THE CAUSES OF MOVEMENT 9 



position in spite of repeated stimulation by blows, whereas tendrils and 

 the leaflets of Oxalis^ for instance, on a repetition of the stimulation, take 

 up a new position of equilibrium. Similarly, if stimuli are repeated on 

 a muscle before relaxation has taken place, the muscle responds to each 

 and remains contracted in a condition of tetanus. 



Furthermore, Mimosa is exceptional in that any shock-stimulus to which 

 the leaflets respond produces the maximal possible movement. Usually, 

 however, as for example in the leaflets of Oxalis, a single blow may act 

 as a sub-maximal stimulus, and the full sinking of the leaflets be produced 

 only by repeated shocks. The existence of a labile condition is not 

 essential for the realization of an irritable movement, and in fact in many 

 cases the latter may not involve an increase in the general activity of 

 growth, but merely its guidance and regulation. Naturally, however, the 

 accumulation of potential energy in the form of high tissue-strains and 

 the like is necessary for the performance of rapid movements. 



Except in those cases where any operative stimulus produces the 

 maximal effect, increasing intensity of excitation produces increasing and 

 more rapid response. This applies to transitory as well as to intermittent 

 and continuous stimulation. Weak heliotropic, geotropic, or photonastic 

 stimulation, for instance, produces a less pronounced curvature than strong 

 stimulation. There is, however, no exact relation between the intensity of 

 the stimulus and the amount of response, or of the sensory excitation. 

 These physiological processes usually increase less rapidly than the stimulus 

 does, so that a greater increase in the intensity of the stimulus is required 

 in a strongly excited organ than in one under weak stimulation to produce 

 the same increase of excitation or response. This rule is well known in 

 animal physiology, and in addition, beyond a certain intensity of stimulus, 

 the response may alter, as when organisms swim towards diffuse light but 

 away from strong sunlight, and hence collect at a definite distance from 

 a local source of illumination. Rooted plants also curve towards a strong 

 source of illumination when far away from it, take on a diaheliotropic 

 position when nearer, and curve away from it when still nearer. 



These effects are the result of a change of tone, which may often be 

 due to the fact that some of the factors involved in sensation are affected 

 more than others by increasing stimulation. This is shown especially well 

 when with increasing concentration a negative osmotropism overcomes 

 a positive chemotropism. 



Every disturbance of equilibrium inducing curvature excites reactions 

 directed towards the restoration of equilibrium. Hence on the removal 

 of a tropic stimulus, the organ affected returns to the original position 

 assumed in virtue of its autotropism, so long as the power of movement 

 is retained. Even in adult organs which have ceased to grow, curvatures 

 may be removed if a power of potential growth resides at the nodes. 



