BULLETIN OF THE BUSSEY INSTITUTION. 323 
die overworked, or, in other words, starved out. Such is the process 
which has taken place in the black spots on the leaf. Here the para- 
site has increased to such an extent as to destroy the proper tissue of 
the leaf, while, in the adjacent green parts, although the mycelium is 
present, it is not in such quantity as to overcome the assimilating power 
of the leaf-cells. It is an important fact that the relative activity of 
the latter and of the mycelium varies with the temperature and moist- 
ure of the surrounding atmosphere. The Peronospora is much more 
easily affected by moisture than the potato plant itself. So long as the 
air is dry, the mycelium grows but slowly, while, unless the dryness is 
excessive, the potato leaves can do their work very well. But suppose 
the temperature to keep equally warm, and the atmosphere to become 
very damp, then the absorbing power of the mycelium is very much 
increased, while the assimilating power of the leaf-cells is little altered. 
Thus it happens that a sudden change from dry weather to moist will 
cause the mycelium to increase so very much beyond the power of the 
potato plant to support it, that, in the struggle for existence, the lat- 
ter blackens and dies. Once in a given plant, then, we see how the 
Peronospora can destroy it; but the question arises, How does it 
get in? 
So far, we have spoken only of the mycelium as found in the leaf; 
but, as the disease advances, it is found in any part of the plant, even 
the tubers, and the description given of it in the leaf will answer for 
it in any part of the plant, except that, in the tubers, it is generally 
a little larger and furnished ‘with more numerous projections than 
elsewhere. When the disease has arrived at a certain point, viz., just 
about the time of the appearance of the spots on the leaves, these 
mycelial threads make their way into the air, and, taking the easiest 
course for this, they generally grow through the breathing-pores. As 
has already been observed, the breathing-pores are more numerous on 
the under surface of the leaves than anywhere else; and it is on this 
part of the plant that we most easily recognize the change. ‘To the 
naked eye it appears like a slight frost on the leaf, and, after the spots 
have begun to appear, we generally find around them, on the under 
surface of the leaf, a ring of frost work, very delicate, however. Under 
the microscope we have the appearance presented in Fig. 1, where, for 
convenience in printing, the leaf.has been inverted, and what appears 
to be the upper surface is, in reality, the lower. On the left hand we 
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