The _ventral part of the archegonium_ (calyptra) continues to grow for some time, and encloses the growing embryo, but when the spores are ripe it is finally ruptured by the sporangium, and remains situated like a sheath (_vaginula_) around its base. The sporangium opens, longitudinally, by _valves_ or _teeth_ (Fig. 194, 195, 197 _b_), very rarely by a lid, or sometimes not at all. _A columella is wanting_ (except in _Anthoceros_, Fig. 194); but on the other hand, a few of the cells lying between the spores are developed into _elaters_ (Fig. 196), _i.e._ spindle-shaped cells with spirally-twisted thickenings, which are hygroscopic, and thus serve to distribute the spores. (They are seen in Fig. 189 _C_, not yet fully developed, as long cells radiating from the base of the sporangium. They are wanting in _Riccia_). [Illustration: FIG. 194.--_Anthoceros lævis_ (nat. size): _K_-_K_ capsules.] [Illustration: FIG. 195.--_Plagiochila asplenioides_: _a_ unripe, and _b_ an open capsule; _p_ involucre. The ventral edge of each leaf is higher than its dorsal edge, and covered by the dorsal edge of the next one.] [Illustration: FIG. 196.--An elater with two spores.] Round the entire archegonium, (or group of archegonia, when several are developed on the same receptacle) a sheath--the _involucre_--is often formed, which persists, and encloses the base of the stalk of the sporangium, together with the sheath of the archegonium (Fig. 195 _p_). In the Marchantiaceæ each archegonium is enclosed in a loose investment, the perigynium, which is developed as an outgrowth from the cells of its stalk. The majority of the Liverworts are found in damp and shady places, pressed to the substratum; a few are found floating in fresh water. Family 1. =Marchantieæ.= This embraces only forms with a thallus, which is more or less distinctly dichotomously branched, in some, one or two rows of thin leaves are situated on its under surface. On the upper surface of the thallus are found large air-chambers. Order 1. =Ricciaceæ.= The sporogonia are, with the exception of a few genera, situated singly on the surface of the thallus, and consist only of a capsule without foot or stalk. They always remain enclosed by the wall of the archegonium (calyptra), and open only by its dissolution. Elaters are not developed. Some genera are found floating like Duckweed.--_Riccia glauca_ grows on damp clay soil. _R. fluitans_ and _R. natans_ float in stagnant waters.