CONSTITUENT FACTORS 269 



typical thermo-geotropic curve and the curve of IMimosa is 

 not so great as appears at first sight, with the exception of 

 the spasmodic fall in the evening. I will presently explain 

 the reason of the sudden fall in the evening, and of the 

 multiple pulsations in the forenoon. 



It is possible to trace a continuity between the typical 

 thermo-geotropic reaction and the characteristic diurnal 

 movement of the leaf of Mimosa which is affected by light. 

 The young leaves which expand at the beginning of spring 

 take some time to become adjusted to the diurnal variation. 

 There are two intermediate stages through which the leaves 

 pass before they exhibit their characteristic diurnal curve. 

 Slow rhythmic pulsations are at first seen to occur during 

 day and night. At the next stage the leaves exhibit the 

 diurnal movement of fall from thermal dawn to thermal 

 noon, and that of erection from thermal noon to thermal 

 dawn next morning, the record being in every way similar 

 to the typical thermo-geotropic curve. It is only at the 

 final stage of development of the leaf that there is the 

 spasmodic fall in the evening, which will be shown to be a 

 characteristic post-maximum after-effect of light. 



The complexity of the diurnal movement of Mimosa 

 arises from the fact that there are three factors whose 

 fluctuating effects are different at various hours of the day. 

 The position at any particular hour results from the algebraical 

 summation of the effects of the following factors : (i) the 

 thermo-geotropic reaction ; (2) the autonomous pulsation of 

 the leaf ; (3) the immediate effect of light ; and (4) its 

 after-effect. I will take up the detailed consideration of 

 the subject in the following order : 



1. The thermo-geotropic reaction. — A crucial experiment 

 is described later which demonstrates the thermo-geotropic 

 effect in the diurnal movement of the leaf. 



2. Autonomous pulsation of Mimosa. — The natural pulsa- 

 tion of the leaf is obscured by the paratonic effect of 

 external stimulation. The occurrence of pulsatory response 

 in the morning record {see fig. 155) led me to search for 



