1916] GRAVES—CHEMOTROPISM IN RHIZOPUS 361 
greater chemotropic stimuli than is the case in A. This agrees 
well with the experiments with turnip juice; in A we have the 
negative chemotropic stimulus working against the positive chemo- 
tropic stimulus, while in B we have both these forces working 
together. 
The fact that no positive curvature appears in the early stage \ 
of B may be accounted for by the weak growth of the hyphae in , 
non-nutrient agar. They thus do not react to the sugar diffusing 
from the upper film, nor have they yet produced sufficient staling 
substances to cause them to turn to the holes leading to the upper 
layer. But with the increasing diffusion of sugar they become in 
time better nourished, develop more of the staling substances, and 
react to this negative chemotropic stimulus as well as to the posi- 
tive stimulus exerted by the sugar. 
In C, although the average number of germ tubes per sq. mm. 
in the lower layer is given in the table as 5, a result of the poor 
germination in the plain agar, in this case among the 11 preparations 
holes were selected which had an approximately equal number of 
hyphae about them in both films. In order to have germ tubes 
of fairly equal length in both lower and upper films, the lower 
films were given a 4 hours’ start before the upper films were added. 
The results, however, are not in line with what we have seen for the 
same experiment with turnip juice (C of table IV), for we would 
expect to find a positive reaction in the lower layer; but, as already 
observed, the spores germinate poorly and the hyphae grow slowly 
in the plainagar. The majority of them germinate only around 
the holes, where the sugar is diffusing through. With this sugar 
diffuse also staling substances from the more vigorously growing 
hyphae above, thus counteracting any stimulus which would other- 
wise be exerted on the hyphae of the lower layer by their own 
staling substance. The positive result in the similar case where 
turnip juice is used is probably due to the greater chemotropic 
activity of the turnip juice; and possibly also to a more rapid 
specific diffusion of the active substance of turnip juice as com- 
pared with cane sugar, as well as to the more vigorous growth of 
the lower layer hyphae which are nourished by the diffusing turnip 
juice. 
