SAPROLEGN I ALES 151 



condition in Didyuchus is properly to be regarded merely as a 

 variation in the process of germination. 



The existence in the Saprolegniaceae of zoospores of two 

 difTerent forms has not been satisfactorily explained. Some 

 authors have suggested that the reniform spore is the primitive 

 type and the ovoid spore a more recent development. In this 

 connection it is of interest to note that the zoospores of the 

 Peronosporales are always of the reniform type. Atkinson 

 (1909) gives, as evidence of the monophyletic nature of the fungi, 

 data indicating that the phenomenon of diplanetism as present 

 in the Saprolegniaceae has gradually evolved through the Chy- 

 tridales and Ancylistales. 



The first sporangium formed from a hypha may be called the 

 primary sporangium. After the escape of the zoospores a second 

 sporangium may be developed, then a third, and so on until 

 several are present. The secondary sporangium and those which 

 follow it arise by one of several different methods. In Sap- 

 rolegnia and Leptolegnia the new sporangium is initiated by the 

 upward growth of the septum at the base of the old one, and 

 soon fills the old cavity more or less completely. The hypha 

 may even grow through the mouth of the old sporangium, and 

 cut off the new sporangium at a considerable distance beyond 

 its tip. Several new sporangia may be developed in turn within 

 the wall of the primary sporangium. This phenomenon has 

 been termed proliferation. In Achlya, Aphanomyces, Pythiopsis, 

 and Thraustotheca cymose branching of the hypha occurs in 

 the development of new sporangia, lateral sporangia growing 

 out below the primary, and proliferation being absent. In Iso- 

 achlya and Protoachlya cymose branching and proliferation may 

 both occur together. Sometimes sporangia are cut off in basi- 

 petal succession {Didyuchus). Rarely intercalary sporangia 

 occur. 



Certain sporangia, after being cut off from the hyphae, may 

 remain quiescent for a considerable period, and later discharge 

 their spores in the usual manner. In such cases the term resting 

 sporangium has been appHed. These are merely ordinary 

 sporangia whose development has been temporarily arrested, 

 perhaps by conditions of the environment. They are sometimes 

 shorter and broader than the typical sporangia. 



The term chlamydospore has been given to another type of 

 non-sexual repnxhictive body formed in many species of the 



