384 “ BULLETIN OF THE BUSSEY INSTITUTION. 
tell us what the insect is, neither do they explain why it is that turnips, 
carrots, and other roots, are attacked by insects, and yet no disease in 
the least resembling the potato rot attacks any of these plants. 
The theory that the disease arises from an exhausted condition of the 
potato plant which has been cultivated too long by cuttings is destroyed 
by the fact that seedlings are affected as well as other plants, and by 
the fact that the disease prevails amongst the wild species of Peru, 
where the potato is indigenous. The theory that it is owing to some 
morbid change in the juices of the potato, and that the Peronospora 
does not cause the disease, but itself lives upon the morbid products, is 
overthrown by the following experiment which has been repeated over 
and over again, and which any one who owns a microscope can easily 
perform: Slice a potato affected by the rot, and let it stay under 
a glass until the cut surface is covered by the cotton-wool-like mass of 
Peronospora. Then slice a sound potato, and place the two halves 
under different glasses. On one sprinkle some of the Peronospora 
spores from the first potato, and, in from twenty-four to forty-eight 
hours, it will become covered with a mass of mycelium and spores, 
which, under the microscope, will be recognized to be those of Pero- 
nospora infestans, and the mycelium will be found running through the 
tuber in all directions, and the tuber becomes rotten at once. The 
other half will remain unchanged, except that the cut surface grows a 
little darker for some days, when a few of the decomposition moulds 
will be found on it, and it will very gradually decay. ~ Inasmuch then 
as we always find the Peronospora mycelium in potatoes affected by 
the rot even before the spots on the leaves appear, and, on the other 
hand, can produce the disease at will in healthy potatoes by sowing 
the spores, we need not suppose that any predisposing morbid change 
in the potato itself precedes the appearance of the Peronospora. 
From what we have seen about the cause of the rot and the knowl- 
edge which we possess of the habits of the Peronospora, it is evident 
that there is no such thing as a specific* against it. Whatever com- 
pletely destroys the fungus will also kill the potato itself, and the farmer 
who purchases “a sure cure” for the rot, may be perfectly certain that 
* By a specific is meant any thing which will not only protect tubers in 
which there is, as yet, none of the Peronospora mycelium, but which will also’ 
prevent the further development ov the mycelium in tubers in which it already 
exists. 
