CHAPTER XXV 



THE DIURNAL MOVEMENT OF THE LEAF OF MIMOSA 



The thermo-geotropic record of diurnal movement of plants 

 described in a previous chapter consists of an up-curve from 

 thermal noon to thermal dawn, and of a down-curve from 

 thermal dawn to thermal noon. The responding organ, 

 whether an inclined stem or a horizontally placed petiole, 

 underwent an erection during the decline of temperature, 

 and a fall with the rise of temperature. The diurnal record 

 of the Mimosa leaf appears, however, to be totally different ; 

 this apparent difference arises from the presence of fresh 

 complicating factors, inasmuch as the leaf is sensitive not 

 only to variation of temperature but also to changes of light. 



Experiment 147. Diurnal record of Mimosa. — I took 

 the diurnal record of Mimosa (fig. 155) for 24 hours, 

 commencing at 2 p.m., which is the thermal noon. The 

 summer and winter records are essentially the same, the 

 only difference being the greater vigour of movement 

 exhibited by summer specimens. The diurnal movement 

 of the leaf is very definite and characteristic, for the curves 

 obtained for several years in succession are found to be 

 quite concordant. The record may conveniently be divided 

 into four phases. 



First phase. — The leaf erects itself after thermal noon 

 until 5 or 5.30 p.m. The temperature, it should be re- 

 membered, is undergoing a fall during this period. 



Second-phase. — There is a sudden fall of the leaf in the 

 evening, which continues till 9 p.m. or thereabout. 



Third phase. — The leaf erects itself till thermal dawn at 

 about 6 a.m. next morning. 



