454 LIFli; MOVEMENTS IN PLANTS 



will be described in a subsequent chapter, the latent period 

 for geotropic excitation, I find, to be sometimes as short as 

 a second. 



PHYSIOLOGICAL CHARACTER OF GEO-ELECTRIC RESPONSE. 



The intensity of the electro-motive variation is found 

 to depend on the physiological vigour of the specimen. 

 The Tropceolum plant, used for most of the above experi- 

 ments, are at 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 174. — In February ihe intensity of electric 

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

 in March that I made quantitative determination of the 

 induced electro-motive force betv/een the iipper and lower 

 contacts on rotation of the specimen from zero to 90^. 

 The E. M. F. was determined by the potentiometer method. 

 I give below the following typical values obtained with 

 two different specimens: — 



Specimen ... ... Induced E. M. F. 



(1) ... ... 12 millivolts. 



(2) 15 „ 



In the most favourable season the induced electro- 

 motive force is likely to exceed the above value very 

 considerably. 



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 electricar reaction were found 

 abolished. The physiological character of the response was 

 also demonstrated by first obtaining the normal electric 

 response in a vigorous specimen ; after death, by immersion 



