Plants

PLANTS. In the most generally used sense, a plant is a member of the lower or vegetable order of living organized things; the term is also popularly applied to the smaller herbaceous plants, thus excluding trees and shrubs. The early use of the word is for a twig, shoot, cutting or sapling, which was the meaniiig of Lat. planta (for plancta, the root being that seen in planus, flat, cf. Gr. irXariis, broad; pianta thus meant a spreading shoot or sucker). Other meanings of plant are derived from the verb to plant (Lat. plantare, to fix in position or place). It is thus used of the fixtures, machinery, apparatus necessary for the carrying on of an in.dustry or business, and in colloquial or slang use, of a swindle, a carefully arranged plot or trap laid or fixed to deceive; cf. also PLANTATION. In the following sections the botanical sense of the word is followed, the term being used generally as opposed to animals.

CLAssIFICATIoN o~ PLANTS

Some account of the history of plant classification and the development of a natural system in which an attempt is made to show the actual relationships of plants, is given in the article BOTANY. The plant world falls into two great divisions, the higher or flowering plants (Phanerogams), characterized by the formation of a seed, and the lower or flowerless plants (Cryptogams), in which no seed is formed but the plants are disseminated by means of unicellular bodies termed spores. The term Cryptogam is archaic, implying a hidden method of reproduction as compared with the obvious method represented by the flower of the Phanerogam; with the aid of a good microscope it is, however, easier to follow the process of fertilization. in many Cryptogams than in the flowering plants. These two great divisions are moreover of unequal value, for the Cryptogams comprise several groups differing from each other by characters as marked as those which separate some of them from the Phanerogams. The following groups or sub-kingdoms are those which are now generally recognized:

I. Thallophyta.

Cryptogams -~ II. Bryophyta.

- LIII. Pteridophyta.

Phanerogams or IV. Spermatophyta.

Thallophyta are the most lowly organized plants and include a great variety of forms, the vegetative portion of which consists of a single cell or a number of cells forming a more or less branched thallus. They are characterized by the absence of that differentiation of the body into root, stem and leaf which is so marked a feature in the higher plants, and by the simplicity of their internal structure. Both sexual and asexual reproduction occur, but there is usually no definite succession of the two modes, marking that alternation of sexual generation (gametophyte) and asexual generation (sporophyte) which characterizes the higher groups. The group has until recent years been regarded as comprising three classes distinguished by well-marked physiological featuresthe Algae (including the Seaweeds) which contain chlorophyll, the Fungi which have no chlorophyll and therefore lead a saprophytic or parasitic mode of life, and the Lichens which are composite organisms consisting of an alga and a fungus living together in a mutual parasitism (symbiosis); Bacteria were regarded as a section of Fungi. Such a system of classification, although convenient, is not the most natural one, and a sketch of the system which better expresses the relationships between the various subdivisions is given here. It has however been deemed advisable to retain the older groups for purpose of treatment in this work, and articles will be found under the headings ALGAE, FUNGI, BACTERIA, and LIcHENs. The study of phylogeny has suggested fourteen classes arranged in the following sequence: (1) Bacteria; (2) Cyanophyceae (Blue-green algae); (3) Flagellatae; (4) Myxomycetes (Slime-fungi); (5) Pendineae; (6) Conjugatae; (7) Diatomaceae (Diatoms); (8) Fleteroconteae; (9) Chlorophyceae (Green Algae); (10) Characeae (Stoneworts); (II) Rhodophyceae (Red Algae); (12) Eumycetes (Fungi);

(13) Phycomycetes (Algal fungi); (I 4) Phaeophyceae (Brown Algae). Bacteria (see BACTERIOLOGY) and Cyanophyceae (see ALGAE), which are often grouped together as Schizophyta, are from points of view of both structure and reproduction extremely simple organisms, and stand apart from the remaining groups, which are presumed to have originated directly or indirectly from the Flagellatae, a group of unicellular aquatic organisms combining animal and plant characteristics which may be regarded as the starting-point of unicellular Thallophytes on the one hand and of the Protozoa on the other. Thus simple forms included in the Heteroconteae, Chlorophyceae and Phaeophyceae show an obvious connection with the Flagellatae; the Peridineae may be regarded as a further developed branch; the Conjugatae and Diatomaceae cannot be directly connected; the origin of the Rhodophyceae is also obscure; while the Characeae are an advanced and isolated group (see ALGAE). The Mycetozoa or Myxomycetes are a saprophytic group without chlorophyll, of simple structure and isolated position. The algal fungi, Phycomycetes, are obviously derived from the Green Algae, while the remaining Fungi, the Eumycetes, appear to have sprung from the same stock as the Rhodophyceae (see FUNGI). Owing to the similarity of structure and mode of life it is convenient to treat the Lichens (q.v.) as a distinct class, while recognizing that the component fungus and alga are representatives of their own classes.

The Bryophyta and Pteridophyta have sprung from the higher Thallophyta, and together form the larger group Archegoniatae, so-called from the form of the organ (archegonium) in which the egg-cell is developed. The Archegoniatae are characterized by a well-marked alternation of gametophyte and sporophyte generations; the former bears the sexual organs which are of characteristic structure and known as antheridia (male) and archegonia (female) respectively; the fertilized egg-cell on germination gives rise to the spore-bearing generation, and the spores on germination give rise directly or indirectly to a second gametophyte.

The Mosses and Liverworts (see BRYOPHYTA) include forms with a more or less leaf-like thallus, such as many of the liverworts, and forms in which the plant shows a differentiation into a stem bearing remarkably simple leaves, as in the true mosses. They have no true roots, and their structure is purely cellular or conducting bundles of a very simple structure are present. The independent plant which is generally attached to the soil by hair-like structures is the sexual generation, the sporophyte is a stalked or sessile capsule which remains always attached to the gametophyte from which it derives the whole or part of its nourishment.

The Ferns and fern-like plants (see PTERIDOP1IYTA) have on the other hand a well developed independent sporophyte which is differentiated into stem, leaf and root with highly organized internal structure including true vascular bundles. In general structure they approach the Phanerogams with which they form collectively the Vascular Plants as contrasted with the Cellular PlantsThallophyta and Bryophyta. The gametophyte is a small thalloid structure which shows varying degrees of independence affording an interesting transition to the next group.

