Bryophytes : Liverworts and Mosses 



469 



most numerous in the tropics. Liverworts may be found along 

 streams, on overhanging rocks, on shaded moist soil, and on 

 trunks of trees. Li the tropics they often occur as epiphytes on 

 the stems and leaves of trees. 



The liverworts are probably descended from plants like the 

 green algae; for it is thought that the simplest plants existed 

 first and that plant life (as well as animal life) had its origin in the 

 water. The liverworts may be considered, therefore, as a group of 

 simple plants that exhibit some of the evolutionary stages through 

 which plants passed in taking up life upon the land. In this re- 

 spect they can be compared to the amphibious (Greek : amphi, 

 double, and bios, life) frogs and salamanders of the animal world. 



Life history of a liverwort. The most common of the aquatic 

 liverworts is Ricciocarpus, a small, heart-shaped thallus which 

 floats on the surface of ponds and lakes. On its lower side are 

 hair-like rhizoids and scales that aid in absorption. On its 



Fig. 286. Some widely distributed liverworts: A, Pellia thallus with antheridia (dots 

 on surface) and a sporophyte; B, archegonial thallus of Anthoceros with sporophytes; C, 

 antheridial thallus of the same ; D and E, land and water forms of Riccia. {After Vdenovsky.) 



