TEMPERATURE ON POLAR EXCITATION 245 



loss of conductivity, for we have seen in a previous chapter 

 that the conductivity is enhanced by a rise of temperature. 

 The pulvinus, again, was found extremely sensitive. Yet, 

 in spite of the high conductivity and motile excitability, the 

 transmitted effect of excitation was often found ineffective 

 at a high temperature. 



Thus the only remaining factor to which the change 

 might be attributed was the excitability of the conducting 

 tissue itself. Taking the parallel case of the animal nerve, 

 it is known that the excitability of a nerve is diminished by 

 local cooling, the stimulus being that due to break induction- 

 shock. But Gotch and Macdonald have made the very 

 interesting observation that the effect is reversed in the case 

 of stimulation by constant current ; here the nerve excita- 

 bility is enhanced by lowering of temperature. By employ- 

 ing a descending current — with the kathode proximal to the 

 contractile muscle — they found that the make-excitation 

 which was ineffective when the nerve was locally warmed 

 to 30° C. became effective when locally cooled to 5° C. 



It occurred to me that the failure of indirect stimulation 

 by the closure of constant current in Mimosa might be due 

 to the depression of excitability of the conducting petiole, 

 in consequence of the high temperature. Should this prove 

 to be the case, then this specific reaction would afford a 

 very striking demonstration of the characteristic similarities 

 in the conducting-tissues of the animal and plant. 



The effect of cold in modifying the excitability of the 

 conducting animal nerve is, as said before, dependent on the 

 mode of stimulation. We shall now see whether this holds 

 good in the case of the plants also. First, in order to 

 determine the effect of cold on the exciting efficiency of the 

 break induction-shock, the two electrodes from the secondary 

 coil were placed on the petiole, one 20 mm. from the pulvinus 

 and the other further away. It has been shown that with 

 a single induction-shock it is the kathode which causes 

 excitation. The electrodes of the secondary coil are so 

 connected with the petiole as to render the proximal contact 



