THE RICCIA GROUP 279 



One must bear in mind these general characters of the bryo- 

 phytes, as the liverworts and mosses are separately taken up 

 and their characters finally summarized by treating the sub- 

 jects under the four heads : 



Class I. The liverworts, or Hepaticce 

 Class II. The mosses, or Musci. 

 The origin and evolution of the bryophytes. 

 Summary of the bryophytes and thallophytes. 



CLASS I. THE LIVEKWOKTS, OE HEPATIC^ 



286. The liverworts. The liverworts grow most luxuriantly 

 , in moist and shaded situations, some forms on the ground, some 



on rocks, and some on trees. There are also certain aquatic 

 liverworts which float on the surface of the water, and a few 

 very simple ones which are entirely submerged like the algae. 

 Thus, although most of the types have the land habit, some show 

 very clearly adaptations for the aquatic life of their ancestors 

 among the algae. The creeping habits of the liverworts probably 

 indicate the way in which land plants arose and became estab- 

 lished first along the margins of streams, ponds, and marshes 

 where algal growths emerged from the water or were left 

 stranded on the wet earth. These first land liverworts naturally 

 clung close to the wet earth in the beginning, until the devel- 

 opment of root-like systems of filaments (rhizoids), which could 

 gather moisture, permitted them to develop upright stems as 

 in the mosses. The forms of the liverworts are various, as will 

 appear in the following brief account of the four orders. 



287. The Riccia group. The simplest liverworts (order Eic- 

 ciales) have a flat plant body (gametophyte), some forms float- 

 ing on the surface of the water and others submerged, while 

 certain types grow close to moist earth. The plant body is a 

 true thallus (Fig. 245, A), and indeed is much simpler than the 

 plant bodies of many thallophytes among the brown and red 

 algae. The lower surface of the thallus bears numerous filaments, 



