41 8 ECOLOGY. 



in a ring around the central vascular bundle grow rapidly so as to 

 unduly strain the central tissue and epidermis, making it brit- 

 tle. In this condition a light puff of wind whirls them away in 

 eddies to the ground. The frosts of autumn assist in the separa- 

 tion of the leaf from the stem, but play no part in the coloration 

 of the leaf. 



As the cold weather of autumn and winter draws slowly on, 

 these trees and shrubs cast off their leaves, and thus get rid of 

 the extensive transpiration surface, or in same cases the dead 

 leaves may cling for quite a long period to the trees. However 

 in the death and fall of the leaves of these deciduous trees and 

 shrubs, or the dying back of the aerial shoots of perennial 

 herbaceous plants, there is a most useful adaptation of the plant 

 to lay aside, for the cold period, its extensive transpiration sur- 

 face. For while the soil is too cool for root absorption, should 

 transpiration go on rapidly, as would happen if the leaf surface 

 remained in a condition for evaporation, the plants would lose 

 all their water and dry up. 



