in days past for mildewed corn, produced, it was said, by
the berberry bushes, which were cut down, and then mildew disappeared
from the corn-fields, so that Rollesby no longer merited its
_sobriquet_. It has already been shown that the corn-mildew (_Puccinia
graminis_) is dimorphous, having a one-celled fruit (_Trichobasis_),
as well as a two-celled fruit (_Puccinia_). The fungus which attacks
the berberry is a species of cluster-cup (_AEcidium berberidis_), in
which little cup-like peridia, containing bright orange pseudospores,
are produced in tufts or clusters on the green leaves, together with
their spermogonia.

De Bary's observations on this association of forms were published in
1865.[Y] In view of the popular belief, he determined to sow the
spores of _Puccinia graminis_ on the leaves of the berberry. For this
purpose he selected the septate resting spores from _Poa pratensis_
and _Triticum repens_. Having caused the spores to germinate in a
moist atmosphere, he placed fragments of the leaves on which they had
developed their secondary spores on young but full-grown berberry
leaves, under the same atmospheric conditions. In from twenty-four to
forty-eight hours a quantity of the germinating threads had bored
through the walls and penetrated amongst the subjacent cells. This
took place both on the upper and under surface of the leaves. Since,
in former experiments, it appeared that the spores would penetrate
only in those cases where the plant was adapted to develop the
parasite, the connection between _P. graminis_ and _AEcid. berberidis_
seemed more than ever probable. In about ten days the spermogonia
appeared. After a time the cut leaves began to decay, so that the
fungus never got beyond the spermogonoid stage. Some three-year-old
seedlings were then taken, and the germinating resting spores applied
as before. The plants were kept under a bell-glass from twenty-four to
forty-eight hours, and then exposed to the air like other plants. From
the sixth to the tenth day, yellow spots appeared, with single
spermogonia; from the ninth to the twelfth, spermogonia appeared in
numbers on either surface; and, a few days later, on the under surface
of the leaves, the cylindrical sporangia of the _AEcidium_ made their
appearance, exactly as in the normally developed parasite, except that
they were longer, from being protected from external agents. The
younger the leaves, the more rapid was the development of the
parasi