Dey.— Studies in the Physiology of Parasitism. V. 307 
tion the spores swell up and become rather constricted in the middle, and 
thus have a more or less dumb-bell shaped appearance, as observed by 
former workers ; they sometimes become septate. A germ-tube comes out 
generally from one end of the spore, but the rare, septate ones produce 
a germ-tube at each end. In a very few cases more than two germ-tubes 
were found to develop from a spore. The germ-tube, as it grows, comes 
in contact with the hard surface of the glass, and as soon as this occurs its 
tip swells out into a dark-brown, thick-walled, spore-like body, the 
appressorium (of Frank) or adhesion organ. Muncie (10) holds that the 
appressorium is not formed until the germ-tube has reached a certain 
length. Observation shows, however, that it is formed whenever the 
germ-tube touches a hard foreign substance; thus it may be produced 
soon after the germ-tube comes out of the spore, so that it lies almost in 
contact with the spore (PI. XXI, Fig. 3 A) ; on the other hand, the germ-tube 
may grow to a considerable length before it is produced (Fig. 3). As to the 
stimulus necessary for its formation, there is no doubt it is the result of 
contact, as Hasselbring (7) has shown in the case of Gloeosporium fructi- 
genum. In this species Hasselbring observed a distinct germ-pore in the 
appressorium, through which a hypha later grows out. In C. Linde - 
muthianum , however, no such germ-pore is to be found, the hypha 
always arising at the point of contact of the appressorium with the glass, 
whatever orientation the appressorium may possess. The young appres- 
sorium is sheathed in a mucilaginous coat (Figs. 4, 4 a). This can clearly 
be demonstrated by staining with very dilute watery gentian violet for 
30 seconds, or by mounting the germinated spores in ‘ collargol ’ (1). By 
means of this sheath the appressorium becomes attached to the surface, so 
that even a fairly strong jet of water does not remove it. In fixed and 
stained preparations this mucilage appears to be reduced by the dehydrating 
agents to irregular granules and threads, analogous to the similar structures 
observed in the germ -tube of Botrytis cinerea by Blackman and Welsford (1). 
Penetration of the Germ-tube into the Host. In following the details 
of penetration by the young ‘ infection hypha ’, freshly picked, young, 
juicy pods of French bean ( Phaseolus vulgaris , vars. Canadian Wonder and 
Brown Dutch) were used. These two varieties were found to be very 
susceptible to the disease. The size of the bean-pods varied from 5 to 
8 cm. long by about § cm. wide. In such young pods the cuticle is thin, 
and so they were likely to prove suitable for infection studies. Young 
leaves were first tried, but as their cuticle and the outer walls of their 
epidermal cells are exceedingly thin, such leaves were found unsuitable for 
observation of the changes occurring in the cuticle and subcuticular layers 
during penetration. Spores were taken from a culture on maize-meal agar 
which had been at 25 0 C. for about a month ; these were stirred thoroughly 
in 1 c.c. of sterile tap-water till the mucilage holding them together in 
