CHAPTER VIII 



SENSITIVITY IN PLANTS AND ANIMALS 



THE second characteristic or capacity exhibited to 

 a greater or less degree by all organisms, we have 

 termed sensitivity, or the power to respond to stimuli 

 from without or from within (p. 7). 



Let us first of all glance, quite briefly, at some of the 

 chief types of stimuli to which organisms are subject. 

 These will be found to be (a) mechanical, i.e., those 

 which might be popularly described as a push or a 

 pull ; (b) chemical, where the stimulus lies not in the 

 mass of the material,, but in the chemical properties 

 it possesses and which are able to induce certain 

 alterations in the form, position or behaviour of the 

 organism ; (c) thermal, where the stimulus is of the 

 nature of a more or less sudden change of tempera- 

 ture ; (d) photic, the access of light or its with- 

 drawal ; and (c) electric, the influence of an electric 

 current or shock. 



Not only the nature, but also the intensity of the 

 stimulus, must be taken into account, for we find 

 that every vital process varies in its activity within 

 wide limits, according to the intensity of the stimulus. 

 Each process goes on best when the stimulus is of a 

 definite intensity, but there is also a minimum, or 

 liminal, intensity at which the process commences, 

 and a maximum intensity at which it ceases. 

 The best response is represented by the optimum 

 intensity of the stimulus, but the optimum bv no 



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