THE EVOLUTION OF PLANT REPRODUCTION 



61 



when it awakens from its dormant state. The origin of the seed 

 habit, which has become the most successful reproductive ad- 

 vance following the independence of the sporophyte, can be 

 traced back to insignificant beginnings. All of the very primitive 

 plants which originated the seed habit have become extinct; 

 but among the fern group there are living a few species which 

 give us a picture as to how the seed habit might have originated. 



Fig. 35. — The little club moss sporophyte is a plant of small size with minute 



overlapping leaves. 



One of these is the little club moss (Selagznella) , whose reproduc- 

 tive cycle includes several noteworthy innovations. 



Most of the ferns, in their asexual generation, produce only 

 one kind of spore, which germinates into a single kind of pro- 

 thallus. But the little club moss sporophyte — a plant of small 

 size, with overlapping leaves covering the branching stems (fig. 

 35) — develops two kinds of sporangia. Some sporangia produce 

 a few large spores, known as megaspores; in other sporangia are 

 formed thousands of very small spores — the microspores. These 

 spores are diff'erent not only in size, but also in behavior. The 

 microspores grow into male prothalli and thus produce only 



