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 



Via. 22. — ^Ebect Shoot of Maple, with Leaves standing otrr on all 

 Sides oe Stem. (Rbditoed. After Kebner.) 



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 



