FUNCTIONS OF LEAVES Rid 
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 ike. 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. 
