FUXCTIOXS OF LEATES 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 lentieels 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.^ 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 drj-ing 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, narro^1"- 

 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. 



IMuch of the starchj-, sugary, and protoplasmic contents 

 of the leaf disappears before it falls. These valuable 

 materials have been al^sorbed bv 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. 



- Except where there is a severe dry season. 



