Brown. — Studies in the Physiology of Parasitism . III. 401 
2. In order to determine if the parasitic action of the germinating 
spores was diminished by removing the toxic substances formed in the 
infection drop, Nordhausen experimented as follows : A strip of filter paper 
was placed in the infection drop and a current of water made to flow 
through the paper. Such an arrangement might be expected to carry 
away a large amount of the toxic substances formed by the drop. Never- 
the less the entrance of the fungus was not in any way prejudiced. From 
this experiment Nordhausen concluded that the toxic substance must 
be very active even in a state of extreme dilution. 
It may be observed, in passing, that the conclusions drawn from the 
experiment just described are in direct contradiction with those drawn from 
his observations on the relative efficiency of large and small infection drops. 
In the light of the above work it was important to determine whether 
the extract employed throughout the present investigation was able to 
penetrate from the outside into the tissue of the host plant. In carrying 
out this series of experiments, two methods of applying the active extract 
were adopted : 
1. In one series the extract was applied in considerable quantities 
contained in a chamber formed by a glass ring which was attached by 
vaseline to the surface of the leaf ; the chamber so formed had a capacity 
of 1-1*5 c * c * This method lent itself readily to the investigation of the 
activity of the extract at the end of the experiment, so that it could be seen 
how far failure of the extract to penetrate the cuticle could be set down to 
its loss of activity by lapse of time, as suggested by de Bary. 
2. As the amount of available air might conceivably be concerned 
in the process, a series of experiments were carried out in which the 
extract was applied to the leaf in the form of drops comparable in size with 
the average infection drop (about 30 drops to the c.c.). 
The extract was tested on wounded as well as on intact leaves, the 
wound being produced by shaving off a small piece of epidermis. Also the 
active extract was compared with extract which had been de-activated 
by heating to 65 0 C. The tests were repeated many times and on the 
leaves and flowers of a large number of plants. 
In the case of the wounded petals and of the great majority of wounded 
leaves a distinct action on the part of the extract spreading outwards from 
the region of the wound could be seen (in a few hours in the case of petals, 
overnight in the case of leaves). Where no action could readily be demon- 
strated under these conditions, the experiments on the corresponding 
uninjured leaves were rejected. It was certain therefore that all the 
unwounded leaves and petals under consideration would show a readily 
noticeable reaction to the presence of the extract if any passage of the 
latter through the cuticle took place. 
In the case of the majority of plants examined, passage of the active 