Sper,nelo phyla are characterized by an extreme reduction of the gametophyte generation. The sporophyte is the plant which is differentiated into stem, leaf and root, which show a wonderful variety 01 form; the internal structure also shows increased complexity and variety as compared with the other group of vascular plants, the Pteridophyta. The spores, as in the heterosporous Pteridophyta, are of two kindsmicrospores (pollen grains) borne in microsporangia (pollen sacs) on special leaves (sporophylls) known as stamens, and macrospores (embryo-sac) borne in macrosporangia (ovules) on sporophylls known as carpels. The fertile leaves or sporophylls are generally aggregated on special shoots to form rioweN which may contain one or both kinds The microspores are set free from the sporangiurn and carried generally by wind or insect agency to the vicinity of the macrospore, which never leaves the ovule. The male gametophyte is represented by one or few cells and, except in a few primitive forms where the male cell still retains the motile character as in the Pteridophyta, is carried passively to the macrospore in a development of the pollen grain, the pollen tube. The Spermatophyta are thus land plants par excellence and have, with the few exceptions cited, lost all trace of an aquatic ancestry. Aquatic plants occur among seed plants but these are readaptations of land plants to an aquatic environment. After fertilization the female cell, now called the oospore, divides and part of it develops into the embryo (new sporophyte), which remains dormant for a time still protected by the ovule which has developed to become the seed. The seed is a new structure characteristic of this group, which is therefore often referred to as the Seed-plants. The seed is set free from the parent plant and serves as the means of dissemination (see FLOWER; POLLINATION; FRUIT, and SEED). The Spermatophyta fall into two classes, Gymncsperms (q.v.) and Angiosperms; the former are the more primitive group, appearing earlier in geological time and showing more resemblance in the course of their life-history to the Pteridophyta. A recently discovered fossil group, the Pteridospermae (see PALAEOBOTANY) have characters intermediate between the Ptendophyta and the more primitive seedplants.

In GY~~1NospERMsso-called because the ovules (and seeds) are borne on an open sporophyll or carpelthe microsporophylls and macrosporophylls are not as a rule associated in the same shoot and are generally arranged in cone-like structures; one or two small prothallial cells are formed in the germination of the microspore; the male cells are in some older members of the group motile though usually passive. The ovule is not enclosed in an ovary, and the usually solitary macrospore becomes filled with a prothallus, in the upper part of which are formed several rudimentary archegonia. The fertilized egg-cell (oospore) forms a filamentous structure, the proernbryo, from a restricted basal portion of which one or more embryos develop, one only as a rule reaching maturity. The embryo consists of an axis bearing two or more cotyledons and ending below in a radicle; it lies in a generally copious food-storing tissue (endosperm) which is the remains of the female prothallus. The plant has a well-developed main root (tap-root) and a single or branched leafy stem which is provided with a means of secondary increase in thickness. The leaves are generally tough-skinned and last for more than one season.

The ANGIOSFERMS, which are much the larger class, derive their name from the fact that the carpel or carpels form a closed chamber, the ovary, in which the ovules are developedassociated with this is the development of a receptive or stigmatic surface on which the pollen grain is deposited. The sporophylls (stamens and carpels) are generally associated with other leaves, known as the perianth, to form a flower; these subsidiary leaves are protective and attractive in function and their development is correlated with the transport of pollen by insect agency (see ANGI0sPERM5; POLLINATION, and FLOWER). The male gametophyte is sometimes represented by a transitory prothallial cell; ,the two male cells are carried passively down into the ovary and into the mouth of the ovule by means of the pollen-tube. The female gametophyte is extremely reduced; there is a sexual apparatus of naked cells, one of which is the egg-cell which, after fusion with a male cell, divides to form a large siispensorial cell and a terminal embryo. Endosperm is formed as the result of the fusion of the second male cell with the so-called definitive nucleus of the embryo-sac (see ANGlOSPERMS). The embryo consists of an axis bearing one (Monocotyledons) or two (Dicotyledons) cotyledons, which protect the stem bud (plumule) of the future plant, and ending below in a radicle. The seed is enclosed when ripe in the fruit, a development of the ovary as a result of fertilization of the egg-cell. (A. B. R.)

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 plantbody 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 w~:chf. ~ B~

ni. n/i ccl.

~ ct FIG. 1.Examples of the differentiation of the cells of plants.

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 colorless tubelets (rhizoids, rh.) into the mud on which it grows. The subaerial part is tubular or ovoid, and contains the chloroplast (clil.). There are several nuclei.

C, Base of the multicellular filamentous Green Alga Chaetomorpha aes-ea. 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., colorless 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 colorless central (medullary) branches, which conduct food substances and support the (ass. con.) 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 colorless.

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 1 7th 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 i9th 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)~ I ill N~V O~V~

S.. _________

FIG. ia.Examples of the differentiation of the tissue of plants.

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 neighborhood of the very thin cross-wall. Note resemblance toHandR.

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. p1.) 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. p1., 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. i.e., Lower epidermis, with st., stoma.

U, Absorbing cell, with process (root-hair) from piliferous layer of root of Phanerogam.

\. Endodermal cell of Phanerogam, with suberized central band on radial and transverse walls.

on the method of association. Such a system is called a tissuesystem, 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 cylin.drical 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 Thailophyta, 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: (1) a mass of protoplasm provided with (2) a nucleus and (3) an assimilating apparatus consisting of a colored protoplasmic body, called a chromatophore, the pigment of which in the pure green forms is chlorophyll, and which may then be called a cliloroplast. The whole of these living structures are covered externally by the dead cell-membrane (fig. I 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 Different Iatypes mainly takes two directions. In the fixed forms tioninAlg~. the cell or cells which attach - the plant to the substratum often have a peculiar form, containing chlorophyll aod constituting a rudimentary fixing organ or rhizoid (fig. 1 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. I 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. I B) is an example parallel with Oedocladiurn; 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 Codzaceae 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, *hich are directed perpendicularly to the surface of the body, ending blindly there, contain abundant chlorophyll, and are the assimilating organs (fig. 1 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 colorless 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 is connected with its neighbors 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 colorless, or nearly colorless, parenchymatous cortex, which acts as a food storage tissue (fig. 1, 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 connection 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 conducting 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, particuTissue t~Jf larly in con nexion with the formation of spores, and ,er~n,~,onthiS may exhibit considerable tissue-differentiation.

