internal tissues. The cells immediately subjacent to the superficial assimilating layer form a colourless, or nearly colourless, parenchymatous cortex, which acts as a food storage tissue (fig. i, G), and surrounds a central medulla of elongated conducting cells. The latter are often swollen at the ends, so that the cross-wall separating two successive cells has a larger surface than if the cells were of uniform width along their entire length. Cells of this type are often called trumpet-hyphae (though they have no connexion with the hyphae of Fungi), and in some genera of Laminariaceae those at the periphery of the medulla simulate the sieve-tubes of the higher plants in a striking degree, even (like these latter) developing the peculiar substance callose on or in the perforated cross-walls or sieve-plates. A specialized con- ducting tissue of this kind, used mainly for transmitting organic substances, is always developed in plants where the region of assimilative activity is local in the plant-body, as it is in practically all the higher plants. This is the case in the Fucaceae, and in a very marked degree in the Laminariaceae in question, where the assimilative frond is borne at the end of an extremely long supporting and conducting stipe. A similar state of things exists in some of the more highly differentiated Red Seaweeds. The tissue developed to meet the demands for conduction in such cases always shows some of the characters described. It is known as leptom, each constituent cell being a leptoid (fig. i, H). In addition to the cell types described, it is a very common occurrence in these bulky forms for rhizoid-like branches of the cells to grow out, mostly from the cells at the periphery of the medulla, and grow down between the cells, strengthening the whole tissue, as in the Rhodophyceae. This process may result in a considerable thickening of the thallus. In many Laminariaceae the thallus also grows regularly in thickness by division of its surface layer, adding to the subjacent permanent tissue and thus forming a secondary meristem. The simpler Fungi, like the simpler Green Algae, consist of single cells or simple or branched cell-threads, but among the higher kinds a massive body is often formed, particu- Tissue Dif- larly in connexion with the formation of spores, and ferentiatlott in Fungi. this may exhibit considerable tissue-differentiation. A characteristic feature of the fungal vegetative plant-body (mycelium) is its formation from independent coenocytic tubes or cell-threads. These branch, and may be packed or inter-woven to form a very solid structure; but each grows in length independently of the others and retains its own individuality, though its growth in those types with a definite external forrn is of course correlated with that of its neighbours and is subject to the laws governing the general form of the body. Such an independent coenocytic branch or cell-thread is called a hypha. Similar modes of growth occur among the Siphoneous Green Algae and also among the Red Seaweeds. A solid fungal body may usually be seen to consist of separate hyphae, but in some cases these are so bent and closely interwoven that an appearance like that of ordinary parenchymatous tissue is obtained in section, the structure being called pseudo parenchyma. By the formation of numerous cross-walls the resemblance to parenchyma is increased. The surface-layer of the body in the massive Fungi differs in character according to its function, which is not constant throughout the class, as in the Algae, because of the very various conditions of life to which different Fungi are exposed. In many forms its hyphae are particularly thick-walled, and may strikingly resemblethe epidermis of a vascular plant. This is especially the case in the lichens (symbiotic organisms composed of a fungal mycelium in association with algal cells), which are usually exposed to very severe fluctuations in external conditions. The formation of a massive body naturally involves the localization of the absorptive region, and the function of absorption (which in the simpler forms is carried out by the whole of the vegetative part of the mycelium penetrating a solid or immersed in a liquid substratum) is subserved by the outgrowth of the hyphae of the surface-layer of that region into rhizoids, which, like those of the Algae living on soil, resemble the root-hairs of the higher plants. The internal tissue of the body of the solid higher Fungi, particularly the elongated stalks (stipes) of the fructifications of the Agarics, consists of hyphae running in a longitudinal direction, which no doubt serve for the conduction of organic food substances, just as do the " trumpet-hyphae," similar in appearance, .though not in origin, of the higher Brown Seaweeds. (In one genus (Lactarius) " milk-tubes," recalling the laticiferous tubes of many vascular plants, are found.) These elongated hyphae are frequently thick-walled, and in some cases form a central strand, which may serve to resist longitudinal pulling strains. This is particularly marked in certain lichens of shrubby habit. The internal tissues, either consisting of obvious hyphae or of pseudoparenchyma, may also serve as a storehouse of plastic food substances. Looking back over the progress of form and tissue-differentiation in the Thallophyta, we find that, starting from the simplest unicellular forms with no external differentiation of the body, we can trace an increase in complexity of organization every-where determined by the principles of the division of physiological labour and of the adaptation of the organism to the needs of its environment. In the first place there is a differentiation of fixing organs, which in forms living on a soft nutrient sub-stratum penetrate it and become absorbing organs. Secondly, in the Algae, which build up their own food from inorganic materials, we have a differentiation of supporting axes from assimilating appendages, and as the body increases in size and becomes a solid mass of cells or interwoven threads, a corresponding differentiation of a superficial assimilative system from the deep-lying parts. In both Algae and Fungi the latter are primarily supporting and food-conducting, and in some bulky Brown Seaweeds, where assimilation is strongly localized, some of the deep cells are highly specialized for the latter function. In the higher forms a storage and a mechanically-strengthening system may also be developed, and in some aerial Fungi an external protective tissue. The " hyphal " mode of growth, i.e. the formation of the thallus, whatever its external form, by branched, continuous or septate, coenocytic tubes (Siphoneae and Fungi), or by simple or branched cell-threads (Red and many Green Algae), in both cases growing mainly or entirely at the apex of each branch, is almost universal in the group, the exceptions being met with almost entirely among the higher Brown Seaweeds, in which is found parenchyma produced by the segmentation of an apical cell of the whole shoot, or by cell division in some other type of meristem. Bryophyta.