1916] GRAVES—CHEMOTROPISM IN RHIZOPUS 359 
the young stage, the effect of the negative chemotropic force is 
very slight where the hyphae are only 4oy long. Later, when 
more staling substance has developed with the growth of the 
hyphae, the percentage of turning toward the film free from staling 
substance is much increased; we have seen in the older prepara- 
tions that when the hyphae attain a considerable length the turning 
may become nearly roo per cent. 
A consideration of C bears out our interpretation of A and B, 
for since both films here contain spores, the amount of staling sub- 
stance is more or less equal throughout the preparation, and the 
force due to a negative chemotropic stimulus, therefore, is prac- 
tically eliminated. Any reaction which occurs should be a positive 
chemotropic one, and, as the tables show, there is a considerable 
turning from the plain agar to the turnip juice agar. That 
extent of this increases with the age of the preparation is not easily 
explained, though it is no doubt partly due to the fact that some 
of the hyphae naturally react more slowly than others.? On the 
other hand, the hyphae in the turnip juice agar show an evident 
repulsion from the plain agar. This repulsion also increases with 
the age of the preparation, although not so markedly. It may 
likewise be accounted for by a slower response of some of the 
hyphae, as well as by the fact that in this film the concentration of 
the turnip juice decreases, through diffusion, at first only in the 
immediate vicinity of the holes; later the decreasing concentration 
extends farther out from the holes and affects more hyphae. 
EXPERIMENTS WITH CANE SUGAR.—Strengths of 2.5, 5, and 10 
per cent cane sugar were tried, the cane sugar used being the ordi- 
nary commercial lump sugar. The following combinations were 
arranged in the experimentation with each percentage of sugar. 
A B C 
Agar-++sugar 
Plain agar Ager-tsuger ‘-+ spores 
Mica plate-> Agar+sugar Plain aga Plain agar 
bey +spores 
In 2.5 per cent cane sugar agar, the hyphae appeared very ill 
nourished, being slender and of slow growth. It seems reasonable 
7It may perhaps be caused also by the continual diffusion of the turnip juice 
farther and farther from the holes into the plain agar, thus acting on more and more 
hyphae. 
