402 A TEXTBOOK OF GENERAL BOTANY 
period of favorable moist conditions for their development. The 
sporophyte of a fern, on account of its structure, can withstand 
conditions that would be fatal to the prothallus. As the game- 
tophytes of Selaginella derive their nourishment from the sporo- 
phyte, they are more independent of external conditions than 
are those of ferns. 
A differentiation of spores into megaspores and microspores has 
occurred in several lines of pteridophytes. This differentiation 
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Fic. 450. Cross section of a portion of a Selaginella leaf. (x 235) 
is found in the peculiar water ferns, and it appears that a similar 
differentiation in ferns, in a past geologic age, led to the develop- 
ment of modern seed plants. 
It is of interest to compare the gametophytes and sporophytes 
of liverworts, ferns, and Selaginella (Fig. 438). In all but the most 
advanced types of liverworts the sporophyte is entirely dependent 
on the gametophyte for nourishment. Even in Anthoceros the 
sporophyte is partially dependent on the gametophyte. In typi- 
cal ferns the gametophyte and sporophyte are both independent 
plants. In Selaginella the gametophyte is dependent on the sporo- 
phyte for nourishment, the reverse of the condition in liverworts. 
Interrelationship of Pteridophyta. The relationship between 
the Filicineae, the Kquisetineae, and the Lycopodineae is a much- 
disputed question. Certainly no one of these classes seems to have 
been derived from a modern representative of another. It would 
appear, however, that all three are descended from a common 
ancestor. 
