OiiAP. VII. SLEEP OF LEAVES. 345 



approacli each other, so that the base of the central leaflet 

 overlaps the bases of the two lateral leaflets. They bend 

 np 60 much that thej press against the stem; and on looking 

 down on one of these 3 oung plants from vertically above, the 

 lower surfaces of the leaflets are visible ; and thus their upper 

 surfaces, in accordance with the general rule, are best protected 

 from radiation. Whilst the leaves on these young plants were 

 thus behaving, those on an old bush in full flower did not sleep 

 «t night. 



Trigonella Crdica resembles a Melilotus in its sleep, which will 

 be immediately described. According to M. Eoyer,* the leaves 

 of Medicago maculata rise up at night, and " se renversent un 

 peu de maniere a presenter obliquement au ciel leur face in- 

 ferieure." A drawing is here given (Kg- 139) of the leaves 

 of M. marina awake and asleep ; and this would almost serve 

 for Gytisus fragrans in the same two states. 



Melilotus (Tribe 3).— The species in this gemis sleep in a 

 remarkable manner. The three leaflets of each leaf twist through 

 an angle of 90°, so that their blades stand vertically at night 

 with one lateral edge presented to the zenith (Pig. 140). We 

 shall best understand the other and more complicated move- 

 ments, if we imagine ourselves always to hold the leaf with the 

 tip of the terminal leaflet pointed to the north. The leaflets in 

 becoming vertical at night could of course twist so that their 

 upper surfaces should face to either side; but the two lateral 

 leaflets always twist so that this surface tends to face the north, 

 but as they move at the same time towards the terminal leaflet, 

 the upper surface of the one faces about N.N.W., and that of 

 the other N.N.E. The terminal leaflet behaves differently, for 

 it twists to either side, the upper surface facing sometimes east 

 and sometimes west, but rather more commonly west than east. 

 The terminal leaflet also moves in another and more remarkable 

 manner, for whilst its blade is twisting and becoming vertical, 

 the whole leaflet bends to one side, and invariably to the side 

 towards which the upper surface is directed; so that if this 

 surface faces the west the whole leaflet bends to the west, until 

 it comes into contact with the upper and vertical surface of 

 the western lateral leaflet. Thus the upper surface of the 

 terminal and of one of the two lateral leaflets is well protected. 



The fact of the terminal leaflet twisting indifferently to either 



* • Annales des So. Nat. Bot." (5th series), ix. 1868, p. 368. 



