Boyle —Studies in the Physiology of Parasitism. VI. 345 
in the amount of mucilage present in both cases. It has already been 
noted that both hyphae and appressoria are surrounded by a thick muci- 
laginous investment when growing in a nutrient fluid. The mucilaginous 
sheath is much less constant and more difficult to demonstrate in the case 
of aerial hyphae. When such hyphae come into contact with the host the 
pressure exerted by the infection tube may dislocate the hyphal tip owing 
to the lack of mucilage. This may give rise to the necessary stimulus 
for the formation of attachment organs. 
Whatever may be the true explanation of the difference in behaviour of 
the fungus in attacking through a liquid medium and through the medium 
of air, it cannot be explained by assuming that in one case the hyphae 
are sufficiently well nourished to secrete the necessary toxin to dissolve the 
cuticle, since penetration can and does take place both by direct infection 
and by penetration after the formation of attachment organs under similar 
conditions of food-supply. As already stated, no evidence could be adduced 
in support of de Bary’s observation that death of the underlying cells took 
place before the cuticle was penetrated. 
Summary. 
The early stages of infection of bean leaves by Sclerotinia Liber tiana 
have been studied. 
The hyphae of the ordinary mycelium and also the appressoria growing 
in turnip juice are surrounded by a mucilaginous sheath. In the case of 
aerial hyphae the mucilaginous sheath cannot always be demonstrated. 
When a hyphal tip comes in contact with any resistant material, such 
as a cover-slip or the host surface, the staining reaction of the wall of 
the tip becomes modified. This modification extends a short distance 
behind the point of contact : it is very strongly marked in the case of 
appressoria. 
From the tip of each hypha which is in contact with the host plant or 
with a glass surface there arises an ‘ infection hypha usually very narrow, 
which, under appropriate conditions, penetrates the host. 
The ‘ infection hypha * shows a normal unmodified wall. 
The cuticle may be markedly indented at the point of contact with the 
fungal hypha. This indentation is due to the pressure exerted by the 
‘ infection hypha ’. 
The invading hyphae are apparently fixed to the cuticle by means of 
the mucilaginous sheath. 
There is no evidence at this stage of the softening or solution or any 
modification of the cuticle or subcuticular layers of the host. 
The rupture of the cuticle by the ‘ infection hypha ’ appears to be due 
solely to mechanical action. 
After the cuticle has been penetrated the tissue beneath rapidly 
