MINUTE STRUCTURE OF LEAVES 175 



187. The Fall of the Leaf. In the tropics trees retain 

 most of their leaves the year round ; a leaf occasionally 

 falls, but no considerable portion of them drops at any 

 one season. 1 The same statement holds true in regard to 

 our cone-bearing evergreen trees, such as pines, spruces, 

 and the like. But the impossibility of absorbing soil-water 

 when the ground is at or near the freezing temperature 

 (Exp. XVII) would cause the death, by drying up, of 

 trees with broad leaf-surfaces in a northern winter. And 

 in countries where there is much snowfall, most broad- 

 leafed trees could not escape injury to their branches from 

 overloading with snow, except by encountering winter 

 storms in as close-reefed a condition as possible. For 

 such reasons our common shrubs and forest trees (except 

 the cone-bearing, narrow-leafed ones already mentioned) 

 are mostly deciduous, that is they shed their leaves at the 

 approach of winter. 



The fall of the leaf is preceded by important changes 

 in the contents of its cells. 



EXPERIMENT XXXVII 



Does the Leaf vary in its Starch Contents at Different Seasons ? 



Collect in early summer some leaves of several kinds of trees and 

 shrubs and preserve them in alcohol. Collect others as they are 

 beginning to drop from the trees in autumn and preserve them in 

 the same way. Test some of each lot for starch as described in 

 Sect. 181. 



What does the result indicate? 



Much of the sugary and protoplasmic contents of the 

 leaf disappears before it falls. These valuable materials 



1 Except where there is a severe dry season. 



