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 or structural unit of the plant body.
An account of the anatomy of plants naturally begins with the cell which is the proximate unit of organic structure. The cell is essentially an individualized mass of protoplasm, of small dimensions, and containing a differentiated protoplasmic body, called a nucleus. Other protoplasmic bodies, known as plastids, and inanimate matter, mainly in solution or suspension, are pres ent. But all cells which are permanent tissue-elements of the plant body possess, in addition, a more or less rigid limiting mem brane or cell-wall, consisting primarily of cellulose or some allied substance excreted by the living cell-body and serving to connect the different cells of a tissue (see below). Upon the characters of constitution, thickness and sculpture of the cell-walls the qualities of a plant-tissue largely depend. The life of all the cells in a plant-body may be prolonged or a number of the cells may die and their protoplasm be removed. Thus a tissue may consist of living encysted units combined with an inanimate framework of cell-walls, enclosing in their cavities solely liquids and gases. In such cases the characters of the adult tissue largely depend on the proportion and distribution, the stature, form and contents of the living and dead units, and on the nature of their walls. It is customary in plant anatomy to speak of the cell-wall with its enclosed cavity as "the cell," and of the contained protoplasm or other substances as cell-contents.
In all but the simplest forms the plant-body is composed of a number of cells associated in more or less definite ways. In the higher or more complicated plants the cells come to differ greatly among themselves so that the adult body is composed of definite systems of units. Each system has 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 developmental, functional or structural characters to which it may be conveniently applied, and accepted by the general resemblance which was thought to exist between the texture of plant-substance and that of a textile fabric before the fundamental constitution of plant-substance was discovered.
It is convenient here to define the two chief types of cell-form which characterize the tissues of the higher plants. The term parenchyma is applied to tissues the cells of which are isodiametric or cylindrical in form. The term prosenchyma is applied to tissues consisting of long narrow cells with pointed ends.
The reader is referred to special articles on the anatomy of the lower plants (Thallophyta: Algae and Fungi, and Bryophytes: mosses and liverworts) in which the differentiation of tissues is comparatively slight. It must suffice here to mention that both Algae and Fungi range from the simplest unicellular forms with no external differentiation of the body to forms of greater stature and complexity of organization. The progress in complexity is closely associated with division of physiological labour among the component cells and with the adaptation of the multicellular organism to the needs of its environment. But whatever are the various states attained by individual genera the majority of the cells of the adult bodies of both Algae and Fungi remain alive and the tissues are essentially parenchymatous.
In accordance with the greater complexity in the conditions of life on land for self-supporting plants, considerably more ad vanced tissue-differentiation is exhibited by the Bryophyta, which are mainly terrestrial plants. In a general way this greater com
plexity consists (I) in the restriction of regular absorption of water to those parts of the plant-body which are in close contact with the soil, and (2) in the more regular evaporation of water from the parts exposed to the air. It is, however, in the higher vascular plants that those two principles find their fuller ex pression, for in the bryophytes water is still absorbed, as, for example, from rain or dew, by the general plant-surface. The lowest liverworts have an extremely simple vegetative structure comparable to that of many of the simpler Algae, for as their bodies are small and normally live in damp air the demands of terrestrial life on them are at a minimum. Thus their bodies con sist of true parenchyma, and the vast majority of the component cells remain alive. Rooting and absorbing thread-like cells (rhi zoids) universally occur on their lower surfaces in contact with the soil, and considerable tissue-differentiation may occur within the body. Thus some possess a distinct assimilative system con sisting of branched chains of thin-walled cells packed with green plastids (chloroplasts) and arising from the bases of large cavities directed towards their upper illuminated surfaces. These cavities are completely roofed by a layer of surface cells pierced by pores, surrounded by special cells, and through which aeration and evaporation are freely maintained. In certain forms in which the body consists of thick midribs and delicale lateral leaf like appendages strands or bundles of long thick-walled fibre-like (prosenchymatous) cells with pointed ends run longitudinally through the midribs, which are devoid of special assimilative tis sue. The walls of these cells are strongly lignified (i.e., consist of woody substance) and are irregularly thick and thin so as to be closely studded by simple pits which are usually arranged in spirals running round the cells and often elongated in the direc tion of the spiral. There is much variety of detail in the differen tiation of the tissues of the liverworts, but the general plan of construction resembles that adopted in the leaves of higher plants. In the mosses the plant-body is generally more elaborate in accordance with its fuller commitment to terrestrial life. There is always a radially organized supporting and conducting axis (stem) bearing laterally throughout the greater part of its length thin, flat assimilating and transpiring appendages (leaves). To the base of the stem or to those parts in contact with the soil are attached branched rhizoids. In some cases the stem possesses a special surface or epidermal layer of cells, 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 and supporting tissue) which forms the outer cortex. This passes gradually into the thinner-walled parenchyma of the inner cortex. The entire cortex is generally alive and its cells often contain reserve foods in the form of starch. The centre of the stem, in the forms which live on soil, is composed of a strand of narrow, elongated, thin-walled, unlignified, dead water-conducting cells (hydroids) which are seldom pitted. This hydrom strand has in most cases no connection with the leaves, but runs straight up the stem and spreads out locally below the reproductive organs. In the aquatic and semi-aquatic forms, and in those mosses which live under conditions of extreme drought the entire plant-surface absorbs water, perpetually in the first two cases, and during rain in the third. In such forms the hydrom strand is either slightly developed or altogether absent. The leaves of most mosses are flat plates, each consisting of a single layer of square or oblong assimilating cells containing chloroplasts.