EXTERNAL STIMULUS AND INTERNAL ENERGY 71 



demonstration would undoubtedly become still more con- 

 vincing, if we should succeed in discovering some mode of 

 response in which the antagonistic effects of internal energy 

 and external stimulus found opposite expressions. 



Such an example, of a very striking character, I have in 

 my work on ' Plant Response ' shown to be found in growth- 

 response. But the same opposition, between the effects of 

 external stimulus and internal energy, I shall now proceed to 

 demonstrate not only by means of growth-response, but also 

 through mechanical and electrical responses. In the case of 

 growth, the responsive expression of the growing organ, under 

 increased turgidity, consists of an expansive elongation. If 

 the organ be growing at a uniform rate, an increase of internal 

 energy will enhance that rate. But when external stimulus 

 acts directly on the growing organ, the normal rate of growth 

 is retarded during the action of stimulus. Thus it will be seen 

 that though the mechanical response of a motile organ, and 

 the movement of a growing organ, appear so different, yet 

 these two expressions are not fundamentally distinct. For 

 while in one, the application of direct stimulus, causing nega- 

 tive turgidity-variation and contraction, induces depression of 

 the leaf, in the other, the same negative turgidity-variation 

 and contraction under external stimulus causes a depression 

 of the rate of growth. And, on the other hand, indirect effect 

 of stimulus, or increase of internal energy, inducing positive 

 turgidity-variation, brings about in the one case the erection 

 of the leaf, in the other an enhancement of the normal rate 

 of growth. This parallelism is displayed in detail in the 

 table given below. 



It is thus understood that the indication of response to 

 external stimulus is the depression of the motile leaf, or 

 depression of the rate of growth, while the effect of increased 

 internal energy is the erection of the motile leaf, or enhance- 

 ment of the rate of growth. 



We shall first deal with the responsive expression to 

 that positive turgidity-variation which is due to the increase 

 of internal energy. One mode of increasing the internal 



