SUMMARY AND CONCLUSION 287 
a special cell, or odgonium, where it is fertilized by the 
much smaller motile spermatozoid. 
Besides this line of Chlorophycexw, which may be as- 
sumed to have given rise to the Bryophytes, there are 
several other groups which have branched off from the 
primitive stock. The most important of these are the 
Siphonez, characterized by the complete suppression 
of division walls in the often large thallus; and the 
two very important groups of marine alge, the red and 
the brown sea-weeds, characterized by the special pig- 
ments developed, as well as other important peculiari- 
ties. It is among these marine alge that there are 
found the largest and most complex of the Thallophytes, 
but this is not always associated with a corresponding 
perfection of the reproductive parts, which may be 
exceedingly primitive. Thus in the giant kelps, often 
hundreds of feet in length, so far as is known only 
non-sexual zodspores of the simplest description are 
developed. 
These great sea-weeds have been profoundly modified 
by their environment and have diverged widely in their 
structure from the primitive fresh-water forms which, in 
another direction, have given rise to the higher plants. 
It is exceedingly unlikely that either the red or the 
brown alge have produced any higher types, but they 
themselves represent the highest expression of their 
respective lines of development. 
The evolution of the sexual cells, z.e. the transition 
from the non-sexual zodspores, first to similar gametes, 
and later to the separate male and female cells, has evi- 
dently been accomplished quite independently in several 
widely separated groups of plants,— e.g. Volvocinee, 
