THE LEAVES 85 



very damp or rainy situations secure this stream by 

 building very large leaves. The banana is a com- 

 mon example, and many other tropical plants show 

 leaves several times as large. 



108. Sleep movements. — If the position of the 

 leaflets of the bean or locust are noticed at sunset or 

 later, it will be seen that their blades are placed 

 nearly vertically. This position is supposed to be a 

 method of preventing the leaf from cooling so rapidly 

 as it would if held horizontal, and also from accumu- 

 lating a layer of dew which would hinder transpira- 

 tion. (See also § 93.) 



109. Velvety surfaces. — If the leaves of some of 

 the species of begonias are examined, they will be 

 found to show an upper surface that is velvety to 

 the touch, and when examined with a hand lens 

 appear to have an immense number of small pro- 

 jections. This is due to the fact that the cells of the 

 upper side of the leaf are all extended in little cones. 

 The cones entrap the rays of sunlight as it were, and 

 refract them so that they warm the leaf and increase 

 the* transpiration over what it would be if the surface 

 were smooth. This device is also exhibited by the 

 petals of violets, pansies, and primulas. 



