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 alge, but there is usually little differ- 
ence in the structure of the two, aside from the repro- 
ductive organs. Thus in Cidogonium, or Vaucheria, 
there is no apparent difference between the plants which 
produce zodspores 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 Chlorophycex, and this is suggested in 
Cidogonium, 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 zodspores which give rise to as 
many new individuals. In Coleochete (see Fig. 10), 
the genus which on the whole approaches most nearly 
to the lower Archegoniate, the germinating resting- 
spore produces a multicellular body, from each of whose 
cells a zodspore is produced which then develops into 
the new plant. 
