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ANATOMY

OF PLANTS and whose cells are ordinarily full of starch. This may he called the axis of such a sporogonium were prolonged downwards into amylom. The stem in this family falls into two divisions, an the soil to form a fixing and absorptive root, the whole structure underoround portion bearing rhizoids and scales, the rhizome, and would become a physiologically independent plant, exhibiting in many though by no means all respects the leading features of | a leafy aerial stem forming its direct upward continuation. The the sporophyte or ordinary vegetative and spore-bearing individual leaf consists of a central midrib, several cells thick, and two wings, in Pteridophytes and Phanerogams. These facts have led to the one cell thick. The midrib bears above a series of closely set, vertilongitudinally-running plates of green assimilative cells over theory, plausible in many respects, of the origin of this sporophyte cal, the wings close in dry air so as to protect the assimilativeand by descent from an Anthoceros-like sporogonium (see Pteri- which transpiring plates from excessive evaporation of water. The midrib dophyta). The facts, however, give us no warrant for asserting a strong band of stereom above and below. In its centre is a homology (i.e. identity by descent) between the tissues of an has Anthocerotean sporogonium and those of the sporophyte in the band-shaped bundle consisting of rows of leptom, hydrom, and amylom cells. This bundle is continued down into the cortex of the higher plants. as a leaf trace, and passing very slowly through the sclerenIn the Mosses the plant-body (gametophyte) is always separable stem chymatous external cortex and the parenchymatous, starchy into a radially organized, supporting and conducting axis (stem) internal cortex to join the central cylinder. The latter has a and thin, and transpiring a ppendages M mosses. (ieaves )_ flat, [porassimilating, tiie histology of the comparatively central strand consisting of files of large hydroids, separated from simple but in many respects aberrant Bog-mosses (Sphagnacese), one another by very thin walls, each file being separated from its see Muscine.®, Ency. Brit. vol. xvii.] The stems of the other neighbour by stout, dark-brown walls. This is probably homomosses resemble one another in their main histological features. logous with the hydroid cylinder in the stems of other mosses. In a few cases there is a special surface or epidermal layer, but It is surrounded by (1) a thin-walled, smaller - celled hydrom usually all the outer layers of the stem are composed of brown, mantle ; (2) a suberized amylom sheath ; (3) a leptom mantle, thick-walled, lignified, prosenchymatous, fibre-like cells forming a interrupted here and there by starch cells. These three concentric peripheral stereom (mechanical or supporting tissue) which forms tissue mantles are evidently formed by the conjoined bases of the the outer cortex. This passes gradually into the thinner-walled leaf traces, each of which is composed of the same three tissues. parenchyma of the inner cortex. The whole of the cortex, stereom As the aerial stem is traced down into the underground rhizome and parenchyma alike, is commonly living and its cells often portion, these three mantles die out almost entirely—the central contain starch. The centre of the stem in the forms living on hydrom strand forming the bulk of the cylinder and its elements soil is occupied by a strand of narrow elongated hydroids, which becoming mixed with thick-walled stereids ; at the same time differ from those of the liverworts in being thin-walled, unlignified, this central hydrom-stereom strand becomes three-lobed, with and very seldom pitted. The hydrom strand has in most cases no deep furrows between the lobes in which the few remaining connexion with the leaves, but runs straight up the stem and leptoids run, separated from the central mass by a few starchy spreads out below the sexual organs or the foot of the sporogonium. cells, the remains of the amylom sheath. At the periphery of the It has been shown that it conducts water with considerable lobes are some comparatively thin-walled living cells mixed with rapidity. In the stalk of the sporogonium there is a similar a few thin-walled hydroids, the remains of the thin-walled hydrom strand, wdiich is of course not in direct connexion with, but mantle of the aerial stem. Outside this are three arcs of large continues the conduction of water from, the strand of the gameto- cells showing characters typical of the endodermis in a vascular phytic axis. In the aquatic, semiaquatic, and xerophilous types, plant; these are interrupted by strands of narrow, elongated, where the whole surface of the plant absorbs water, perpetually thick-walled cells which send branches into the little brown scales in the first two cases and during rain in the last, the hydrom borne by the rhizome. The surface layer of the rhizome bears strand is either much reduced or altogether absent. In accord- rhizoids, and its whole structure strikingly resembles that of the ance with the general principle already indicated, it is only where typical root of a vascular plant. In Atrichum undulatum the absorption is localized {i.e., where the