172 PLANT-LIFE 



demonstration that the Phanerogams have evolved 

 from heterosporous Cryptogams, and even the latter, 

 in their turn, may have evolved from homosporous 

 forms. In point of fact, heterospory originating in 

 cryptogamic forms, is carried to its logical culmination 

 in the Flowering plants. That this is so will appear from 

 the facts which we shall now adduce. 



Flowering plants are also characterized as Sperma- 

 phytes, or seed-producing plants. A fertile seed is 

 resultant upon a sexual act which is fundamentally the 

 same as that occuring in all sexual plants. The em- 

 bryonic plant is in the seed, which is virtually a resting- 

 stage in which adverse conditions can be tided over, 

 and by means of which the species can be propagated in 

 a suitable environment. We must not conclude that 

 because we have to do with " seeds " in Flowering plants, 

 we have ceased to do with spores. The pollen grains 

 of Phanerogams are the equivalents of the microspores 

 in the Cryptogams; the ovules from which seeds 

 develop correspond to the megasporangia of the Pteri- 

 dophytes, and the embryo-sac, in the ovule, is, practi- 

 cally, the megaspore. Even a prothallus, although 

 much reduced, is traceable in the development of the 

 megaspore within the ovule ; and in certain Phanerogams, 

 the Cycads and Cone-bearers, a rudimentary prothallus 

 is formed in the germination of the pollen grain. Thus 

 there is an alternation of generations in the Flowering 

 plants, although it is not so apparent as in the Arche- 

 goniates. In the former the sexual generation is 

 highly specialized and greatly reduced, and it is the 

 asexual generation, the sporophyte, which is most palp- 

 ably the " plant." 



