CHAPTER XXXVI 



EFFECT OF UNILATERAL STIMULATION ON AUTONOMOUS 



TORSION 



The experiments already described show how diverse are 

 the influences which modify the autonomous torsional 

 movement of plants. I describe some of the important 

 factors in operation during the course of the day and their 

 individual effects. 



Effect of variation of turgor. — An increase of turgor 

 induces an enhancement of the normal rate of torsion, 

 while a diminution causes a retardation culminating in a 

 reversal. Turgor may be increased {a) by enhanced rate 

 of ascent of sap, or [h) by increased moisture in the air 

 (dependent on the direction of the wind) ; diminished turgor 

 is, on the other hand, produced by drought due to tran- 

 spiration increased by dry air, by high temperature, and 

 by strong sunlight. 



Effect of variation of temperature. — Rise of temperature 

 up to an optimum enhances the rate, while increasing cold 

 induces a retardation. A rapid variation of temperature 

 acts as a stimulus and induces a reversal of direction. 



Effect of stimulation. — The following is true of all modes 

 of stimulation. Feeble stimulation causes an enhancement 

 in the normal rate, while strong stimulation causes a 

 retardation culminating in a reversal. The effect of stimu- 

 lation is modified by its point of application, direct stimu- 

 lation producing one effect, indirect stimulation giving rise 

 to an effect precisely the opposite. 



Effect of light.— The intensity of light during the day 

 undergoes continuous variation. It increases from dawn 

 to noon and dechnes from noon to evening. The effect of 



