GEOTROPIC AND PHOTIC STIMULATION 417 



in a characteristic manner depending upon the rate of 

 revolution of the plane of junction of the two diverse longi- 

 tudinal halves of the anisotropic stem which necessarily 

 accompanies torsion. If the rate of revolution be such that 

 the flank A is always uppermost, then the autonomous torsion 

 will be accelerated by gravitational reaction. The movement 

 of the stem as a whole will then be composed of (i) the cir- 

 cumnutation, say, anti-clockwise, due to the torsion of the 

 erect portion of the stem, and (2) the circumnutation of the 

 overhanging portion round a more or less inclined axis. 

 This compound movement is not unlike planetary motion 

 in which, in addition to the revolution round its own axis, 

 the planet describes an elliptical path round the sun. 



But if the revolution of the plane of junction does not 

 keep pace with the torsional movement of the stem, a lag 

 will occur, the result of which will be an alternate accelera- 

 tion and retardation of autonomous torsion by gravi- 

 tational reaction, according as the A or the R flank of the 

 stem faces upwards. An oscillatory torsional movement, 

 positive and negative, will then take place. 



Combined Effects of Geotropic and Photic 



Stimulation 



Having ascertained the individual effects of stimulation 

 by light and by gravity of the different sides of the organ, it 

 is possible to predict their effects in different combinations, 

 taking examples which exhibit strongly contrasted effects. 

 Since the stimulus of gravity is unchangeable in direction, 

 the stimulus of light is superposed on it either from above 

 or from below. The horizontal stem is adjusted in darkness 

 till the rate of torsion is maximum, that is, in the A position 

 in which autonomous torsion and geotropic reaction are con- 

 cordant, represented by the symbol T -f G. By means of 

 two inclined mirrors, a beam of light from an arc-lamp is 

 then thrown upon it alternately from above and from 



below. In the first case, the action of light co-operates 



2 E 



