RESPONSE TO STIMULUS OF LIGHT 403 



(fig. 236), where the leaf, after its excitatory fall, was re- 

 erected, on cessation of stimulus, above its original height. 

 This, as we saw, was due to a certain portion of the. incident 

 stimulus becoming latent, and thus increasing the internal 

 energy. 



We shall next take up the subject of the occurrence of 

 positive response, as sometimes induced by light. This may 

 be the result of various different causes. There is one fact, 

 however, in connection with the action of light which it is 

 important to bear in mind. Thus, if we subject the lower 

 half of the pulvinus of Mimosa, for instance, to the action of 

 sunlight, its responsive fall will be gradual, unlike the sudden 

 depression caused by thermal or mechanical stimulation. 

 This is because light, usually speaking, constitutes a stimulus 

 of only moderate intensity. We have seen that a stimulus 

 which falls below a certain critical level of excitatory intensity 

 will evoke positive, instead of negative response. We have 

 also seen that from a sub-tonic tissue the positive response is 

 more easily obtained than from one which is highly ex- 

 citable. Now, as the excitatory efficiency of a mechanical 

 stimulus is very great, and as that stimulus is also incapable 

 of finely graduated decrease, it follows that, in order to ex- 

 hibit positive response under such stimulation, it is necessary 

 that the tissue stimulated should be extremely depressed, or 

 even moribund. Under such conditions I have shown 

 (p. 83) that it is possible under feeble stimulus to obtain 

 positive response, which, under stronger, will pass into the 

 normal negative. 



The stimulus of light, then, whose action is very moderate, 

 discriminates more finely between tonic gradations of the 

 tissue than can other forms of stimulus. If this tonic con- 

 dition be very favourable, and the excitability high, the re- 

 sponse will be by normal galvanometric negativity. If the 

 tonic condition, however, be less favourable, the response is 

 liable to be positive. This latter fact will be very strikingly 

 demonstrated, in a later chapter, by experiments carried out 

 on nerves. It will there be shown that while highly excitable 



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