PTERIDOPHYTES 
323 
completes the life cycle, as the asexual spores develop the 
prothallium again. 
In contrasting this life history with that of Bryophytes 
several important diiferences 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 
FIG. 294. Archegonium of Pteris at the time of fertilization, showing tissue of gam- 
etophyte (A), the cells forming the neck (B), the passageway formed by the dis- 
organization of the canal cells (C), and the egg (D) 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. 
