CHAP. VII. SLEEP OF LEAVES. 391 



leaves rise so much at night that they may almost be said to 

 sleep. 



titrephium floribundum* (Graminese). The oval leaves are 

 provided with a pulvinus, and are extended horizontally or 

 declined a little beneath the horizon during the day. Those 

 on the upright culms simply rise up vertically at night, so 

 that their tips are directed towards the zenith. (Fig. 164.) 



Fig 164. 



Strephiurn floribundum : culms with leaves during the day, and when asleep 

 at night. Figures reduced. 



Horizontally extended leaves arising from much inclined or 

 almost horizontal culms, move at night so that their tips 

 point towards the apex of the culm, with one lateral margin 

 directed towards the zenith; and in order to assume this 

 position the leaves have to twist on their own axes through an 

 angle of nearly 90. Thus the surface of the blade always stands 

 vertically, whatever may be the position of the midrib or of the 

 leaf as a whole. 



The circumnutation of a young leaf (2 3 inches in length) was 

 traced during 48 h. (Fig. 165). The movement was remarkably 

 simple; the leaf descended from before 6.40 A.M. until 2 or 

 2.50 P.M., and then rose so as to stand vertically at about 6 P.M., 

 descending again late in the night or in the very early morning. 



* A. Brongniart first observed la Soc. Bot. de France,' torn. vii. 

 that the leaves of this plant and 1860, p. 470. 

 of Marsilea sleep : see ' Bull, de 



