32 



A TEXT-BOOK OF BOTANY 



Motile leaves also shift their positions throughout the day 

 in reference to light; and at night a very characteristic 

 position is assumed, once called a sleeping position, but 

 better night position. The contrast between the day and 

 night positions of leaves such as those of the sensitive plants, 

 and even of the common white clover, is quite striking (Fig. 

 28). These night positions, produced by the withdrawal 

 of light, may be induced by placing plants in darkness; and 

 experiments will show that the power is more common than 

 is generally supposed. Just what it means is not clear. 

 The suggestion has been made that the night position is a 

 protection against danger from the loss of heat, but it may 

 have no such meaning. 



(10) Rain is a menace to leaves, for if the water soaks 

 in and fills up the air spaces and stomata, communication 

 with the air is cut off; hence leaves shed water with remark- 

 able promptness, partly by their positions, partly by their 

 structure. Some of the structures 

 that prevent the rain from soaking 

 in are a smooth epidermis, a cuticle, 

 a waxy deposit, felt-like coverings, 

 overlapping scales, etc. In the 

 rainy tropics it is very common for 

 the sunken veins and ribs of the 

 leaves to form a sort of drainage 

 system for carrying off water, the 

 main channel lying along the midrib, 

 which terminates in a long, spout- 

 like point (Fig. 319). 



18. Fall of leaves. Many shrubs 



FIG. 29. Diagrammatic sec- 

 tion through a node of horse- 

 chestnut, showing the posi- /. , 



tionof the cuttin g -off layer and trees of temperate regions lose 

 () and the vascular bundle their leaves at the approach of 



(6) not cut through. 



winter, or even earlier, putting out 

 This is called the 

 While 



new leaves in the following spring. Tnis is 

 deciduous habit , and it is an adaptation to climate, 



