1916] GRAVES—CHEMOTROPISM IN RHIZOPUS 367 
results with solutions of equal osmotic pressure. Now, as regards 
the osmotic pressure of turnip juice, W. Brown, working at this col- 
lege, has determined that it is equal to that of a 14 per cent solution 
of cane sugar. It will be recalled that in the preparation of the 
turnip juice agar the turnip juice was diluted to half-strength, and 
its osmotic pressure must therefore be equal to about a 7 per cent 
solution of cane sugar. But our experiments show that even a 
10 per cent solution of cane sugar is far less effective for causing 
a reaction than the turnip juice. It is clear, therefore, that osmo- 
tropism cannot be of great importance. 
THE DISTRIBUTION OF A FUNGUS IN ITs Host.—While the results 
given in this paper apply only to Rhizopus nigricans, there is abun- 
dant evidence from similar work carried on with Penicillium and 
Botrytis that the same conditions exist there also. It is quite 
likely that similar chemotropic reactions obtain in the majority 
of the fungi. If this be the case, it is possible that the distribution 
of a fungus in its host may depend mainly, not on a positive chemo- 
tropic reaction, but on the dominant negative chemotropic stimulus 
of its own staling products. 
Summary 
The following results apply to Rhizopus nigricans Ehrenb. in 
‘particular, but experimental evidence is at hand that the general 
principles involved apply also to Botrytis cinerea Pers. and Penicil- 
lium no. 24 Thom. Most of the data have been derived from work 
with two layers of medium separated by a perforated mica plate. 
1. The fungus shows a marked negative chemotropic reaction 
to a medium in which it has been growing for some time. 
2. The hypothesis brought forward by CLark and Furton, 
that this negative chemotropism is a reaction of the fungus toward 
its “staling substances,” is conclusively substantiated by the 
following evidence: (a) the hyphae in one layer of medium turn 
away from another layer without spores if it contains their own 
staling products; (6) the hyphae always show a marked turning 
from the medium in which they are growing to any medium which 
is free of hyphae, no matter what the composition of that medium 
may be, unless the second medium contains the substances which 
exert a negative chemotropic stimulus; (c) when approximately 
