168 THE EVOLUTION OF PLANTS 



the cavity is partitioned up by bands of sterile 

 tissue. 



A vast number of small spores are formed 

 in the sporangia set apart for them, and even 

 the megaspores are rather numerous, number- 

 ing from forty upwards in the megasporangium. 



The subsequent development is much the 

 same as in Selaginella; the antheridium of the 

 microspore here gives rise to only four sper- 

 matozoids, which have numerous cilia, as in the 

 Ferns, instead of only two, as in other Lycopods. 



The female prothallus makes no attempt 

 to grow out of the megaspore, and only be- 

 comes exposed by the splitting open of the 

 spore-membrane. Fertilisation takes place as 

 in Selaginetta, an embryo is developed and the 

 cycle is complete. 



The two genera of living heterosporous Ly- 

 copods are manifestly very different; probably 

 they represent two distinct lines of descent, 

 which have run their course independently ever 

 since the Palaeozoic period. The reasons for 

 this view will now become apparent. 



The geological history of the Selaginella type 

 can be traced back with certainty to the Car- 

 boniferous epoch. The Tertiary and Mesozoic 

 records are of little importance, except as attesting 

 the continuous existence of the group, but when 

 we get back to the Palaeozoic age the evidence 



