Dey. — Studies in the Physiology of Parasitism. V. 309 
In material on which spores have been growing for 24-40 hours, 
nothing beyond this preliminary stage is to be observed. The activities 
of the young germ-tube during this period are confined to the production 
of the appressorium, for the development of the infection hypha is never 
observed earlier than forty-eight hours after sowing, while the appressoria 
may be formed within twenty-four hours. Muncie held that the incuba- 
tion period (between sowing and penetration) varied from 3^ to 6 days, 
according to the amount of moisture present and the temperature. 
Edgerton was not able to obtain definite signs of infection earlier than four 
and a half days after sowing (8). These workers, of course, did not observe 
the very earliest stages of infection investigated here. 
The next stage in infection is the development of a slight protuberance 
on that part of the appressorium in contact with the cuticular surface. 
This protuberance pushes in the cuticle still farther, so that a distinct 
hollow is formed on the surface (Fig. 5 a). From this protuberance there 
now develops a very fine peg-like outgrowth, the ‘ infection hypha ’, which 
stains homogeneously, and in which the distinction of wall and cavity 
could not be observed. 1 As the result of the development of the ‘ infection 
hypha 5 a rupture is caused in the relatively inelastic cuticle (Fig. 6). 
A very careful and searching study was made for evidence of disorganiza- 
tion or swelling or any other change in the cuticle at this stage or later, 
but no such evidence was obtainable. C. Lindemuthianum forms then no 
special substance which is capable of producing softening or chemical 
change in the cuticle. These facts prove that, as in the case of B . cinerea , 
the f infection hypha ’ obtains entrance by breaking or rupturing the cuticle 
of the host as the result of the pressure exerted by the development of 
the ‘ infection hypha ’ arising from the appressorium. In sections this 
rupture is indicated by a break in the continuity of the cuticle near the 
peg (Figs. 7, 8, 10, 14, 15). The perforation to be observed is considerably 
wider than the ‘ infection hypha’ itself. This may be explained by the fact 
that the epidermis of the bean-pod is no doubt in a state of tension, owing 
to the turgescence of the inner tissue. 2 When the ‘ infection hypha ’ enters 
through the cuticle and brings about softening and disorganization of the 
subcuticular layers, the cuticle is free to contract at that particular spot, 
and consequently the aperture is widened. 
Once the cuticle is ruptured, the pressure due to growth in length of 
the germ-tube forces the appressorium a little farther into the surface of 
the pod ; this is well seen in Fig. 15. In the same figure it is to be noted 
that the penetrating peg, which is at first exceedingly fine, swells into 
a hypha of normal size soon after it reaches the softer cellulose layers. 
1 The relation of the ‘ infection hypha ’ to the inner and outer layers of the appressorium could 
not be determined. 
2 That the outer tissues of the fruit are so stretched can be well demonstrated in the cucumber. 
