1 1 6 Brown. — Studies in the Physiology of Parasitism. VIII. 
of a large number of discoloured spots. In some cases the first attack 
might be very localized, only one spot appearing for a considerable time, 
in which case the rise in conductivity in the drop subsequent to attack took 
place more gradually (No. 6 of spore-containing drops in the preceding 
table). 
By the time infection can be detected macroscopically it has already 
progressed to some extent ; the first stages of attack can only be 
demonstrated microscopically. This is readily done by taking a thin 
surface section of the petal and viewing it from above in the field of the 
microscope. Penetration, when it has taken place, is shown by the different 
appearance of the portion of the hypha within the tissue, the margins 
being more hazy than those of the hyphae external to the tissue, and also 
by the necessity of focusing at a lower level. Further, the internal 
portion of an invading hypha appears to follow a direction entirely 
independent of the original (external) hypha, and so the point of penetration 
is almost invariably marked by a sharp bend in the hypha when the latter 
is looked at from a direction at right angles to the leaf surface. 
In experiments of the above type it was repeatedly proved that 
infection had already taken place in cases where there was no visible 
macroscopic discoloration and where there was no evidence whatever of 
any tendency towards an increased rate of exosmosis into the spore- 
containing drops as compared with the controls. Infection could thus 
always be demonstrated by ocular means before any effect on exosmosis, 
due to the presence of the fungus, appeared ; 1 in other words, the rate of 
exosmosis into the spore-containing drops is the same as into the spore-free 
drops until such time as penetration has taken place in the case of the 
former. According to the view of £ action in advance of penetration ’ put 
forward by de Bary, the rapid exosmosis of nutrient substances from the 
host cells takes place antecedent to penetration, and is in fact responsible 
for the germination of the fungal spores and for their attack. It is clearly 
shown, however, by the preceding data that penetration has already taken 
place before the rapid exosmosis due to killing of the host cells has begun. 
In fact, it is only some time after infection that any divergence in the rates 
of exosmosis between the spore-free and the spore-containing drops takes 
place. This lag would be explained on the basis of the time required for 
the invading hyphae to kill the adjacent cells of the host and for the 
soluble contents of the killed cells to diffuse out into the infection drop. 
These results are thus in complete agreement with the views put 
forward in the earlier papers of this series and in opposition to the view of 
de Bary on this subject. 
1 Indeed, in one very fortunate case, penetration of the cuticle was seen to have taken place 
while the host cell immediately underlying the point of penetration had not yet lost its colour and. 
was still plasmolysable. 
