398 BOTANICAL GAZETTE [ DECEMBER 
close contact of the spores could prevent the toxic effect of the 
chemical. 
Generally speaking, Botrytis required a stronger solution to 
kill than did the Macrosporium, while Uromyces required much 
greater strength than either of them. This might have been 
expected from the relative thickness of their walls, and is further 
illustrated by table XIX, which gives the strength of solution 
required to prevent completely the growth of spores. Excep- 
tions to this generalization occur, however, as has already been 
pointed out, which indicate a selective difference. Some fungi 
on experimentation gave with some chemicals a sharp and defi- 
nite killing point. Others were gradually weakened or the 
number of germinating spores decreased as the strength of the 
solution increased. This second phase is particularly well illus- 
trated in the cultures of Uromyces. With each chemical a 
strength could be found which just perceptibly injured the fungus, 
and another which prevented most spores from germinating. 
This is what has previously been given as the killing strength. 
At this strength one of two things occurred: either about the 
usual number grew, but grew only slightly and in stunted or 
often distorted manner; or, in each culture, nearly all the spores 
failed utterly to grow, while one or a half dozen in the hundreds 
of a culture would grow and grow usually vigorously and appa- 
rently uninjured. In order to kill these few persistent spores 
much greater strength was required. Thus, in potassium chro- 
mate, for Uromyces, ,", killed or prevented most growth. But 
some spores grew even in .“,, and ,%, was necessary to inhibit 
growth completely. On the other hand ., did weaken growth 
somewhat. So there is established a wide range between the 
weakest solution injuring the fungus and the weakest solution 
surely preventing growth. 
Table XX shows the variation in range of susceptibility. 
In this table two concentrations are named for each fungus: 
At the weaker normal growth took place, while the stronget 
completely inhibited growth. 
It is here to be noticed that Uromyces gives a remarkable 
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