—The Bryophyta [including the Liverworts (Hepaticae) and Mosses (Musci)], the first group of mainly terrestrial plants, exhibit considerably more advanced tissue differentiation, in response to the greater complexity in the conditions of life on land. In a general way this greater complexity may be said to consist (I) in the restriction of regular absorption of water to those parts of the plant-body embedded in the soil, (2) in the evaporation of water from the parts exposed to the air (transpiration). But these two principles do not find their full expression till we come, in the ascending series, to the Vascular Plants. In the Bryophytes water is still absorbed, not only from the soil but also largely from rain, dew, &c., through the general surface of the subaerial body (thallus), or in the more differentiated forms through the leaves. The lowest Hepaticae have an extremely simple vegetative structure, little more advanced than that found in some of the higher Green Algae and very much simpler than in the large Red and Brown Seaweeds. The plant-body (thallus) is always small and normally lives in very damp air, so that the demands of terrestrial life are at a minimum. It always consists of true parenchyma, and is entirely formed by the cutting off of segments from an apical cell. A sufficient description of the thallus of the liverworts will be i found in the article BRYOPHYTA. We may note the universal Liver- occurrence on the lower surface of the thallus of fixing worts. and absorbing rhizoids in accordance with the terrestrial life on soil (cf. Oedocladium among the Green Algae). The Marchantiaceae (see article BRYOPHYTA) show considerable tissue-differentiation, possessing a distinct assimilative system of cells, consisting of branched cell threads packed with chloroplasts and arising from the basal cells of large cavities in the upper part of the thallus. These cavities are completely roofed by a layer of cells; in the centre of the roof is a pore surrounded by a ring of special cells. The whole arrangement has a strong resemblance to the lacunae, mesophyll and stomata, which form the assimilative and transpiring (water-evaporating) apparatus in the leaves of flowering plants. The frondose (thalloid) Jungermanniales show no such differentiation of an assimilating tissue, though the upper cells of the thallus usually have more chlorophyll than the rest. In three genera—Blyttia, Symphyogyna and Hymenophytumthere are one or more strands or bundles consisting of long thick-walled fibre-like (prosenchymatous) cells, pointed at the ends and running longitudinally through the thick midrib. The walls of these cells are strongly lignified (i.e. consist of woody substance) and are irregularly but thickly studded with simple pits (see CYTOLOGY), which are usually arranged in spirals running round the cells, and are often elongated in the direction of the spiral (fig. i, I). These cells are not living in the adult state, though they sometimes contain the disorganized remains of protoplasm. They serve to conduct water through the thallus, the assimilating parts of which are in these forms often raised above the soil and are comparatively remote from the rhizoid-bearing (water-absorbing) region. Such differentiated water-conducting cells we call hydroids, the tissue they form hydrom. The sporogonium of the liverworts is in the simpler forms simply a spore-capsule with arrangements for the development, protection and distribution of the spores. As such its consideration falls outside the scheme of this article, but in one small and peculiar group of these plants, the Anthoceroteae, a distinct assimilating and transpiring system is found in the wall of the very long cylindrical capsule, clearly rendering the sporogonium largely independent of the supply of elaborated organic food from the thallus of the mother plant (the gametophyte). A richly chlorophyllous tissue with numerous intercellular spaces communicates with the exterior by stomata, strikingly similar to those of the vascular plants (see below). If the axis of such a sporogonium were prolonged downwards into the soil to form a fixing and absorptive root, the whole structure would become a physiologically independent plant, exhibiting in many though by no means all respects the leading features of the sporophyte or ordinary vegetative and spore-bearing individual in Pteridophytes and Phanerogams. These facts, among others, have led to the theory, plausible in some respects, of the origin of this sporophyte by descent from an Anthoceros-like sporogonium (see PTERIDOPHYTA). But in the Bryophytes the sporogonium never becomes a sporophyte producing leaves and roots, and always remains dependent upon the gametophyte for its water and mineral food, and the facts give us no warrant for asserting homology (i.e. morphological identity) between the differentiated tissues of an Anthocerotean sporogonium and those of the sporophyte in the higher plants. Opposed to the thalloid forms are the group of leafy Liverworts (Acrogynae), whose plant-body consists of a thin supporting stem bearing leaves. The latter are plates of green tissue one cell thick, while the stem consists of uniform more or less elongated cylindrical cells. The base of the stem bears numerous cell-filaments (rhizoids) which fix the plant to the substratum upon which it is growing. In the Mosses the plant-body (gametophyte) is always separable into a radially organized, supporting and conducting axis (stem) Mosses. and thin, flat, assimilating, and transpiring appendages (leaves). To the base of the stem are attached a number of branched cell-threads (rhizoids) which ramify in the soil, fixing the plant and absorbing water from soil. (For the histology of the comparatively simple but in many respects aberrant Bog-mosses (Sphagnaceae), see BRYOPIYTA.] The stems of the other mosses resemble one another in their main histological features. In a few cases there is a special surface or epidermal layer, but usually all the outer layers of the stem are composed of brown, thick-walled, lignified, prosenchymatous, fibre-like cells forming a peripheral stereom (mechanical