186 



PRACTICAL COURSE IN BOTANY 



moisture, by means of grooves, channels, and taper-pointed 

 leaves, which act as natural gutters and drain pipes. 



203. The fall of the leaf. This is, in effect, an adjust- 

 ment to change of temperature, but that it is not directly due 

 to cold is shown by Exp. 75, and also by the fact that leaves 

 in the tropics and those of evergreens, while they do not fall 

 at stated periods like the bulk of the foliage in the temperate 

 zones, are cut off just the same and replaced by new ones, 



whenever, for any 

 reason, they are un- 

 able to perform their 

 function. In cold 

 climates they fall at 

 the approach of 

 winter, not because 

 the frost loosens 

 them, but because 

 the roots are not able 

 to absorb enough 

 moisture to supply 

 them with material 

 for making food. 

 The needles and the 

 scale-leaves charac- 

 teristic of evergreens 

 in cold regions are 

 enabled to persist indefinitely by reason of their contracted 

 surface. This prevents the dissipation of moisture and affords 

 no lodging for the accumulations of sleet and snow that 

 would otherwise cumber and perhaps break the boughs with 

 their weight. Trees and shrubs that shed their leaves in win- 

 ter are said to be deciduous, from a Latin word meaning " to 

 fall." Can you mention some advantages of the deciduous 

 habit to a plant with broad, expanded leaves, growing in 

 a cold climate? 



The mechanical means by which the leaf fall is accorn- 



FIG. 254. Fallen leaves. Notice how they cover 

 the ground with a warm mulch, protecting the soil 

 from denudation, and the roots and seeds from frost. 



