RESPONSE OF THE TWISTING STEM 413 



same as those observed with the anisotropic pulvinus, but 

 with the difference that the torsion induced by the unilateral 

 action of light on the different sides of the stem is algebraically 

 summated with its autonomous torsion. This is made clear 

 on inspection of lig. 225, in which the shading indicates 

 the more excitable half. The straight arrows indicate the 

 direction of the light incident on the four sides, N, A, N', 

 and R. The arrow T indicates the natural torsion. Light 

 incident on the neutral sides N or N' induces no torsional 

 effect, but there may be a slight rectilinear movement up or 

 down due to positive phototropism. The torsion induced 

 by light falling upon either flank, A or R, is maximum, and 

 is algebraically summated with the natural torsion. That 

 induced by light falling on flank A co-operates with the 

 natural torsion : that induced by light falling on flank R 

 opposes the natural torsion and may even overpower it, so 

 that it is not only retarded but the direction is reversed. 



The results obtained show that there is no ground for 

 the assumption that twining plants possess a specific sensi- 

 bility to light different from that of ordinary plants. The 

 variation of torsional movement due to the action of uni- 

 lateral light on different sides of the organ results from the 

 differential excitability of the organ, the characteristics of 

 which conform to the Law of Torsional Response in all 

 anisotropic organs. 



A few words may be added in regard to the conditions 

 for securing the best result. These are : (i) quick determina- 

 tion of the maximum position A, and (2) completion of the 

 whole series of observations for N, R, and N' within a short 

 time. For the plane of junction is not stationary but 

 slowly revolving, and can only be regarded as stationary 

 for a relatively short time. It is also desirable to take only 

 a short length of the stem for the observation of torsional 

 movement, so that the plane of junction of the two halves 

 of the organ is vertical and straight. 



I shall next show that the effect of unilateral stimulation 

 by gravity is essentially similar to that induced by light. 



