ANGLE AND REACTION 323 



petiole of Tropaeolum is also a few seconds, a value which is 

 quite consonant with the idea of particles inducing excita- 

 tion by their fall through an exceedingly short distance. 

 In less sensitive organs the latent period is protracted for 

 reasons that have already been given (p. 284). 



Physiological Character of Geo-Electric Response 



The intensity of the electromotive variation is found 

 to depend on the physiological vigour of the specimen. 

 The Tropaeolum plants often employed in my experiments 

 are in the best condition of growth in Calcutta in February ; 

 after this the plants begin to decline in March and die off 

 by the end of April. 



Experiment 178. — In February the intensity of electric 

 response was nearly double of that in March ; it was only 

 in March that I could construct an accurate potentiometer 

 for quantitative determination of the induced electromotive 

 force between the upper and lower contacts on rotation of 

 the specimen from zero to 90°. I give below the following 

 typical values obtained with two different specimens : 



Specimen Induced E.M.F. 



(i) . . . . . .12 millivolts 



[2) ...... 15 ,, 



Effect of Age. — While a young petiole gave the above 

 value, an old specimen from the same plant exhibited no 

 response. The plants were in a dying condition in April 

 and all indications of electric reaction were found to be 

 abolished. 



Relation between Angle of Inclination and 

 Geotropic Excitation 



In the Method of Axial Rotation, experimental condi- 

 tions are ideally perfect. In the neutral position the 

 sides A and B are both parallel to the vertical lines of 

 gravity, and are little affected by geotropic reaction. As 



