i6 



might thoroughly infect the waters of a considerable 

 stream. 



But it is possible that no trace of Saprolegnia may be 

 present either on living or dead organic bodies in a mass 

 of fresh water, and that no Saprolegnia may be imported 

 from without; and that, nevertheless, after a time, these 

 bodies may be attacked by the fungus. This arises from 

 the circumstance that Saprolegnia ferax, like other Sapro- 

 legnice, has two modes of reproduction, the one sexless, by 

 zoospores, such as have been described, the other sexual, 

 by oospores, which answer to the seeds of ordinary plants. 

 Reproduction by zoospores is the ordinary and constant 

 process, that by oospores takes place with less regularity. 

 I have never seen an oospore upon Saprolegnia growing on 

 salmon, and there are some sources of difficulty in the 

 identification of these bodies which lead me to entertain 

 a doubt whether any other observer has done so. Mr. 

 Murray, however, has found them abundantly on Sapro- 

 legnia growing on dace. Saprolegnia may be cultivated 

 on flies for many months without the appearance of 

 oospores, and then, in a solitary specimen, they may 

 present themselves in great numbers. 



These oospores are spherical bodies provided with a thick 

 coat. Once formed, they pass into a quiescent condition, in 

 which they may remain for many months. Sooner or later, 

 however, they awake to new activity, and their contents 

 either break up into zoospores which are set free and roam 

 about until they reach an appropriate nidus, or grow 

 directly into a Saprolegnia. Thus it is possible that the 

 Saprolegnia in a pond or stream, having given rise to 

 oospores which fall to the bottom and remain quiescent, 

 may die away and leave no apparent trace of its existence, 

 and yet, months afterwards, the oospores may germinate, 



