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. [Illustration: 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.) [Illustration: Fig. 542. Fructification of a Jungermannia, magnified; its cellular spore-stalk, surrounded at base by some of the leaves, at summit the 4-valved spore-case opening, discharging spores and elaters. 543. Two elaters and some spores from the same, highly magnified.] [Illustration: Fig. 544. One of the frondose Liverworts, Steetzia, otherwise like a Jungermannia; the spore-case not yet protruded from its sheath.]