The Fungus of Salmon Disease. 5 9 



when the seeds are ripe they are expelled or liberated from the 

 sporange into the surrounding water, where their distribution 

 becomes at once general. These spores seem to possess the 

 power of moving at the instant of their liberation, but their 

 energy is almost expended when they are free, and their volun- 

 tary motion only carries them a very short distance, when they 

 settle down and become quiescent, unless carried away by cur- 

 rents. This spore, at an undetermined, or, more properly, 

 varying time after liberation, has again the power of voluntary 

 motion, imparted by flagella, as in the first case. The first 

 form of zoospore is ovoid with its pair of flagella from the front 

 or narrow end ; the second form is uniform with an anterior 

 and posterior fiagellum diverging from the hillum. The 

 existence of these two forms constitutes the phenomenon of 

 diplanetism. The zoospores seem able to germinate at very 

 short notice, growing immediately after liberation if suitable 

 food be near, but their vitality does not seem to have been very 

 carefully studied. 



The continuance of the reproduction of the Saprolegnieae is 

 provided for, under prolonged unfavourable conditions, by another 

 kind of spore that is far more resisting, being able to retain its 

 unimpaired vitality and readiness to germinate even after a 

 lapse of several months. These latter seeds or spores are called 

 oospores, and they are developed, matured, and rendered suit- 

 able to carry on their function in an oosporangium, which is 

 only supposed to be produced under exceptional conditions, of 

 an unfavourable nature to the ordinary life of the plant. In 

 laboratory practice of cultivation they are hardly ever visible, 

 except in cold weather, and after studied departure from 

 the usual methods of culture. It is said that oospores not only 

 retain their vitality for a long time, but that a certain period of 

 torpidity is necessary to them before germination. 



It is easy to understand that such a provision as oospore 

 formation is quite necessary, and that without it the fungus of 

 salmon disease might become extinct, except in such climates 

 as would favour by temperature a continuous development 



