PROPAGATION OF A STIMULUS ; AFTER-EFFECT, ETC. 597 



The case is very similar with geotropic cftects: if an erect growing stem (e.g. of 

 Dipsacus) is allowed to remain for one or two hours in a horizontal position, here 

 also the curvature is as yet scarcely noticeable. If the shoot is now placed erect, 

 and thus brought into a position where the stimulation due to gravitation ceases, the 

 stimulation already introduced during the horizontal position nevertheless goes 

 on acting; the stem continues to curve for several hours, the concavity becoming 

 more and more pronounced on that side which was turned upwards in the previous 

 horizontal position. And no doubt in the case of every phenomenon of irritability 

 there is not only a propagation in space of the stimulus, but also an after-effect in 

 time, although not always so evident as in these cases. 



As regards the explanation of these phenomena, it may be said quite gene- 

 rally, first, respectfng the after-effect in time, that every effect whatever in Nature 

 occupies a certain time — in fact Time itself is nothing more than the course of 

 natural phenomena. When a cannon is fired off at 1000 meters distance, the flash 

 of light is seen almost simultaneously, because the movements of light travel so 

 exceedingly rapidly'* the report however is not perceived until several seconds later, 

 because the waves of sound travel much more slowly. The interval of time lying 

 between cause and effect depends on the properties of the medium in which the 

 effect is propagated. And the same is the case with the phenomena of irritability ; 

 it depends on the irritable structure of an organ whether the external stimulus or 

 shock sets it in motion suddenly — i.e. in a very short time — or whether the given 

 shock requires a longer time for its action. If the leaf of a Mimosa is irritable in a 

 high degree, a vibration acts almost instantaneously, and the same is the case with a 

 leaf of DioncEa; but if these organs are feebly — i. e. less— irritable, then the movement 

 caused by the shock is slow. Or, as we may also say, the more unstable the 

 equilibrium in the molecular structure of the irritable organ is, the more rapidly will 

 any shock acting as a stimulus set the organ in motion. Nevertheless it may also 

 depend upon other circumstances how much time intervenes between the external 

 influence and the subsequent visible stimulation : the more complicated the mutually 

 acting causes and effects are which finally bring about the visible stimulation move- 

 ment the longer will be the time consumed in the matter. Since then the effects of 

 stimulation in general usually depend upon very complicated chains of causes, they also 

 as a rule make their appearance slowly, and this is especially true of most of the stimu- 

 lations occurring in plants. It was this very slowness which caused the general fact 

 of the irritability of plants to be so long overlooked, and subsequently under-estimated. 



If the effects induced by contact, vibration, alterations of light and tem- 

 perature, &c. occurred with the same rapidity in plants as do the corresponding 

 stimulations in animals, plants would appear no less irritable than animals ; and if we 

 imagine the stimulations which are continually occurring in plants to take place a 

 hundred times as rapidly as they really do, our gardens, fields, and meadows would 

 present very strange and uncanny movements. 



The rapidity with which stimuli call forth their corresponding effects is in 

 animals, as a rule, much greater. Between the opening of the closed eye and the 

 perception of light scarcely any time at all seems to pass, and in like manner a sudden 

 \vDunding of the finger seems to call forth its reflex motion instantaneously. But no 

 "physiologist doubts that in both cases a small interval of lime elapses, and compared 



