395 
the Physiology of Parasitism . //. 
The swelling of the cellulose subcuticular layer appears to stretch 
the cuticle. Possibly this facilitates the entry of other hyphae which find 
less resistance in the ‘ thinned out ’ cuticle, for in the drop infections 
the number of hyphae entering increases rapidly after the first few have 
penetrated the cuticle and caused the swelling of the subcuticular layer of 
the wall. The softening of the cellulose layers which underlie the cuticle 
Text-fig. 2. 
For explanations see p. 396 . 
would also markedly reduce the resistance which that membrane could offer 
to perforation by mechanical pressure. 
When once a number of hyphae have entered the leaf other hyphae are 
seen to enter through the stomatal apertures, though first infection of the leaf 
was never observed to be brought about in this way. How far this is due 
to the filling of the intercellular spaces of the leaf with fluid, either from the 
liquid on the leaf or by the cell sap exuding from dead cells, is still uncertain. 
The question also as to how far the penetration of the leaf is due 
to chemotropic stimuli arising from slight exudations of substances through 
the cuticle, or how far it is due to a contact stimulus, still requires further 
investigation. 
As the hyphae penetrate through the epidermis, the cells of the palisade 
parenchyma become affected . 1 First, the nuclei move upwards towards the 
epidermis (Fig. 22), then gradually they begin to disintegrate, the chloro- 
plasts swell, and starch almost disappears from the affected region. The 
1 There was no clear evidence of the movement of the nuclei of the epidermal cells towards 
the outer walls as a response to the development of the fungus on the leaf in the manner described 
by Ritter (7). The nuclei of the epidermal cells of normal leaves are sometimes found in close 
proximity to the outer wall. 
