6 AQUATIC PHYCOMYCETES 



large, oil globule. These spores are monoplanetic (monomorphic) and 

 they usually undergo a short rest period, or pause, immediately after 

 emergence from the sporangium. Their movement, as Cohn (1853) 

 long ago pointed out, is highly erratic, frequently being a hopping punc- 

 tuated by periods of amoeboid creeping and changes of shape. Other 

 features of the group were noted by Scherffel. Among them are the 

 gleaming cytoplasm and the resting spores formed by direct transfor- 

 mation — either sexually or asexually — of a cell (the spore usually 

 not resting within a "mother cell" or container). 



Characteristic of the members of the "Saprolegniineen-Peronospo- 

 rineen" series are the biflagellate, bilaterally symmetrical zoospore, 

 the more granular cytoplasm, and a resting spore borne within the lu- 

 men of a larger container. The secondary zoospore is somewhat kidney- 

 like or grape-seed-like in shape and bears a shallow longitudinal groove 

 from which two nearly equal, oppositely directed fiagella arise. The 

 plasma, in contrast to that of the chytrid zoospore, is granular, no large 

 single globule being formed. The swimming spores appear to lack the 

 capacity for pronounced amoeboid movement. Scherffel considered 

 the vacuolar phenomena occurring in the sporangium during formation 

 of the zoospores in the "Saprolegniineen-Peronosporineen" series to 

 be a further distinction between the two groups, but cytological work 

 (Karling, 1937b; Hillegas, 1940) has not substantiated this difference. 



Within the Chytridiales, as here delimited, two series of forms are 

 recognized, the Inoperculatae and the Operculatae, whose members 

 frequently parallel each other in their methods of development and 

 in their thallus structure (Sparrow, 1935b). In the first of these series, 

 the Inoperculatae, the zoosporangium opens upon the dissolution of 

 the discharge papilla, forming on the sporangium wall or at the tip 

 of the discharge tube a pore for the liberation of the zoospores. In 

 the second, the Operculatae, this pore is formed after the circumscissile 

 dehiscence of a well-defined operculum or cap. The cap is a definite 

 specialized morphological structure and not merely a torn-off portion 

 of the sporangium wall or tip of the discharge tube. 1 Furthermore, 

 the type of discharge is constant in a population of a given fungus 

 and in the sporangia subsequently formed by its zoospores. The further 

 interrelationships of members of the Chytridiales are indicated by the 



1 For a discussion of the "endooperculum", see p. 63. 



