394 Blackman and Weis ford. — Shi dies in 
be observed where penetration of the cuticle is delayed to a somewhat later 
stage. In these cases the tip of the germ tube or hypha swells up and 
spreads out on the surface of the leaf (Figs. 9, 11, 14) and in some cases 
subsidiary swellings may be produced (Figs. 19, 20). These swellings are, 
of course, of the nature of simple appressoria. From the swollen hypha 
where it is closely applied to the leaf a peg-like hyphal outgrowth now 
appears which pierces the cuticle and pushes its way into the wall of the 
epidermal cell (Figs. 14, 19). Here also there is no sign of any softening or 
other chemical action on the cuticle as a careful study of preparations treated 
with scharlach red shows ; there can be no doubt that, as in the case just 
described, the perforation of the cuticle is due solely to mechanical action, 
i. e. to the pressure exerted by the peg-like outgrowth. For the exercise 
of such pressure the hypha from which the outgrowth proceeds must 
be held firmly to the cuticle ; this is achieved by a mucilaginous in- 
vestment of the apex of the germ tube or of the appressorium. Apart 
from this adhesion to the cuticle due to the mucilaginous investment, 
the germ tube is always firmly pressed against the cuticle. Such a 
pressure is no doubt brought about by an extension in length of the 
germ tube, which is, at the same time, fixed at its basal end ; the basal 
end, in the case of a young germ tube like that of Figs. 6, 7, may be 
practically the spore. 
Brown ( 1 ) in the first of these studies shows that there is no evidence 
for the secretion by B. cinerea of a special toxic substance other than a cell- 
wall-dissolving enzyme. This is fully borne out by our observations, for in 
no case were the epidermal cells seen to be adversely affected in any way 
before the penetration of the cuticle by the germ tubes. We are quite 
unable to confirm the observations of Nordhausen ( 7 ), who described the 
killing of epidermal cells below the infection drop even when the conidia 
were only in the very early stages of germination. 
Even after penetration of the cuticle has occurred the first obvious 
change in the epidermal cells is usually the swelling of the walls, while 
disorganization of the protoplast follows later. It is clear that the 
hypothesis of a crystalloidal toxin which can diffuse through the uninjured 
cuticle and kills the cells beneath, as assumed by Nordhausen ( 7 ) and 
Smith ( 9 ), is quite untenable. These results are fully supported by 
Brown’s study of the physiological conditions in the ‘ infection drop ’ 
published in the same number of this journal (Brown, 2 ). 
Changes produced after penetration. As already stated, immediately 
after the cuticle is penetrated and the hyphal ingrowth reaches the sub- 
cuticular layer of the wall, this layer begins to swell up. As the swelling 
gradually increases this layer becomes laminate in structure (Fig. 19). The 
swelling may be so great that the lumen of the epidermal cell below may be 
almost completely obliterated. 
