MESOFHYTE ASSOCIATIONS. 245 



gradually developed until the leaf is entirely separated 

 from the stem except by a woody strand or two, which is 

 easily broken (see Fig. 208). In this way the scar which 

 remains has really been formed before the leaf falls. 



In this process of sloughing off leaves, the plant cannot 

 afford to lose the living substance present in the working 

 leaves. This substance, during the preparation for the 

 fall, has been gradually withdrawn into the permanent 

 parts of the plant. 



It will be noticed that in general deciduous leaves are 

 thin, exceedingly variable in form, and in a general hori- 

 zontal position, nor do they have the firm, leathery texture 

 of the xerophyte leaves. All this indicates great leaf ac- 

 tivity, for, the necessity of protection being removed, the 

 leaf is not impeded in its work by the development of pro- 

 tective structures. 



One of the most prominent features associated with the 

 deciduous habit is the autumnal coloration. The vivid 

 colors which appear in the leaves of many trees, just before 

 the time of falling, is a phenomenon which has attracted a 

 great deal of attention, but although it is so prominent, the' 

 causes for it are very obscure. It will be noticed that this 

 autumnal coloration consists in the development of various 

 shades of two typical colors, yellow and red. These colors 

 are often associated together in the same leaf, and some- 

 times a leaf may show a pure color. 



The two colors hold a very different relation in the leaf 

 cell. It is known that the yellow is due to the breaking 

 down of chlorophyll, so that the chloroplasts, which are 

 green when active, become yellow when disorganizing, and 

 finally bleach out entirely. That yellow may indicate a 

 post mortem change of chlorophyll may be noticed in con- 

 nection with the blanching of celery, in which the leaves 

 and upper part of the stem may be green, the green may 

 shade gradually into yellow, and finally into the pure white 

 of complete blanching. 



