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 Eollesby 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 

 (j33cidium berbcridis), 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 BOW 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 an 

 Berlin," Jan. 1865 ; Summary, in " Journ. Hoy. Hort. Soc., London," vol. i. 

 n.s. p, 107. 



