Brown. — Studies in the Physiology of Parasitism. VIII. 1 1 1 
these old spores are sown in water on Cerens they attack readily enough, 
and for the reason that they obtain the nutrient necessary for their germina- 
tion by passive exosmosis from the plant. 
In a second experiment use was made of the fact that a certain con- 
centration of carbon dioxide in the atmosphere inhibits the germination of 
Botrytis spores when sown in water, whereas the effect is relatively much 
less when the spores are sown in nutrient. Young spores of Botrytis were 
sown in water on petals of Rose and Cereus, in the one case in air, in the 
other in an atmosphere composed of 80 per cent, air and 20 per cent, 
carbon dioxide. In air attack readily took place in both cases, but in the 
carbon dioxide atmosphere only the Cereus was attacked. Even after 
three days the spores on Rose in the latter case were ungerminated. These 
results show that in Cereus the exosmosis of nutrient is sufficient to 
stimulate the germination of the spores in spite of the antagonistic effect of 
the carbon dioxide, whereas in the case of Rose the amount of exosmosis 
is insufficient for this purpose. 
A third type of experiment was based on the fact that spores of 
Botrytis which germinate in pure water can be inhibited from doing so by 
sowing them sufficiently densely. This inhibiting effect is removed by the 
addition of a sufficiency of nutrient. Thus, while the effect of increasing 
the density of spore sowing is to diminish the amount of germination when 
the spores are sown in water, no such effect is obtained (at any rate within 
the same limits of spore concentration) when the spores are sown in nutrient. 
In the case of Cereus petals which give high exosmosis figures, 
increase of spore concentration has no appreciable effect on the time of 
establishment of attack. In cases where the amount of exosmosis is much ' 
less, increase of concentration of spores leads to delay in the time of attack. 
Thus in an experiment with petals of Sweet-pea the following figures were 
obtained : 
Relative Density Number of . I ul P er °I Percentage of 
s ^ . infections after . s J 
of spore sowing. inoculations. ' hours infections . 
i/io 57 49 86 
1 53 30 57 
10 41 1 2.5 
These results are readily interpreted on the lines of the higher rate of 
exosmosis from Cereus and the smaller rate from Sweet-pea. 
The effect of increased density of spore sowing was not so clearly 
marked as was anticipated. Though only one of the forty-one inoculations 
with the densest spore suspension had taken in twenty hours, nevertheless 
they all showed infection ultimately. Total inhibition was expected on 
the ground that when a drop of the spore suspension was added to a drop 
of water which had lain on the petal for several days no germination took 
place. A more striking illustration of this discrepancy was met with in 
