go BOTANICAL GAZETTE [FEBRUARY 
with Mucor mucedo and Aspergillus niger; the values given by him 
for these forms are to be found in columns I and III below. It was 
impracticable to use Asperillgus niger in the present study, and 
' Sterigmatocystis nigra was substituted with the understanding that 
the two forms are not always distinguished. The present results 
for Mucor mucedo and Sterigmatocystis nigra are given below in 
columns II and IV respectively. ' 
In the control cultures, where distilled water was used in the 
tubes, the effect was the same as for the majority of chemical sub- 
stances; that is, 10-30 per cent. of the hyphae turned toward the 
tubes. The same amount of positive turning was observed in the 
case of all the strongly toxic compounds used. Even with a 0.05 per 
cent. solution of mercuric chlorid and a 1 per cent. solution of copper 
sulfate, which completely inhibited germination within a radius of 
eight to twelve tube diameters from the openings, the hyphae not 
only grew across the diffusion areas, but 10-30 per cent. of those 
approaching the openings turned toward them and grew for a con- 
siderable distance into the tubes. 
Although four concentrations of cane sugar ranging from 20 to 
0.1 per cent. and four concentrations of meat extract ranging from 
10-0.01 per cent. were used, no definite relation between the strength 
of stimulus and that of response was apparent. 
Two corresponding series were made with ten representative 
compounds; in one series sugar-beet agar was the culture medium; 
in the other, distilled-water agar. No difference in the behavior 
of the hyphae due to a difference in the media in which they grew 
could be observed. 
Tests with mica plates —Thin sheets of mica were cut into pieces 
about 25X16™™; these were perforated with a needle, the holes 
being 0.1-0.15™™ in diameter, and about 2™™ apart. Covers of 
suitable size were cut from glass 1™™ thick. A layer of gelatin or 
agar was placed on a cover, a mica plate was placed on this just 
before it hardened, and a second layer was placed above the plate. 
The chemical to be tested was made up in double strength solution 
in redistilled water, and one volume of this was added to one volume 
of gelatin or agar, also made up double strength in redistilled water. 
It was usually found convenient to have the layer containing the 
