246 RESEARCHES ON IRRITABILITY OF PLANTS 



the kathode, and therefore the point of excitation. Records 

 are then taken under the same stimulus alternately, (1) 

 with the excited point as warm as the general temperature 

 of the room, which was 30 C, and (2) with the tempera- 

 ture lowered to about 5 C. by the application of cooled 

 water. It is essential that the cooling should be effected 

 gradually, for sudden variation of temperature of itself 

 causes excitation. 



The series of records in fig. 122 shows the result. It is 

 seen that while the excitation is effective at H, H, H, when 



Fig. 122. — Effect of cold on excitability to induction-shock : h, c, 

 alternate effective and ineffective excitation at moderately 

 warm and low temperatures respectively. Testing stimulus, 

 a single break induction-shock was maintained constant. 



the stimulated point is warm, it becomes ineffective at c, c, 

 when the excited point is cooled. 



This proves that in the conducting-tissue of the plant 

 lowering of temperature depresses the excitatory efficiency 

 of a break induction-shock. 



For studying the effect of cold on the exciting efficiency 

 of the constant current, I next made suitable electrical 

 connection with two points on the petiole, the proximal 

 kathode, k, being at a distance of 10 mm. from the pulvinus. 

 Excitation was produced by the make of the descending 

 current. An e.m.f, was applied which caused minima,! 



