(103 ) 
are characteristic of chemical systems and that they are of general 
occurrence in nature. Later investigations suggest that an area, 
sharply defined by pressure and temperature, may exist for every 
substance or system of substances, within which it can be in the 
state of false equilibrium in consequence of these passive resistances !). 
Though these passive resistances probably decrease rapidly on the 
limits, yet there is no reason to suppose, that they should decrease 
to zero beyond this area. 
If we now stimulate a reflex-apparatus as it occurs in nature, 
the stimulus increase must in the first place reach a definite 
finite value, large enough to overcome the passive resistances in the 
system, before this stimulus is capable to bring about a change in 
this system. In consequence of the imperfect isolation, the increase 
of the stimulus per unit of time must in the second place exceed 
a definite finite value in order to bring about a measurable change 
in the system. If we have to deal with a thermal effect, then by 
conduction, heat will pass from the reflex-apparatus to the surroun- 
ding medium and be lost as measurable heat in the reflex-apparatus. 
The same holds good for the pressure, as only semipermeable walls 
occur in nature. In physiology the phenomenon that the increment 
of the stimulus, both as regards its absolute value as its veiocity, 
must exceed a finite value in order to cause a measurable effect, 
is known under the name of the “threshold”. 
The elements, which compose the threshold value, belong therefore 
to two groups, such as arise from passive resistances in the chemical 
system, and such as originate in the imperfect isolation of the reflex- 
apparatus from the surrounding medium. 
For the higher plants, where this latter group of elements fails 
in the threshold value, the latter is remarkably small as compared 
to the whole interval over which the stimulus can be extended. 
From the experiments of geotropic stimulation of CZAPEK *), where 
a centrifugal force expressed in grams is considered as measure of 
the stimulus, appears that while the whole interval of stimulation 
extends over 40 grams, the threshold value lies near 1 milligram. 
The refractory period is due to the existence of a threshold in 
a system which is in a modified physical and chemical state in 
consequence of an immediately preceding stimulus. Since this state 
is partly determined by the stimulus which preceded, the refractory 
period is also partly determined by this stimulus in its characteristic 
1) Van ’r Horr, Vorlesungen. 1898. Heft I. p. 206. 
*) Czarek, Jahrb, f, Wiss, Botanik, 1898, Bd, 32, p. 193, 
