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 (Riccia irkhocar pa), 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 
(Fig. 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 
