xxxvi.] POTATO DISEASE, IL PASSIVE STATE. 301 



mouth downwards on any flat surface, and a saucer of 

 water placed underneath to keep the air humid by evapor- 

 ation. The result invariably is, that the Peronospora 

 mycelium within the potato leaves gives rise to an enor- 

 mous number of oospores or resting-spores ; and as the 

 leaves gradually decay, the decayed material swarms with 

 the Artotrogus hydnosporus of Montagne, the resting 

 condition of the potato fungus. 



During the early autumn of 1875 another important 

 fact in regard to the potato fungus came to light. On 

 making a rigid examination of every part of diseased potato 

 plants for oospores, we found them in great abundance 

 in the old exhausted seed tubers. In every other part of 

 the potato plant the oogonia were rare ; but in the old 

 sets the oogonia sometimes swarmed in myriads. The 

 explanation of this fact may be that the tubers were in 

 this instance planted with the perennial mycelium of the 

 potato fungus in their tissues. As this mycelium on 

 starting into growth could not produce conidia, being 

 underground, it spent itself in the tuber by a vast pro- 

 duction of oospores. There is no more certain position 

 for lighting on large colonies of resting-spores than in the 

 old exhausted seed tubers belonging to potato plants 

 destroyed by the Peronospora, or in the old diseased and 

 damaged tubers that are left in the field to rot, or are 

 incorporated in dung heaps as manure. 



It is curious that at the very time when we were 

 making the above observations, Dr. Sadebeck of Berlin 

 found a parasite, named by him Pythium Equiseti, first 

 upon Equisetum arvense, L., and afterwards (as he at first 

 thought) upon living potato plants near Coblenz. 

 Whether the Pythium upon the Equisetum was the same 

 as the parasite upon the potatoes is uncertain, as Professor 

 Sadebeck could not transfer the parasite from one plant to 

 the other, neither could we do so on repeating his experi- 

 ments. Our impression is that the parasites are distinct. 

 P. Equiseti, Sdbk., was described and illustrated by us, 



