Boyle. — Studies in the Physiology of Parasitism. VI. 343 
unaffected. This is evidently due to the fact that the palisade cells are in 
closer contact and hence allow a more rapid diffusion of the lethal 
substance or substances than the cells of the spongy parenchyma, which are 
in contact at comparatively few points. The vesicle increases in size and 
eventually sends branches along the epidermal wall of the host. Meanwhile 
the original small breach is widened by the contraction of the cuticle, 
rendered possible by the collapse of the underlying cells, and the ruptured 
ends are pushed inwards by the continued growth of the fungus (Figs. 16, 
1 7, 18, 19). Very rarely were the hyphae observed to grow downwards 
into the mesophyll cells of the leaf immediately after penetration. 
Even when the fungus has been growing in the epidermal layer for 
some time and the tissues of the leaf are completely disorganized, the 
staining reactions of the cuticle indicate that it has undergone no chemical 
change (Figs. 17, 18, 19). 
As was observed by Blackman and Welsford, and also by Dey, the 
number of infections increases rapidly after the cuticle is once broken. 
This may be due to a chemotropic stimulus or to the fact that most of the 
hyphae in the infection drop are about the same stage of development, and 
hence it would be expected that many would penetrate about the same 
time. The stomatal apertures do not appear to have any special attraction 
for the fungal hyphae, as only in one case was the fungus observed to enter 
by this channel, and in that particular case an appressorium was formed 
over a stoma, one of the appressorial branches entering through the 
stomatal aperture while the others penetrated in the normal manner 
(Fig. 20). 
Appressoria. 
It has already been noted that Busgen emphasized the importance of 
appressoria in bringing the fungus and host into close contact. His 
assumption, however, that these organs serve for the accumulation and 
penetration of toxic substances into the host can no longer be accepted. 
More recently Hasselbring ( 9 ), working on the formation of appressoria by 
the anthracnoses, found that in that group the food factor was an important 
one. When grown in nutrient media he observed that the germ tubes lost 
their power of reacting to contact stimuli by the formation of appressoria. 
In this connexion the views of de Bary ( 2 ), working on the infection of bean 
plants by Sclerotinia Libertiana , are worthy of note. Since the attachment 
organs formed by this fungus vary considerably in size, it is possible that 
different workers may interpret these organs differently. It will therefore 
add to a clear understanding of the experiments to be described to quote 
de Bary’s description of appressoria : 1 ‘ . . . the mycelium forms short 
branches on which arise tufts of secondary branches which, becoming 
1 (4) 1. c., p. 21 . 
