70 BRITISH PLANTS 
tendency is to avoid the direction of strongest light 
(p. 46). 
Sleep-Movements in Leaves (nyctitropism).—In some 
plants leaves change their position periodically in rela- 
tion to light. The trifoliate leaves of the wood-sorrel 
close together at night (Fig. 24). The three leaflets droop 
into a vertical position round the petiole, forming a kind 
of moist chamber with the lower surfaces of the leaves 
facing inwards. In the clover the two basal leaves rotate 
aay 
SSS") see 
NY 
Iiq. 22.—Erercr SHoot or Marie, witH LEAVES STANDING OUT ON ALL 
Sms or Stem. (Repucep. § Arrer KERNER.) 
so as to bring their under surfaces together in a vertical 
plane, while the terminal leaflet comes down over the top, 
closing the chamber thus formed. The position assumed 
at night is one which tends to diminish the loss of water 
from the lower surfaces of the leaves. On a clear night 
the earth quickly cools, and with it the plant. Absorption 
is therefore diminished, but this is counteracted by the 
night-position assumed by the leaves. The day-position 
is of no service to a plant at night, and so they move into 
