External Anatomy. The formation of external organs depends on the unequal growth of the cells composing the body, or of definite groups (if them. Thus arise lobes or segments having their own special forms. In the simplest algæ and fungi the external segmentation of the body is slight or wanting. The entire body may be represented by a single spherical or oblong cell. Sometimes these cells are branched. In certain desmids the branching is elaborate, and so symmetrical as to make the body an object of great beauty. In all these cases, however, the branch has, in itself, a structure precisely like the main body. In somewhat more complex plants the body consists of a row or filament of cells. Among these plants it is very common to find branches arising which are themselves branched, and repeat in all essential characters the main axis. Other plants have their cells arranged in the form of a flat plate. This plate may become more extended in one direction, producing a band-like form. If branching occurs, it is likely to take place in the plane of the flattening. The branch may be essentially like the main axis, or it may take on a special form. Sometimes the more extended growth occurs in several directions, when the body becomes more or less regularly lobed. Unequal growth of any part of the flat body will produce a fluted or frilled form.
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An alga, showing nearly undifferentiatod body; terminal and sexual cells only unlike the rest.
It is only when the plants become massive, so that some cells are exposed on the surface and others hidden in the interior, that marked dissimilarity arises. Then the external parts are likely to be differentiated from the internal, because of the unlike conditions under which the two exist. However little or much the body may be lobed, there will be an unequal exposure to light, and the side best illuminated, whether of organ or whole plant, will take on a different structure from the shaded one. Thus the whole body of liverworts and the leaves of seed plants become dorsiventral. Other factors also determine the mode of growth; e.g., an erect position and the consequent exposure of the body to the loss of water demands organs for absorption, for conduction, and for protection against excessive evaporation; again, the cells in the interior, removed from the air, must be supplied with it by the development of an aërating system.
Thallophytes. The vegetative body of the lower plants is the gametophyte, i.e., the phase in the life history which produces sex organs. (See Alternation of Generations.) The external anatomy of the gametophyte is therefore first considered. Among the algæ and fungi there is little differentiation of the body, because it is usually slender, and all parts are equally fitted to carry on independently the life work. Among the highest algæ, the lower part of the body is often so constructed as to form (a) hold-fasts, by which the plant is anchored to the surface on which it grows; (b) a roundish stalk of greater or less length; and (c) thinner flattened parts, which expose a large surface to the water and light, and so imitate leaves. (See Alqæ.) Among the higher fungi the body is segmented into a nutritive portion, the mycelium, which usually ramifies extensively through the substratum, and a reproductive part, which rises into the air and produces spores. The aërial part of the body may be club-shaped, umbrella-like, diffusely branched, spherical, etc. See Fungi.
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A thallus of a liverwort, Blasia, showing a simple ribbon-like body, with lobed edge.
Bryophytes. Among the liverworts the body is either a flat ribbon, more or less branched, or is segmented into a roundish axis with thin, scale-like outgrowths on upper and under surfaces, the upper ones being relatively large, conspicuous, and green, whence they are called leaves; while the under ones are small, inconspicuous, and pale. From various parts of the body on the under side arise hold-fasts in the form of slender hairs (rhizoids). Similar out-
