POTATO-PtTNGtrs. 113 



development — they require no second one. Sometimes the species 

 parasitic on one host-plant attaches itself to another host, but only par- 

 tially carries out its development in it, say as far as the formation of 

 conidia ; but it should be noted that this is a phenomena altogether 

 different from that of metcecia. 



LiFE-HISTORT OF THE PoTATO-FtJIfGUS. 



3. Let me now try to explain, from known facts and from the 

 observations that have just been made, what is known of the life- 

 history of the Potato-fungus, and what yet remains to be discovered. 

 In the first place, it is generally known that when the Potato-fungus 

 has once made its appearance in the fields in summer, its progress, so 

 far as concerns the formation, mode of distribution, and germination 

 of the conidia, is the same as that described above in connection with 

 the Peronospore(B. It is further known that the conidian growth is 

 not only found in the foliage, but that it extends to the tubers in the 

 ground, and there developes the mycelium, which is the immediate 

 cause of that injury to the tubers which is so dreaded. On the decay 

 of the foliage, and when the tubers have been gathered, the Fungus 

 disappears from the field, and does not appear again till next summer ; 

 generally not when the young foliage appears, but about the time of 

 the blossoming of the Potato, or even later. The question then arises, 

 "Where does it remain in the period between its disappearance and its 

 reappearance ? How and where does it winter, and how does it pass 

 from its winter quarters to the foliage of the Potato ? 



In the first place, it is clearly established that the conidia of the 

 Potato-fungus have a very short existence, and are incapable of living 

 through the winter. From what is certainly known about all other 

 Feronosporece and also about Saprolegniece, we can look only to oospores, 

 which endure throughout winter, and to perennial mycelium for 

 maintaining the life of the Fungus. It has long been known that 

 perennial mycelium frequently occurs in the tubers, and to this point 

 I shall again recur. Oospores have not been observed in the Phyto- 

 phthora ; but from analogy it may be taken as certain that they 

 may somewhere occur. The discovery of them would at once fill up 

 the gap both in the morphology of the Fungus and in the practically 

 important question of how it hibernates. And, accordingly, ever 

 since the oospores of a Peronospora were discovered, innumerable 

 searches have been made for those of Phytophthora. I have myself 

 looked for them for fifteen years, and on every opportunity have 

 searched for them in the stalks, leaves, flowers, fruit, and tubers of the 

 Potato. In July of the present year (1875), when the Fungus ap- 

 peared in this district in sad abundance, I obtained a very large 

 amount of material for study, and at the same time secured the 

 kindly assistance of two botanists experienced in researches of this 

 kind. Dr. Rostafinski and Dr. Stahl. But again only negative results 

 were arrived at. 



4. From the researches on the tubers it seemed possible that by 

 other methods the oospores in the Potato might be discovered. It is 

 known that in tubers which are well-ripened and comparatively free 

 from water the mycelium is capable of living and vegetating for a 

 long time, even until well into the following summer. If we cut 



