Brown.-— Studies in the Physiology of Parasitism . IX. 289 
Tests of unbruised leaves were run simultaneously, but only in the 
following cases was an increased germination noted : 
Old spores . 
Control 0-07 
Ruta 0.25 
Choisya 0*35 
Apple 0*22 
A large number of experiments have been carried out along the lines of 
the one above described, and, while the results vary in degree in different 
experiments, the general result is convincing, viz. that certain volatile sub- 
stances given off from plant tissues— especially when the latter are bruised 
—have a distinct effect on the germination of fungal spores. In the 
majority of cases the effect was found to be one of increased germination, but 
in some instances a distinct retardation, amounting in some cases to a com- 
plete inhibition, was produced. The most striking illustration of the latter 
so far observed is that of the onion. Tests have been made of leaves of 
Allium Schoenoprasum and of bulb scales of the ordinary onion, and in all 
cases it has been found that the addition of a small piece of onion tissue 
has a strongly repressing effect on the germination of Botrytis spores. 
This result is in agreement with the statements of Walker (5) on this 
point. 
It is impossible at present to assert that one particular plant produces 
greater stimulation than some other one, as no means has been devised for 
regulating or measuring the intensity of the stimulating factor. Quite apart 
from questions of the varying amount of tissue and the varying amount of 
cell-bruising in different cases, there is also the different volatility of the 
various volatile compounds, their various solubilities in water, &c. In 
practice it has been found that sometimes a particular plant gives the most 
striking result, in other experiments some other plant. But while no exact 
rule can be laid down in this respect, one can say broadly that some plants 
give good stimulation, others less so. Good material for showing stimula- 
tion are bruised leaves of Choisya , Rtita , and apple, though equally good 
results have been at times obtained with leaves of Eucalyptus, Amomum, and 
some others. It will be noticed that most of the plants tested are of an 
aromatic nature. The effect, however, is not confined to such oily plants, 
for distinct stimulation of growth has been obtained with bruised leaves of 
broad bean. In this case it was found that a considerable amount of tissue 
was required. On the other hand, only small stimulation has yet been seen 
in the case of lavender leaves. 
Table I brings out an interesting experimental point, viz. that the 
stimulative effect is more readily discernible when the spores are of feeble 
germinative capacity. The ideal condition is when the spores in the control 
experiment remain ungerminated, in which case any stimulative effect is 