A characteristic feature of the fungal vegetative plantbody (mycelium) is its formation from independent coenocytic tubes or cell-threads. These branch, and may be packed or interwoven 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 form is of course correlated with that of its neighbors 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 parenchyrna. 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 resemble the 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 everywhere determined by the principles of the division of physiological labor 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 substratum 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 (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 found in the article BRYOIHYTA. We may note the universal Li occurrence on the lower surface of the thallus of fixing ver and absorbing rhizoids in accordance with the terrestrial Worts. 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 generaBlyttia, Symphyogyna and Hymenophytum there are one or more strands or bundles consisting of long thickwalled 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-capstile 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 sporo-. gonium 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 Ptericlophytes 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 spore gonium 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)

M and thin, flat, assimilating, and transpiring appendages osses. (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. IFor the histology of the comparatively simple but in many respects aberrant Bog-mosses (Sphagnaceae), see BRYOPHYTA.] 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. 1, J). The hydrom strand has in most cases no connection 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 connection with, but continues the conduction of water from, the strand of the gametophytic axis. In the aquatic, semi-aquatic, and xerophilous types, 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 waterabsorbing 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 blindlythe 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 welldeveloped 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. 1, 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 sclernchymatous 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 neighbor 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 entirelythe central hydrom strand forming the bulk of the cylinder and its elements becoming mixed with thick-walled stereids; 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 plan.t; these are interrupted by strands ofnarrow, 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 Cat harinea 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 somewhat 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 labor 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 connection 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 absorptive 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, (1) on the one hand, a specialization of a surface layer regulating the immediate relations of the plant with its surroundings. In the typically submerged Alg~ 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 preeminently absorptive, and in the latter protectiveprovision 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 tissu~ of the primitive thallus, but which often undergoes further different,iation 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 connection. 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. B

These serve not only to protect the plant against slight P dermis. 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. 1, 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 A nihoceros. 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 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 considerably 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 elongated cell projecting above the general level of the lairs, 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 contents 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 hydaihodea, the wide distribution and variety of which have been revealed by recent research. These are special organs, ?Iydathodes. 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 connection 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-sloma, 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. types of glands also exist, either in connection 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.l 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 tht term epidermis, is fundamentally different from the epidermis of the stem. In correspondence with its water-absorbing epidermis function it is not cuticularized, but remains usually thinof Root, walled; the absorbing surface is increased by its cell~

being produced into delicate tubes which curl round and adher~ firmly to particles of soil, thus at once fixing the root firmly in thi soil, and enabling the hair to absorb readily the thin films of watei ordinarily surrounding the particles (fig. I, U). The root-hair end~ blindly and is simply an outgrowth from a surface cell, havin~ no cross-walls. It corresponds in function with the rhizoid of i Bryophyte. At the apex of a root, covering and protecting th~ delicate tissue of the growing point, is a special root-cap consistinf of a number of layers of tissue whose cells break down into mucilagi towards the outer surface, thus facilitating the passage of the ape~ as it is pushed between the particles of soil.

The cortex, as has been said, is in its origin the remains of th~ primitive assimilating tissue of the plant, after differentiatioi of the surface layer and the conducting system. I Cortex. consists primitively of typical living parenchyma; bu its differpotlistion mov he esctremelv vsred, sinr-p in the rnmnle~

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, Mesophyli. 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 cellsi.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 01 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, --------

ph/i ~ Ce ..----.- FIG. iTransverse Sections of Leavei.

A Dorsiventraf leaf. B. Isobilateral leaf.

c/i, epidermis; st stoma; me,, mesophyil; pal, palisade; spa, spongy tissue; Isp, inteicellular space; wi., water tissue; x, xylem; p/i, phioem; Phil, phloeoterma; sri, scierenchyma.

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 waterstorage tissue. The cortex of a young stem is usually green, and plays a more or less important part in the assimilative function. It alse always possesses a well-developed lacunar system communicating with the external air through stomata (in the young stem) or lenticel~ (see below). This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through whicF 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 largel) in providing the stereom of the plant. In the leaf-blade this sometimes aopears as a layer of thickened subepidermal cells, tht hypoderm, often also as subepidermal bundles of sclerenchymatou~ fibres, or as similar bundles extending right across the leaf from mu epidermis to the other and thus acting as struts. Isolated celh (idioblasts), thickened in various ways, are not uncommonly founc 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, a1 extremely frequent type of mechanical tissue is collenchyma. This consists of elongated cells with cellulose walls, which are locall~ thickened along the original corners of the cells, reducing the lumer to a cylinder, so that a number of vertical pillars of cellulose con nected by comparatively thin walls form the framework of th~ tissue. This tissue remains living and is usually formed quiti early, just below the epidermis, where it provides the first periphera support for a still growing stem or petiole. Sclerenchyma may bi formed later in various positions in the cortex, according to loca needs. Scattered single stereids or bundles of fibres are no imnrornmnn in the rnrtev of the root The innermost layer of the cortex, abutting on the central cylinder of the stem or on the bundles of the leaves, is called the jthloeoterma, and is often differentiated. In the leafPhloeo- blade it takes the form of special parenchymatous erma. 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 chioroplasts. 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 .s system, such as we saw among the highest mosses, YS em. 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 connection 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. 1-lence we must conclude that the conducting system of the Ptcridophytes 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. 732) 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 either thin-walled with lignified (woody) spiral (fig. I, p.) or annular thickenings, or with thick lignified walls, incompletely perforated by pits (fig.i, 9.) (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. I, 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. I, 0.) 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 endwall occupied by a group of perforations, a sseve-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 the term for a morphologically defined tissue system, i.e. the leptom 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 sistercells 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 sporophyteroot, stem and leafand 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), Aflangehowever, the peripheral mantle of phloem is interrupted, ~7ii ~. the xylem coming to the surface of the cylinder ,~dS;

along (usually) two or (sometimes) more vertical lines. c eun~7 Such an arrangement of vascular tissue is called radial, ~

and is characteristic of all roots (figs. 3 and In). 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 neighboring 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 must Phanerogams and of some Pteridophytes. When the protoxylems have an intermediate position the stele is inesarch (many Pteridophytes and some of the more primitive Phanerogams). In many cases externai 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 rout into the stem, the phloem becomes continuous round the xylem. At the same time the ~ ~

stele becomes more bulky, all its elements increas- ~ ~ 0 ing in number (fig. 4). Soon a bundle goes off to ~ the first leaf. This consists of a few xylem elements, e a a segment of phloem, pericycle, and usually an arc of h~s 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 throughout the axis. In others a central parenchyma or primetive pith a new region of the primitive stelar conjunctiveappears in the centre of the xylem. In most ferns internal p/deem appears instead of a ~arenchymatous pith (fig. 5). Sometimes this condition, that of the amphiphloic 110 plostele, 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 endodernlis, 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 mesocyclethese 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 per pa / -..-

~ ~~C5 ph ,

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FIGS. ft1.Types of Stole in Vascular Plants. Ftc. 3.Dlarch stele of root of a F stem of young Fern. Fin. 6.Sofenostele of stem of Fern showing detachment of lea Malonia. vie. ~.Tricyclic dictyostele of Dan,,na. Fec. io.Diarch haplostele of .5 Lyco podium. Fio. i3.Typical siphonostele of dicotyledn. FIG. i4.Stele of mr Explanation of Lettering: st. stele; mit. meristele; it. leaf-trace; 1.1. leaf-gap; cor cell; per. pericycle; ph. phfoein; sacs. mesocyde; x. xylem; px. protoxylem; ins. metu starchy xylem-parenchyma, which, when the xylem is bulky, usually appear among the tracheids, the phloem also often being penetrated by similar bands of phloem-parenchyma.)

