MUSOINE^K. 



189 



In contradistinction to the Leafy-Mosses, stress must be laid on 

 the well-marked dorsiventrality of the vegetative organs ; i.e. the 

 very distinct contrast between the dorsal side exposed to the light 

 and the ventral side turned to the ground. Veins are never found 

 in the leaves. 



The ventral part of the archegonium (calyptra) continues to grow 

 for some time, and encloses the growing ambryo, but when the 

 spores are ripe it is finally ruptured by the sporangium, and re- 

 mains situated like a sheath (vagimtla) around its base. The 

 sporangium opens, longitudinally, by valves or teeth (Fig. 194, 195, 

 197 6), 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 



FIG. 194. Anthtceros Imvit 

 (nat. size) : K-K capsules. 



FIG. 196. An 

 elater with two 

 spores. 



FIG. 195. Plagiochila asplenioides : a unripe, atid 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. 



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 (7, not yet fully developed, 

 as long cells radiating from the base of the sporangium. They 

 are wanting in Riccia). 



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^). In the MarchantiaceaB each 

 archegonium is enclosed in a loose investment, the perigynium, which is 

 developed as an outgrowth from the cells of its stalk. 



