BULLETIN OF THE UNITPD STATES FISH COMMISSIOX. 443 



developc sporangia, and horizontal branches, which are driven, like sub- 

 soil ploughs, iuto the middle layer of cells. The zoosporaiigia emit 

 multitudes of zoospores, many of which are deposited on the epidermis 

 in the neighborhood of the first; and, penetrating it in the same way, 

 add to the SaproJegnia i)lantatiou. Thus the disease constantly 

 spreads centrifugally ; aud, as the oldest and most luxuriant growth of 

 Saprolefinia is in the centre, so is the mechanical destruction of the epi- 

 dermis first effected there. But it is in this region, also, that the greatest 

 number of root-hyphfP penetrate the derma. They cannot fail to inter- 

 fere with the nutrition of the tissues which they traverse ; in fact, their 

 ramifications are often so close-set that the proper tissues of the super- 

 ficial layer of the derma almost disai^pear. Sooner or later, therefore, 

 necrosis sets in, and then ulcerative sloughing takes place, resulting in 

 an open sore. ISTo doubt the morbid process thus described is accele- 

 rated and intensified by the irritation caused by the innumerable small 

 grains of sand and other foreign bodies entangled by the mycelium. 

 But that the primary cause of all the mischief is the parasitic fungus 

 does not appear to be open to doubt. If it were otherwise, the structural 

 alteration of the skin should precede the fungus and not follow it, as it 

 actually does. 



In fact, the Saprolegnia is the cause of the salmon disease exactly as 

 the closely allied fungus Peronospora is the cause of the potato disease. 

 In symptoms, progress, and results, there is the closest analogy between 

 the tAvo maladies. Peronospora, like Saprolegnia, gives rise to spores 

 which may be ciliated aud actively locomotive, or may germinate with- 

 out passing through an active stage. When these spores germinate on 

 the surface of a healthy potato plant, their hyphte perforate the walls of 

 the cells with which they are in contact, and then ramify, as a mycelium, 

 in the inner substance of the plant, carrying- destruction wherever they 

 go. The mycelium gives oft' hyphre which pass through the stomates to 

 the surface ; and there they throw oft" abundant spores, which repeat 

 the process until the whole plant is destroyed. Even the tubers are 

 invaded ; bat, in them, the mycelium becomes quiescenton the ap])roach 

 of the winter season, to break out again, in full vigour, if the tubers are 

 planted in the following- spring. Moreover, there is as much uncertainty 

 about the occurrence of antheridia and oosporangia, and of any sexual 

 method of reproduction, in the Peronospora of the potato, as in the Sap- 

 rolegnia while it infests the salmon. 



There is a great deal of reason to believe that the Saprolegnia growing 

 on salmon is killed by salt water ; and that the injured skin may heal 

 and become covered with a new epidermis when a diseased salmon 

 enters the sea. But the discovery that the root-hyphte of the Sapro- 

 legnia ramify in the derma, where the sea water cannot reach them, 

 raises a carious and important question. It becomes possible that a 

 diseased salmon returning to the sea may regain a healthy epidermis 

 and appear perfectly sound ; but that, like a potato-tuber invaded by 