In the other groups of Pteridophytes internal phloem is not found and an internal endodermis but rarely. The centre of the S~hooo- stele is however often occupied by a large-celled pith resembling the cortex in structure, the cortex and pith ~ together being classed as ground tissue. To this type of steIn having a ground-tissue pith, whether with or without internal phloem, is given the name siphonostele to distinguish it from the solid haplostele characteristic of the root, the first-formed portion of the stem, and in the more primitive Pteridophytes, of the whole of the 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 Dktyosteb. haplostele, the segments of inner endodermis, pericycle, phloem and ~ Pig. 6.

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F,~./4. ~ F~.g rn. FIG. 4.Haplostele of stem of young Fern. Fxo. s.Amphiphloic haplostele of -trace and leaf-gap. FIG. 7.Dictyostele of Fern. Fiu. 8.Tricycfic solenostele of eloginell~. Fzo. ix.Tristefic stein of Selaginella. Fie. 12.Modified hapfostele of ocotyledn. FIG. if.Poiyarch root of Veratrum (a monocotyfedon).

cortex; p.t. peristelar tissue; p.1. peristefar lacuna; end. endoderruis; p.c. passage ylem; p. pith; ccl. p. sclerised pith; c. cainbium; p.m.r. primary, medullary, ray.

I mesocycle joining with the corresponding outer segments to form a nearly concentric structure. For this reason a stem in which as pot ystelic, the term stele being transferr~d from the primary I central cylinder of the i~xis and applied to the vascular strands just described. In this use the term loses, of course, its morphoI logical value, and it is better to call such a segment of a broken-up I stele a meristele, the whole solenostele with overlapping leaf-gaps being called a dictyostele. The splitting up of the vascular tube I into separate strands does not depend wholly upon the occurrence I of leaf-gaps. In some forms other gaps (perforations) appear in the vascular tube placing the pith and cortex in communication.

In other cases the leaf-gaps are very broad and long, the meristeles separating them being reduced to comparatively slender strands, while there is present in each gap a network of fine vascular threads, some of which run out to the leaf, while others form cross-connections between these leaf-trace strands and also with the main cauline meristeles. Finally the cauline meristeles themselves may be resolved into a number of fine threads. Such a structure may be spoken of as a dissected dictyostele.

In some solenostelic ferns, and in many dictyostelic ones additional vascular strands are present which do not form part of the primary vascular tube. They usually run freely in the pith and Polycycly. join the primary tube in the neighborhood of the leaf-gaps. Sometimes a complete internal vascular cylinder, having the same structure as the primary one, and concentric with it, occurs in the pith, and others may appear, internal to the first (Matonia, Saccoloma). Junctions of the first internal cylinder are made with the primary (external) cylinder at the leafgaps, and of the second internal cylinder with the first in the same neighborhood (fig. 8). In dictyostelic ferns similar internal (dictyostehc) cylinders are found in some forms, and occasionally a large series of such concentric cylinders is developed (Marattiaceae) (fig. 9). In such cases the vascular system is said to be polycyclic in contrast with the ordinary monocyclic condition, These internal strands or cylinders are to be regarded as peculiar types of elaboration of the stele, and probably act as reservoirs for water-storage which can be drawn upon when the water supply from the root is deficient.

The vascular supply of the leaf (leaf-trace) consists of a single strand only in the haplostelic and some of the more primitive siphonostelic forms. In the microphyllous groups Leaf.trace of Pteridophytes (Lycopodiales and Equisetales) in and Petlolar which the leaves are small relatively to the stem, the Strands, single bundle destined for each leaf is a small strand whose departure causes no disturbance in the cauline stele. In the megaphyllous forms, on the other hand, (Ferns) whose leaves are large relatively to the stem, the departure of the correspondingly large trace causes a gap (leaf-gap) in the vascular cylinder, as already described. In the haplostelic ferns the leaf-trace appears as a single strand with a tendency to assume the shape of a horseshoe on cross-section, and this type is also found in the more primitive solenostelic types. In the more highly developed lorms, as already indicated, the leaf-trace is split up into a number of strands which leave the base and sides of the leaf-gap independently. In the petiole these strands may increase in number by branching, and thotigh usually reducible to the outline of the primitive horseshoe, more or less elaborated, they may in some of the complex polycylic dictyostelic types (Marattiaceae) be arranged in several concentric circles, thus imitating the arrangement of strands formed in the stem. The evolution of the vascular structure of the petiole in the higher ferns is strikingly parallel with that of the stem, except in some few special cases.

There is good reason to believe that the haplostele is primitive in the evolution of the vascular system. It is found in most of P all I of those Pteridophytes which we have other reasons for e considering as primitive types, and essentially the same Ontogeny type is found, as we have seen, in the independently with developed primitive conducting system of the mossPh.~logeny. stem. This type of stern is therefore often spoken of as protoslelic. In the Ferns there is clear evidence that the amphiphloic haplostele or protostele succeeded the simple (ectophloic) protostele in evolution, and that this in its turn gave rise to the solenostele, which was again succeeded by the dictyostele. Polycycly was derived independently from monocycly in solenostelic and in dictyostelic forms. In the formation of the stem of any fern characterized in the adult condition by one of the more advanced types of vascular structure all stages of increase in complexity from the haplostele of the first-formed stem to the particular condition characteristic of the adult stem are gradually passed through by a series of changes exactly parallel with those which we are led to suppose, from the evidence obtained by a comparison of the adult forms, must have taken place in the evolution of the race, There is no more striking case in the plantkingdom of the parallel between ontogeny (development of the individual) and phylogeny (development of the race) so well known in many groups of animals.

