196 A TEXT-BOOK OF BOTANY 



independent. In fact, the prothallium is so small, and the 

 leafy sporophyte becomes relatively so large, that the 

 dependence of the latter upon the former is a very small 

 item in the life-history. 



111. Alternation of generations. The contrast between 

 the alternating generations of mosses and the same genera- 

 tions in ferns is striking. In mosses the gametophyte is 

 the conspicuous phase in the life-history, with its prostrate 

 filaments and leafy branches; while in ferns the gameto- 

 phyte (prothallium) is a very inconspicuous phase in the 

 life-history, being seen only by those who know what to 

 look for, and resembling a very small simple liverwort. 

 In the mosses the sporophyte is at most only a stalked 

 spore-case, attached to the gametophyte and dependent 

 upon it for nourishment; while in ferns the sporophyte is 

 a large, independent, leafy plant, with vascular system 

 and roots. 



The formula for the life-history of a fern may be written 

 as follows: 



G(prothallium) > O (leafy plant) O GrH%> O S O G, 



etc. 



