342 Boyle . — Studies in the Physiology of Parasitism . IV. 
afford any evidence of their being softened or dissolved at this stage, as was 
described by de Bary ( 4 ). 
The cuticle appears to be ruptured by purely mechanical means. 
When infection takes place without the formation of a large appres- 
sorium the essential feature of penetration, viz. the mechanical rupturing 
of the cuticle, is the same. The pressure exerted on the surface of the 
host, apparently rendered possible by the mucilaginous investment sur- 
rounding the tips of the hyphae, may be very considerable. (Fig. 9). As 
in the case of appressorial penetration an ‘ infection hypha ’ is sent out 
which pierces the cuticle (Figs. 13, 15). 
When the barrier offered by the cuticle is overcome the ‘ infection 
hypha’ swells into a vesicle in the epidermal cell (Figs. 12, 13, 14, 15, 16). 
Immediately the continuity of the cuticle is broken the walls of the host 
cells near the point of penetration show signs of being chemically altered. 
The vesicle is surrounded by a bright halo probably indicating solution of 
the cell contents (Figs. 13, 15). In some instances the subcuticular layer 
of the epidermal wall is markedly swollen (Figs. 6, 15), and in extreme 
cases the swelling of the cellulose walls may completely obliterate the 
cavity of the epidermal cell (Fig. 15). The swelling of the subcuticular 
layer appears to be an irregular feature, as no such swelling could be 
detected in several instances after penetration had taken place (Figs. 12, 14). 
In such cases, however, in addition to the alteration in the staining capacity 
of the layer, its distinctly laminate structure indicated that the wall had 
undergone some chemical change. While the vesicle is still confined to 
the epidermal cell the chloroplasts of the palisade cells underneath appear 
to lose their coherence (Fig. 15). They swell up and become converted 
into a darkly staining uniformly granular mass. Meanwhile the nuclei of 
the palisade cells beneath the point of attack move towards the top of the 
cells, as was observed by Blackman and Welsford in the case of infection by 
Botrytis cinerea . Later, they also lose their identity, and are indistinguish- 
able in the general disorganized mass of cell contents. In hand-cut 
sections of fresh material the affected areas appear as brownish or blackish 
patches extending for some distance on either side of the point of attack, 
indicating that death of the cells has taken place. In fixed and stained 
preparations the affected areas can be easily recognized owing to their 
increased capacity for absorbing stains. 
Death of the cells extends some distance beyond the limits of the 
invading hypha, due either to enzymes secreted by the fungus or to the 
products of disorganized cells. The lethal substance or substances appear 
to diffuse more rapidly along the palisade cells of the mesophyll than into 
the spongy parenchyma, as the chloroplasts of the palisade cells for some 
distance on either side of the point of infection are swollen or disorganized, 
while those of the spongy parenchyma immediately underneath are still 
