small millets, in
countries where these are cultivated for food, are liable to attacks
from allied parasites. Ergot attacks wheat and rice as well as rye,
but not to such an extent as to have any important influence upon the
crop. Two or three other species of fungi are sometimes locally
troublesome, as _Dilophospora graminis_, and _Septoria nodorum_ on
wheat, but not to any considerable extent. In countries where maize is
extensively grown it has not only its own species of mildew
(_Puccinia_), but also one of the most enormous and destructive
species of _Ustilago_.

A singular parasite on grasses was found by Cesati in Italy, in 1850,
infesting the glumes of _Andropogon_.[a] It received the name of
_Cerebella Andropogonis_, but it never appears to have increased and
spread to such an extent as was at first feared.

Even more destructive than any of these is the potato disease[b]
(_Peronospora infestans_), which is, unfortunately, too well known to
need description. This disease was at one time attributed to various
causes, but long since its ascertained source has been acknowledged to
be a species of white mould, which also attacks tomatoes, but less
vigorously. De Bary has given considerable attention to this disease,
and his opinions are clearly detailed in his memoir on _Peronospora_,
as well as in his special pamphlet on the potato disease.[c] One sees
the cause of the epidemic, he says, in the diseased state of the
potato itself, produced either accidentally by unfavourable conditions
of soil and atmosphere, or by a depravation that the plant has
experienced in its culture. According to these opinions, the
vegetation of the parasite would be purely accidental, the disease
would be independent of it, the parasite would be able frequently even
to spare the diseased organs. Others see in the vegetation of the
_Peronospora_ the immediate or indirect cause of the various symptoms
of the disease; either that the parasite invades the stalks of the
potato, and in destroying them, or, so to speak, in poisoning them,
determines a diseased state of the tubercles, or that it introduces
itself into all the organs of the plant, and that its vegetation is
the immediate cause of all the symptoms of the disease that one meets
with in any organ whatever. His observations rigorously proved that
the opinions of the latter were those only which were well founded.
All the alterations seen on examining spontaneous individuals are
f