596 LIFE MOVEMENTS IN PLANTS 



the light was turned off. The after-effect was a persis- 

 tence of previous movement followed by recovery (Fig. 

 218). 



After-effect at maximu'tn : Experiment 228. — Continued 

 action of light for 18 minutes induced maximum positive 

 curvature as seen in the upper part of the curve becoming 

 horizontal. On the stoppage of light, there was a recovery 

 to the original position of equilibrium (Fig. 219). 



After-effect (it post-ma.vimnm : Experimetit 229. — A 

 fresh specimen of plant was taken for this experiment; it 

 exhibited maximum positive curvature after an exposure 

 of 20 minutes ; continuation of light for a further period 

 of 17 minutes produced complete neutralisation. Stoppage 

 of light at this point, gave rise to a rapid down-move- 

 ment (Fig. 220) below the equilibrium position. 



The experiments that have been described show that 

 the rapid fall of the leaf of Mitm'sa in the afternoon is 

 due to 'over-shooting' which is the after-effect of pro- 

 longed action of light. 



We are now in a position to give a full explanation 

 of the different phases of diurnal movement of the leaf 

 of Mimosa. The fall of the leaf commences from its 

 highest position at thermal-dawn at 6 a.m. in the 

 morning and continued till the thermal-noon at 2 p.m. 

 This is the therrao-geotropic reaction due to rise of tempera- 

 ture In the forenoon the phototropic action is positive, 

 and the fall of the leaf, due to rise of temperature, is 

 brought about in opposition to the action of light. The 

 temperature begins to fall after 2 p.m. and the leaf 

 begins to erect itself, and in the absence of any dis- 

 turbing factor would have continued its up-movement till 

 next morning. But light undergoes a rapid diminution 

 after 5 p.m. and the after-effect of light is an 'over- 

 shooting' ^in a downward direction. This fall continues 



