278 CHAP. XXV. DIURNAL MOVEMENT OF MIMOSA 



Experiment 152. After-effect at maximum. — Continued 

 exposure to light for 18 minutes induced maximum posi- 

 tive curvature, as seen in the upper part of the curve 

 becoming horizontal. On the withdrawal of light, there 

 was recovery to the original position of equilibrium 



(fig. 162). 



Experiment 153. After-effect at post-maximum. — A fresh 

 specimen of the plant was taken for this experiment ; it 

 exhibited maximum positive curvature after an exposure of 

 20 minutes ; continued exposure for a further period of 

 17 minutes produced complete neutralisation, as indicated 

 by return to normal position. Withdrawal of light at this 

 point gave rise to a rapid down-movement (fig. 163) below 

 the equilibrium position. 



It is now possible to give a full explanation of the 

 different phases of diurnal movement of the leaf of Mimosa. 

 The fall of the leaf from its highest position commences 

 at thermal dawn at 6 a.m. in the morning and continues till 

 thermal noon at 2 p.m. ; this is the thermo-geotropic reaction 

 due to rise of temperature. In the forenoon the photo- 

 tropic reaction is positive, and the fall of the leaf, due to rise 

 of temperature, is effected in opposition to the response to 

 light. As the temperature begins to fall after 2 p.m., the 

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

 ing factor would continue its up-movement till next morning. 

 But light undergoes rapid diminution after 5 p.m., the after- 

 effect of which is manifested as an ' overshooting ' of the 

 leaf in a downward direction. This fall continues till about 

 9 P.M., after which the leaf erects itself under the thermo- 

 geotropic action of falling temperature, the maximum 

 erection being attained at the thermal dawn at about 6 a.m. 

 next morning. 



The peculiarity of the diurnal curve of Mimosa has been 

 shown to be due to the sensitiveness of the leaf to both 

 light and variation of temperature. This conclusion is veri- 

 fied by experiments with other plants similarly sensitive to 

 both photic and thermal variation. Nothing could be more 



