412 CH. XXXVI. TORSION UNDER UNILATERAL STIMULATION 



on the flanks by A and R. l^he neutral sides are N, N'. 

 If a stimulus, light for example, acts from above on N, an up- 

 movement due to positive phototropism will occur ; if from 

 below, a more pronounced down-movement due to the 

 greater excitability of the lower half. No torsional move- 

 ment will, however, occur on stimulation of N or N'. But 

 the results will be different when stimulation is applied 



L L 



i 1 



Diagrammatic representation of torsional response of anisotropic 

 organs to unilateral stimulation by light l. 



Ideal transverse sections of the organs: n, n', ventral and dorsal 

 sides ; A, R, flanks ; a-r, plane of junction of the two diverse 

 longitudinal halves, of which the more excitable is shaded. 



Fig. 224. Pulvinus of Mimosa: Stimulation of sides n, n', 

 evokes no torsional response ; stimulation of flank a induces 

 positive (anti-clockwise) torsion (left curved arrow) ; stimu- 

 lation of flank R induces negative (clockwise) torsion (right 

 curved arrow). 



Fig. 225. Twisting Stem (supposed erect) : Normal torsion, t, un- 

 affected by stimulation of sides n, n' ; increased on stimulation 

 of flank A, decreased on stimulation of flank r. 



laterally to either flank A or R. Lateral stimulation of 

 flank A induces a torsional response against the hands of 

 the clock, the less excitable upper half being moved so as 

 to face the stimulus. This will be distinguished by a 

 positive sign. Lateral stimulation of the opposite flank R 

 induces a negative or clockwise torsion. 



t 



Torsional Response of the Twisting Stem 



The effects of unilateral stimulation by light on the 

 autonomous torsion of the twisting stem are much the 



