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On the Physiology of Parasitism. 
penetration of the cuticle to take place, actually takes place only 
when penetration has been fully established. 
A microscopic investigation (No. 11 of the series) carried out 
with the usual technique confirmed these results in every way. It 
was shown that in the case of the attack of the fungus on leaves of 
broad bean, the underlying tissue was wholly unaffected up to the 
time when penetration had actually taken place. 
It is clear therefore that the de Bary hypothesis is untenable, 
at least in the present case, and that fungi such as Botrytis cinerea 
exert no influence whatever upon the host tissue until the harrier 
offered by the outer cuticularised layers of the latter has been passed. 
The process of penetration is thus conceived as follows:— 
When spores of Botrytis cinerea are placed in a drop of water on 
the surface of a plant, the sources of energy required to enable 
them to penetrate the cuticular barrier consist simply of that stored 
within the spores themselves and of that which diffuses passively 
from the uninjured plant into the water drop. The phenomenon, 
therefore, falls to be investigated along three distinct lines : (1) An 
investigation of the germinating capacity of the fungal spores in 
pure water and in nutrients. (2) An investigation of the rate of 
escape of nutrient materials from plant tissue into water placed in 
contact with its outer surface. (3) An investigation of the physics 
of cuticular resistance on the one hand and of the power possessed 
by fungi of penetrating membranes on the other. An investigation 
of the existence and nature of tropic forces also calls for treatment 
in this connection. These investigations are still incomplete in detail, 
but enough work has been carried out to indicate the lines along 
which advance will be made. 
(1) On the germinating capacity of fungal spores in water and 
nutrients. 
The necessity for this investigation was only brought home at 
a comparatively late stage in the research, so that only a limited 
number of data are yet to hand. The object in view was in the 
first instance a purely practical one— viz., to control the amount 
of spore germination for the general experimental work. 
The method of experiment consisted in the preparation of 
spore suspensions in water hy means of filtration through fine muslin. 
The water was purified by repeated centrifuging and renewal until 
the conductivity of the suspension equalled that of distilled water. 
