THE CAUSES OF MOVEMENT 7 



The resulting movement affords no indication as to the mode of 

 perception, and no movement at all may occur when two opposed stimuli 

 neutralize each other, or when the resulting attempts at movement are 

 similar and of opposite kinds. If, however, one stimulus preponderates, 

 and a movement results, the same amount of energy will be expended as 

 when a similar movement is produced by a single stimulus. 



The resultant reaction due to conjoint stimuli is neither quantitatively 

 nor qualitatively the sum of their separate actions. This is still the case 

 when the stimuli are of like kind, for since the power of reaction is always 

 limited, the superposition of a supra-maximal stimulus upon a sub-maximal 

 one may produce little or no additional response. Hence also with stimuli 

 progressively increasing in intensity, the later responses do not increase in 

 proportion to the increases of excitation. 



A satisfactory solution of problems of this kind is not at present 

 possible, although sufficient is known to show that the mechanism of 

 irritable perception and response is not always the same. It is clear 

 that a changed response to a particular stimulus must be due to some 

 change in the mode of perception if the responding mechanism is unaltered. 

 Even when the percipient organ is distinct from the responding region, 

 however, any agency which affects the former may cause modifying 

 influences to radiate from it to the responding mechanism. Hence a -tonic 

 stimulus which primarily acts on the percipient organ alone may indirectly 

 modify the character of the curving zone, so that the capacities of both 

 perception and response are altered. 



Without doubt a change of irritability is in many cases largely or 

 entirely the result of alterations in the sensory and related processes. 

 Modifications of irritability very commonly take place during the life of 

 an organ, so that a particular tropic stimulus does not always produce 

 the same result. It is not however certain whether, for instance, the lack 

 of response to shaking in an etherized plant of Mimosa is the result of an 

 inhibition of the power of perception, of induction, or of motion. 



Similarly it is difficult or impossible to say whether in a particular case 

 two simultaneous stimuli fuse in the act of perception, or whether they act 

 singly upon the motor mechanism. The former appears to be usually the 

 case when two dissimilar tropic stimuli act conjointly, whereas a fusion of 

 this kind does not appear to occur between tropic and photonastic or 

 contact stimuli. In all cases, however, it must be remembered that the 

 independence of the processes of sensation and response is only relative, and 

 that a modification of the one is certain to react upon the other. 



A perceptible response is in all cases only produced when the stimulus 

 reaches a certain minimal intensity, while between stimulation and 

 response a latent period of variable duration always intervenes. The 

 resulting movement is nearly always gradually accelerated to a maximum, 



