GRAVES: CHEMOTROPIC REACTIONS IN RHIZOPUS 325 



Although Rhizopus nigricans Ehrenb. was the species mainly 

 worked with, Botrytis cinerea Pers. and Penicillium No. 24 Thom^ 

 were also used, and there is evidence at hand that the results given 

 for Rhizopus are applicable to them also. 



In work of this sort the methods are all-important, especially the 

 method of estimating the intensity of the chemotropic reaction as 

 expressed in the directions assumed by the hyphae. The method 

 finally adopted for this, as well as an account of all of the technique, 

 will be published later: it will suffice to say here that the method is 

 based on the doctrine of chances; i. e., considering a large number 

 of hyphae, if all external conditions are equal, as many hyphae 

 should turn in one direction as another. Any deviation will 

 indicate a reaction to a disturbing force. 



For making the preparations, the perforated mica plates were 

 adopted which were used by Miyoshi and also by Fulton and Clark, 

 with the holes, however, spaced farther apart; as preliminary 

 experimental work had shown that with the holes nearer together 

 the diffusion was fairly rapid. The perforated plates separated 

 two layers of medium, and various combinations of spores and 

 chemical substances in these two layers were tried, as will be 

 shown later. As to the external conditions under which the 

 experiments were carried on, all inequalities of moisture content 

 in the medium and in the surrounding atmosphere were as far as 

 possible eliminated, and the preparations were incubated in a 

 dark chamber at a constant temperature. 



Experiments with the ''staling substance." — Very early in the 

 work, while testing various percentages of sugar and glucose, I 

 obtained a complete confirmation of the results of Clark and of 

 Fulton in that the hyphae always turned towards the sporeless 

 layer, whether it contained sugar or not. The more hyphae 

 present, the stronger was the turning. If the spores were sown 

 in both layers, no turning resulted in either layer, or it was not 

 nearly as pronounced. According to the hypothesis of Fulton 

 and Clark, above stated, this phenomenon was probably due to a 

 negative chemotropic reaction to some substance excreted by the 

 mycelium of the fungus itself. 



In order to prove this hypothesis, the problem was now to 



1 Thorn, C. Cultural studies of species of Penicillium. U. S. Dept. Agr. Bur. An. 

 Ind. Bull. 118. 1910. 



