DIURNAL MOVEMENT OF MIMOSA 581 



the withdrawal of light. I shall, however, in the course of 

 this paper show, that the characteristic movements occur 

 even after complete removal of the sub-petioles. In the 

 following experiment, carried out with the intact plant, 

 the effect of possible variation of weight is completely 

 eliminated. In spite of this, the diurnal movement ex- 

 hibited its characteristic phases including sudden movement 

 in the evening. 



The experiment I am going to describe will exhibit 

 the diurnal curve obtained by an entirely different 

 method, and will clearly exhibit the thermo-geotropic 

 effect, as well as the immediate and after-effect of light. 



DIURNAL VARIATION OF GEOTROPIC TORSION. 



I have shown that the pulvinus of Mimosa, subjected 

 laterally to the action of stimulus of gravity, exhibits 

 a torsional response. When the Mimosa plant is laid 

 sideways, so that the plane of separation of the upper 

 and lower halves of the pulvinus is vertical, geotropic 

 stimulus acts laterally on the two halves of the differ- 

 entially excitable pulvinus. When the less excitable upper 

 half is to the left of the observer (see Fig. 179), the 

 responsive torsion under geotropic stimulus will be clock-wise, 

 the less excitable upper half of the pulvinus being thereby 

 made to face the vertical lines of gravity. When the plant 

 is turned over to the other side (the less excitable upper 

 half being now to the right of the observer) the induced 

 tor3ion will be counter clock-wise. The response is there- 

 fore determined by the directive action of stimulus of 

 gravity. Light has also been shown to give rise to 

 torsion (p. 400). Light acting in the same direction 

 as the stimulus of gravity, i.e., from above, enhances the 

 rate of torsion, the curve of response being due to the 

 joint effects of light and gravity. 



