Aquatic herbs, contained in the genera Nitella and Chara, always growing under water, with slender jointed stems, surrounded at the joints by whorls of tubular leaves or branches, which are either membranaceous and transparent, as in _Nitella_; or brittle, and more or less encrusted with carbonate of lime, as in _Chara_, Stonewort. The organs of reproduction are formed in the axils of the branches, and consist of transparent globules, and hard, spiral nuculas, which appear to be formed of twisted leaves, the points of which often form a kind of crest. Young plants are only produced by the nuculas. ORDER CCXVI.—MUSCI.—THE MOSS TRIBE. [Illustration: FIG. 149.—CRYPTOGAMOUS PLANTS.] The Mosses have fibrous roots, and slender wiry stems, densely covered with leaves, which are very small, and laid over each other like scales (see _a_ in _fig._ 149). The theca (_g_) is urn-shaped, and it is produced singly; in most cases, on a long, slender, wiry stem, called a seta, which signifies a bristle, but sometimes without any stalk. It always springs from a tuft of leaves, differing both in size and shape from ordinary leaves, which form what is sometimes called the perichætium. Among these may occasionally be seen a few stalks, resembling the Lichen called Cup-moss, which terminates in a kind of cup, and thickened at the base. The cups and upper parts soon die away, and the thicker part left among the leaves swells, and in time rises on a stalk of its own, carrying away one of the leaves with it on its head. This is the theca, and the leaf it carried away, and which resembles an extinguisher, is called the calyptra, and it remains on till the sporules are nearly ripe. When the calyptra falls, the theca is found to be covered with a little lid called the operculum; which also falls off in time, and shows the mouth or stoma of the theca. This mouth is sometimes naked, and sometimes covered with a kind of film; but generally it is surrounded by a row of long, slender, hair-like teeth called the peristome or fringe. When there are two rows of these hair-like teeth, the inner ones, which are finer than the others, are called the cilia; and the number of both the cilia and the teeth is always some number that can be divided by four. In the cavity of the theca is a central axis called the columella, and around that are found the sporules, kept together by the lining of the theca, which forms a kind of open bag. This is the usual construction of all the numerous genera of mosses; but in some kinds, as for example in the Hair-moss (_Polytrichium_), in addition to the theca, a number of granules are found among the leaves, which are said to be capable of producing young plants. ORDER CCXVII.—HEPATICÆ.