CHAPTER VI 



EXTERNAL STIMULUS AND INTERNAL ENERGY 



Hydraulic transmission of energy in living tissue -True meaning of tonic con- 

 dition Opposite expressions of internal energy and external stimulus seen in 

 growth-response Parallelism between responses of growing and motile organs 

 Increased internal energy caused by augmentation of temperature finds 

 expression in enhanced rate of growth ; erection of motile leaf ; curling move- 

 ment of spiral tendril ; and galvanometric positivity External stimulus induces 

 opposite effect in all these cases Sudden variation of temperature, acting as a 

 stimulus, induces transient retardation of growth ; depression of motile leaf ; 

 uncurling movement of spiral tendril ; and galvanometric negativity Laws of 

 mechanical and electrical response. 



We have seen in the last chapter that external stimulus, 

 directly applied, induces in an excitable vegetable tissue a 

 contractile response, with concomitant galvanometric nega- 

 tivity. An increase of internal energy, on the other hand, 

 was there seen to give rise to expansion, and galvanometric 

 positivity. These two factors of external stimulus and internal 

 energy are thus seen to be antagonistic in their general 

 expressions. 



But while stimulus from outside, impinging on an excit- 

 able area, thus caused an expenditure of energy at that area, 

 by inducing excitatory response there, yet it was also found 

 that, by its indirect effect, it brought about an increase of 

 energy in neighbouring tissues. By the sudden contraction, 

 and expulsion of water from the excited area, energy was 

 transmitted hydraulically, and the consequent positive tur- 

 gidity-variation caused an erectile response of a neighbour- 

 ing motile organ. In the simple case in which the point of 

 receptivity was at a distance from that of response, it was 

 seen that these two effects of stimulus, direct and indirect, 

 were easily discriminated from one another. But as the 



