512 



LIFE MOVEMENTS IN PLANTS 



force of the magnet will be diminished under rise of 

 temperature with coneomitapt diminution of deflection of 

 the needle and the torsion of the wire. Fall of tempera- 

 ture, on the other hand, will cause an increase of deflec- 

 tion and of torsion. The physical illustration given above 

 will help us to understand how the physiological effect 

 of variation of temperature may bring about changes in 

 geotropic curvature and torsion, 



TROPIC EQUILIBRIUM UNDER VARYING INTENSITIES OF 

 STIMULUS. 



The following experiment will show that the position 

 of tropic equilibrium is not fixed but subject to variation 

 under changes of effective stimulation. 



Experiment 198. — I have explained how a maximum 

 tropic curvature Is Induced under continued action of light. 

 Employing the pulvlnus of Erythrlna indica I applied 

 light on the upper half of the pulvinus : (1) of medium 



Fig. iSo.^Eifect of variaiiou of intensity of light on pliototiopic eqii.libriiim. 

 Increase of intensity of light from L to L' produces hii increased positive cur- 

 vature and a new state of balance. Diminished intensity of light / brings about 

 a new balance at a lower level. The cessation of light (f within a circle) 

 restores the notnial position of the organ. 



intensity L, (2) of strong intensity L'', and (;>} of feeble 

 intensity /. The source of light was an arc lamp ; he 



