272 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA 



it recovered its normal activity after a period of 3 hours 

 or so. The diurnal record of the leaf was commenced shortly 

 after i P.M. ; it will be noticed that the leaf, though deprived 

 of the weight of its sub-petioles, still exhibited a sudden 

 fall at about 5 p.m. (fig. 158). The fall of the leaf cannot 



therefore be due to increased 

 mechanical moment. The 

 effect of weight was, more- 

 over, eliminated in another ex- 

 periment on torsional response 

 (Experiment 149), which also 

 exhibited a sudden movement 

 of the leaf after 5 p.m. 



Pfeffer, in his * Entste- 

 hung der Schlafbewegung ' 

 (1907), has offered another 

 explanation of the sudden fall 

 of the leaf of Mimosa. This, 

 according to him, is not the 

 direct effect of diminished in- 

 tensity of light in the evening, 

 but is due to the release of 

 the leaf from the phototropic 

 action of light, which, so long 

 as it is sufficiently intense, 

 holds the leaf in the normal 

 position with its upper surface 

 at right angles to the incident rays. 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 (i) that the tension 

 continuously decreases till the evening ; and (2) that as soon 

 as the phototropic restraint which holds the leaf up is 

 removed, it falls down in accordance with the prevailing 

 diminished tension. 



Fig. 158. Recordof leaf of Mimosa 

 after removal of sub-petioles. 

 The leaf fell up to 2 .30 p.m., and 

 then rose till 5 p.m., after which 

 there was a spasmodic fall. 



Successive dots at intervals of 

 15 minutes. 



