Page:EB1911 - Volume 21.djvu/761

Rh cavity as “the cell,” and the contained protoplasm or other substances, if present, as cell-contents. This is in accordance with the original use of the term “cell,” which was applied in the 17th century to the cavities of plant-tissues on the analogy of the cells of honeycomb. The use of the term to mean the individualized nucleated mass of living protoplasm, which, whether with or without a limiting membrane, primitively forms the proximate histological element of the body of every organism, dates from the second quarter of the 19th century. For a more detailed description of the cell see 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 on the method of association. Such a system is called a tissue-system, the word tissue being employed for any collection of cells with common structural, developmental, or functional characters to which it may be conveniently applied. The word is derived from the general resemblance of the texture of plant substance to that of a textile fabric, and dates from a period when the fundamental constitution of plant substance from individual cells was not yet discovered. It is convenient here to define the two chief types of cell-form which characterize tissues

of the higher plants. The term parenchyma is applied to tissues whose cells are isodiametric or cylindrical in shape, prosenchyma tissues consisting of long narrow cells, with pointed ends.

. 1a.—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 neighbourhood of the very thin cross-wall. Note resemblance to H and R.

L, Optical section of cell of parenchyma in the same moss. Embedded in the protoplasm are a number of starch grains.

M, Part of elongated stereid of a Moss. Note thick walls and oblique slit-like pits with opposite inclination on the two sides of the cell seen in surface view.

N, One side of the end of hydroid (tracheid) of a Pteridophyte (fern), with scalariform pits.

O, Optical section of two adjacent leptoids (sieve-tube segments) of Pteridophyte, with sieve plates (s. pl.) on oblique end-wall and side-walls.

P, Part of spiral hydroid (tracheid) of Phanerogam (Flowering Plant).

Q, Three segments of a “pitted” vessel of Phanerogam.

R, Optical section of leptoid (sieve-tube segment) of Phanerogam, with two proteid (companion) cells. s. pl., sieve-plate.

S, Optical section of part of thick-walled stereid of Phanerogam, with almost obliterated cavity and narrow slit-like oblique pits.

T, Part of vertical section through blade of typical leaf of Phanerogam. u.e., Upper epidermal cells, with (c) cuticle. (p) Assimilating (palisade) cell. sp., Assimilating (spongy) cells with large lacunae. l.e., Lower epidermis, with st., stoma

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

V, Endodermal cell of Phanerogam, with suberized central band on radial and transverse walls. 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 and the (q.v.) (the two divisions of the Thallophyta, which is the lowest of the four great groups into which the plant-kingdom is divided) have their bodies each composed of a single cell. In the Algae such a cell consists essentially of: (1) a mass of protoplasm provided with (2) a nucleus and (3) an assimilating apparatus consisting of a coloured protoplasmic body, called a chromatophore, the pigment of which in the pure green forms is chlorophyll, and which may then be called a chloroplast. The whole of these living structures are covered externally by the dead cell-membrane (fig. 1 A) It is from such a living and assimilating cell, performing as it does all the vital functions of a green plant, that, according to current theory, all the different cell-forms of a higher plant have been differentiated in the course of descent.

Among the Green Algae the differentiation of cells is comparatively slight. Many forms, even when multicellular, have all their

cells identical in structure and function, and are often spoken of as “physiologically unicellular.” The cells are commonly joined end to end in simple or branched filaments. Such differentiation as exists in the higher types mainly takes two directions. In the fixed forms the cell or cells which attach the plant to the substratum often have a peculiar form, containing chlorophyll and constituting a rudimentary fixing organ or rhizoid (fig. 1 C). In certain types living on damp soil, the rhizoids penetrate the substratum, and in addition to fixing the plant absorb food substances (dissolved salts) from the substratum (fig. 1 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. 1 B) is an example parallel with Oedocladium; Bryopsis, with Draparnaldia. In Caulerpa the imitation of a higher plant by the differentiation of fixing, supporting and assimilating organs (root, stem and leaf) from different branches of the single cell is strikingly complete. In the Siphoneous family of Codiaceae the branches of the primitive cell become considerably interwoven one with another, so that a dense tissue-like structure is often produced. In this we get a further differentiation between the central tubes (branches of the primitive cell), which run in a longitudinal direction through the body, possess little or no chlorophyll, and no doubt serve to conduct food substances from one region to another, and the peripheral ones, which are directed perpendicularly to the surface of the body, ending blindly there, contain abundant chlorophyll, and are the assimilating organs (fig. 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 colourless branches frequently grow out from some of the cortical cells, and, pushing among the already-formed threads in a longitudinal direction, serve to strengthen the thallus by weaving its original threads together. The cells belonging to any given thread may be recognized at an early stage of growth, because each cell