588 LIFE MOVEMENTS TN PLANTS 



half, and the neutralisation is due to the full contraction 

 of the upper half antagonised by slight contraction of the 

 lower half. The horizontal position of the leaf under light 

 is therefore the result of balance of the two antagonistic 

 reactions. If the incident light be very strong, the more 

 intense transmitted excitation induces greater contraction of 

 the lower half, and bring about a resultant down-move- 

 ment {cf. p. 331). 



Let us consider the effect of daily variation of light on 

 Mimosa ; we have here to take account both of intensity 

 and duration. The intensity of light is seen to undergo a 

 continuous increase which reaches a climax at noon ; it 

 then begins to decline slowly and the diminution of inten- 

 sity of light is very abrupt after 5 p.m. 



Under natural conditions the following phototropic effects 

 are observed during the course of the day : light acting 

 from above induces an up-movement of the leaf ; but 

 this is opposed by the thermo-geotropic fall of the leaf due 

 to rise of temperature. As the two opposing effects are nearly 

 balanced, any fluctuation of the relative intensity of the 

 two gives rise to the pulsatory movements often seen in 

 the forenoon ; the Mimosa leaf has moreover an autono- 

 mous movement of its own. Under continued action of light 

 neutralisation begins to take place after 1 p.m. {cf. 

 Expt. 135). Later in the day the phototropic effect may 

 become negative ; reversal into this negative takes place 

 under the joint action of intensity and duration of light ; 

 it takes place earlier under strong, and later under feeble, 

 light. 



THE EVENING SPASMODIC FALL OF THE LEAF, 



I shall now deal with the difficult problem of the sudden 

 fall of the leaf after 5 p.m. Pfeffer regarded this sudden 

 fall in the evening as due to the increased mechanical 

 moment of the secondary petioles moving forward on the 



