248 THE PTERIDOPHYTA 



in the other two, and the sporogonium of the Moss 

 is also much more highly fitted for life in com- 

 paratively dry air. But it is only in the Fern that 

 the spore-bearing generation is free living, completely 

 adapted to terrestrial life, while the prothallus is on 

 the same level of development as the thallus of Pellia, 

 and like it must have damp soil on which to grow 

 and a film of liquid water in which the sperms can 

 swim for fertilisation. The Fern, therefore, though a 

 terrestrial plant, is still tied, so far as its sexual repro- 

 duction is concerned, to semiaquatic conditions, un- 

 doubtedly a heritage from the aquatic algae from which 

 it is .descended. 



The majority of Ferns live in damp shady places, 

 where they are more likely to find suitable conditions 

 for the production of prothalli from their spores and 

 the occasional free water necessary for the liberation 

 and swimming of the sperms. But some depend for 

 their propagation almost entirely on vegetative repro- 

 duction, and these can live in comparatively dry 

 habitats. The common bracken fern (Pteridium aqui- 

 linum) is a good example. It flourishes on rather 

 dry sandy soil, its underground stems spreading far 

 and wide and continually sending up new fronds, 

 while the older parts gradually die off. It can and 

 does produce prothalli from its spores when they fall 

 in a suitable damp spot, but this is in many localities 

 a rare possibility, and the great bulk of this vigorous, 

 aggressive, rapidly spreading plant is produced vege- 

 tatively. 



The Small-Leaved Cone-Bearing Pteridophytes. The 

 Clubmosses (Lycopods) and Horsetails (Equiseta) differ 

 widely from the Ferns in general appearance, though 

 the broad outlines of their life history aie essentially 



