AUTONOMOUS TORSION 433 



this region has been indirectly stimulated by a transmitted 

 impulse (p. 369). The stimulus of gravity is perceived at the 

 root-tip and the responsive movement takes place at the 

 distant growing region. In contrast with this is the fact 

 that the growing region of the shoot is both sensitive and 

 responsive to geotropic stimulation. As the effects of 

 direct and indirect stimulation on growth are antithetic, 

 the responses of shoot and root cannot but be of opposite 

 sign. There is thus no necessity for postulating two different 

 irritabilities for the shoot and the root ; the difference of 

 response is not due to any inherent quality, but to the mode 

 of stimulation, direct in the one case, indirect in the other. 

 Difference between effects of geotropic and photic stimula- 

 tion of the root. — In phototropism, the energy of light 

 directly stimulates the responding cells of the growing 

 region, causing a curvature towards the stimulus. In 

 geotropism, the force of gravity is in itself inoperative ; 

 it is only through the weight of the cell-contents of the 

 perceptive tissue that stimulation is effected at the root-tip. 

 The impulse there initiated travels to and indirectly stimu- 

 lates the growing region which responds by a curvature 

 away from the incident stimulus. 



Autonomous Torsion 



It has been shown that the twisting growing stem is 

 anisotropic, two longitudinal halves at any given moment 

 being unequally active, like the two halves of the pulvinus 

 of Mimosa (p. 376). The plane of demarcation separating 

 the two physiologically diverse halves in Mimosa is fixed ; 

 but in the stem the plane slowly revolves, passing from 

 segment to segment (p. 393). The resulting torsional growth 

 brings about the movement of circumnutation (p. 381). 



Effect of variation in the rate of ascent of sap. — Enhance- 

 ment of the rate of ascent induces an increase in the rate 

 of torsion, while depression of ascent or plasmolytic with- 

 drawal of water induces retardation of the rate, culminating 

 in actual reversal of the direction of torsion (p. 386). 



2 F 



