EFFECT OF STIMULATION ON TORSION 435 



in an inverted position thus offers an important confirmation 

 of the statoHth-theory. 



Effect of Unilateral Stimulation on Autonomous 



Torsion 



Action of light. — The effects of hght on different sides 

 of the twisting organ are unequal. There are two sides, 

 N, N', which are neutral, exhibiting no responsive variation 

 of torsion under stimulation. The two flanks A and R, at 

 right angles to N, N^ are, however, highly sensitive, stimula- 

 tion of A inducing maximum acceleration, while stimulation 

 of R causes maximum retardation (p. 410). All the varied 

 results are included under the Law of Torsional Response 

 that has been established : A differentially growing organ 

 laterally stimulated undergoes a torsion by ivhich the less 

 excitable side is made to face the stimulus. The i^iduced 

 torsional movement is algebraically summated with that of the 

 existing autonomous torsion. 



Action of stimulus of gravity. — The effects under geotropic 

 stimulation are precisely similar to those of photic stimulation 

 — that is to say, there are two sides, N, N', which are neutral, 

 whereas the effect of stimulation of flank A causes a maxi- 

 mum enhancement of the rate of torsion, and stimulation of 

 flank R induces a maximum retardation culminating in 

 reversal. 



Combined effects of geotropic and photic stimulation. — 

 The experimental results prove that the effect of geotropic 

 stimulation is algebraically summated with that of photic 

 stimulation, thus inducing a variation in the rate of torsion 

 (p. 415). Light, when sufficiently strong, acting in op- 

 position to gravitational stimulus, may thus induce a 

 reversal of the direction of torsion. 



In conclusion it may be stated summarily that the 

 experimental evidence adduced in the foregoing chapters, 

 as to the nature and conditions of the growth-movements, 

 makes it possible to explain their mechanism in a simple 

 and comprehensive manner. The growth-movements are 



