PTEKIDOPHYTES 



323 



completes the life cycle, as the asexual spores develop the 

 prothallium again. 



In contrasting this life history with that of Bryophytes 

 several important differences are discovered. The most, 

 striking one is that the sporophyte has become a large, 

 leafy, vascular, and independent structure, not at all re- 

 sembling its representative (the sporogonium) among the 

 Bryophytes. 



Also the gametophyte has become much reduced, as 

 compared with the gametophytes of the larger Liverworts 

 and Mosses. It seems to have resumed the simplest liver- 

 wort form. 



212. The gametophyte. — The prothallium, like a simple 

 liverwort, is a dorsiventral body, and puts out numerous 



Pig. 294. Archegonium of Ptens at the time of fertilization, showing tissue of gam- 

 etophyte {A), the cells forming the neck (iJ), the passageway formed by the dis- 

 organization of the canal cells (C), and the egg (Z>) lying exposed in the venter. 

 —Caldwell. 



rhizoids from its ventral surface (Fig. 293). It is so thin 

 that all the cells contain chlorophyll, and it is usually short- 

 lived. 