or supporting tissue) which forms the outer cortex. This passes gradually into the thinner-walled parenchyma of the inner cortex. The whole of the cortex, stereom and parenchyma alike, is commonly living, and its cells often contain starch. The centre of the stem in the forms living on soil is occupied by a strand of narrow elongated hydroids, which differ from those of the liverworts in being thin-walled, unlignified, and very seldom pitted (fig. i, J). The hydrom strand has in most cases no connexion with the leaves, but runs straight up the stem and spreads out below the sexual organs or the foot of the sporogonium. It has been shown that it conducts water with considerable rapidity. In the stalk of the sporogonium there is a similar strand, which is of course not in direct connexion with, but continues the conduction of water from, the strand of the gametophytic axis. In the_aquatic, semi-aquatic, and xerophiloustypes, where the whole surface of the plant absorbs water, perpetually in the first two cases and during rain in the last, the hydrom strand is either much reduced or altogether absent. In accordance with the general principle already indicated, it is only where absorption is localized (i.e. where the plant lives on soil from which it absorbs its main supply of water by means of its basal rhizoids) that a water-conducting (hydrom) strand is developed. The leaves of most mosses are flat plates, each consisting of a single layer of square or oblong assimilating (chlorophyllous) cells. In many cases the cells bordering the leaf are produced into teeth, and very frequently they are thick-walled so as to form a supporting rim. The centre of the leaf is often occupied by a midrib consisting of several layers of cells. These are elongated in the direction of the length of the leaf, are always poor in chlorophyll and form a channel for conducting the products of assimilation away from the leaf into the stem. This is the first indication of a conducting foliar strand or leaf bundle and forms an approach to leptom, though it is not so specialized as the leptom of the higher Phaeophyceae. Associated with the conducting parenchyma are frequently found hydroids identical in character with those of the central strand of the stem, and no doubt serving to conduct water to or from the leaf according as the latter is acting as a transpiring or a water-absorbing organ. In a few cases the hydrom strand is continued into the cortex of the stem as a leaf-trace bundle (the anatomically demonstrable trace of the leaf in the stem). This in several cases runs vertically downwards for some distance in the outer cortex, and ends blindly—the lower end or the whole of the trace being band-shaped or star-shaped so as to present a large surface for the absorption of water from the adjacent cortical cells. In other cases the trace passes inwards and joins the central hydrom strand, so that a connected water-conducting system between stem and leaf is established. In the highest family of mosses, Polytrichaceae, the differentiation of conducting tissue reaches a decidedly higher level. In addition to the water-conducting tissue or hydrom there is a well-developed tissue (leptom) inferred to be a conducting channel for organic substances. This leptom is not so highly differentiated as in the most advanced Laminariaceae, but shows some of the characters of sieve-tubes with great distinctness. Each leptoid is an elongated living cell with nucleus and a thin layer of protoplasm lining the wall (fig. i, K). The whole cavity of the cell is sometimes stuffed with proteid contents. The end of the cell is slightly swollen, fitting on to the similar swollen end of the next leptoid of the row exactly after the fashion of a trumpet-hypha. The end wall is usually very thin, and the protoplasm on artificial contraction commonly sticks to it just as in a sieve-tube, though no perforation of the wall has been found. Associated with the leptoids are similar cells without swollen ends and with thicker cross-walls. Besides the hydrom and leptom, and situated between them, there is a tissue which perhaps serves to conduct soluble carbohydrates, and whose cells are ordinarily full of starch. This may be called amylom. The stem in this family falls into two divisions, an underground portion bearing rhizoids and scales, the rhizome, and a leafy aerial stem forming its direct upward continuation. The leaf consists of a central midrib, several cells thick, and two wings, one cell thick. The midrib bears above a series of closely set, vertical, longitudinally-running plates of green assimilative cells over which the wings close in dry air so as to protect the assimilative and transpiring plates from excessive evaporation of water. The midrib has a strong band of stereom above and below. In its centre is a band-shaped bundle consisting of rows of leptom, hydrom and amylon cells. This bundle is continued down into the cortex of the stem as a leaf-trace, and passing very slowly through the sclerelchymatous external cortex and the parenchymatous, starchy internal cortex to join the central cylinder. The latter has a central strand consisting of files of large hydroids, separated from one another by very thin walls, each file being separated from its neighbour by stout, dark-brown walls. This is probably homologous with the hydrom cylinder in the stems of other mosses. It is surrounded by (i) a thin-walled, smaller-celled hydrom mantle; (2) an amylom sheath; (3) a leptom mantle, interrupted here and there by starch cells. These three concentric tissue mantles are evidently formed by the conjoined bases of the leaf traces, each of which is composed of the same three tissues. As the aerial stem is traced down into the underground rhizome portion, these three mantles die out almost entirely—the central hydrom strand forming the bulk of the cylinder and its elements becoming mixed with thick-walled steroids; at the same time this central hydromstereom strand becomes three-lobed, with deep furrows between the lobes in which the few remaining leptoids run, separated from the central mass by a few starchy cells, the remains of the amylom sheath. At the periphery of the lobes are some comparatively thin-walled living cells mixed with a few thin-walled hydroids, the remains of the thin-walled hydrom mantle of the aerial stem. Outside this are three arcs of large cells showing characters typical of the endodermis in a vascular plant; these are interrupted by strands of narrow, elongated, thick-walled cells, which send branches into the little brown scales borne by the rhizome. The surface