Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/82

Rh 70 MUSCINE^E [MOSSES. lose forms (including Blasia and Fossombronia), as well as Haplomitrium Hookcri. In Riella and Sph&rocarpus the capsule is without elaters, but possesses sterile cells mor phologically corresponding to them. All the others have elaters. (b) Acrogynous. The apex or the apical cell itself is employed in the formation of the archegonium. All the forms are foliose, except Haplomitrium, which also, in the develop ment of its leaves, does not correspond with the foliose forms, but with Fossombronia. II. Antlwcerotese. Thallose forms. The archegonia, immediately on their first formation, are sunk into the tissue of the thallus. The antheridia stand in closed cavities. The formation of the sporogonium shows no distinction between stalk and capsule, but is pod-shaped and fixed by its swollen foot into the thallus. At maturity the sporogonium opens with two valves, between which the columella becomes visible. At its base it possesses a long and permanent growth. II. MOSSES (Musci or Musd Frondosi). The Mosses proper play a much larger part in the economy of nature than the Liverworts ; they occur in much greater quantity and are more widely distributed, the con ditions of their production not being confined within such narrow limits. While the Liverworts, for the most part, thrive only in localities that are not too dry, though some forms that grow upon bark, such as Radula and Frullania, can even endure drought, Mosses, on the other hand, have an almost ubiquitous character. Many grow in water (Conomitrium, Fontinalis, &c.), in swamps (various kinds of Hypnum and others), on dry rocks (Grimmia, Andre&a), on roofs, in fields, and on trees, in short, under favour able circumstances, a growth of Mosses develops itself almost everywhere. This is connected with the fact that very many have the capacity of enduring drought, and of suspending their growth while it continues and resuming it again in moist weather. Besides, a few, such as Hypnum cupressiforme, Ceratodon, Barbula ruralis, can grow upon any kind of substratum. Others grow by preference on organic substrata : as Buxbaumia indusiata on decayed trunks, Splachnum on old cowdung or on a damp humus- soil, Orthotrichum and the species of Neckera on the firm bark of trees, Phascum in manured fields, a great number like Grimmia and Andreaea on rocks. As regards the chemical quality of the substratum, we can distinguish be tween Mosses that live on chalk (Seligeria, the species of Gymnostomum, and some Hypna) and those that avoid chalk (Andresea, Dicranum). The species of Polytrichum and Thuidium abietinum are fond of sandy soils, Ephem- erum, Fissidens taxifolius, and others of loamy soils, while Archidium phascoides grows on muddy ground. This wide extension of the Leaf -mosses is rendered possible by their uncommonly great capacity for reproduction. Not only is the formation of spores in most species very extensively carried on, but we have to add to this the manifold pro duction of asexual means of multiplication, such as gemmae, &c. The sexual as well as the asexual generation of Mosses attains to a higher grade of perfection than that of Liverworts. (1) The Sexual Generation. The extent of the sexual generation of the Moss-plants proper varies within wide limits. While the stem of Ephemerum and some species of Phascum appears in the form of a small bud, often scarcely visible to the naked eye, there are species of more than 4 inches in length, as Sphagnum, Fontinalis, and the tropical species Spiridens. In every case, however, we have here a really leafy stem ; no thallose forms exist. The leaves are simple and always small. They either consist of one layer in their whole extent, or are traversed by a mid-rib of several layers, which is often of rather complex structure, and consists of thick-walled and thin- walled tissue-elements, of which the former give mechanical firmness to the leaf, while it is principally in the latter that the conveyance to the stem of the matter formed in the leaf takes place. In the species of Polytrichum the middle nerve occupies the greatest part of the leaf, and is covered with lamellae, which consist of cells bearing chloro phyll. Instead of these lamellae, we find in some other Mosses simple rows of cells, as in Pharomitrium subsessile. A peculiar structure of the leaf is also found in Leucobryum glaucum and Sphagnum. The first -named Moss forms thick greenish-white turfs in damp spots. A transverse section through a leaf shows it to be composed of two (or three) layers of cells. Above and below is a layer of large cells devoid of contents, the membrane of which is per forated in various places, so that the cavities of the indi vidual cells communicate with each other ; thus arises a system of capillary tubes which suck up water and retain it like a sponge. Between the angles of every two of these colourless cells we find inserted a cell containing chlorophyll. The colourless cells are filled with air instead of water, and thus the whole plant has a whitish appear ance (Leucobryum), while the green hue comes out at the approach of moisture. The leaves of Sphagnum possess a similar structure. In these too are perforated cells devoid of contents, and surrounded by the meshes of cells contain ing chlorophyll. The inner wall of the empty cells devoid of chlorophyll is for the most part set with peculiar thickenings of annular or spiral form (fig. 9,/), which give FIG. 9. Sphagnum aciMfolium. A. A part of the surface of the leaf, seen from above. It consists of tubular cells containing chlorophyll cl, and large empty cells, which are thickened on their inner surface by spiral bands /, and pierced in some places with openings I. B. Transverse section of the leaf ; cl, cells containing chlorophyll ; Is, the large empty cells. (After Sachs.) it the necessary firmness, as in the vessels of higher plants. In this case too the empty perforated cells are intended for the purpose of absorbing water ; and the same end is attained by cells of similar structure, which form the rind of the stem. The stem of Mosses is distinguished by its slight but uniform thickness, which for the most part does not exceed that of a thick thread. Compared with this deli cacy, the compact, firm, and tough quality of the corre sponding forms in higher plants is so much the more strik ing. It depends on the fact that the exterior layers of cells in the stem have thickened cell- walls, generally of a brown colour, while the central parts have thin walls. The differ entiation of the tissues reaches its highest point in Polytri chum, Hookeria, Splachnum, and others. Strings of sepa rated cells pass from the leaves into the stem, and apply themselves to its central tissue, which is of different conformation from the rest of the tissue of the stem, a circumstance which requires mention because this is the first indication of the fact of such extensive occurrence