Brown . — Studies in the Physiology of Parasitism . III. 405 
In all the preceding experiments, the spores were sown in a medium of 
sufficient strength to enable germination of all the spores present to take 
place. It may be thought that in such a case there might be no killing 
antecedent to penetration, there being sufficient nutriment already present 
in the infection drop ; but that the formation of the toxin might be called 
forth by a condition of starvation in the infection drop. This hypothesis is 
a priori improbable ; the following experiments were made to settle this 
point : 
Suspensions of spores of equal density were made in water and in a 
Ifl III I'll 7H III 7VI 
series of media of strength — 5 — 3 —5 — ? ^2 where m represents 
s 1 4 8 16 32 64 ^ 
a glucose-peptone medium (glucose 1 per cent., peptone o -3 per cent). 
Within this range of strength of medium all variations were obtained from 
very poor to fairly vigorous germination. Drops of these suspensions were 
placed on petals of rose and on leaves of broad bean. In all cases dis- 
coloration was produced, but in the case of sowings in water on bean leaf only 
very slowly. When discoloration was established over the surface of con- 
tact of the original drop with the leaf, the drop was slightly displaced so 
as to include a small portion of the adjacent surface. In no case, however, 
was any discoloration of the newly included surface shown. 
The above experiments show conclusively that there is no accumula- 
tion in the infection drop of a substance which can diffuse through the 
cuticle and bring about death of the underlying cells. 
B. On the presence of a soluble oxalate in the infection drop . 
The evidence already brought forward under (A) applies also to this 
special case. As considerable importance has been attached by various 
writers 1 to the part played by oxalates in conditioning parasitic attack, 
it may not be considered superfluous to bring forward certain additional 
evidence on this point. 
In the infection drop, as has already been stated, the first appearance 
of discoloration of the underlying tissue (rose petal, bean leaf) was observ- 
able in about twelve hours from sowing. Now in parallel experiments 
ft 
it was determined that, in the case of both plants, a concentration — 
40 
of oxalic acid when placed in the form of drops on the surface produced no 
noticeable action within a period of twelve hours. The same statement 
fl 
applies to a concentration — of potassium oxalate. If oxalic acid or 
oxalate be the toxic substance concerned, the concentration of the same in 
11 PI 
the infection drop must at least equal — or — in the respective cases. In 
1 1 40 30 r 
1 See especially R. E. Smith : Bot. Gaz., vol. xxxiii, 1910, p. 385. 