layer of the rhizome bears rhizoids, and its whole structure strikingly resembles that of the typical root of a vascular plant. In Catharinea ANATOMY] undulata the central h^ %drom cylinder of the aerial stem is a loose tissue, its interstices being filled up with thin-walled, starchy parenchyma. In Dawsonia superba, a large New Zealand moss, the hydroids of the central cylinder of the aerial stem are mixed with thick-walled stereids forming a hydrom-stereom strand some-what like that of the rhizome in other Polytrichaceae. The central hydrom strand in the seta of the sporogonium of most mosses has already been alluded to. Besides this there is usually a living conducting tissue, sometimes differentiated as leptom, forming a mantle round the hydrom, and bounded externally by a more or less well-differentiated endodermis, abutting on an irregularly cylindrical lacuna; the latter separates the central conducting cylinder from the cortex of the seta, which, like the cortex of the gametophyte stem, is usually differentiated into an outer thick-walled stereom and an inner starchy parenchyma. Frequently, also, a considerable differentiation of vegetative tissue occurs in the wall of the spore-capsule itself, and in some of the higher forms a special assimilating and transpiring organ situated just below the capsule at the top of the seta, with a richly lacunar chlorophyllous parenchyma and stomata like those of the wall of the capsule in the Anthocerotean liverworts. Thus the histological differentiation of the sporogonium of the higher mosses is one of considerable complexity; but there is here even less reason to suppose that these tissues have any homology (phylogenetic community of origin) with the similar ones met with in the higher plants. The features of histological structure seen in the Bryophytic series are such as we should expect to be developed in response to the exigencies of increasing adaptation to terrestrial life on soil, and of increasing size of the plant-body. In the liverworts we find fixation of the thallus by water-absorbing rhizoids; in certain forms with a localized region of water-absorption the development of a primitive hydrom or water-conducting system; and in others with rather a massive type of thallus the differentiation of a special assimilative and transpiring system. In the more highly developed series, the mosses, this last division of labour takes the form of the differentiation of special assimilative organs, the leaves, commonly with a midrib containing elongated cells for the ready removal of the products of assimilation; and in the typical forms with a localized absorptive region, a well-developed hydrom in the axis of the plant, as well as similar hydrom strands in the leaf-midribs, are constantly met with. In higher forms the conducting strands of the leaves are continued downwards into the stem, and eventually come into connexion with the central hydrom cylinder, forming a complete cylindrical investment apparently distinct from the latter, and exhibiting a differentiation into hydrom, leptom and amylom which almost completely parallels that found among the true vascular plants. Similar differentiation, differing in some details, takes place independently in the other generation, the sporogonium. The stereom of the moss is found mainly in the outer cortex of the stem and in the midrib of the leaf. Vascular Plants.—In the Vascular Plants (Pteridophytes, i.e. ferns, horse-tails, club mosses, &c., and Phanerogams or Flowering Plants) the main plant-body, that which we speak of in ordinary language as "the plant," is called the sporophyte because it bears the asexual reproductive cells or spores. The gametophyte, which bears the sexual organs, is either a free-living thallus corresponding in degree of differentiation with the lower liverworts, or it is a mass of cells which always remains enclosed in a spore and is parasitic upon the sporophyte. The body of the sporophyte in the great majority of the vascular plants shows a considerable increase in complexity over that found in the gametophyte of Bryophytes. The principal new feature in the external conformation of the body is the acquirement of " true " roots, the nearest approach to which in the lower forms we saw in the " rhizome " of Polytrichaceae. The primary root is a downward prolongation of the primary axis of the plant. From this, as well as from various parts of the shoot system, other roots may originate. The root differs from the shoot in the characters of its surface tissues, in the absence of the green assimilative pigment chlorophyll, in the arrangement of its vascular system and in the mode of growth at the apex, all features which are in direct relation to its normally subterranean life and its fixative and absorptive733 functions. Within the limits of the sporophyte generation the Pteridophytes and Phanerogams also differ from the Bryophytes in possessing special assimilative and transpiring organs, the leaves, though these organs are developed, as we have seen, in the gametophyte of many liverworts and of all the mosses. The leaves, again, have special histological features adapted to the performance of their special functions. Alike in root, stem and leaf, we can trace a three fold division of tissue systems, a division of which there are indications among the lower plants, and which is the expression of the fundamental conditions of the evolution of a bulky differ- Tissue entiated plant-body. From the primitive uniform Systems. mass of undifferentiated assimilating cells, which we may conceive of as the starting-point of differentiation, though such an undifferentiated body is only actually realized in the thallus of the lower Algae, there is, (i) on the one hand, a specialization of a surface layer regulating the immediate relations of the plant with its surroundings. In the typically submerged Algae and in submerged plants of every group this is the absorptive and the main assimilative layer, and may also by the production of mucilage be of use in the protection of the body in various ways. In the terrestrial plants it differs in the subterranean and subaerial parts, being in the former pre-eminently absorptive, and in the latter protective—provision at the same time being made for the gaseous interchange of oxygen and carbon dioxide necessary for respiration and feeding. This surface layer in the typically subaerial " shoot " of the sporophyte in Pteridophytes and Phanerogams is known as the epidermis, though the name is restricted by some writers, on account of developmental differences, to the surface layer of the shoot