The stele of most Lycopods is amore or less modified protostele, but in the genus Lyco podium a peculiar arrangement of the xylem Ab and phloem is found, in which the latter, instead of being erran confined to a peripheral mantle of tissue, forms bands Stelar running across the stele and alternating with similal Systems of bands of xylem (fig. 12). In Selaginella the stelar systeim h do~ shows profounder modifications. In some forms we find ji vtes. a simple protostele, exarch-polyarch in one species (S. spinosa), exarch-diarch in several (fig. 10). In other species, however, a peculiar type of polystely is met with, in which the original diarch stele gives rise to se-called dorsal and ventral stelar cords which at first lie on the surface of the primary stele, but eventually at a higher level separate from it and form distinct secondary steles resembling the primary one. Similar cords may be formec on. and may seoarate from, these secondary steles. thus ~ivinr ris to a series of steles arranged in a single file (fig. Ii). ln the creeping stem of one species (S. Lyallii) a polycyclic solenostele is found exactly parallel with that of the rhizome of ferns. The gaps in the outer tubular stele, however, are formed by the departure of aerial branch-traces, instead of leaf-traces as in the ferns. The first formed portion of the stern in all species of Selaginella which have been investigated possesses an exarch haplostele. The stele of Equisetum is of a very peculiar type whose relations are not completely clear. It consists of a ring of endarch collateral bundles1 surrounding a hollow pith. The protoxylem of each is a leaftrace, while the metaxylem consisting of a right and a left portion forms a quite distinct cauline system. All the metaxylems join at the nodes into a complete ring of xylem. The whole stele may be surrounded by a common external endodermis; sometimes there is an internal endodermis in addition, separating the bundles from the pith; while in other cases each bundle possesses a separate endodermis surrounding it. At the nodes the relation of the endodermis to the bundles undergoes rather complex but definite changes. It is probable that this type of stele is a modification of a primitive protostele, in which the main mass of stelar xylem has become much reduced and incidentally separated from the leaftraces.

During recent years a number of fossil (Carboniferous and Permian) plants have been very thoroughly investigated in the light of modern anatomical knowledge, and as a result it has become st i s clear that in those times a large series of plants etisted ear ys intermediate in structure between the modern ferns tern of Cycaand the modern Gymnosperms (especially Cycads), dofiices.

and to these the general name Cycadofilices has been applied. We now know that many at least of the Cycadofilices bore seeds, of a type much more complex than that of most modern seed plants, and in some cases approximating to the seeds of existing Cycads. Among the Cycadofilices a series of stages is found leading from the primitive fern-protostele to the type of siphonostele characteristic of the Cycads which agrees in essentials in all the Spermophytes. The main events in this transition appear to have been (I) disappearance of the central xylem of the protostele and replacement by pith, leading to the survival of a number of (mesarch) collateral bundles (see below) at the periphery of the stele; (2) passage from mesarchy to endarchy of these bundles correlated with a great increase in secondary thickening of the stele. The leaves of the more primitive members of this series were entirely fern-like and possessed a fern-like vascular strand; while in the later members, including the modern Cycads, the leaf bundles, remaining unaffected by secondary thickening, are mesarch, while those of the stem-stele have become endarch. Besides the types forming this series, there are a number of others (Medulloseae and allied forms) which show numerous, often very complex, types of stelar structure, in some cases polystelic, whose origin and relationship with the simpler and better known types is frequently obscure. Among the existing Cycads, though the type of vascular system conforms on the whole with that of the other existing seed-plants, peculiar structures are often found (e.g. indications of polystely, frequent occurrence of extra-stelar concentric bundles, anomalous secondary thickening) which recall these complex types of stelar structure in the fossil Cycadofilices.

The typical structure of the vascular cylinder of the adult primary stem in the Gyrnnosperms and Dicotyledons is, like that of the higher ferns, a hollow cylinder of vas- Structure of cular tissue enclosing a central parenchymatous pith. the Stele In But, unlike the ferns, there is in the seed-plants no in- s d I ~ ternal phloem (except as a special development in ee pan $~$~ certain families) and no internal endodermis. The xylem and phloem also, rarely form perfectly continuous layers as they do in a solenostelic fern. The vascular tissue is typically separable into distinct collateral bundles (figs. 13, 23), the xylem of which is usually wedgeshaped in cross-section with the protoxylem elements at the inner extremity, while the phloem forms a band on the outer side of the xylem, and separated from it by a band of conjunctive tissue (mesodesm). These collateral bundles are separated from one another by bands of conjunctive tissues called primary medullary rays, which may be quite narrow or of considerable width. When the pith is large celled, the xylems of the bundles are separated from it by a distinct layer of conjunctive tissue called the endocycle, and a similar layer, the pericycle, separates the phloem from the cortex. The inner layer of the cortex (phloeoterma) may form a well-marked endodermis, or differ in other ways from the rest of the cortex. The pericycle, medullary rays, endocycle and mesoderm all form parts of one tissue system, the external conjunctive, and are only topographically separable. The external conjunctive is usually a living comparatively small-celled tissue, whose cells are consider ably elongated in the direction of the stem-axis and frequently contain abundant starch. Certain regions of it, particularly thi whole or part of the pericycle, but sometimes also the endocycle are typically converted into thick-walled hard (scierenchymatous, tissue usually of the prosenchymatous (fibrous) type, which v important in strengthening the stem, particularly in enablingi to resist bending strains. The relatively peripheral position ii the stem of the pericycle is important in this connexmon. Variou~ secondarv meristems f see o. 7~tf) also arise in the external coniunctive Most of the collateral bundles of this spermophytic type of siphonostele are leaf-trace bundles, i.e. they can be traced upwards from any given point till they are found to pass out of the cylinder, travel through the cortex of the stem and enter a leaf. The remaining bundles (compensation bundles) which go to make up the cylinder are such as have branched off from the leaf-traces, and will, after joining with others similarly given off, themselves form the traces of leaves situated at a higher level on the stem. Purely cauline vascular strands (i.e. confined to the stem) such as are found in the dictyosteles of ferns are rare in the flowering plants. The leaf trace of any given leaf rarely consists of a single bundle only (unifascicular); the number of bundles of any given trace is always odd; they may either be situated all together before they leave the stele or they may be distributed at intervals round the stele. The median bundles of the trace are typically the largest, and at any given level of the stem the bundles destined for the next leaf above are as a whole larger than the others which are destined to supply higher leaves. Leaf-gaps are formed in essentially the same way as in the ferns, but when in the case of a plurifascicular trace the bundles are distributed at intervals round the cylinder it is obvious that several gaps must be formed as the different bundles leave the stele. The gaps, are, however, often filled as they are formed by the development of external conjunctive tissue immediately above the points at which the bundles begin to bend out of the stele, so that sharply defined open gaps such as occur in fern-steles are but rarely met with in flowering plants. The constitution of the stele of a flowering plant entirely from endarch collateral bundles, which are either themselves leaf-traces or will form leaf-traces after junction with other similar bundles, is the great characteristic of the stem-stele of flowering plants. These collateral bundles are obviously highly individualized. The external conjunctive tissue is often arranged in relation to each bundle separately, the pericyclic fibres for instance, already referred to, being cften confined to the bands of pericyclic tissue abutting on the phloem of each bundle, while the Cortex and pith frequently form rays in the intervals between the adjacent bundles.

