104 EVOLUTION OF PLANTS 



charged the disintegrated cells of the canal traversing 

 the neck, and thus cleared the passage to the egg-cell 

 within the venter. The spermatozoids enter the open 

 archegonium and make their way to the central cell, 

 where one of them penetrates the egg-cell, thus effect- 

 ing its fertilization. 



The necessity of water for the effecting of fertiliza- 

 tion is significant, as it would seem to be a reversion 

 to the aquatic condition of the algal ancestors of the 

 Archegoniates. 



ALTERNATION OF GENERATIONS 



The alternation of sexual and non-sexual individuals is 

 met with in many algse, but there is usually little differ- 

 ence in the structure of the two, aside from the repro- 

 ductive organs. Thus in (Edogonium, or Vaucheria, 

 there is no apparent difference between the plants which 

 produce zoospores and those which bear the sexual cells ; 

 and sometimes, at least, the formation of one sort of 

 reproductive cells or the other is entirely a question of 

 nutrition. 



In the higher Chlorophyceae, and this is suggested in 

 QEdogonium, it will be remembered that the spore pro- 

 duced as the result of fertilization does not at once grow 

 into a plant like the parent, but there is first a division 

 of its contents into four zoospores which give rise to as 

 many new individuals. In Coleochsete (see Fig. 10), 

 the genus which on the whole approaches most nearly 

 to the lower Archegoniatae, the germinating resting- 

 spore produces a multicellular body, from each of whose 

 cells a zoospore is produced which then develops into 

 the new plant. 