of Angiosperms, and by others extended to the surface layer of the whole plant in both these groups. On the other hand, we have (2) an internal differentiation of conducting tissue, the main features of which as seen in the gametophyte of Bryophytes have already been fully described. In the Vascular Plants this tissue is collectively known as the vascular system. The remaining tissue of the plant-body, a tissue that we must regard phylogenetically as the remnant of the undifferentiated tissug of the primitive thallus, but which often undergoes further differentiation of its own, the better to fulfil its characteristically vital functions for the whole plant, is known, from its peripheral position in relation to the primitively central conducting tissue, as (3) the cortex. Besides absorption, assimilation, conduction and protection there is another very important function for which provision has to be made in any plant-body of considerable size, especially when raised into the air, that of support. Special tissues (stereom) may be developed for this purpose in the cortex, or in immediate connexion with the conducting system, according to the varying needs of the particular type of plant-body. The important function of aeration, by which the inner living tissues of the bulky plant-body obtain the oxygen necessary for their respiration, is secured by the development of an extensive system of intercellular spaces communicating with the external air. In relation to its characteristic function of protection, the epidermis, which, as above defined, consists of a single layer of cells has typically thickened and cuticularized outer walls. Epidermis. These serve not only to protect the plant against slight mechanical injury from without, and against the entry of smaller parasites, such as fungi and bacteria, but also and especially to prevent the evaporation of water from within. At intervals it is interrupted by pores (stomata) leading from the air outside to the system of intercellular spaces below. Each stoma is surrounded by a pair of peculiarly modified stomata. epidermal cells called guard-cells (fig. r, T), which open and close the pore according to the need for transpiration. The structure of the stomata of the sporophyte of vascular plants is fundamentally the same as that of the stomata on the sporogonium of the true mosses and of the liverwort Anthoceros. Stomata are often situated at the bottom of pits in the surface of the leaf. This arrangement is a method of checking transpiration by creating a still atmosphere above the pore of the stoma, so that water vapour collects in it and diminishes the further outflow of vapour. This type of structure, which is extremely various in its details, is found especially, as we should expect, in plants which have to economize their water [ANATOMY 734 supply. The stomata serve for all gaseous interchange between the plant and the surrounding air. The guard-cells contain chlorophyll, which is absent from typical epidermal cells, the latter acting as a tissue for water storage. Sometimes the epidermis is consider-ably more developed by tangential division of its cells, forming a many-layered water-tissue. This is found especially in plants which during certain hours of the day are unable to cover the water lost through transpiration by the supply coming from the roots. The water stored in such a time supplies the immediate need of the transpiring cells and prevents the injury which would result from their excessive depletion. The epidermis of a very large number of species bears hairs of various kinds. The simplest type consists simply of a single Hairs. elongated cell projecting above the general level of the epidermis. Other hairs consist of a chain of cells; others, again, are branched in various ways; while yet others have the form of a flat plate of cells placed parallel to the leaf surface and inserted on a stalk. The cells of hairs may have living con-tents or they may simply contain air. A very common function of hairs is to diminish transpiration, by creating a still atmosphere between them, as in the case of the sunk stomata already mentioned. But hairs have a variety of other functions. They may, for instance, be glandular or stinging, as in the common stinging nettle, where the top of the hair is very brittle, easily breaking off when touched. The sharp, broken end penetrates the skin, and into the slight wound thus formed the formic acid contained by the hair is injected. Mention may be made here of a class of epidermal organ, the hydathodes, the wide distribution and variety of which have been revealed by recent research. These are special organs, Hydathodes. usually situated on foliage leaves, for the excretion of water in liquid form when transpiration is diminished so that the pressure in the water-channels of the plant has come to exceed a certain limit. They are widely distributed, but are particularly abundant in certain tropical climates where active root absorption goes on while the air is nearly saturated with water vapour. In one type they may take the form of specially-modified single epidermal cells or multicellular hairs without any direct connexion with the vascular system. The cells concerned, like all secreting organs, have abundant protoplasm with large nuclei, and sometimes, in addition, part of the cell-wall is modified as a filter. In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants) ; in this last case the cells in question are known as epithem. The epithem is penetrated by a network of fine intercellular spaces, which are normally filled with water and debouch on one or more intercellular cavities below the epidermis. Above each cavity is situated a so-called water-stoma, no doubt derived phylogenetically from an ordinary stoma, and enclosed by guard-cells which have nearly or entirely lost the power of movement. The pores of the water-stomata are the outlets of the hydathode. The epithem is frequently surrounded by a sheath of cuticularized cells. In other cases the epithem may be absent altogether, the tracheal strand debouching directly on the lacunae of the mesophyll. This last type of hydathode is usually situated on the edge of the leaf. Some hydathodes are active glands, secreting the water they expel from the leaf. [Many other types of glands also exist, either in connexion with the epidermis or not, such as nectaries, digestive glands, oil, resin and mucilage glands, &c. They serve the most various purposes in the life of the plant. but they are not of significance in relation to the primary vital activities, and cannot be dealt with in the