Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/117

STEM IN ACOTYLEDONrf.] BOTANY 107 resembles much that of a Monocotyledon. One peculiarity, however, must be noticed. The spiral, annular, and reti culated vessels at the inner side of the bundle soon dis appear, and an air-canal is formed in the bundle. This canal must be distinguished from the vallecular canals found in the cortex, with which they alternate. Round the bundles a single layer of usually thickened and coloured cells forms a sheath, and this may either run continu ously round the stem, and thus shut off completely the medullary from the cortical portion of the stem, or may invest separately each bundle, in which case the medullary and cortical portions of the stem are continuous. In some cases a layer of thickened cells is found within the vascular bundles. The cortical portion is divisible into two layers, an inner layer of parenchymatous cells with air canals, the vallecular canals, and an outer layer of thickened cells immediately underlying the epidermis. The structure of the rhizome resembles that of the aerial stem, differing only in the fact that the thickened cells of the outer cortical layer are dark coloured, and that the central canal of the internodes is often absent. In Catamites, a fossil plant which nourished abundantly during the Carboniferous Period, we have a plant which in all particulars is closely allied to Equisetum of the present day. But whilst the young stem of Calamites presented a zone of vascular bundles separating it into a medullary and cortical portion (just as in an Equisetum), new vessels were added to the exterior of the pre-existing bundles as the plant grew, so that a series of woody wedges were formed, increasing in growth to an enormous extent. D- In Lycopodiaceae, or Club Mosses, the stem presents differences from that in Ferns and Equisetaceae. In most members of the family there is an axial cylinder formed of either one or more nbro-vascular bundles. In some cases (Selaginella) the fibro-vascular bundles remain separate and do not form a cylinder. They are surrounded by a mass of prosenchymatous cellular tissue, the cells of which may be thin-walled, leaving no intercellular spaces, or the cell- walls may be much thickened. Immediately surrounding the central axial bundle in Selaginella are large air lacunae, which are wanting in Lycopodium. The fibro-vascular bundles consist of tissues very similar to what occurs in Ferns, and the bundles are all cauline. In the leaves, bundles arise which pass into the stem and coalesce with the cauline bundles. In Isoetes we find a most anomalous structure. Here there are no internodes in the stem ; consequently, it is much shortened. The central axial fibro-vascular bundle is very poorly developed, and round it a meristem is formed, which, by adding new parenchyma outside, increases the diameter of the stem. Thus we have here a thickening ring, but it differs from that structure in Dicotyledons, inasmuch as it is not formed by a union of cambium cells of the fibro-vascular bundles. It resembles more the thickening ring formed in Dracaena, Yucca, and other Monocotyledons. In Rhizocarpea? the structure of the stem is very simple, ! - consisting of an axial fibro-vascular bundle surrounded by parenchyma full of air-cavities. In plants with Acotyledonous stems the mode of branch ing is various. In Lycopodiacese we have an example of a dichotomous branching, and the same is also seen in Ferns. In the Equisetaceae a mode of branching occurs een nowhere else in the Vegetable Kingdom. The branches arise from deeply-seated lateral buds, which, upon developing, break through the superincumbent tissue ; these have been termed endogenous buds. In Characece the branching is of the normal monopodial type. Illogens. Amongst Thallogens, as a rule, there is no evident distinction between stem and leaf. The vegetative organs consist of a number of cells separate or combined into a more or less flattened expansion termed the thallus. In this group are included all the Algae, Fungi, and Lichens, and many Hepaticae. The vegetative structures vary much in each of these groups. In many Hepaticae, as Mar- ohantia, the vegetative structure is of a thalloid nature, and appears as a flattened, broad, dichotomously-branched plate, composed of several layers of cells. The upper and lower layers consist of colourless cells, and form a sort of epidermis. The upper epidermal layer has stomata, while the lower gives off root-like filaments, which attach it to the stratum on which it grows. The stomata are, by repeated division of the guard-cells, converted into canals. The central layer of cells contains green matter, and between the cellules there are large intercellular spaces, which com municate with the stomata on the upper surface. In what are usually termed the foliose Hepaticae, such as Junger- manniae, a distinct stem is visible bearing leaves. All are cellular, no differentiation occurring beyond a compacting of the peripheral layers of cells. The mode of branching is usually dichotomous. In Fungi the vegetative organs consist of a mass of Fungi, cellular filaments, anastomosing and branching in various ways, constituting the mycelium or spaivn (fig. 95, m), from which diversely-shaped portions of cellular tissue are developed, forming receptacles for the support of the reproductive organs. The mycelium creeps along in or upon the stratum whence it is nourished, sending up here and there a receptacle. The struc tural elements which form these are colour less cellular filaments, or hyphce, divided usually by many transverse septa. These sometimes combine to fcrm a dense mass Fig. 95. of parenchyma. In some cases the hypha? A species of Mould- fungus (Botrytis), consisting of a mycelium m,bear- ing a septate cellular stalk s, which branches at the iipex, each division bearing a rounded spore c. of the mycelium are densely interwoven, and form masses of definite shape, the outer layer of which (pseudo-parenchyma- tous) forms a hard shell or skin; such a mass is termed a sclerotiwm. The repro ductive receptacle formed from the myce lium is also composed of hyphae. When there is only a single hypha the reproductive bodies are borne on the ends of its branches ; but more usually the branches of the hyphse bearing the reproductive bodies unite to form a flattened expansion, the hymenium. As they contain no chlorophyll, Fungi take up assimilated matter from other organisms, and therefore are frequently parasites. In Algae the simplest forms present nothing but a cell- Algae, wall, containing a coloured protoplasmic substance, as in some Nostocaceae. In some cases firmness is attained by deposition of silica as in Diatoms, or by lime carbonate as in Acetabularia. Lignification never occurs. More usually, however, the cells are combined into a tissue, but the forms which this may assume and the modes of combination are more various than in any other class of plants, linear masses, strings, globes, laminae, &c., being formed. There may be in some cases a slight differentiation into distinct organs. Thus, in some of the higher forms, as Fucaceae, a distinct stem is formed, from which flattened thalli resembling leaf structures arise, and at the base root -like structures (rhizoids) are formed. These parts, however, have none of the internal structures which characterize higher plants, but consist entirely of cellular tissue. At most there is a condensation of cellular tissue at the periphery, forming an epidermis, and a similar condensation in the axis. The mode of branching amongst Algae is either monopodial or dichotomous. In Lichens the thallus (fig. 9G) consists of a hyphul Lichens, clement of anastomosing and interlacing filaments, amongst which are distributed rounded unicellular coloured bodies,

the yonidia (fig. 97, </). These gonidia are either arranged