358 PRACTICAL COURSE IN BOTANY 



their allies. The sudden and violent break in the succession 

 of vegetable life that accompanies the appearance of the 

 pteridophytes (412) is probably to be explained by the 

 development of a land flora and the necessity of adaptation to 

 life in a new medium. The fact that no living cell, whether 

 vegetable or animal, can absorb nourishment except in a 

 liquid form, seems to point to an aquatic origin more or less 

 remote for all life. This inference is further strengthened, 

 in the case of plants, by the fact that even in so highly or- 

 ganized a group as the pteridophytes, fertilization cannot 

 take place except in water. Such a requirement would 

 manifestly be a great disadvantage to land plants, and one 

 of the first steps in response to the demands of a new habitat 

 would be to get rid, as far as possible, of the primitive game- 

 tophyte with its outgrown adaptations to a liquid medium, 

 and to transfer the greater part of the work of reproduction 

 to the asexual generation, in which the problem of fertiliza- 

 tion did not have to be directly met, the asexual spores ger- 

 minating without it. The greater the number of these 

 produced, the better the chance that at least some of the 

 gametes developed from them would meet the difficult con- 

 ditions of fertilization, and the survival of the species be 

 assured. At the same time, in order to meet the requirements 

 of terrestrial life successfully, and to provide for continuing 

 the sexual generation, correlative changes would have to 

 take place in the gametophyte by which the increasing 

 uncertainty of fertilization due to structural changes in the 

 sporophyte, and the absence of a liquid medium for the con- 

 veyance of free swimming antherozoids would be avoided. 

 This necessity has been met by the development of the pollen 

 tube, which bores its way to the egg cell, carrying with it the 

 generative cells, which in seed plants have taken the place 

 of the more primitive antherozoids. With the concomitant 

 reduction of the gametophyte and development of the seed 

 habit, the adaptation to land conditions has been made 

 complete. 



