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 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, even the gametophore being suppressed, and 

 represented, if at all, by a rudiment. The conspicuous 

 leafy branch of the Mosses, commonly called " the moss 

 plant," corresponds to nothing in the Pteridophytes, there- 

 fore, except possibly the rudiment referred to, the prothal- 

 lium representing only the protonema part of the gameto- 

 phyte of the true Mosses. 



This reduction of the gametophyte seems to be associ- 

 ated with the fact that the chlorophyll work has been trans- 

 ferred 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 the 

 Pteridophytes both generations are independent green 

 plants, the leafy sporophyte remaining attached to the 

 small gametophyte only while beginning its growth (Fig. 

 Ill, /y). 



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." 



