EFFECT OF TEMPERATURE ON ELECTRICAL RESPONSE 189 



i*s below a certain thermotonic minimum, the effect of a rise 

 of temperature will be to enhance the amplitude of response 

 by removing molecular sluggishness. This fact has been 

 illustrated in the gradually heightened mechanical response 

 of the autonomous pulsation of Desmodium gyrans when a 

 plant artificially cooled was allowed to return to the normal 

 temperature of the room (fig. 120). If similarly a plant 

 tissue be first cooled and then allowed to return to the 

 surrounding temperature, its electrical responses to suc- 

 cessive uniform stimuli being recorded throughout, a stair- 

 case increase of response will be observed during the return. 

 When the temperature, however, is raised above a certain 

 ptimum,' a depression of the amplitude of response begins, 

 not by the depression of excitability, but by the increasing 

 force of recovery due to an augmentation of the internal 

 factor. True depression only takes place when the plant 

 approaching a condition of heat-rigor. 



One very curious effect of temperature-variation which 

 has been touched upon is the marked increase of sensi- 

 tiveness which often makes its appearance as its after-effect. 

 This is seen exemplified in the record given in fig. 124, 

 showing the effect of a cyclic variation of temperature 

 on Eucharis lily. In another experiment with Scotch kale ? 

 the response at the temperature of 30 C. was eleven 

 divisions, and at 50 C. eight divisions, during the thermal 

 ascent. During the descent, however, the amplitude at 

 50 C. was sixteen, and at 30 C. twenty-three divisions. 

 The sensitiveness was thus doubled. This enhancement 

 may be due in part to the increased molecular mobility 

 consequent on the annealing effect, as it were, of temperature- 

 variation. But it may also be regarded as partly due to the 

 difference of the antagonistic forces which the excitatory 

 response has to overcome during ascent and descent. During 

 the thermal ascent, the opposing expansive force is being 

 rapidly accelerated. During the thermal descent, on the 

 other hand, this is no longer the case, for the force of re- 

 covery is now undergoing a diminution. 



