590 LIFE MOVEMENTS IN PLANTS 



leaf in the normal position with its upper surface at right 

 angles to the incident rays. Thus, on being set free from 

 the strong action of light, the leaf moves in accordance 

 with the preceding condition of tension ; and as this is low 

 the leaf falls, soon to rise again as the tension increases in 

 prolonged darkness. 



The above explanation presupposes: (1) that the tension 

 was continuously decreasing till the evening, and (2) that as 

 soon as the phototropic restraint which held the leaf up 

 was removed it fell down in accordance with the prevailing 

 tliminished tension. 



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 (Fig. 210). As the 

 natural tendency of the leaf was to erect itself, the removal 

 of phototropic restraint cannot therefore induce a move- 

 ment of fall. 



As regards the factor of light, the effect in the after- 

 noon is a down-movement on account of transverse con- 

 duction of excitation ; but the leaf is prevented from exhibit- 

 ing this down-movement by the thermo-geotropic up-move- 

 ment due to fall of temperature after the thermal noon. I 

 shall presently describe experiments on the pure effect of 

 light, which will show that the action of continued photic 

 stimulus induces a down-movement of the leaf in the 

 afternoon. 



The results of experiments that have been described show 

 that the sudden fall of the leaf in the evening could not 

 be due to : 



(1) increased mechanical moment, 



(2) the natural tendency of the leaf to fall towards 



evening against phototropic action by which the 

 leaf is held up. 



