FUNCTIONS OF LEAVES 141 
of such combustible substances as oil, starch, and sugar.! 
In ordinary leafy plants the leaves (through their stomata) 
are the principal organs for absorption of air, but much air 
passes into the plant through the lenticels of the bark. 
168. 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.2?, 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 
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. 
Much of the starchy, sugary, and protoplasmic contents 
of the leaf disappears before it falls. These valuable 
materials have been absorbed by the branches and roots, 
to be used again the following spring. 
The separation of the leaf from the twig is accomplished 
by the formation of a layer of cork cells across the base of 
1 The necessity of an air supply about the roots of the plant may be shown 
by filling the pot or jar in which the hydrangea was grown for the transpira- 
tion experiment perfectly full of water and noting the subsequent appearance 
of the plant at periods from twelve to twenty-four hours apart. 
2 Except where there is a severe dry season. 
