Mosses are also propagated by gemmæ or buds. They are produced in many situations, sometimes on distinct organs, sometimes on the tips of the leaves, or on rootlets which grow on various parts of the plants, and which in some of the mosses form a dense woolly or silky mass of a bright yellow or brown colour varying to purple. On the fibres of this mass, green cells appear, which are developed into reproductive buds. Almost every cell on the surface of a moss is capable of forming, by continued division, a cellular nodule, which falls off and gives rise to a germ which grows into a new plant. These nodules are generally situated at the extremity of the leaves, or on the leaves themselves, while pro-embryo fibres spring from the leaf cells of many mosses. M. von Mohl observes that since the cells in the different parts of mosses are capable of being developed into a bud or embryonic confervoid structure producing a bud, it follows that in these plants, notwithstanding their rather complex structure, the subordination of the individual cell to the purposes of the whole plant is still but small; and even here individual life readily acquires the preponderance. This facility of reproduction possessed by the various individual parts of the plants, accounts for the extensive tracts over which mosses spread themselves; moreover, some are diœcious, and as the spores might not always be fertilized, the gemmæ ensure the continued existence of the species.
Mosses are divided into five principal groups, differing exceedingly in importance, and in comparative numbers. They are chiefly distinguished by the position of their fruit. The Pleurocarpi have their fruit lateral, whether on the stem or branches. They comprise thirteen tribes; many of them are found in the southern hemisphere, but a considerable number, especially of the Hypnei, Drepanophyllei, and Hookeriei, are European. The Cladocarpi are characterised by having their urns seated on the tops of very short lateral branches, and by their double axis of growth. The Acrocarpi are distinguished by their main stems ending in fruit. They comprise twenty-seven tribes, and embrace genera and species having a wide geographical range. In the Syncladei the branches of the plant are fasciculate; this group comprises the Sphagnums. The Schistocarpi are remarkable for their fruit splitting into valves, and consist of the tribe Andræaceæ. The various tribes depend on the structure of the urns and leaves, as well as on the natural habits of the plants. In this numerous class of plants only a few remarkable for peculiarity of structure can be mentioned.
The species of the acrocarpous tribe Phascei are exceedingly numerous, and contain the simplest of all mosses; they grow on newly turned-up soil, and are chiefly annual. Their leaves generally have nerves, and are bordered by large cells. The urns, which are either sessile, or upon a short stalk, have not a trace of peristome, and sometimes have a columella, sometimes not. The spores are large compared with those of other mosses.
The tribe Dicranei contains numerous species, some of which are the commonest of mosses in Europe. They are easily known by their single peristome of sixteen teeth divided half-way down. The leaves are extremely crisp and convolute, and the hood is spoon-shaped. The Leucobryum is remarkable for pallid leaves; it has three layers of cells, a narrow layer of green cells embedded in the centre of the leaf with a broader layer of colourless cells on each side, whose cell walls are perforated with large round openings, as in [fig. 50] b. The mosses of this order live on sandstone rocks, shady banks, and trunks of trees. [Fig. 50] shows the microscopic structure of the leaves of various mosses.
In the tribe Grimmiei we have frequently a sessile urn, with a single peristome, and a mitre-shaped hood. The leaves, which are dark green, have minute hexagonal perforated cells on their upper surface, and a white nerve projecting from their extremity.
In the tribe Polytrichei the mouth of the urn is mostly closed by a flat membrane and a hood rough with silky hairs. The leaves are sheathing at their base and spreading at their tips; except in a few cases they are rigid; and the nerve often exhibits lamelliform folds. The Polytrichum dendroides contains scalariform ducts, and starch granules.
The Bryei are of variable size, but a number of European species are among the finest of mosses, on account of their large leaves and beautiful double-toothed peristome, the great distinction of the group. The leaves are margined, toothed, and composed of a loose reticulation of large rhomboidal cells; in the genus Timmia they clasp the stem at the base and spread widely at the tip. There are thirty-three British species of the genus Bryum, many of which with their abundant urns are extremely ornamental.
Fig. 50. Microscopic structure of leaves of mosses:—a, Octoblepharum albidum; b, Leucobryum glaucum; c, Sphagnum latifolium; d, Hypopterygium Smithianum; e, Eucamptodon perichætialis; f, Andræa subulata; g, Campylopus lamellinervis.
The tribe Splachnei contain many of the most singular and beautiful of the whole class of mosses. They have large-celled diaphanous leaves and a straight urn, with the spores radiating from the columella. The urn has a swelling at its base, often of greater dimensions than the urn itself. In the Splachnum vasculosum it is purple and very large, but nothing in comparison of that organ in S. luteum and rubrum, which are the pride of hyperborean Europe and America. The enormous size of the swelling, the variety of colouring, the singularity and elegance of form, and in some cases the unusual dimensions, make the species objects of great interest. The common S. ampullaceum, when growing in abundance on the shallow peaty banks of some mountain stream where cattle come to drink, is scarcely exceeded in beauty by any cryptogam. Species of this order are abundant in the two hemispheres, but the same species rarely occur in both. Their habits, too, are different, for while those in the north only grow on manure, those in the south grow on the trunks of fallen trees. Three genera occur in Great Britain. Gemmæ are found in the axils of the leaves in most species of this group.