In some cases this individualization is carried ftirther, the cortex and pith becoming continuous between the bundles which appear as isolated strands em- Aberrant bedded in a general \ ,L.~/ ~ Typesof ground-tissue. Each J~ 1 1 / Stelein bundle has its own ~ investment of tissue P corresponding with external conjunctive, and now called peridesm. The bundles sometimes keep their arrangement s v in a ring corresponding with the stele, though the continuous cylin 0 der no longer exists (species of - Ranunculus). This condition is ~t1 known as astely. In some astelic L- ~ stems (Nymphaeaceae) the number of bundles is greatly increased and they are scattered throughout the ground tissue. A polystelic con g dition arises in some members of this order by the association of collateral 8 bundles round common centres. A

similar phenomenon is seen in two r widely separated genera of flowering plants: Primula Auricula and Gunnera (Halorageae).

The monocotyledons, one of the primary divisions of angiosperms, typically possess large Monocoty- leaves with broad Iedonous sheathing bases containType. ing a very great number of bundles. This results in the number of bundles present at any (Sachs.). - given level of the stem being enor c1c~Gd i~ ~e5~Y~. ~ mously increased. These bundles cotyledon. are scattered in a definite though not r. Annuiar vessel, superficially obvious order through & I,~iterceiiuIar canal, the conjunctive tissue of the stele, 1. Pitted ~ which occupies nearly the whole vv. Sieve-tubes with accompanying corn- diameter of the stem, the cortex sd.p~cierizedperid~sm. being reduced to a very narrow p. Surrounding narenchyrna. outer layer, or disappearing altogether cells a of the bundie are parenchy- (fig. 3). The mass of conjunctive matous, i marks the mner side of tissue is developed as a large-celled the bun ~ ground-tissue, and round each ,bundle there is a peridesm which rs often fibrous (fig. 16). It is possible to suppose that this condition is derived from the astelic condition already referred to, but the evidence on the whole leads to the conclusion that it has ansen byan increase in the number of the bundles within the stele, the individuality of the bundle asserting itself after its escape from the original bundle-ring of the primitive cylinder.

In the stems of many water-plants various stages of reduction of the vascular system, especially of the xylem, are met with, and very often this reduction leads to the formation of a compact stele in which the individuality of the separate Reduced bundles may be suppressed, so that a closed cylinder lmpbost~h1c of xylem surrounds a pith. The phloem is generally Type. unreduced, and there is normally a well marked endoderinis (fig. 17).

Fio. i 7.Transverse section of the stele of the stem of a water-plant (Naias);

1. intercellular channel representing xylem; ph. phloem; e. endodermis.

In other cases the reduction goes much further, till the endodermis eventually comes to surround nothing but an intercellular channel formed in place of the stelar tissue.

In the blade of a typical leaf of a vascular plantessentially a thin plate of assimilating tissuethe vascular system takes the form of a number of separate, usually branching and anastomosing strands. These, with their associated Stelar stereom, form a kind of framework which is of great Tissueol Leaf and importance in supporting the mesophyll; but also, and R~t.

chiefly, they provide a number of channels, penetrating every part of the leaf, along which water and dissolved salts are conveyed to, and elaborated food-substances from, the mesophyll cells. The bundle-system is of course continuous with that of the petiole and stem. The leaf-bundles are always collateral (the phloem being turned downwards and the xylem upwards), even in Ferns, where the petiolar strands are concentric, and they have the ordinary mesodesm and peridesm of the collateral bundle. The latter is often sclerized, especially opposite the phloem, and to a less extent opposite the xylem, as in the stem. As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sieve-tubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath.

The structure of the stele of the primary Fin. i&Vertical section of a Palm-stem showing the root as it is found in most Pteridophytes vascular bundle,, Jr. curving and many Phanerogams has been already inwards and then outwards. described. The radial structure is characteristic of all root-steles, which have in essential points a remarkably uniform structure throughout the vascular plants, a fact no doubt largely dependent on the very uniform conditions under which they live. While the stele of the primary root in both Gymnosperms and Angiosperms is usually diarch or tetrarch, the large primary root-steles of many adventitious roots are frequently polyarch, sometimes with a very large number of protoxylems. Such a stale seldom has the centre filled up with xylem, this being replaced by a large-celled pith, so that a siphonostelic structure is acquired (fig. 15). Sometimes, however, the centre of a bulky root stale has strands of metaxylem (to which may be added strands of metaphioem) scattered through it, the interstices being filled with conjunctive. The conjunctive of a root-stele possessing a pith is often sclerized between the pith and the pericycle. Sometimes all the parenchyma within the stele undergoes this change. In the roots of some palms and orchids a polystelic structure obtains.

In certain families of Angiosperms a peculiar tissue, called laticiferous tissue is met with. This takes the form of long usually richly branched tubes which penetrate the other tissues of the plant mainly in a longitudinal direction. They possess a delicate Laticiferous layer of protoplasm, with numerous small nuclei lining Tissue the walls, while the interior of the tube (corresponding with the cell-vacuole) contains a fluid called latex, consisting of an emulsion of fine granules and drops of very various substances suspended in a watery medium in which various other substances (salts, sugars, rubber-producers, tannins, alkaloids and various enzymes) are dissolved. Of the suspended substances, grains of caoutchouc, drops of resin and oil, proteid crystals and starch grains may be mentioned. Of this varied mixture of substances some are undoubtedly plastic (i.e. of use in constructing new plant-tissue), others are apparently end-products of metabolism, in other words excrela, though they are not actually cast out from the plant-body. The relation ~ of the laticiferous tissue to the assimi I lating cells under which they often end, and the fact that where this tissue is / richly developed the conducting paren ~ chyma of the bundles, and sometimes also 4 the sieve-tubes, are poorly developed, as well as various other facts, point to the conclusion that the laticiferous system has an important function in conducting plastic substances, in addition to acting as an excretory reservoir. As a secondary function we may recognize, in certain cases, the power of closing wounds, which results from the rapid coagulation of exuded latex in contact with the air. The use of certain plants as rubber-producers (notably Hevea b~-a~iliensis, the Pam rubber-tree) depends on this property. The trees are regularly tapped and the coagulated latex which exudes is collected and worked up into rubber. Opium is obtained from the latex of the opium poppy (Pa paver somniferum), which contains the alkaloid morphine.

- Laticiferous tissue is of two kinds:

(1) lati.ciferous cells (coenocytes) (fig. 9) which branch but do not anastomose, and the apices of which keep pace in their growth with that of the other tissues of -, the plant (Anocynaceae, most Eunhorbi(Alter Haberlandt. From vinm,, - -

Try/-Book of Bc/any, by 11cr- aceae, &c.); t2) ~aticzferous vessels ~fig. 20) mission,) which are formed from rows of menFie. ifA portion of a lactici- stematic cells, the walls separating the lerous coenocyte dissected out s cells breaking down, so that a network ihe leaf of a Euphorbia (Xi20). of laticiferous tubes arises (Papaveraceae, Hevea, &c.). In some cases (Allium, Convolvulaceae, &c.) rows of cells with latex-like contents occur, but the walls separating the individual cells do not break down.

