428 BOTANY OF THE LIVING PLANT 



Algae, where sexual evolution is incomplete, the gametes can germinate 

 without fusion. If the whole gametangnuii could do this, the result 

 would be what is seen in the azygospores. 



Comparing the Phycomycetes as a whole with the Siphonaccous 

 Algae, the Zygosporeae would be referable in origin to the more 

 primitive Siphoneae, in which the differentiation of sex was less 

 advanced. The Oosporeae, on the other hand, would be referable to 

 more advanced types, in which sexual differentiation extended to the 

 gametangia themselves. Probably the Zygomycetes and Oomycetes 

 represent two distinct lines of evolution, of which the Oomycetes 

 started subsequently to the advance in sexual differentiation. 



