1916] GRAVES—CHEMOTROPISM IN RHIZOPUS 357 
Since varying conditions of light, heat, and moisture had been 
eliminated, it is obvious that only chemotropic forces were at work 
here. 
The turning toward the holes in preparations B and C was very 
marked, but in A it was not nearly so pronounced. The only 
difference between B and C lay in the distance from the hole at 
which the curvature became noticeable in the same period of time 
(8.5 hours). In C, the reaction was apparent at a distance of 3-4 
diameters of the hole, counting from its margin; but in B, the turn- 
ing could be observed as far as 10 diameters from the hole. In B 
and C the turning in all cases was 100 per cent, while in A it varied 
from 60 to go per cent. 
In C, since the turnip juice was everywhere of practically the 
Same concentration, the only force exerted must be due to the staling 
substances produced by the hyphae, that is, a negative chemotropic 
force. In B, however, where we have the most marked turning 
of all, we can fairly assert that the turnip juice in the sporeless 
layer is the cause of the additional stimulus, and is, therefore, a 
positive chemotropic force. This reasoning was corroborated by 
the condition in A, where in the lower film the positive chemotropic 
force, due to the turnip juice, would be working against the nega- 
tive chemotropic force due to the staling substance, with the result- 
ing decrease in the amount of turning. 
In these experiments the hyphae had been allowed to grow 
to a considerable length, none being under 300, since it was 
reasoned that with long hyphae the negative chemotropic stimu- 
lus would be much greater than with short ones. On the 
other hand, the: positive chemotropic force should be exerted 
just as strongly on short hyphae as on long ones. In view of 
this, younger stages were tried, with results such as are set forth 
in table IV. 
The combinations A and B correspond to those given previously, 
but C and D are new. D represents the control, for here conditions 
were made as much alike as possible in both upper and lower films. 
The turnip juice being everywhere practically the same in amount, 
no positive chemotropic force can be acting. If the number and 
length of the hyphae were also equal in both films, the turning on 
_ both sides should be no more pronounced in one direction than in 