plant lives on soil from central hydrom cylinder of the aerial stem is a loose tissue, its which it absorbs its main supply of water by means of its basal interstices being filled up with thin-walled, starchy parenchyma. rhizoids) that a water-conducting (hydrom) strand is developed. In Dawsonia superba, the finest of all known mosses, the hydroids The leaves of most mosses are flat plates, each consisting of a of the central cylinder of the aerial stem are mixed with thicksingle layer of square or oblong assimilating (chlorophyllous) walled stereids forming a hydrom-stereom strand somewhat like cells. In many cases the cells bordering the leaf are produced that of the rhizome in other Polytrichaceae. The central hydrom strand in the seta of the sporogonium of into teeth, and very frequently they are thick-walled so as to form a supporting rim. The centre of the leaf is often occupied most mosses has already been alluded to. Besides this there is by a midrib consisting of several layers of cells. These are usually a living conducting tissue sometimes differentiated as elongated in the direction of the length of the leaf, are always leptom forming a mantle round the hydrom, and bounded expoor in chlorophyll and form a channel for conducting the ternally by a more or less well-differentiated endodermis, abutting products of assimilation away from the leaf into the stem. This on an irregularly cylindrical lacuna ; the latter separates the is the first indication of a conducting foliar strand or leaf bundle central conducting cylinder from the cortex of the seta, which, like and forms an approach to leptom, though it is not so specialized the cortex of the gametophyte stem, is usually differentiated into as the leptom of the higher Phseophyceae. Associated with the an outer thick-walled stereom and an inner starchy parenchyma. conducting parenchyma are frequently found hydroids identical in Frequently, also, a considerable differentiation of vegetative tissue character with those of the central strand of the stem, and no occurs in the wall of the spore-capsule itself, and in some of the doubt serving to conduct water to or from the leaf according as higher forms a special assimilating and transpiring organ situated the latter is acting as a transpiring or water-absorbing organ. just below the capsule at the top of the seta, with a richly lacunar In a few cases the hydrom strand is continued into the cortex of chlorophyllous parenchyma and stomata like those of the wall of the stem as a leaf-trace bundle (the anatomically demonstrable the capsule in the Anthocerotean liverworts. Thus the histolotrace of the leaf in the stem). This in several cases runs vertically gical differentiation of the sporogonium of the higher mosses is downwards for some distance in the outer cortex, and ends one of considerable complexity ; but there is here even less reason blindly—the lower end or the whole of the trace being band-shaped to suppose that these tissues have any homology (phylogenetic or star-shaped so as to present a large surface for the absorption of community of origin) with the similar ones met with in the higher water from the adjacent cortical cells. In othercases the trace passes plants. inwards and joins the central hydrom strand, so that a connected water-conducting system between stem and leaf is established. The facts of histological structure in the BryoIn the highest family of mosses, Polytrichaceae, the differentiation of conducting tissue reaches a decidedly higher level. In phytic series are all such as we should expect to be addition to the water-conducting tissue or hydrom there is a well- developed in response to the exigencies of increasing developed tissue inferred to be aconducting channel for nitrogenous adaptation to terrestrial life on soil, and of increasing size substances or leptom, not indeed so highly differentiated as in. the most advanced Laminariacese, but showing some of the characters of the plant-body. In the liverworts we find fixation of of sieve-tubes with great distinctness. Each leptoid is an the thallus by water-absorbing rhizoids; in certain forms elongated living cell with nucleus and a thin layer of protoplasm with a localized region of water-absorption the development lining the wall. The whole cavity of the cell is sometimes stuffed of a primitive hydrom or water-conducting system ; and in 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 others with rather a massive type of thallus the differentiaexactly after the fashion of a trumpet-hypha. The end-wall is tion of a special assimilative and transpiring system. In usually very thin, and the protoplasm on artificial contraction the more highly developed series, the mosses, this last commonly sticks to it just as in a sieve-tube, though no perforation division of labour takes the form of the differentiation of of the wall has been found. Associated with the leptoids are special assimilative organs, the leaves, commonly with a similar cells without swollen ends and with thicker cross walls. Besides the hydrom and leptom, and situated between them, there midrib containing elongated cells for the ready removal of is a tissue which probably serves to conduct soluble carbohydrates, the products of assimilation ; and in the typical forms with