22, BULLETIN 127, U. S. DEPARTMENT OF AGRICULTURE. 
27° C. and in hanging drops between 14° and 27°. In no case were 
any formed at the high temperature of 31°. The point to be noted 
here is limited appressorium formation in the hanging drops of the 
nutrient medium. 
To account for certain of these phenaenena it seems quite reason- 
able to assume that a rather abundant oxygen supply is one of the 
essentials for appressorium formation. It is likely that the spores 
about the extreme edge of a drop would be better supplied with oxy- 
gen than those in the interior. Exposed drops have an abundant 
oxygen supply. In Van Tieghem cells, where the oxygen supply is 
limited, it is quite likely that the demand for oxygen would be greater 
and hence the supply more quickly used up in the nutrient medium 
than in pure water, since more rapid growth occurs in the former. 
With this fungus, then, the presence of food material does not 
seem to inhibit appressorium formation. The contact stimulus is 
apparently necessary as a rule, and a liberal oxygen supply seems to 
be favorable to appressorium formation. The function of appressoria 
will be considered later in connection with the relation of the a 
to the host tissue. 
PATHOGENICITY. 
Whether or not we accept Edgerton’s (13) basis of species distinc- 
tion by host range, it is essential from the phytopathological stand- 
point to know the exact host range of this fungus. 
Under general field conditions the disease has been observed com- 
monly on cucumbers, watermelons, muskmelons, and Lagenaria 
gourds. Experimental evidence of the identity a the fungus in 
these cases is furnished by the cross inoculations made by Sheldon 
(46), who secured infection of cucumber, watermelon, muskmelon, 
and gourd plants with a strain isolated from watermelon, and by 
Carsner,' who secured infection of cucumber plants with a strain 
from muskmelon. 
During the course of this work, successful cross inoculations have 
been made from watermelon to cucumber, from cucumber to water- 
melon and muskmelon, and from muskmelon to cucumber. Leaf 
infection of vigorous plants was secured in all cases. Using the fun- 
gus from cucumber, successful cross inoculations have been made in 
the field upon eight varieties of muskmelon, upon Cucumis anguria, 
C. melo var. fleruosus, C. melo var. dudaim and C. dipsaceus, and upon 
Trichosanthes colubrina and Benincasa cerifera. In these inoculations 
a spore suspension was applied with an atomizer. Out of three series 
made upon about 40 varieties on August 4, 10, and 13, 1916, suc- 
cess was obtained only in the inoculations of August 13. 
Out of a large variety of cucurbits exposed to natural infection by 
drainage water under conditions which practically precluded the pos- 
1Carsner, E. Op. cit. 