The body of a vascular plant is developed in the first place by repeated division of the fertilized egg and the growth of Develop- the products of division. The body thus formed ment of is called the embryo, and this develops into the adult Primary plant, not by continued growth of all its parts as in an animal, but by localization of the regions of cell-division and growth, such a localized region being called a growing-point. This localization takes place first at the two free ends of the primary axis, the descending part of which is the primary root, and the ascending the primary shoot. Later, the axis branches by the formation of new growing-points, and in this way the complex system of axes forming the body of the ordinary vascular plant is built up. In the flowering plants the embryo, after developing up to a certain point, stopf growing and rests, enclosed within the seed. It is only or germination of the latter that the development of the embryc into the free plant is begun. In the Pteridophytes, on the other hand, development from the egg is continuous.

The triple division of tissues is laid down in most cases at 1 very early period of developmentin the flowering plants usuall3 before the resting stage is reached. In many Pteridophytes thi first leaf is formed very early, and the first vascular strand i! developed at its base, usually becoming continuous with the cylinde of the root; the strand of the second leaf is formed in a similar wa~ and runs down to join that of the first, so that the stem stele is forme. by the joined bases of the leaf-traces. In other cases, however, continuous primitive stele is developed, extending from the primar stem to the primary root, the leaf-traces arising later. This i correlated with the comparatively late formation and small development of the first leaves. The evidence scarcely admits of a decision as to which of these methods is to be regarded as primitive in descent. In the seed-forming plants (Phanerogams) one or more primary leaves (cotyledons) are already formed in the resting embryo. In cases where the development of the embryo is advanced at the resting period, traces run from the cotyledons and determine the symmetry of the stele of the primitive axis, the upperpart of which often shows stem-structure, in some respects at least, and is called the hypocoty- ledonary stem or hypocotyl, while the lower part is the primary root .~-,

(Alter Sachs. From Vines T sf-Book of Botany, by permission.)

Fin. 2o.Laticiferous vessels from the cortex of the root Scoyzonera hispanica, tangential secf ion.

A, Slightly magnified. B, A small portion highly magnified.

(radicte). In other cases the root structure of the stele continues up to the cotyledonary node, though the hypocotyl is still to be distinguished from the primary root by the character of its epidermis. On germination of the seed the radicle first grows out, increasing in size as a whole, and soon adding to its tissues by cell division at its apical growing-point. The hypocotyl usually elongates, by its cells increasing very greatly in the longitudinal direction both in number and size, so that the cotyledons are raised into the air as the first foliage-leaves. Further growth in length of the stem is thenceforward confined to the apical growing point situated between the cotyledons. In other cases this growing-point becomes active at once, there being little or no elongation of the hypocotyl and tbe cotyledon or cotyledons remaining in the seed.

The structure of the growing-points or apical meristems varies much in different cases. In most Pteridophytes there is a single large apical cell at the end of each stem and root axis.

This usually has the form of a tetrahedron, with its points base occupying the surface of the body of the axis and its apex pointing towards the interior. In the stem, segments are successively cut off from the sides of the tetrahedron, and b~ their subsequent division the body of the stem is produced. In the root exactly the same thing occurs, but segments are cut off alsc from the base of the tetrahedron, and by the division of thes~ the root-cap is formed (fig. 21). In both stem and root early walli separate the cortex from the stele. The epidermis in the stair and the surface layer of the root soon becomes differentiated froit the underlying tissue. In some Pteridophyte stems the apical eel is wedge-shaped, in others prismatic; in the latter case segment~

- are cut off from the end of the prism turned towards the body o the stem. In other cases, again, a group of two or four prismatIl cells takes the place of the apical cell. Segments are then cut if from the outer sides of these initial cells. In most of the Phanerogams the apical (or primary) merislem, instead of consisting of a single apical cell or a group of initials, is stratifiedi.e. there is .~.. ,,,f.e \\~0.\~.~J \\ y~ \ \j/f / ~/ X 1W! ~ff ~)\ /\y\ \ \~J~/// <~ ~

i..

0,,. ~ec _ ~ i:

.., -, .~ .. -~: ~

~ ~ -V ~ -~ ~

1 a4~

O~ .~

0 0. Cc QO 0 o .,1e o~9 O ,o 0.0 ~ oS 5 5

~ ~,~o. Qo - ~ ~Ci,, SC ~

-- Co ,,,,o Oo,,.ei 0.

~ l,~ ~s~ ~

0~1~0G. ~

~ e.

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cASter Strasburger. From vines Text-Book of Botany, by permission.)

Fio. 21.Median Longitudinal Section through the Apex of the Root of Pteris cretica.

t. Apical cell, p. Wall marking limit between the plerome k, initial segment of root-cap. P and the pleriblem Pb.

k,o. Outermost layer of root-cap. c. Wall marking the inner limit of the outer cortex.

more than one layer of initials (fig. 22). Throughout the Angiosperms the epidermis of the shoot originates from separate initials, which never divide tangentially, so that the young shoot is covered by a single layer of dividing cells, the dermatogen. Below this are (After Dc Bary. From Vines Text-Book of Botany, by permission.)

Fin, 22.Median Longitudinal Section of the Growing, Point of the Stem of HiPPurii vulgaris, showing a many-layered meristem.

5, Rudiment of reaf; d, dermatogen.

