Though the number of species of mosses is far greater than of liverworts, the group offers much less diversity of form. The sexual generation is always a leafy plant, which is not developed directly from the spore but is borne on a well-marked and usually filamentous protonema. The general course of the life-history and the main features of form and structure will be best understood by a brief account of a particular example.Fig. 11. - Funaria hygrometrica.
A, Leafy shoot (g) bearing a young sporogonium enclosed in the calyptra (c).
B, Similar plant with an almost mature sporogonium; s, seta; f, capsule; c, calyptra.
C, Median longitudinal section of a capsule, with the seta gradually widening into the apophysis at its base; d, operculum; p, peristome; a, annulus; c, columella; s, archesporium; h, air-space between the spore-sac and the wall of the capsule.
(From Goebel's Pflanzenmorphologie, by permission of W Engelmann)
Funaria hygrometrica is a moss of very common occurrence even in towns on the soil of paths, at the foot of walls and in similar places. The small plants grow closely crowded in tufts, and consist of short leafy shoots attached to the soil by numerous fine rhizoids. The latter, in contrast to the rhizoids of liverworts, are composed of rows of elongated cells and are branched. The leaves are simple, and except for the midrib are only one layer of cells thick. The structure of the stem though simple is more complicated than in any liverwort. The superficial cells are thick-walled, and there is a central strand of narrow cells forming a water-conducting tissue. The small strand of elongated cells in the midrib of the leaf runs down into the stem, but is not usually connected with the central strand. The sexual organs are developed in groups at the apices, the antheridial group usually terminating the main axis while the archegonia are borne on a lateral branch. The brown tint of the hair-like paraphyses mixed with antheridia (fig. 15) makes the male branch conspicuous, while the archegonia have to be carefully looked for enclosed by the surrounding leaves (fig. 16, B). The sporogonium developed from the fertilized ovum grows by means of a two-sided apical cell (fig. 16 A), and is at first of uniform thickness.
After a time the upper region increases in diameter and forms the capsule, while the lower portion forms the long seta and the foot which is embedded in the end of the stem. With the growth of the sporogonium the archegonial wall, which for a time kept pace with it, is broken through, the larger upper part terminated by the neck being carried up on the capsule as the calyptra, while the basal portion remains as a tubular sheath round the lower end of the seta (cf. figs. 16, C, and fig. 11, A, B). The seta widens out at the base of the capsule into a region known as the apophysis. The peripheral cells of the seta are thick-walled, and it has a central strand of elongated conducting cells. In the epidermis of the apophysis functional stomata, similar to those of the higher plants, are present and, since cells containing chlorophyll are present below the superficial layers of the apophysis and capsule, the sporogonium is capable of independent assimilation. The construction of the capsule will be best understood from the median longitudinal section (fig. 11, C). The central region extending between the apophysis and the operculum is composed of sterile tissue and forms the columella (c). Immediately around this is the layer of cells from which the spores will be developed (s), and the layers of cells on either side of this form the walls of the spore-sac, which will contain the spores.
Between the wall of the capsule, which is composed of several layers of cells, and the spore-sac is a wide intercellular space (h) bridged across by trabeculae consisting of rows of chlorophyll-containing cells. At the junction of the operculum (d) with the rest of the capsule is a circle of cells forming the annulus (a), by help of which the operculum is detached at maturity as a small lid. Its removal does not, however, leave the mouth of the capsule wide open, for around the margin are two circles of pointed teeth forming the peristome. These are the thickened cell-walls of a definite layer of cells (p), and appear as separate teeth owing to the breaking down of the unthickened cell-walls. The numerous spores which have been developed in the spore sac can thus only escape from the pendulous capsule through narrow slits between the teeth, and these are closed in damp air. The unicellular spores when supplied with moisture germinate (fig. 12) and give rise to the sexual generation. A filamentous protonema is first developed, some of the branches of which are exposed to the light and contain abundant chlorophyll, while others penetrate the substratum as brown or colourless rhizoids.
The moss-plants arise from single projecting cells, and numerous plants may spring from the protonema developed from a single spore.Fig. 12. - Funaria hygrometrica. (After Goebel.)
A, Germinating spores. s, Wall of spore; v, vacuole; w, rhizoid.
B, Part of a developed protonema. h, Creeping filament with brown walls from which the filaments of chlorophyll-containing cells (b) arise; k, young moss-plant; w, its first rhizoid.
The majority of the mosses belong to the same great group as Funaria, the Bryales. The other two subdivisions of the Musci are each represented by a single genus. In the Andreaeales the columella does not extend to the upper end of the capsule, and the latter opens by a number of lateral slits. The Sphagnales also have a dome-shaped spore-sac continued over the columella, and, though their capsule opens by an operculum, they differ widely from other mosses in the development of the sporogonium as well as in the characters of the sexual generation. The three groups are described separately below, but some more general features of the mosses may be considered here.