SLEEP OF LEAVES 167 



to cut off all sources of loss of water, during the 

 months when on account of the extreme coldness of 

 the soil, or the great heat and dryness of the air, 

 the roots would be unable to obtain from the ground 

 sufficient water for the leaves. 



4. If one goes out on any moonlit night, one 

 cannot fail to be struck with the different appear- 

 ance of a great many of our common plants whether 

 herbs, shrubs or trees owing to a change in the 

 position of the leaves or leaflets. 



The Tamarind and the Rain-tree (piTHECOLO- 

 BIUM SAM AN) so commonly planted along roads, are 

 good instances of this. Though by day their shade 

 is dense, at night the moon shines easily through. 

 In the Tamarind this is due to the leaflets closing 

 up along the rachides of the pinnae, and turning so 

 that each faces sideways, and the edge, not the flat 

 side, turns upwards. In the Rain-tree, the leaflets 

 fall downwards, so that they also face sideways, 

 not up and down. The rachis of each pinna falls a 

 little also. 



In ARACHIS HYPOG^EA (the Ground-nut) the leaflets 

 turn up a little so as to face sideways rather than 

 upwards. 



A very large number of plants with compound 

 leaves behave in a similar manner, the leaflets moving 

 so as to assume a position in which they are ver- 

 tical instead of being horizontal. 



If we examine the leaves of these plants we shall 

 nearly always find that the short stalk of each 

 leaflet and of each pinna or leaf, is swollen at the 

 base, and that this swollen pulvinus is the motile 



