124 ELEMENTS OP BOTANY. 



The amount of oxygen absorbed and of carbonic acid given 

 off, is, however, so trifling compared witli the amount of each 

 gas passing in the opposite direction, while fixation of carbon 

 is going on in sunlight, that under .such Circumstances it is 

 difiicult 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. CJiemioal 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. Te,st some of each lot for starch as described in § 153. 



What does the result indicate ? ^ 



* Except wliere there is a srvrrc dry season. 



