PHYSIOLOGICAL RESPONSE IN PLANT AND ANIMAL 743 



the windmill, with its alternate movements, would afford a 

 very excellent illustration of the alternate day and night 

 phases in the nyctitropic movement of the plant. 



Now, with regard to the living machine, similarly, a full 

 insight into its action can only be obtained if we are able 

 to disentangle the two opposite factors, of internal energy 

 and external stimulus, and follow them into their responsive 

 expressions, at the same time recognising that the principle 

 of the conservation of energy must hold good in the living 

 system as in the non-living. External energy acting on the 

 plant performs work on it, a part of this incident energy being 

 taken and held latent ; and, in virtue of the latent energy so 

 conserved, work is performed by the plant. Of this internal 

 work, besides the potential chemical energy which is accumu- 

 lated, the maintenance of suction, growth, and autonomous 

 movements may be cited as examples. 



Fundamental unity of physiological response in plant 

 and animal. — Having now seen the intimate connection 

 which exists between the physical and the physiological, and 

 having also seen that the molecular response of the inorganic 

 is not altogether different from that of the plant, it remains 

 only to glance at the physiological continuity of response as 

 between plant and animal ; and here we shall find that 

 there is hardly any phenomenon of irritability, observed in 

 the case of animal tissues, which is not also to be discovered 

 in some simple form in the case of the plant. These resem- 

 blances, moreover, are so numerous and so detailed as to lead 

 us inevitably to the conclusion that we have to deal in the 

 two cases with a single identical phenomenon. 



In the longitudinal response of a radial vegetable organ 

 we have seen how similar is responsive contraction in animal 

 and plant ; and that this similarity extends even to charac- 

 teristic details is seen when we compare the records of the 

 Kunchangraph in the case of the plant, with those of the 

 Myograph in that of the animal. The two are alike in the 

 exhibition of a latent period, which is prolonged by cold and 

 reduced by warmth. The staircase increase of the respon- 



