Proceedings 29 
cies of plants, many of them common weeds of some hop 
gardens. Now, are these latter dzo/ogicad/y distinct, ze, are 
they capable or not of passing over on to the Hop and 
infecting it ? If they are diologically distinct, farmers need 
not fear the presence of mildewed weeds in the hop garden, 
while if not biologically distinct, then farmers must be 
taught the necessity of cutting down certain weeds. 
Returning to the question, Do biologic species exist in 
the Erysiphacee ? \et us first review the existing evidence 
on the subject. 
The experiment of taking the conidia of a mildew grow- 
ing on one host-plant, and sowing them on the leaves of 
another plant has been performed by three botanists. 
1. Prof. Magnus, a well-known German botanist, took 
conidia of the Hop mildew from a hop leaf and placed 
them on the leaves of the Dandelion. After a few weeks, 
an Ojsdium was observed on the Dandelion leaves, due 
according to Magnus, to infection by the conidia of the 
Hop mildew. This experiment is unsatisfactory and quite 
inconclusive for two reasons. It is not stated that after the 
Dandelion leaves were sown with the conidia of the Hop 
they were guarded from subsequent infection by the conidia 
from other sources, and secondly, the mildew that is known 
to occur in Nature on the Dandelion, is zever the Hop 
mildew. Dandelions are common in or near hop-gardens, 
so that if the Hop mildew were able to live on them, speci- 
mens would almost certainly have been collected on them. 
2. Iwanowsky observed an Ozdium on the leaves of some 
Composite, especially Znula Helenium and Lappa tomentosa. 
He took the conidia of this and sowed them on the leaves 
of the Tobacco plant (Vicotiana Tabacum). Ina little 
time an Ozdium appeared on the Tobacco, caused, accord- 
to Iwahowsky, by infection from the conidia of the Com- 
posite. No details are given of the experiment. We are 
not told if the Tobacco plants were guarded against in- 
fection from other sources. If we accept the conclusion, 
the experiment shows that an O7diim can pass from the 
_ leaves of plants belonging to the Comfosite to a plant 
belonging to the So/anacea. 
