368 BOTANICAL GAZETTE [NOVEMBER 
equal amounts of mycelium occur in two similar layers of medium, 
no turning of the hyphae from the one to the other results. 
3. These staling substances are formed as a result of the vital 
activities of the fungus itself. They consist probably of excreted 
products of metabolism. 
4. The staling substances appear to be either thermolabile or 
volatile, for boiling a solution containing them reduces markedly 
their negative chemotropic influence. 
5. Positive chemotropism toward the substances tested (turnip 
juice, cane sugar, and glucose) also exists; but under ordinary 
conditions of growth this positive chemotropism is very much 
weaker than the negative chemotropism previously mentioned. 
6. Turnip juice exerts a much stronger positive chemotropic 
stimulus than the other simple chemical substances tested, much 
stronger than, for example, 5 per cent cane sugar. This suggests 
the possibility that plant juices in general evoke a stronger positive 
chemotropic response than the simple chemical substances hereto- 
fore experimented with. The nature of the attractive substance 
or substances in turnip juice has not yet been determined. 
7. It is impossible in the present stage of our knowledge to 
compare accurately the strength of two chemotropic stimuli; but, 
using the number of hyphae turning as a test of the relative action 
of the two stimuli, we certainly may conclude that in comparison 
with the stimulus exerted by negative chemotropism, that due to 
positive chemotropism is very much less. For example, in a given 
preparation with two layers of media separated by a perforated 
mica plate, and with the hyphae at a certain stage of growth, when 
go-100 per cent of them are turning toward the holes from plain 
agar to turnip juice agar, the stimulus due to the positive chemo- 
tropic effect of the turnip juice has, very approximately, one-thi 
of the effect of the stimulus due to the negative chemotropism from 
the staling products of the fungus itself. In other words, positive — 
chemotropism is responsible at this stage and under these conditions 
for about one-quarter of the whole reaction. With the sugars the 
part played by positive chemotropism is very much less and is 
easily overlooked. 
8. In contrast to the increase in strength of the negative 
chemotropic stimulus with the age of the preparation, the 
