The intensity of the electro-motive variation is found to depend on the physiological vigour of the specimen. The _Tropæolum_ plant, used for most of the above experiments, 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 the 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 between the upper 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 electrical 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 in boiling water, the plant gave no electric response to geotropic stimulus. EFFECT OF DIFFERENTIAL EXCITABILITY OF THE ORGAN. I have hitherto described the geo-electric effect of radial and isotropic organs. The induced E. M. F. at 90° was found practically the same whether A was above and B below, and _vice versâ_. In the mechanical response of the pulvinus of _Mimosa_, the geotropic excitability was, however, found to be greater in the lower half than in the upper (p. 440). I wished to investigate the question of differential geotropic excitability anew, by means of electric response. _Experiment 175._--Electric connections with the galvanometer were made with the upper and lower halves of the pulvinus, the organ being placed in the vertical or neutral position. The angle of inclination was then increased to 90° in the positive and negative directions alternately. TABLE XXXIV.--DIFFERENCE OF GEO-ELECTRIC RESPONSE OF UPPER AND LOWER HALVES OF THE PULVINUS OF _Mimosa_. +---------------------------------------------------------+ | Specimen. | Position of particular | Induced E. M. F. | | | half of pulvinus. | | +-----------+--------------------------+------------------+ | (1) | { Upper half above | 23 millivolts. | | | { Lower half above | 30 " | | | | | | (2) | { Upper half above | 16 " | | | { Lower half above | 29 " | +---------------------------------------------------------+