GEOTROPIC TORSIONAL RESPONSE 665 



pulvinus I find to be very sensitive to geotropic action. 

 Under normal conditions the leaflets of this plant place 

 themselves horizontally. I now took one of these, say the 

 terminal, and adjusted the plant so that its lateral edge was 

 vertical. The dorsal and ventral halves of its pulvinus were 

 thus equally subjected to geotropic action. This experiment, 

 it is to be noted, was carried out in a dark room, where 

 the only stimulus acting was geotropic. It will be seen, from 

 the first part of the left-hand record in fig. 271, that under 

 geotropic stimulus a torsional response was here obtained, 

 which was exactly similar to that given under the action of 

 light in fig. 270. The angular movement was in this case 

 at the rate of about "11° per minute, and it was the dorsal 

 surface of the organ that was eventually turned upwards — 

 that is to say, so as to face the lines of force of gravity. 

 This experiment, then, conclusively demonstrates the fact 

 that the two halves of the dorsi-ventral organ are unequally 

 sensitive to geotropic stimulus, and that it is the upper which 

 is the less sensitive. 



Modification of torsional geotropic response by arti- 

 ficial variation of differential excitability. — The fact that 

 the upper half is, under normal conditions, the less excitable 

 to geotropic stimulus is capable of further and striking 

 demonstration, by the tests of reversal, and acceleration of 

 torsional response. I have already explained, as will be 

 remembered, that if the existing difference of excitability 

 were reversed, the direction of torsional response would also 

 be reversed ; and that if, on the contrary, the difference were 

 by any means increased, the rate of torsional movement would 

 be correspondingly accelerated. In the first part of fig. 271 

 is seen the natural torsional response of the terminal leaflet 

 of Erythrina indica. The excitability of the lower half of 

 the pulvinus was now abolished, by the local applica- 

 tion of chloroform, at the point marked with an upward 

 arrow, and the torsional response to geotropic stimulation 

 was thus found to be reversed, from plus '11° to minus '05° 

 per minute. Hence it was now the lower half of the pul- 



