CHYTRIDIINEAE 65 



in the course of an hour and that many remain in the 

 sporangia and perish. Thus in disease caused by parasites 

 akin to Chytridians we may find extensive areas of necrosis 

 composed of dead parasites. 



Olpidium brassicae. This parasite attacks seedling cab- 

 bages causing damage which resembles that due to Pyihium 

 and which is included by gardeners under the name of 

 " damping off." The sporanges have sporoducts resembling 

 those of O. saprolegniae. Zoospores are uniciliate. Multiple 

 infection occurs. 



Olpidium viciae. Kusano described conjugation of zoo- 

 spores in 1912. The zoospores develope into ordinary 

 sporangia, the zygotes into resting spores ; later findings 

 (1920) in a Synchytrium accord with this observation (Fig. 22). 



Nuclear Processes in Olpidians. The development and 

 sexuality of some species of Olpidiopsis have been examined 

 by J. T. Barrett. The zoospores are bi-flagellate, Fig. 16, 

 a, b. The sexual process is by zygospore, usually in the form 

 of the prickle-sporangium of Cornu, but occasionally sporangia 

 are smooth. 



When first liberated the zoospores swim actively for 

 from 2 to 5 minutes. They then become attached to the 

 slide and a rocking motion begins, the flagella becoming 

 shorter. After 7 to 10 minutes they cease to move for a 

 space of 5 to 20 minutes and one or two contractile vacuoles 

 appear in them and a second period of movement begins. 

 This sequence of events has been compared to the diplanetism 



5 