limits of the present article.] The typical epidermis of the shoot of a land plant does not absorb water, but some plants living in situations where they cannot depend on a regular supply from the roots (e.g. epiphytic plants and desert plants) have absorptive hairs or scales on the leaf epidermis through which rain and dew can be absorbed. Some hydathodes also are capable of absorbing as well as excreting water. The surface layer of the root, sometimes included under the term epidermis, is fundamentally different from the epidermis Epidermis of the stem. In correspondence with its water-absorbing e id function it is not cuticularized, but remains usually thin- of Root. walled; the absorbing surface is increased by' its cells being produced into delicate tubes which curl round and adhere firmly to particles of soil, thus at once fixing the root firmly in the soil, and enabling the hair to absorb readily the thin films of water ordinarily surrounding the particles (fig.', U). The root-hair ends blindly and is simply an outgrowth from a surface cell, having no cross-walls. It corresponds in function with the rhizoid of a Bryophyte. At the apex of a root, covering and protecting the delicate tissue of the growing point, is a special root-cap consisting of a number of layers of tissue whose cells break down into mucilage towards the outer surface, thus facilitating the passage of the apex as it is pushed between the particles of soil. The cortex, as has been said, is in its origin the remains of the primitive assimilating tissue of the plant, after differentiation Cortex. of the surface layer and the conducting system. It consists primitively of typical living parenchyma; but its differentiation may be extremely varied, since in the complex A B Ftc. z.—Transverse Sections of Leaves. A, Dorsiventral leaf. B. Isobilateral leaf. ep, epidermis; st, stoma; mes, mesophyll; pal, palisade; spo, spongy tissue; intercellular space; w.t., water tissue; x, xylem; ph, phloem; phlt, phloeoterma; scl, sclerenchyma. while those which are cylindrical or of similar shape (centric leaves) have it all round. The leaves of shade plants have little or no differentiation of palisade tissue. In fleshy leaves which contain a great bulk of tissue in relation to their chlorophyll content, the central mesophyll contains little or no chlorophyll and acts as water-storage tissue. The cortex of a young stem is usually green, and plays a more or less important part in the assimilative function. It also always possesses a well-developed lacunar system communicating with the external air through stomata (in the young stem) or lenticels (see below). This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through which air can pass to the deeper lying tissues. The cortex of the older stem of the root frequently acts as a reserve store-house for food, which generally takes the form of starch, and it also assists largely in providing the stereom of the plant. In the leaf-blade this sometimes appears as a layer of thickened subepidermal cells, the hypoderm, often also as subepidermal bundles of sclerenchymatous fibres, or as similar bundles extending right across the leaf from one epidermis to the other and thus acting as struts. Isolated cells (idioblasts), thickened in various ways, are not uncommonly found supporting the tissues of the leaf. In the larger veins of the leaf, especially in the midrib, in the petiole, and in the young stem, an extremely frequent type of mechanical tissue is collenchyma. This consists of elongated cells with cellulose walls, which are locally thickened along the original corners of the cells, reducing the lumen to a cylinder, so that a number of vertical pillars of cellulose connected by comparatively thin walls form the framework of the tissue. This tissue remains living and is usually formed quite early, just below the epidermis, where it provides the first peripheral support for a still growing stem or petiole. Sclerenchyma may be formed later in various positions in the cortex, according to local needs. Scattered single stereids or bundles of fibres are not uncommon in the cortex of the root. bodies of the higher plants its functions are numerous. In all green plants which have a special protective epidermis, the cortex of the shoot has to perform the primitive fundamental function of carbon assimilation. In the leafy shoot this function is mainly localized in the cortical tissue of the leaves, known as mesophyll, Mesophym which is essentially a parenchymatous tissue containing chloroplasts, and is penetrated by a system of intercellular spaces so that the surfaces of the assimilating cells are brought into contact with air to as large an extent as possible, in order to facilitate gaseous interchange between the assimilating cells and the atmosphere. At the same time the cells of the mesophyll are transpiring cells—i.e. the evaporation of water from the leaf goes on from them into the intercellular spaces. The only pathways for the gases which thus pass between the cells of the mesophyll and the outside air are the stomata. A land plant has nearly always to protect itself against over-transpiration, and for this reason the stomata of the typical dorsiventral leaf (fig. 2, A), which has distinct upper and lower faces, are placed mainly or exclusively on the lower side of the leaf, where the water vapour that escapes from them, being lighter than air, cannot pass away from the surface of the leaf, but remains in contact with it and thus tends to check further transpiration. The stomata are in direct communication with the ample system of intercellular spaces which is found in the loosely arranged mesophyll (spongy tissue) on that side. This is the main transpiring tissue, and is protected from direct illumination and consequent too great evaporation. The main assimilating tissue, on the other hand, is under the upper epidermis, where it is well illuminated, and consists of oblong cells densely packed with chloroplasts and with their long axes perpendicular to the surface (palisade tissue). The intercellular spaces are here very narrow channels between the palisade cells. Leaves whose blades are normally held in a vertical position possess palisade tissue and stomata on both sides (isobilateral leaves) (fig. 2, B), since there is no difference in the illumination and other external conditions, The innermost layer of the cortex, abutting on the central the term for a morphologically defined tissue system, i.e. the leptom cylinder of the stem or on the bundles of the leaves, is called the Ph/nee- hloeoterma, and is often differentiated. In