CHAP. V tl. 



SLEEP OF LEAVES. 



365 



Mimosa the pulvinus has been greatly reduced, we do not 

 know. Nevertheless, it deserves notice that the reduction of 

 the leaflets in these two genera has apparently been effected by 

 a different process and for a different end ; for with the Mimosa 

 the reduction of the inner and basal leaflets was necessary from 

 the want of space; but no such necessity exists with Desmo- 

 dium, and the reduction of its lateral leaflets seems to have 

 been due to the principle of compensation, in consequence of 

 the great size of the terminal leaflet. 



Uraria (Tribe 6) and Centrosema (Tribe 8). The leaflets of 

 Uraria lagopus and the leaves of a Centrosema from Brazil 

 both sink vertically down at night. In the latter plant the 

 petiole at the same time rose 16^. 



Amphicarpcea monoica (Tribe 8). The leaflets sink down ver- 

 tically at night, and the petioles likewise fall considerably. 



Fig. 151. 



Amphicarpcea monoica : circumnutation and nyctitropic movement of leaf 

 during 48 h. ; its apex 9 inches from the vertical glass. Figure reduced 

 to one-third of original scale. Plant illuminated from above; temp. 

 17-18 C. 



A petiole, which was carefully observed, stood during the day 

 25 above the horizon and at night 32 below it; it therefore 

 fell 57. A filament was fixed transversely across the terminal 

 leaflet of a fine young leaf (2J inches in length including the 



