ZOOSPORES IN THE SAPROLEGNIEJ:. 435 



In Leptomitus and Saprolegnia the flagella are easily- 

 seen even in the sporange. In these forms the zoospores, 

 instead of assembling in a hollow sphere at the mouth of the 

 sporange, swim away freely in all directions for a few minutes, 

 and then encyst after the fashion described for Achlya. In 

 all these genera the cyst opens after a few hours and the 

 zoospore leaves in a different form, kidney shaped, with two 

 flagella diverging from the notch, one anterior (tractellum) and 

 one posterior (pulsellum). This phenomenon of two distinct 

 mobile conditions to the zoospore separated by an interval of 

 rest, has received the name of Diplanetism. It is obvious 

 that Achlya is also diplanetic. 



We have now to consider the full explanation of the outrush, 

 which study has already led us to regard as really due to 

 a vis k fronte, an attraction outside the sporange. No 

 expulsive matter could produce the exit of the last few zoo- 

 spores nor effect the acceleration of their movement near their 

 mouth. Now, Engelmann and Pfeffer have by their brilliant 

 researches familiarised us with the action of chemical 

 stimuli. The swarming of the bacteria at one stage, evidently 

 due to such a stimulus, led me to undertake this research, and 

 I must invoke the theory again at this point. Saprolegniese 

 are among the most aerobic of plants ; their culture only suc- 

 ceeds when the water in which they grow is kept constantly 

 oxygenated. When the oxygen is used up, the hyphse and 

 young sporanges become deformed ; the mature sporanges 

 open by the disappearance of the end wall of the 

 beak; but the zoospores remain inside; they encyst there 

 and form the so-called "Dictyuchus state," which never 

 occurs in well aerated cultures of the above three genera. 



It is obvious then that oxygen dissolved in the ambiant 

 water exercises the stimulus which is the true source of the 

 liberation of the zoospores. That such a stimulus is sufificient 

 to account for the squeezing out is obvious from the observation 



tigator, it is astonishing that such excellent observers should have denied the 

 existence of the flagella, without exhausting every means of ascertaining if 

 they were there. 



