132 
to form with pure water such a diffusion-zone, that the 
object distinctly reacts upon it, is called the threshold for 
the chemotacticum. 
When comparing the definition of the threshold for a 
chemotacticum with the way, in which a threshold is fixed 
with the phototropical experiments, one perceives, how 
unsuspecting one acts with the chemotactical experiments. 
It has been known for a long time, that to acquire a just 
visible phototropical curvation, the time during which the 
stimulus must act, is often much shorter, than the time 
necessary to bring about the further changes, which lead 
to the curvation. Ît is said that the time of presentation 
is shorter than the time of reaction, that the curvation 
is formed by an after action. The complications that crop 
up when one goes on stimulating till the reaction appears, 
are very hard to judge. One has to face so-called changes 
of ,condition”’. The plants used for experiments grow 
continually more insensible during the stimulation that it 
is even possible, that the reaction changes of sign by this. 
Though it becomes more and more probable of late that 
these complications are not so incomprehensible, as they 
seemed before (Arisz, 1914), it is necessary to reckon 
with them. Supposing that with the chemotactical pheno- 
mena we have to do with processes, which may be 
compared to those of phototropism, it is clear, that in 
our experiments we do not observe the phenomena in 
their simplest forms. Pfeffer pointed to the possible 
connection between chemotactical phenomena and photo- 
tropical ones: , Überhaupt liegen hier analoge Beziehungen 
vor. Die in anderen Reizwirkungen, z.B. im Heliotropismus” 
(W. Pfeffer 1888, pag. 624). 
They try to learn more about perception, by working 
with combinations of chemotactica. They think to find 
out by this whether the stimulation, exercised by the 
composing elements of the mixture are percipiated every 
