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. 
