366 BOTANICAL GAZETTE [NOVEMBER 
Further work with plant juices is much to be desired. These 
juices of course contain a large variety of substances, the combined 
effect of which is evidently (at least in turnip juice) much more 
powerful than that of cane sugar. This is perhaps one of the reasons 
why Futton, working with only simple substances like the sugars 
and various salts, was unable to demonstrate the existence of the 
very weak positive chemotropism which these substances may 
cause. The fact should also be emphasized that the hyphae grew 
much more vigorously in turnip juice than in the sugars, and 
probably on that account reacted better as well as much earlier. 
It is reasonable to associate a stronger reaction with a healthier 
growth 
THE GRADIENT OF DIFFUSION IN ITS RELATION TO CHEMO- 
TROPISM.—Mrvosui (8) claimed that when hyphae are placed 
between two concentrations of a substance, the concentrations 
being kept constant, no chemotropic reaction occurred unless a 
definite ratio existed between the two concentrations. Thus a 
fungus between o.1 and o.3 per cent of a sugar solution would not 
react; but with o.1 and 1.0 per cent, a positive curvature occurred 
toward the 1 per cent. Similarly a 0.5 per cent solution must have 
a 5 per cent solution as an antithesis, if a reaction is to ensue. In 
other words, the conditions for a sufficient stimulus to produce 4 
turning are in accordance with Weber’s law. In our own experi- 
ments, in the cases of the hyphae turning toward the holes, it is 
difficult to conceive how this law can apply. The hyphae are so 
slender that the difference in concentration of the diffusing sub- 
stance on opposite sides of a hypha, assuming that the latter is 
growing tangentially to the hole, could hardly be in the proportion 
of ro to 1, given by MrvyosuHt as requisite for a reaction. JOST (6) 
has voiced a similar criticism. As a matter of fact, it is quite 
evident from our work that the hyphae must react to vastly smaller 
differences in concentration than this. We must admit, however, 
that the external factors which bring about chemotropic reaction 
are still somewhat obscure. Possibly there is a reaction to the 
direction of the diffusion current. 
Osmotropism.—It is possible that osmotropism, as suggested 
by Poropxo (9), may play some small part in the reactions, but 
it cannot be an important factor, for if it were, we should get similar 
