DIURNAL THERMONASTIC MOVEMENT 549 



hiuul, the direction of movement be determined by 

 the paratonic effect of external stimulus, gravity for 

 example, an up-movement due to fall of temperature will 

 continue to be an up-movement, whether the plant be 

 held in its normal or inverted position. The responsive 

 movement in relation to the plant will, however, be 

 different ; the closure movement will, on inversion, be 

 reversed into a movement of opening. The reversal of 

 closure into an opening movement or vice versa will thus 

 be a test of the paratonic effect of external stimulus. 



We may thus distinguish thermonastic from thermo- 

 geotropic action by the following tests : 



1. Thermonastic movements are, generally speaking, due 

 to differential growth, and are therefore characteristically 

 present in growing organs. Thermo-geotropic action is 

 independent of growth. 



2. Thermonastic moveme'nts take place in relation to 

 the plant, and is not determined by external force of 

 a directive nature. Opening or closing movement will 

 remain unchanged after inversion of the plant. But thermo- 

 geotropic reaction being determined by the external stimu- 

 lus of gravity, becomes reversed on inversion of the plant. 

 Closure movement is thus converted into opening move- 

 ment, and vice versa. 



I shall now take up the diurnal movement due lo 

 variation of growth induced by change of temperature. Of 

 this the flower of Nyniphcea furnishes an example. 



DIURNAL MOVEMENTS OF Nymphcea. 



The flower of Nymphcea remains closed during the day 

 and opens at night. Figures 195 and 196 are from 