the leaf- terma. lade it takes the form of special parenchymatous sheaths to the bundles. The cells of these sheaths are often distinguished from the rest of the mesophyll by containing little or no chlorophyll. Occasionally, however, they are particularly rich in chloroplasts. These bundle sheaths are important in the conduction of carbohydrates away from the assimilating cells to other parts of the plant. Rarely in the leaf, frequently in the stem (particularly in Pteridophytes), and universally in the root, the phloeoterma is developed as an endodermis (see below). In other cases it does not differ histologically from the parenchyma of the rest of the cortex, though it is often distinguished by containing particularly abundant starch, in which case it is known as a starch sheath. One of the most striking characters common to the two highest groups of plants, the Pteridophytes and Phanerogams, is the vascular possession of a double (hydrom-leptom) conducting system. system, such as we saw among the highest mosses, but with sharply characterized and peculiar features, probably indicating common descent throughout both these groups. It is confined to the sporophyte, which forms the. leafy plant in these groups, and is known as the vascular system. Associated with it are other tissues, consisting of parenchyma, mainly starchy, and in the Phanerogams particularly, of special stereom. The whole tissue system is known as the stelar system (from the way in which in primitive forms it runs through the whole axis of the plant in the form of a column). The stelar system of Vascular Plants has no direct phylogenetic connexion with that of the mosses. The origin of the Pteridophyta (q.v.) is very obscure, but it may be regarded as certain that it is not to be sought among the mosses, which are an extremely specialized and peculiarly differentiated group. Furthermore, both the hydrom and leptom of Pteridophytes have marked peculiarities to which no parallel is to be found among the Bryophytes. Hence we must conclude that the conducting system of the Pteridophytes has had an entirely separate evolution. All the surviving forms, however, have a completely established double system with the specific characters alluded to, and since there is every reason to believe that the conditions of evolution of the primitive Pteridophyte must have been essentially similar to those of the Bryophytes, the various stages in the evolution of the conducting system of the latter (p. X32) are very useful to compare with the arrangements met with in the former. The hydroid of a Pteridophyte or of a Phanerogam is characteristically a dead, usually elongated, cell containing air and water, and Tissue either thin-walled with lignified (woody) spiral (fig. 1, P.) Elements. or annular thickenings, or with thick lignified walls, in- completely perforated by pits (fig.', Q.) (usually bordered pits) of various shapes, e.g. the pits may be separated by a network of thickenings when the tracheid is reticulate or they may be transversely elongated and separated by bars of thickening like the rungs of a ladder (scalariform thickening). When, in place of a number of such cells called tracheids, we have a continuous tube with the same kind of wall thickening, but composed of a number of cells whose cross walls have disappeared, the resulting structure is called a vessel. Vessels are common in the Angiospermous group of Flowering Plants. The scalariform hydroids of Ferns (fig. 1, N.) have been quite recently shown to possess a peculiar structure. The whole of the middle lamella or originally formed cell-wall separating one from another disappears before the adult state is reached, so that the walls of the hydroids consist of a framework of lignified bars with open communication between the cell cavities. The tracheids or vessels, indifferently called tracheal elements, together with the immediately associated cells (usually amylom in Pteridophytes) constitute the xylem of the plant. This is a morphological term given to the particular, type of hydrom found in both Pteridophytes and Phanerogams, together with the parenchyma or stereom, or both, included within the boundaries of the hydrom tissue strand. The leptoid of a Pteridophyte (fig. 1, o.) is also an elongated cell, with a thin lining of protoplasm, but destitute of a nucleus, and always in communication with the next cell of the leptom strand by perforations (in Pteridophytes often not easily demonstrable), through which originally pass strings of protoplasm which are bored out by a ferment and converted into relatively coarse " slime strings," along which pass, we must suppose, the organic substances which it is the special function of the leptoids to conduct from one part of the plant to another. The peculiar substance called callose, chemically allied to cellulose, is frequently formed over the surface of the perforated end-walls. The structure formed by a number of such cells placed end to end is called a sieve-tube (obviously comparable with a xylem-vessel), and the end-wall or area of end-wall occupied by a group of perforations, a sieve-plate. When the sieve-tube has ceased to function and the protoplasm, slime strings, and callose have disappeared, the perforations through which the slime strings passed are left as relatively large holes, easily visible in some cases with low powers of the microscope, piercing the sieve-plate. The sieve-tubes, with their accompanying parenchyma or stereom, constitute the tissue called phloem. This is found in Pteridophytes and Phanerogams with its associated cells, and is entirely parallel with the xylem. The sieve-tubes differ, however, from the tracheids in being immediately associated, apparently constantly, not with starchy parenchyma, but with parenchymatous cells, containing particularly abundant proteid contents, which seem to have a function intimately connected with the conducting function of the sieve-tubes, and which we may call proteid-cells. In the Angiosperms there are always sister-cells of sieve-tube segments and are called companion-cells (fig. I, R.). The xylem and phloem are nearly always found in close association in strands of various shapes in all the three main organs of the sporophyte—root, stem and leaf—and form a connected tissue-system running through the whole body. In the primary axis of the plant among Pteridophytes and many Phanerogams, at any rate in its first formed part, the xylem and phloem are associated in the form of a cylinder (stele), with xylem occupying the centre, and the phloem (in the upward-growing part or primary stem) forming a mantle at