THE PTERIDOPHYTA 243 



structure most highly adapted to subaerial life below 

 the level of the Vascular Plants themselves. And 

 finally, in one of the highest families of Mosses we 

 have underground shoots which have many of the 

 characters of true roots. But in their totality the 

 features described above are characteristic of and — 

 apart from a few aberrant forms adapted to special 

 conditions of life — are universally found only in the 

 Vascular Plants, which include the Pteridophytes and 

 the Seed Plants. 



The Pteridophyta (Ferns, Clubmosses and Horse- 

 tails). — Important as these plants are to the botanist, 

 it is beyond the scope of this book to devote to them 

 any special study. All the vegetative characters of 

 the Vascular Plants can for our purposes be more 

 conveniently considered in connexion with the Seed 

 Plants. But some knowledge of the outline of the 

 life histories of Pteridophytes is essential to under- 

 standing the later phases of adaptation to terrestrial 

 life which are characteristic of the Seed Plants. 



In the Mosses and Liverworts we have two kinds of 

 reproduction which regularly alternate with one another 

 in the life history — ^the sexual process of fertilisation, 

 which can only be carried out in water owing to the 

 fact that the sperms are motile swimming cells, and 

 the process of spore formation and dispersal, which 

 takes place in air. This emphasises the incomplete- 

 ness of adjustment of these plants to terrestrial (sub- 

 aerial) life. In the Pteridophyta, which include three 

 or more divergent stocks or phyla of plants originating 

 so far back in early geological time (certainly before 

 the Devonian rocks were laid down) that we know 

 nothing of their actual origin, we still find these two 

 alternating types of reproduction, in spite of the 



