446 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



of dead insects and otlier such organic matters iu a river must tend to 

 favour the growth and multiplication of any Saprolcgnue which it con- 

 tains, and hence to increase the liability to infection of the salmon 

 which ascend it. 



And that this is no mere hyi)othetical deduction is very well shown 

 by a remarkable case which was carefully investigated by Goeppert* 

 nearly 30 years ago. 



A peculiar water-mould, commonly known as Leptomitus lacteus, but 

 which is so closely allied to Saprolegnia that Pringsheim i^laces it in 

 that genus, is widely spread in running waters, where it grows on all 

 sorts of dead organic substances. 



A factory for making a spirit from turnips was established near 

 Schweidnitz in Silesia, and the refuse was poured into an affluent of the 

 river Westritz, which runs by Schweidnitz. The result was such a 

 prodigious growth of Leptomitus that the fungus covered some 10,000 

 square feet at the bottom of the stream with a thick white layer, com- 

 l)ared to sheep's fleeces, choked up the pipes, and rendered the water 

 of the town uudrinkable. Scattered hyi^hse of this Leptomitus may some- 

 times be found among those of Saprolegnia, growing on fresh-water 

 fishes ; and the two forms are altogether so similar, that conditions analo- 

 gous to those which stimulate the growth of the one may safely be as- 

 sumed to favor that of the other. 



Brefeld lias pointed out that there is no better medium for the culture 

 of fungi of all sorts than an infusion of dung ("mistdecoct"). Land 

 under high cultivation undoubtedly supi)lies the waters iu its neighbor- 

 hood with something that nearly answers to an infusion of dung; and 

 this must be taken into account in discussing the possible factors of 

 salmon disease. 



Again, it is known with respect to many of the common moulds, such 

 as Fenicillium and Mucor, which are habitally sapropliytes (that is to 

 say, live on decaying organic matter, as 8a]^rolegnia does), that they 

 flourish in certain artificial solutions containing salts of ammonia. It 

 is quite possible, though whether the fact is so will have to be exiDcri- 

 mentally determined, that Saprolegnia is capable of living under the 

 same conditions. Fungi are also extremely sensitive to slight differ- 

 ences in the acidity or alkalinity of water, so that even apparently in- 

 significant changes in this respect may come into play as secondary 

 conditions of salmon disease. Hence, although there is not the slight- 

 est ground for regarding "pollutions," whether they arise from agricul- 

 tural or from manufacturing industries, as primary causes of salmon 

 disease, they may have a most important secondary influence; they may 

 in fact determine whether, in any river, the disease shall be sporadic or 

 epidemic. 



But of all the conditions which determine the increase of Saprolegnia^ 

 and, therefore, multiply the chances of infection of healthy fish, the 



* " Botanische Zeituug," XI, p. 163. 1853. 