the periphery (fig. 4). In the downward growing part of the axis (primary root), Arrange-however, the peripheral mantle of phloem is interrupted, Tent to the xylem coming to the surface of the cylinder Stran: along (usually) two or (sometimes) more vertical lines. theCendstral Such an arrangement of vascular tissue is called radial, Cylindr. and is characteristic of all roots (figs. 3 and lo). The cylinder is surrounded by a mantle of one or more layers of parenchymatous cells, the pericycle, and the xylem is generally separated from the phloem in the stem by a similar layer, the mesocycle (corresponding with the amylom sheath in mosses). The pericycle and mesocycle together form the conjunctive tissue of the stele in these simplest types. When the diameter of the stele is greater, parenchymatous conjunctive tissue often occupies its centre and is frequently called the pith. In the root the mesocycle, like the phloem, is interrupted, and runs into the pericycle where the xylem touches the latter (fig. 3). The whole cylinder is enclosed by the peculiarly differentiated innermost cell-layer of the cortex, known as the endodermis. This layer has its cells closely united and sealed to one another, so to speak, by the conversion of the radial and transverse walls (which separate each cell from the other cells of the layer), or of a band running in the centre of these, into corky substance (fig. 1, v.), so that the endodermal cells cannot be split apart to admit of the formation of intercellular spaces, and an air-tight sheath is formed round the cylinder. Such a vascular cylinder is called a haplostele, and the axis containing it is said to be haplostelic. In the stele of the root the strands of tracheids along the lines where the xylem touches the pericycle are spiral or annular, and are the xylem elements first formed when the cylinder is developing. Each strand of spiral or annular first-formed tracheids is called a protoxylem strand, as distinct from the metaxylem or rest of the xylem, which consists of thick-walled tracheids, the pits of which are often scalariform. The thin-walled spiral or annular tracheae of the protoxylem allow of longitudinal stretching brought about by the active growth in length of the neighbouring living parenchymatous cells of a growing organ. During the process the thin walls are stretched and the turns of the spiral become pulled apart without rupturing the wall of the tracheid or vessel. If the pitted type of tracheal element were similarly stretched its continuously thickened walls would resist the stretching and eventually break. Hence such tracheae are only laid down in organs whose growth in length has ceased. The stele is called monarch, diarch, . . . polyarch according as it contains one, two, . . or many protoxylems. When the protoxylem strands are situated at the periphery of the stele, abutting on the pericycle, as in all roots, and many of the more primitive Pteridophyte stems, the stele is said to be exarch. When there is a single protoxylem strand in the centre of the stele, or when, as is more commonly the case, there are several protoxylem strands situated at the internal limit of the xylem, the centre of the stem being occupied by parenchyma, the stele is endarch. This is the case in the stems of most Phanerogams and of some Pteridophytes. When the protoxylems have an intermediate position the stele is mesarch (many Pteridophytes and some of the more primitive Phanerogams). In many cases external protophloem, usually consisting of narrow sieve-tubes often with swollen walls, can be distinguished from metaphloem. As the primitive stele of a Pteridophyte is traced upwards from the primary root into the stem, the phloem becomes continuous round the xylem. At the same time the Evolution stele becomes more bulky, all its elements increas- efthe ing in number (fig. 4). Soon a bundle goes off to stele In the first leaf. This consists of a few xylem elements, pterida a segment of phloem, pericycle, and usually an arc of phytes. endodermis, which closes round the bundle as it detaches itself from the stele. As the stele is traced farther upwards it becomes bulkier, as do the successive leaf-bundles which leave it. In many Pteridophytes the solid haplostele is maintained through-out the axis. In others a central parenchyma or primitive pith—a new region of the primitive stelar conjunctive—appears in the centre of the xylem. In most ferns internal phloem appears instead of a parenchymatous pith (fig. 5). Sometimes this condition, 736 that of the amphiphloic haplostele, is maintained throughout the adult stem (Lindsaya). In the majority of ferns, at a higher level, after the stele has increased greatly in diameter, a large-celled true pith or medulla, resembling the cortex in its characters, and quite distinct from conjunctive, from which it is separated by an internal endodermis, appears in the centre. These successive new tissues, appearing in the centre of the stele, as the stem of a higher fern is traced upwards from its first formed parts, are all in continuity with the respective corresponding external tissues at the point of origin of each leaf trace (see below). Where internal phloem is present this is separated from the internal endodermis by an endocycle or " internal pericycle," as it is sometimes called, and from the xylem by an Internal mesocycle—these two layers, together with the outer mesocycle and pericycle, constituting the conjunctive tissue of the now hollow cylindrical stele. (The conjunctive frequently forms a connected whole with bands of [ANATOMY axis. The type of siphonostele characteristic of many ferns, in which are found internal phloem, and an internal endodermis separating the vascular conjunctive from the pith, is known as a solenostele. The solenostele of the ferns is broken by the departure of each leaf-bundle, the outer and inner endodermis joining so that the stele becomes horseshoe-shaped and the cortex continuous with the pith (fig. 6). Such a break is known as a leaf-gap. A little above the departure of the leaf-bundle the stele again closes up, only to be again broken by the departure of the next leaf-bundle. Where the leaves are crowded, a given leaf-gap is not closed before the next ones appear, and the solenostele thus becomes split up into a number of segments, sometimes band-shaped or semilunar, sometimes isodiametric in cross-section (fig. 7). In the latter case each segment of the solenostele frequently resembles a Dictyoste/y. haplostele, the segments of inner endodermis, pericycle, phloem and dt. px
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