248 
Researches into the Nature of the Potato-Fungus. 
second one. Sometimes the species parasitical on one host 
plant attaches itself to another host, but only partially carries 
out its development in it, say as far as the formation of conidia ; 
but it should be noted that this is a phenomenon altogether 
different from that of metoecia. 
LiFE-HISTOKY OF THE POTATO-FUNGUS. 
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, are the same 
as that described above in connection with the Peronosporeoe. 
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 develops 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 re-appearance ? How antt 
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 Peronosporece and also about Saprolegnieoe, 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 observ ed in the Phytophtliora ; but 
from analogy, it may be taken as certain that they mayv some- 
where 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 bow it hibernates. And, accordingly, 
ever since the oospores of a Pcronospora were discovered, 
innumerable searches have been made for those of Phytophtliora. 
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 
