PTERIDOPHYTES 



131 



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 is much less prominent than the 

 gametophytes of the larger Liverworts and Mosses. If 

 Ferns have been derived from the Liverworts, therefore, it 

 is probable that they came from those with very simple 

 bodies rather than from those in which the gametophyte 

 had become large and complex. The conspicuous leafy 

 branch of the Mosses, commonly called " the moss plant," 

 corresponds to nothing in the Pteridophytes, the prothal- 

 lium representing only the protonema part of the gameto- 

 phyte of the true Mosses. 



The small size of the gametophyte seems to be associ- 

 ated with the fact that the chlorophyll work has been 

 transferred to the sporophyte, which hereafter remains the 

 conspicuous generation. The " fern plant " of ordinary 

 observation, therefore, is the sporophyte ; while the " moss 

 plant " is a leafy branch of the gametophyte. 



Another important contrast indicated is that in Bryo- 

 phytes the sporophyte is dependent upon the gametophyte 

 for its nutrition, remaining attached to it ; while in most 

 of the Pteridophytes both generations are independent 

 green plants, the leafy sporophyte remaining attached to 

 the small gametophyte only while beginning its growth 

 (Fig. Ill, B). 



Among the Ferns some interesting exceptions to this 

 method of alternation have been observed. Under certain 

 conditions a leafy sporophyte may sprout directly from the 

 prothallium (gametophyte) instead of from an oospore. 

 This is called apogamy, meaning " without the sexual act." 



