430 CHAP. XXXVII. GENERAL REVIEW 



stems and by adult leaves of plants. In all these cases an 

 erectile movement is exhibited from thermal noon to thermal 

 dawn, and a movement of fall from thermal dawn to thermal 

 noon. The predominant effect of variation of temperature 

 on the movement is demonstrated by the fact of the abolition 

 of the movement under constant temperature (p. 240). The 

 effect of the stimulus of gravity in inducing the anisotropy 

 which determines the characteristic movement, is proved by 

 the effect on the diurnal record of inversion of the plant 

 (p. 244). The action of thermo-geotropism as an inde- 

 pendent factor is proved by its persistence even after the 

 abolition of transpiration (p. 248). The thermo-geotropic 

 movement is in many ways analogous to the thermonastic 

 movement, (p. 249). A wider generalisation of thermonasty 

 is reached by the inclusion under it of the movements of 

 full-grown organs which have been rendered anisotropic 

 by the stimulus of gravity. 



Diurnal movement due to alternation of light and darkness. 

 The effect of light on the leaflet of Cassia alata is predomi- 

 nant as compared with that of temperature. The leaflets 

 begin to close when light is undergoing rapid diminution 

 after 5 p.m., the closure being completed by 9 p.m. This is 

 also partially due to the after-effect of light. The leaflet 

 remains closed till 5 a.m. next morning, after which it begins 

 to open with the dawning light and becomes fully expanded 

 by 9 a.m. The large terminal leaflet of Desmodium exhibits 

 diurnal movements similar to those of Cassia (p. 214). 



Diurnal movement of leaf of Mimosa.—The complexity 

 of the diurnal movement of the leaf arises from the fact 

 that it represents the algebraical sum of the effects of three 

 fluctuating factors : (i) the thermo-geotropic action ; 

 (2) the immediate effect of light ; and (3) its after-effect. 

 With the exception of a small part of the curve in the 

 evening, the diurnal curve of the plant is essentially similar 

 to the typical thermo-geotropic curve, the leaf exhibiting 

 an erectile movement from thermal noon to thermal dawn, 

 and a fall from thermal dawn to thermal noon (p. 269). 



