POLYMORPHISM. 199 
The other scries of phenomena grouped together under the 
name of polymorphism relate to forms which are removed from 
each other, so that the mycelium is not identical, or, more 
usually, produced on different plants. The first instance of this 
kind to which we shall make reference is one of particular 
interest, as illustrative of the old popular creed, that berberry 
bushes near corn-fields produced mildewed corn. There is a 
village in Norfolk, not far from Great Yarmouth, called “ Mil- 
dew Rollesby,” because of its unenviable notoriety 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 
(Puceinia 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 
(Aicidium 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 pub- 
lished in 1865.* In view of the popular belief, he determined 
to sow the spores of Puccinia graminis on the leaves of the ber- 
berry. 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 atmo- 
spheric 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 
* “Monatsbericht der Koniglichen Preuss, Acad. der Wissenschaften au 
Berlin,” Jan. 1865; Summary, in ‘‘ Journ. Roy. Hort. Soc., London,” vol. i. 
n.s. p, 107. 
