SUMMARY AND CONCLUSION 287 



a special cell, or oogonium, where it is fertilized by the 

 much smaller motile spermatozoid. 



Besides this line of Chlorophyceee, 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 

 Siphonese, characterized by the complete suppression 

 of division walls in the often large thallus; and the 

 two very important groups of marine algse, 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 algse that there are 

 found the largest and most conrplex 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 zoospores 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 algae have produced any higher types, but they 

 themselves represent the highest expression of their 

 respective lines of development. 



The evolution of the sexual cells, i.e. the transition 

 from the non-sexual zoospores, 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. Volvocinese, 



