Seed-Plants 123 



living seed-plants, makes it pretty certain that the 

 existing seed-bearing plants have not all arisen from 

 the same stock. 



Selaginella. Of the existing pteridophytes one 

 genus of club-bosses shows a remarkably close ap- 

 proach to the seed-bearing condition, and illustrates 

 very beautifully the intermediate stage between the 

 typical pteridophytes and the lowest seed-bearing 

 plants. In these club-mosses (Fig. 14, A) we have 

 seen that the germination of the megaspore is al- 

 most entirely completed while the spores are still 

 contained within the sporangium, and the growing 

 gametophyte is nourished by food substances de- 

 rived directly from the cells of the sporophyte, and 

 not from materials stored within the spore itself, as 

 is the case in the other pteridophytes. The final 

 stages, however, including fertilization, are com- 

 pleted after the spores are set free, and as in the 

 lower pteridophytes, water is necessary to convey 

 the sperms to the open archegonium. 



If we examine the " flowers " of one of the lower 

 seed-plants, such as a fir or pine (Fig. 15, B), we 

 shall find that they are composed of closely set 

 scale-like leaves arranged in a cone which is 

 very much like that of the club-mosses. These 

 cones are of two kinds, one bearing megasporangia 

 like those of Selaginella, and usually denominated 

 " Ovules," the other bearing the microsporangia 

 or " Pollen-sacs." In the pine there are two ovules 

 borne upon each scale of the young cone, and 



