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Nature and Cause of the Potato Disease. 
These four kinds of fungi are all that I have observed^ and 
they appear the effect of different causes. The first kind is an 
external fungus, growing only on the outward parts of vegetable 
matter. It is readily produced by placing a piece of potato, 
wood, or any similar substance, in a dark damp cellar for some 
days or weeks, as the case may be, when it will appear in white 
patches on the substance thus treated. It has no sensible action 
on the substance it attacks, for it will grow on sound pieces of 
potato without injuring their texture. I have submitted pieces 
of potatoes to its action for five consecutive months, and have 
found them perfectly sound and uninjured at the end of that 
period. 
The second kind, however, is of a different character, althoush 
it appears and may sometimes be seen growing up from among 
the first ; yet the two cannot long thrive together, for the second 
requires moisture, and the first compaiative dryness. If much 
moisture be present, putrescence exists, and the second kind ad- 
vances while the first recedes; but if it be tolerably dry, the first 
flourishes and the second dies away. The second is the offspring 
of putrescence, and it flourishes only in such kind of matter. It 
is formed of a single footstalk terminated by a head. The foot- 
stalk is simply a tubular vessel, and the head a hollow globe 
seated on the apex of the footstalk. As the fungus arrives at 
maturity, the globe or ball changes from white to a light brown 
colour, at which period an insect of the apode or eel tribe may 
be discovered in it, such as is found in some diseased plants, and 
represented in plate 1. In last November and December I 
made some experiments as to the propagation of this fungus in 
the potato, by inoculation and by the seed, if it contains it, by 
crushing the heads on pieces of potato, but without success. 
Soon afterwards, however, I discovered that it might be produced 
by putrescence, and that without any difficulty. All that is re- 
quired to generate it in a potato is to first bring the potato to a 
])utrescent state, and place it in a damp and moderately warm 
atmosphere, where little or no light is admitted; and in the course 
of a few weeks it makes its appearance. I have closely examined 
the globules or heads for seeds, but have in no instance discovered 
any, although I have employed a most powerful compound 
arrangement, using an object glass of 480 diameters. As this 
kind of fungus appears only with putrescence, and seems to con- 
tain no seed, it may probably be produced by the eel-like insect 
that inhabits the head, in a way something analogous to the gene- 
ration of nut-galls. This idea is probably correct, and seems 
strengthened by the presence of the insect, the absence of seed, 
and iliat it cannot be propagated. Putrescence may be induced 
in a sound potato by immersing it in water for some weeks, and 
