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one by itself or as one stimulus. Is the first suggestion 
the case, then they speak of various stimuli, a stimulus 
for substance a and one for substance b, which have no 
influence on each other. They have understood the fact, 
that this can be concluded only, if first the threshold is 
fixed of the substances a and b, after that they are com- 
bined in such a way, that of substance a as well as of 
substance b a too small concentration is present to bring 
about a reaction by itself. (Pfeffer, 1888, Kniep, 1905). 
So it is the question whether we can call up the two 
different stimuli. In phototropism and geotropism we work 
in the same way if we will find out whether different 
stimuli can be called up there. They have even succeeded 
there in combining a grantational stimulus with a light- 
stimulus by a simple addition. In that case, however, 
other things have to be taken into account besides the 
threshold (C. E. B. Bremekamp, 1915). It has come to 
light that the time which elapses between the beginning 
of the stimulation and the reaction, even if we work with 
threshold-values is shorter with a geotropical process than 
with a phototropical process. They say that the reaction 
times are not the same. If both stimuli are applied in 
such an order, that for both the reaction should begin 
at the same moment, a simple addition takes place. When 
they had not yet paid attention to the reaction-times, it 
was accepted, as it was proved by Mrs. C. J. Rutten 
Pekelharing (1910) that a light-stimulus and a granta- 
tional-stimulus could not be simply called up. 
The possibility is not excluded, that two chemotactical 
stimuli do not make their influence felt in the same short 
time. If so we have to face the same difficulties with the 
addition of them, as we have with the addition of light- 
and grantational-stimuli. Though it is a matter of course 
to suggest, that the reaction-times in our chemotactical 
experiments are very short, in principle it does not make 
