EXTERNAL STIMULUS AND INTERNAL ENERGY 75 



temperature had the opposite effect that, namely, of 

 inducing galvanometric positivity. 



It is thus seen that while the characteristic effect of 

 external stimulus on an excitable tissue is to cause a nega- 

 tive turgid ity-variation, that of increased internal energy 

 is to induce a positive turgidity-variation. The former of 

 these, or negative turgidity-variation, finds electrical expres- 

 sion in galvanometric negativity ; in a motile organ by the 

 fall of the leaf, and in a growing organ by retardation of the 

 rate of growth. The latter, or increase of internal energy, 

 on the other hand, is expressed electrically by galvanometric 

 positivity ; mechanically, by erection of the leaf ; and in a 

 growing organ by an acceleration of the normal rate of 

 growth. 



We thus arrive at the following laws of response of 

 isotropic organs : 



1. Effective stimulation induces contraction and galvano- 

 metric negativity, 



2. Increase of internal energy induces expansion and 

 galvanometric positivity. 



