798 



AQUATIC PHYCOMYCETES 



thallic species. At all stages in their development before actual fertiliza- 

 tion the antheridia and the oogonia can, by alterations in the environ- 

 ment, be made to reverse the sequence of their protoplasmic changes and 

 hence to become again vegetative in character. Further, if sex organs 

 are placed under conditions favorable for sporangial formation they 

 may, if the process of fertilization has not been initiated, be converted 

 into sporangia and produce functional zoospores. Also, in certain 

 groups of Saprolegnia, in which species are differentiated mainly on the 

 number of oogonia supplied with antheridia, this character can be 

 modified considerably in a single form by changing the temperature at 

 which growth takes place. A number of other valuable observations are 

 made in Schlosser's excellent paper. 



Between 1936 and 1949 J. R. Raper conducted a remarkable series 

 of investigations on species of Achlya. He proved (1939b, 1940a) that 

 in A. ambisexualis and A. bisexualis hormones play an essential part in 

 the initiation and coordination of the stages of the sexual process. Emer- 

 son (1950) reviewed his work as follows: 



[Raper] demonstrated conclusively that at least four distinct hormones are 

 involved. Table II summarizes succinctly the action of these substances. 



TABLE [II] 



The Action of Hormones in Coordinating the Sexual Processes of 



Gynandromictic Achlya Sps.* 



* From Raper [1939b]. 