the initials of the cortex and central cylinder Whether these art always in layers which remain separate is not known, but it is certair that in many cases such layers cannot be distinguished. This however, may be due to irregularity of division and displacement of the cells by irregular tensions destroying the obvious layerec arrangement. In some cases there is a perfectly definite line o:

separation between the young cylinder (plerome) and young corte1 (periblem), the latter having one or more layers of initials at th actual apex. This clear separation between periblem and pleromi is mostly found in plants whose stem-apex forms a naked cone the leaves being produced relatively late, so that the stele of th young stem is obvious above the youngest leaf-traces (fig. 22). When the leaves are developed early, they often quite overshadow thi actual apex of the stem, and the rapid formation of leaf-tissui disturbs the obviousness of, and perhaps actually destroys, th~ stratified arrangement of the shoot initials. In this case also the differentiation of leaf-bundles, which typically begins at the base of the leaf and extends upwards into the leaf and downwards into the stem, is the first phenomenon in the development of vascular tissue, and is seen at a higher level than the formation of a stele. The latter is produced (except in cases of complete astely where a cylinder is never formed) after a number of leaf-traces have appeared on different sides of the stem so as to form a circle as seen in transverse section, the spaces intervening between adjacent bundles becoming bridged by small-celled tissue closing the cylinder. In this tissue fresh bundles may become differentiated, and what remains of it becomes the rays of the fully-formed stele. Many cases exist which are intermediate between the two extreme types described. In these the stele becomes obvious in transverse section at about the same level as that at which the first leaf-traces are developed. Where a large-celled pith is developed this often becomes obvious very early, and in some cases it appears to have separate initials situated below those of the hollow vascular cylinder. In some cases where there is apparently a well-marked plerome at the apex, this is really the young pith, the distinction between the stelar and cortical initials, if it exists, being, as is so often the case, impossible to make out. The young tissue of the stelar cylinder, in the case of the modified siphonostele characteristic of the dicotyledonous stem, differs from the adjoining pith and cortex in its narrow elongated cells, a difference produced by the stopping of transverse and the increased frequency of longitudinal divisions. This is especially the case in the young vascular bundles themselves (desmogen strands). The protoxylem and protophloem are developed a few cells from the inner and outer margins respectively of the desmogen strand, the desmogenic tissue left over giving rise to the segments of endocycle and pericycle capping the bundle. Differentiation of the xylem progresses outwards, of the phloem inwards, but the two tissues never meet in the centre. Sometimes development stops altogether, and a layer of undifferentiated parenchyma (the mesodesm) is left between them; or it may continue indefinitely, the central cells keeping pace by their tangential division with the differentiation of tissue on each side. In this case the formation of the primary bundle passes straight over into the formation of secondary tissue by a cumbium, and no line can be drawn between the two processes. The differentiation of the stelar stereom, which usually takes the form of a sclerized pericycle, and may extend to the endocycle and parts of the rays, takes place in most cases later than the formation of the primary vascular strand. In the very frequent cases where the bundles have considerable individuality, the fibrous pericyclic cap very clearly has a common origin from the same strand of tissue as the vascular elements themselves. In such cases it is part of the peridesm or sheath of elongated narrowcelled tissue surrounding the individual bundle.

The separation of layers in the apical meristem of the root is usually very much more obvious than in that of the stem. The outermost is the caiyptrogen, which gives rise to the root-cap, and in Dicotyledons to the piliferous layer as well. The periblem, one cell thick at the apex, produces the cortex, to which the piliferous layer belongs in Monocotyledons; and the plerome, which is nearly always sharply separated from the periblem, gives rise to the vascular cylinder. In a few cases the boundaries of the different layers are not traceable. The protoxylems and the phloem strands are developed alternately, just within the outer limit of the young cylinder. The differentiation of metaxylem follows according to the type of root-stele, and, finally, any stereom there may be is developed. Differentiation is very much more rapidi.e. the tissueo are completely formed much nearer to the apex, than is the case in the stem. This is owing to the elongating region (in which proto. xylem and protophloem alone are differentiated) being very much shorter than in the stem. The root hairs grow out from the cells of the piliferous layer immediately behind the elongating tegion.

The branches of the stem arise by multiplication of the cells 01 the epidermis and cortex at a given spot, giving rise to a protuber ance, at the end of which an apical meristem is established. Thi vascular system is connected in various ways with that of th(parent axis by the differentiation of bundle-connections across thi cortex of the latter. This is known as exogenous branch-formation In the root, on the other hand, the origin of branches is endogenous The cells of the pericycle, usually opposite a protoxylem strand divide tangentially and give rise to a new growing-point. The ne~ root thus laid down burrows through the cortex of the mother-root and finally emerges into the soil. The connections of its stele witl that of the parent axis are made across the pericycle of the latter Its cortex is never in connection with the cortex of the parent, but with its pericycle. Adventitious roots, arising from stems, usuall) take origin in the pericycle, but sometimes from other parts of th Conjunctive.

In most of the existing Pteridophytes, in the Monocotyledons and in annual plants among the Dicotyledons, there is n further growth of much structural importance in the ~ d ~

tissues after differentiation from the primary men- Tissues. stems. But in nearly all perennial Dicotyledons, in all dicotyledonous and gymnospermous trees and shrubs and in fossil Pteridophytes belonging to all the great groups, certain layers of cells remain meristematic among the permanent tissues, or after passing through a resting stage reacquire menstematic properties, and give rise to secondary tissues. Such meristematic layers are called secondary meristems. There are two chief secondary meristems, the cambium and the phellogen. The formation of secondary tissues is characteristic of most woody plants, to whatever class they belong. Every great group or phylum of vascular plants, when it has become dominant in the vegetation of the world, has produced members with the tree habit arising by the formation of a thick woody trunk, in most cases by the activity of a cambium.

The camb-ium in the typical case, which is by far the most frequent, continues the primary differentiation of xylem and phloem in the desmogen strand (see above), or arises in the resting mesodesm or mesocycle and adds new (secondary) xylem and phloem to the primary tissues. New tangential walls arise in the cells which are the seat of cambial activity, and an initial layer of cells is established which cuts off tissue mother-cells on the inside and outside, alternately contributing to the xylem and to the phloem. A tissue mother-cell of the xylem may, in the most advanced types of Dicotyledons, give rise to(I) a tracheid; (2) a segment of a vessel; (3) a xylem-fibre; or (4) a vertical file of xylem-parenchyma cells. In the last case the mother-cell divides by a number of horizontal walls. A tissue mother-cell of the phloem may give rise to (i) a segment of a sieve-tube with its companion cell or cells; (2) a phloem fibre; (3) a single phloem-parenchyma (cambiform) cell, or a ve~rtical file of short parenchyma cells. At celtain points the cambium does not give rise to xylem and phloem elements, but cuts off cells on both sides which elongate radially and divide by horizontal walls. When a given initial cell of the cambium has once begun to produce cells of this sort it continues the process, so that a radial plate of parenchyma cells is formed stretching in one plane through the xylem and phloem. Such a cell-plate is called a medullary ray. It is essentially a living tissue, and serves to place all the living cells of the secondary vascular tissues in communication. It conducts plastic substances inwards from the cortex, and its cells are frequently full of starch, which they store in winter. They are accompanied by intercellular channels serving for the conduction of oxygen to, and carbon dioxide from, the living cells in the interior of the wood, which would otherwise be cut off from the means of respiration. The xylem and phloem parenchyma consist of living cells, fundamentally similar in most respects to the medullary ray cells, which sometimes replace them altogether. The parenchyma is often arranged in tangential bands between the layers of sievetubes and tracheal elements. The xylem parenchy