Page:EB1911 - Volume 21.djvu/763

Rh A sufficient description of the thallus of the liverworts will be found in the article We may note the universal

occurrence on the lower surface of the thallus of fixing and absorbing rhizoids in accordance with the terrestrial life on soil (cf. Oedocladium among the Green Algae). The Marchantiaceae (see article ) 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 genera—Blyttia, Symphyogyna, and Hymenophytum—there are one or more strands or bundles consisting of long thick-walled 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, which are usually arranged in spirals running round the cells, and are often elongated in the direction of the spiral (fig. 1, 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-capsule 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 sporogonium 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 Pteridophytes 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 ). But in the Bryophytes the sporogonium 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)

and thin, flat, assimilating, and transpiring appendages (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. [For the histology of the comparatively simple but in many respects aberrant Bog-mosses (Sphagnaceae), see .] 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 connexion 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 connexion 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 water-absorbing 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 blindly—the 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 well developed 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 sclerenchymatous 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 neighbour by stout, dark-brown walls. This is probably homologous with the hydrom cylinder in the stems of other mosses. It is surrounded by (1) 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 entirely—the central hydrom strand forming the bulk of the cylinder and its elements becoming mixed with thick-walled stereids; at the same time this central hydrom-stereom 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 plant; these are interrupted by strands of narrow, 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 Catharinea