DISPROPORTIONALITY BETWEEN STIMULUS AND EFFECT. 589 



nomena of irritability, viz. ihe disproportionality existing between the external siimultis 

 and the ultimate action. In the cases of movements of parts of plants due to 

 imbibition and desiccation as just considered, there is an easily intelligible propor- 

 tionality between cause and effect : when a certain quantity of water penetrates into 

 the walls of cells, the latter become distended according to the volume of water, and 

 upon this the movement depends. In like manner the bending, of a flexible haulm 

 of straw, or of a woody branch, corresponds to the pressure which acts from 

 without and gives rise to the curvature, and we have the simplest case of such purely 

 mechanical effects, for example, when an elastic ball is driven against another One of 

 equal size and elasticity ; as is well known, the former comes to rest because it has 

 given up the whole of its momentum to the latter. All this has no resemblance to 

 phenomena of irritability; on the contrary, a very characteristic point of the latter 

 lies in the fact that neither the quantity nor the quality of a phenomenon of irritability 

 need have any kind of similarity or proportionality to the stimulus, and it is 

 simply on this that the peculiarly wonderful and even startling effects of irritability 

 depend ; and here probably we have the reason why until quite recently the pheno- 

 mena of irritability, which are at bottom identical with those of life, were set off 

 against those of the rest of nature as something entirely different, and regarded as the 

 effects of a special force — -vital force. A calm criticism of the prevailing circum- 

 stances, however, leads to an entirely different conclusion. The conspicuous 

 inequality in kind and in degree between the cause and the effect arises rather 

 from the fact that in the living irritable organ a series of causes are already in 

 existence, which taken together with the external stimulus give rise to the effect. 

 The disproportionality between cause and effect in irritability is simply and only 

 apparent, not real, as will be brought forward still more clearly from subsequent 

 considerations. In the first place, however, the main fact with which we are con- 

 cerned may be illustrated by a few other examples. 



If growing stems or petioles are illuminated from one side for some time, they 

 generally become curved in such a manner that the apex bends over towards the 

 source of light. There is no doubt that it is the rays of light which effect this 

 curvature, and it is just as little doubtful that the mechanical energy of these light- 

 rays would be far too insignificant to cause such a curvature of the parts of the 

 plant ; on the contrary, very peculiar mechanisms must exist in the latter which, on 

 being stimulated by the rays of light, bring about this curvature. Still more striking 

 in this connection are the geotropic curvatures. If an upward-growing stem is laid 

 horizontally, it curves upwards ; if the same is done with a primary root, its apex 

 curves downwards. The simple change of position of the axis of growth of these 

 organs with regard to the radius of the earth and the direction of gravitation effects 

 changes in the growth in length, which stand in no comprehensible mechanical 

 relation whatever to the other effects of gravitation. That it is here, however, 

 entirely and simply a matter of adjustment conditioned by the organisation of the 

 plant, follows at once from the above-named circumstance, that the one part of the 

 plant becomes curved upwards and the other downwards when its position with 

 respect to the radius of the earth has been altered. 



Similarly we meet with the disproportionality between cause and effect in cases 

 where, by means of a slight touch on the under side of the motile organ, a leaf of 



