(“102 ) 
tact of 6 a minute; for a tact of 10 a minute the successive ele- 
vations decrease distinctly in height. The duration of the cycle of 
metabolism must therefore be longer than 6 seconds, while 10 
seconds is sufficient for it under the given circumstances. When. 
the cycle is complete, the reflex apparatus has returned to its previous 
state and the relation between the masses, the energy and the entropy 
of the system of the assimilation products to those of the system of 
the dissimilation products has returned to its former constant value. 
If we now drop our premise of thermal, mechanical and chemical 
isolation of the reflex-apparatus, which state is never realized in 
nature, the nature and the state of the surrounding medium, in 
regard to which the isolation is imperfect, are to be taken into 
account. For the higher animals the blood may be considered as the 
surrounding medium, whereas for the lower animals the haemolymph 
fluid takes this place. For plants where a surrounding fluid medium 
fails and the cells are for the greater part separated by wails of cellu- 
lose, the degree of isolation of every tissue-element is very complete. 
Both, with regard to its chemical nature and to its physical condition, 
the surroundiug medium has for all animals the property to be 
variable only within very narrow limits. For approximation I shall 
neglect these variations. Further we introduce the passive resistances 
of the physiological systems in our consideration. It is a general 
characteristic of these passive resistances that they prevent a certain 
kind of motion or change, in whatever way the initial state of the 
system may be modified and to whatever external agencies of force, 
and heat, it may be subjected. Possibly these impediments exist only 
within certain limits, but always within such limits, which allow 
finite variations in the values of all the quantities which express 
the initial state of the system or the mechanical or thermal influences 
acting on it, without producing the change in question 3). 
As a proof of the existence of these passive resistances in physiological 
systems we can adduce the very great number of experiments on 
the influence of water upon the metabolism in plants. Perfectly 
dry seeds, mosses and lichens®), which do not respire, can be 
exposed to variations of temperature of more than 100°C., without 
producing any modification in these systems, either temporal or 
lasting, notwithstanding these tissues in turgescent state are not 
distinguished by a high power of resistance against variations 
of temperature *). We know of these passive resistances that they 
i) Gapas: lesen ps allie 
9) Prerrer. Pflanzenphysiologie Ister Bd. 1897. p. 576. 
5) Prerrer. |. c. 2ter Bd. 1901. p. 288, 
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