124 ELEMENTS OF BOTANY. 
The amount of oxygen absorbed and of carbonic acid given 
off, is, however, so trifling compared with the amount of each 
gas passing in the opposite direction, while fixation of carbon 
is going on in sunlhght, that under such circumstances it is 
difficult to observe the occurrence of respiration at all. In 
ordinary leafy plants the leaves (through their stomata) are 
the principal organs for absorption of air, but a good deal of 
air passes into the plant through the lenticels of the bark. 
°158. 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. 
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 ($ 142) would cause the 
death, by drying up, of trees with broad leaf-surfaces in a 
northern winter. And in countries where there is much snow- 
fall, 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. 
‘159. Chemical Changes in the Leaf before its Fall. — 'The 
fall of the leaf is preceded by important changes in the 
contents of its cells. 
‘Experiment 31.— 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 § 159. 
What does the result indicate ? 
1 Except where there is a severe dry season. 
