1916] GRAVES—CHEMOTROPISM IN RHIZOPUS ‘307 
outside of his prescribed 1.5 diameters from the center of the hole, 
for in all probability his films were thicker than this. 
THE RATE OF DIFFUSION THROUGH THE PERFORATED MICA 
PLATES.—In view of the researches of BROWN and EscomBeE (2) 
on diffusion through perforated membranes, it was realized that 
it was essential to obtain definite data as to the rate of diffusion 
through the perforated mica plates. Experiments were set up in 
which the rates of diffusion of cane sugar through 121-holed plates 
and through 16-holed plates were compared for various periods of 
time. Without going into the details of the experiments, it may be 
stated that in either case, although diffusion did not proceed at such 
a rate as to reduce markedly the difference in concentration in the 
two films, yet diffusion through the 16-holed plate was naturally 
slower, and on this account plates so perforated were chosen for 
the work. 
Experimental work 
It is not our purpose to present a detailed account of all the 
experiments. In the following the most important results are out- 
lined, using the data from representative tests. In all cases given, 
the results have been repeatedly verified. Altogether over 40,000 
hyphae were examined and allocated to their 4 classes in the manner 
described above. 
EXPERIMENTS WITH ‘‘STALED’’ SOLUTIONS.—Very early in the 
work, while various percentages of sugars were being tested for 
their power of inducing a positive chemotropism, it became quite 
evident that the hyphae would always turn toward the holes leading 
to the other agar layer, regardless of whether sugar was present in 
it or not, provided it contained no germinating spores. It was also 
significant that the more hyphae there were present in the one 
film, the stronger was the turning from it toward the film without 
spores. Furthermore, if the spores were sown in both layers, no 
turning resulted in either layer, or it was not nearly so pronounced. 
This was quite in line with the experience of FuLTon and CLARK, 
and according to their hypothesis, previously stated, was probably 
due to a negative chemotropic reaction to some substance or sub- 
stances excreted by the hyphae themselves. In accordance with 
