2IO 



A TEXTBOOK OF THEORETICAL BOTANY 



since they are often encountered mixed with Saprolegnia it will be necessary to 

 say something about them (Fig. 202). 



In the genus Achlya the zoospores, instead of being discharged from the 

 zoosporangium in a series, hke bullets from a machine gun, are ejected in 

 a cluster, forming a large mass around the apex of the zoosporangium. Here 

 they immediately encyst and lose their flagella, thus passing into the second 



Fig. 202. — Different modes of discharge and further development of zoospores in 

 Saprolegniaceae. A, Pythiopsis. B, Saprolegtiia. C, Aphanomyces. D, Thrausto- 

 theca. 



phase of a diplanetic condition. After a rest of several hours they emerge 

 with reniform bodies and swim away with lateral flagella. 



In the genus Dictyuchus, which may also be found in hemp-seed cultures, 

 the zoospores are so closely packed in the zoosporangium that they assume a 

 hexagonal pattern. In this genus they do not emerge through an apical pore 

 but by numerous pores all over the surface of the sporangium. They are 

 kidney-shaped with lateral flagella. Thus in this genus the encysted stage 

 is passed through within the zoosporangium. 



In the genus Thranstotheca the condition is somewhat similar, except that 

 the spores are liberated by the breakdown of the wall of the sporangium in 

 ^ non-motile spherical state and soon become reniform with lateral flagella. 



