The Origin of Land Plants 109 



is developed, so that the gametophyte is to some 

 extent self-supporting, while in other forms the 

 growth is either entirely at the expense of material 

 stored up in the spore, or in Selaginella, as we 

 shall presently see, the gametophyte is nourished 

 directly from the tissues of the sporophyte, thus re- 

 versing the relation of sporophyte to gametophyte 

 as compared with the lower archegoniates. 



In Selaginella germination takes place in the 

 megaspore while it is still in the sporangium, and 

 long before it has reached its full size. At this time 

 the spore contains but little protoplasm, and the sub- 

 sequent growth of the gametophyte, which until it 

 is nearly fully grown is included entirely in the 

 spore, is mainly at the expense of the sporophyte. 

 The cells of the sporangium walls remain active, 

 and through them material is drawn from the sporo- 

 phyte for the nourishment of the developing gameto- 

 phyte inside the growing spore. The gametophyte 

 may be said, therefore, to grow parasitically upon 

 the sporophyte, thus reversing the conditions which 

 obtain in the lower archegoniates, where it is the 

 sporophyte which derives its nourishment from the 

 tissue of the large green gametophyte in which it is 

 enclosed. The long retention of the germinating 

 spore within the sporangium of Selaginella is a 

 further step in the direction of seed formation. 



The Origin of Seeds. Much light has been 

 thrown upon the history of the seed plants by the 

 extensive studies of the Paleozoic fossils, which 



