222 STRUCTURE AND LIFE HISTORIES 



OTHER FORMS 



203.'Riccia. — About 4,000 species of liverworts have 

 been described, and it is, of course, possible here to refer 

 to only a very few of the forms, chosen because they 

 illustrate some special idea or step in the evolutionary 

 development of plants. In addition to the forms already 

 mentioned, attention should be called to the genus Riccia, 



Fig. 168. — A liverwort (i?ic;(;Jo/»'Jc/toc(i>-^o), X about 35. Cross-section 

 of the thallus, showing young sporogonium in the enlarged venter of the 

 archegonium. (After M. A. Howe.) 



which is of interest because of its aquatic mode of life, 

 and also because of its extremely simple sporophyte — the 

 simplest sporophyte, in fact, of all plants that possess 

 archegonia (Archegonates) . The fertilized egg of Riccia 

 develops a sporophyte which has only fertile cells (spores) , 

 except for a wall, one cell thick, enclosing the spores 

 (Fi§. 168). In fact, the sporophyte has been reduced to 

 only a very simple spore-case, of short duration; it never 

 projects beyond the venter of the archegonium. Spores 

 are formed in the usual way, as described for the forms 



