Fig. 527. Single plant of Physcomitrium pyriforme, magnified. 528. Top of a leaf, cut across; it consists of a single layer of cells.
[499.] Mosses or Musci. The pale Peat-mosses (species of Sphagnum, the principal component of sphagnous bogs) and the strong-growing Hair-cap Moss (Polytrichum) are among the larger and commoner representatives of this numerous family; while Fountain Moss (Fontinalis) in running water sometimes attains the length of a yard or more. On the other hand, some are barely individually distinguishable to the naked eye. Fig. [527] represents a common little Moss, enlarged to about twelve times its natural size; and by its side is part of a leaf, much magnified, showing that it is composed of cellular tissue (parenchyma-cells) only. The leaves of Mosses are always simple, distinct, and sessile on the stem. The fructification is an urn-shaped spore-case, in this as in most cases raised on a slender stalk. The spore-case loosely bears on its summit a thin and pointed cap, like a candle-extinguisher, called a Calyptra. Detaching this, it is found that the spore-case is like a pyxis ([376]), that is, the top at maturity comes off as a lid (Operculum); and that the interior is filled with a green powder, the spores, which are discharged through the open mouth. In most Mosses there is a fringe of one or two rows of teeth or membrane around this mouth or orifice, the Peristome. When moist the peristome closes hygrometrically over the orifice more or less; when drier the teeth or processes commonly bend outward or recurve; and then the spores more readily escape. In Hair-cap Moss a membrane is stretched quite across the mouth, like a drum-head, retaining the spores until this wears away. See Figures [527]-[541] for details.
500. Fertilization in Mosses is by the analogues of stamens and pistils, which are hidden in the axils of leaves, or in the cluster of leaves at the end of the stem. The analogue of the anther (Antheridium) is a cellular sac, which in bursting discharges innumerable delicate cells floating in a mucilaginous liquid; each of these bursts and sets free a vibratile self-moving thread. These threads, one or more, reach the orifice of the pistil-shaped body, the Pistillidium, and act upon a particular cell at its base within. This cell in its growth develops into the spore-case and its stalk (when there is any), carrying on its summit the wall of the pistillidium, which becomes the calyptra.
Fig. 529. Mnium cuspidatum, smaller than nature. 530. Its calyptra, detached, enlarged. 531. Its spore-case, with top of stalk, magnified, the lid (532) being detached, the outer peristome appears. 533. Part of a cellular ring (annulus) which was under the lid, outside of the peristome, more magnified. 534. Some of the outer and of the inner peristome (consisting of jointed teeth) much magnified. 535. Antheridia and a pistillidium (the so-called flower) at end of a stem of same plant, the leaves torn away (♂, antheridia, ♀, pistillidium), magnified. 536. A bursting antheridium, and some of the accompanying jointed threads, highly magnified. 537. Summit of an open spore-case of a Moss, which has a peristome of 16 pairs of teeth. 538. The double peristome of a Hypnum. 539-541. Spore-case, detached calyptra, and top of more enlarged spore-case and detached lid, of Physcomitrium pyriforme (Fig. [527]): orifice shows that there is no peristome.
[501.] Liverworts or Hepatic Mosses (Hepaticæ) in some kinds resemble true Mosses, having distinct stem and leaves, although their leaves occasionally run together; while in others there is no distinction of stem and leaf, but the whole plant is a leaf-like body, which produces rootlets on the lower face and its fructification on the upper. Those of the moss-like kind (sometimes called Scale-Mosses) have their tender spore-cases splitting into four valves; and with their spores are intermixed some slender spiral and very hygrometric threads (called Elaters) which are thought to aid in the dispersion of the spores. (Fig. [542-544].)