114 Brown. — Studies in the Physiology of Parasitism. VIII. 
Here, apart from slight irregularities, the amount of germination runs 
more or less inversely to the conductivity. The leaves had not been treated 
to a preliminary washing, so that the source of the conducting material 
cannot be definitely stated. It may have come mainly from the interior 
of the leaf or may have been present to a large extent as some kind of 
incrustation on the surface. 
Though the present experiment relates to a case where there is no 
question of parasitic attack (it was found that only the most meagre attack 
took place even when the spores were sown in nutrient), nevertheless it is 
interesting as showing that spores when placed in distilled water on the 
surface of a leaf come under the influence of substances which are able to 
exercise an important effect on their germination. 
It will be noticed incidentally that the correlation which was shown to 
exist between the figures for conductivity and for germinative capacity 
does not hold as from plant to plant. A drop of a given conductivity 
obtained from Bean leaves has a much smaller effect in stimulating 
germination than one of the same conductivity obtained from Cereus petals. 
Thus, apart from quantitative differences, there are qualitative differences 
between the substances diffusing out in the two cases. 
Effect of the Presence of Spores in the Drop on the 
Rate of Exosmosis. 
In an earlier paper of this series it was shown that the active principle 
obtained from Botrytis spores is unable to diffuse through the cuticle. 
Blackman and Welsford confirmed this result by showing that the first 
visible change in the cells of the host plant took place only after actual 
penetration of the cuticle had been effected. On the basis of these 
observations the conclusion was drawn that the fungus has no effect on 
the host until penetration of the cuticle has taken place. The presence or 
absence of spores in the drop would thus be immaterial as regards the rate 
of exosmosis into the drop so long as infection was in abeyance. This 
deduction has been strikingly confirmed by the use of the conductivity 
method. 
The method of experimenting was as follows. Flowers of Sweet-pea 
proved to be most satisfactory for this purpose. They furnished very 
uniform conductivity figures and being symmetrical one side could be used 
as a control to the other. Drops of water were laid on the one side (e. g. 
the right wing and the right half of the standard), and drops of spore- 
containing water on the other side (left wing and left half of standard). 
Examination of the drops was begun an hour or so before infection 
began, the time required for this being ascertained by preliminary 
experiment. The conductivity of each drop was then measured at hourly 
intervals, the drops being replaced as far as possible on the part of the 
