580 LIFE MOVEMENTS IN PLANTS 



algebraical summation of the following- factors: (1) the 

 thermo-geotropic action, (2) the immediate effect of photic 

 stimulus and (3) the after-effect of light. The leaf of 

 Mimosa has, moreover, as I sliall show, an autonomous 

 movement of its own. I shall take up the full consider- 

 ation of the subject in the following order : 



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

 will be described which demonstrates the effect of thermo- 

 geotropism in the diurnal movement of the leaf of 

 Mimosa. 



2. Autonomous pulsation of Mimosa. — The natural 

 pulsation of the plant is obscured by the paratonic effect 

 of external stimuli. I shall explain, the method by which 

 the natural pulsation of the leaf becomes fully revealed. 



3. The itmnediate effect of light. — This is not con- 

 stant, but will be shown to undergo a definite variation 

 with the intensity and duration of light. A very great 

 difficulty in the study of eft'ect of daylight at different 

 parts of the day is introduced on account of the absence 

 of any reliable recorder for measurement of fluctuation 

 of light. I shall describe a device which gives a con- 

 tinuous record of photic variation for the whole day. 



4. The after-effect of light. — The spasmodic fall of 

 the leaf of Mimosa towards the evening presents the 

 most difficult problem for solution. I shall first describe 

 the diurnal movement of another plant which presents 

 characteristics similar to those of Mimosa. I shall also 

 demonstrate the various after-effects of light at different 

 parts of the day. These results will offer the fullest 

 explanation of the sudden fall of the leaf towards 

 evening. 



As regards tho sudden fall of the leaf about evening, 

 Pfeffer regarded it as due to increased mechanical 

 moment of the secondary petioles moving forward on 



