214 



DESMODIUM. 



[CH. VIII 



horizon instead of 30° as at first. These changes are 

 graphically represented in the Power of Movement in 

 Plants, p. 334, fig. 135. The ordinates represent the 

 angle made with the vertical by the leaflet under obser- 

 vation ; a fall in the curve thus represents a drop of the 

 leaflet, 0° representing a vertically dependent position. 

 The dotted line represents temperature, and is to be read 

 in connection with the numbers 76° F....90°F. on the right 

 hand of the diagram. 



(262) Autonomous movements: Desmodium gyrans. 



The leaf of Desmodium gyrans represented in 

 consists of one large and a pair of very 

 minute leaflets. It is these which execute 

 the movements for which the plant is 

 celebrated. The point of each leaflet de- 

 scribes a rough sort of circle or ellipse, but 

 the movement being exceedingly jerky and 

 irregular the circular course is not obvious. 

 The movement does not occur at all unless 

 the temperature' is about 22° C, and may 

 be remarkably stimulated by a higher 

 temperature, thus at 40° C. the circles are 

 made at the rate of about two in three 

 minutes. The experiment may be performed 

 by immersing a leaf in cool water which is 

 gradually heated. 



fig. 43 



1 This is only true of full-grown plants. Seedlings move at lower 

 temperature : see Power of Movement in Plants, p. 362. 



