io8 Plant Life and Evolution 



sporous types which once existed are now quite 

 extinct. This tendency towards heterospory in so 

 many unrelated groups is an interesting case of 

 parallel development, and may be compared to the 

 separate evolution of sexual cells in widely separate 

 groups of algse, or the development of a similar 

 fibre-vascular system in such probably unrelated 

 types as the ferns and club-mosses. There are four 

 quite distinct existing families of heterosporous 

 pteridophytes, two of which show unmistakable evi- 

 dence of being derived from two types of 

 " homosporous " ferns. One of the others is unmis- 

 takably related to the club-mosses; the second is of 

 somewhat doubtful affinity. 



The evolution of heterospory in so many unre- 

 lated groups of pteridophytes is of importance in 

 connection with the question of the origin of seeds, 

 the structures which especially distinguish the high- 

 est plant types. The separation of the male and 

 female gametophytes, and the reduction of these, are 

 the first steps in the evolution of the seed. In all 

 of the existing pteridophytes, except the genus Se- 

 laginella, the germination of the spore and the de- 

 velopment of the gametophyte takes place after the 

 spores are shed, although it may be said that in 

 some of the water- ferns the spores do not become 

 free from the sporangium, which is detached with 

 the spores inside it. In some of the water ferns the 

 reduction of the female gametophyte is less than 

 in the other heterosporous forms and chlorophyll 



