33 6 ECOLOGY. 



here and there the older leaves of certain shrubs lose more or 

 less of the green color and take on livelier tints. With the 

 disintegration of the chlorophyll bodies, other colors, which in 

 some cases were masked by the green, are uncovered. In other 

 cases decomposition products result in the formation of new 

 colors. These coloring substances to some extent absorb the 

 sun's rays, so that much of the nitrogenous substances in the 

 leaf may not be destroyed, but may pass slowly back into the 

 stem and be stored for future use. 



504. Fall of the leaf. — The gorgeous display of color, then, 

 which the leaves of many trees and shrubs put on is one of the 

 many useful adaptations of plants. While this is going on in 

 deciduous trees, the petiole of the leaf near its point of attach- 

 ment to the stem is preparing to cut loose from the latter by 

 forming what is called a separative layer of tissue. At this 

 point the cells in a ring around the central vascular bundle 

 grow rapidly so as to unduly strain the central tissue and 

 epidermis, making it brittle. In this condition a light puff of 

 wind whirls them away in eddies to the ground. The frosts of 

 autumn assist in the separation 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 some 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 transpira- 

 tion surface. 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. 



