Boyle. — Studies in the Physiology of Parasitism . VI. 341 
be seen although the walls of the individual branches appear to be thickened 
(Figs. 3, 5). It would thus appear that the contact stimulus induces the 
formation of a uniformly thick wall round the appressorial hyphae. When 
further growth is about to take place the thickened walls become locally 
thinned out, possibly at the first points of contact of the appressorial 
branches with the cover-slip, and ‘ infection hyphae ’ emerge — giving the 
effect of germ pores when viewed from above. In support of the view that 
these differentiated areas are secondary formations, and not ‘ germ pores it is 
noticed that when hanging-drop cultures are examined microscopically 
these areas come into focus before the outline of the appressorial hyphae 
containing them. This is apparently due to the appressorial branches 
being pushed downwards by the pressure exerted by the emerging ‘ infection 
hyphae ’ against the cover-slip so that the former come into focus at a lower 
level. In no case could the hyaline areas and the branches of the appres- 
sorium containing them be brought into focus at the same time, as would 
be the case if they were really germ pores. Further evidence in support 
of the view that the germ-pore appearance is due to ‘ infection hyphae ’ 
emerging from the appressorium will be alluded to later when dealing with 
the formation of these organs on the leaf surface. The fact that these 
hyaline areas are not to be seen in young appressoria may perhaps account 
for their not having been recorded by previous workers. 
Penetration. 
The first preliminary to penetration is the firm adhesion of the fungus 
mycelium to the host surface. This appears to be effected by means of 
the mucilaginous sheath. When the tip of a hypha comes into contact 
with any resistant material the character of the outer wall undergoes some 
change. This modified wall extends a short distance behind the point of 
contact and is easily recognized in fixed and stained preparations by its 
greater affinity for stains and also by the fact that its refractive index is 
different from that of the unaltered wall of the hypha. This modification 
is much more marked in the walls of appressorial hyphae, and before 
‘ infection hyphae * are produced the thickening is continuous over the tips 
of the branches (Figs. 11, 18). 
The appressoria adhere firmly to the cuticle by means of the muci- 
laginous sheath. When penetration is about to take place the thickened 
walls of the appressorial hyphae become locally thinned out over a small 
area, and in longitudinal section a bright halo is noticed inside the thin 
portion of the wall as though the latter were being dissolved (Fig. 12). 
The modified wall over this area disappears and the small infection hypha 
presses firmly against the cuticle, which may become markedly indented 
(Figs. 10, 11, 12, 14). Although carefully looked for, it has not been 
observed that the staining reactions of the cuticle or subcuticular layers 
