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—s (leaty plant) —o—G_2> o—S—o—G, 
ete. 
