GEOTROPIC TORSION 273 



Referring to the first point, an inspection of the diurnal 

 curve of Mimosa shows that the leaf had no natural tendency 

 to fall towards the evening. There was, on the contrary, 

 a movement of erection, on account of fall of temperature 

 after the thermal noon [cf. fig. 155). As the natural ten- 

 dency of the leaf was to erect itself, removal of phototropic 

 restraint cannot be the cause of the movement of fall. 



The following experiment not only exhibits the diurnal 

 curve of an intact plant, but also clearly demonstrates 

 the thermo-geotropic effect, as well as the immediate and 

 the after-effect of light. 



Diurnal Variation of Geotropic Torsion 



I have already demonstrated the torsional response 

 under unilateral stimulation by light. I shall also show in a 

 later chapter that similar torsional response is obtained under 

 the stimulus of gravity (p. 307). 



When a Mimosa plant is laid down sideways, so that the 

 plane of junction between the upper and the lower halves of 

 the pulvinus is vertical, geotropic stimulus acts laterally on 

 the two differentially excitable halves of the pulvinus. When 

 the less excitable upper half is to the left of the observer, the 

 responsive torsion under geotropic stimulus is clockwise, 

 the less excitable upper half of the pulvinus being thereby 

 made to face the vertical lines of gravity. When the plant 

 is turned over to the other side (the less excitable upper 

 half being now to the right of the observer), the induced 

 torsion is counter-clockwise. It has been shown that lateral 

 stimulation by light gives rise to torsion ; when light acts in 

 the same direction as the stimulus of gravity, i.e. from above, 

 there is an enhancement of the rate of torsion, the resulting 

 curve of response being due to the joint effects of light and 

 gravity. 



Experiment 149. — I obtained a 24 hours' record of the 

 variation of torsional response in the leaf of Mimosa, com- 

 mencing with thermal noon at 2 p.m. It is to be borne in 



