MECOPHYTE SOCIETIES. 



241 



where the separation is to occur, a cleavage region is gradu- 

 ally developed until the leaf is entirely separated from the 

 stem except by a woody strand or two, which is easily 

 broken (see Fig. 200). 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 prep- 

 aration for the fall, has been grad- 

 ually withdrawn into the perma- 

 nent parts of the plant. 



It will be noticed that in 

 general deciduous leaves are thin, 

 exceedingly variable in form, and 

 in a general horizontal position, 

 nor do they have the firm, leathery 

 texture of the xerophyte leaves. 

 All this indicates great leaf ac- 

 tivity, for, the necessity of pro- 

 tection being removed, the leaf is 

 not impeded in its work by the 

 development of protective struc- 

 tures. 



One of the most prominent 

 features associated with the de- 

 ciduous habit is the autumnal col- 

 oration. 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 sometimes a leaf 

 may show a pure color. 



FIG. 200. A section through the 

 base of a leaf of horse-chest- 

 nut preparing to fall off at 

 the end of the growing sea- 

 son. A cleavage plate (*) has 

 developed between the woody 

 bundle (6) and the surface. 

 Presently this reaches the 

 surface, and only the woody 

 strand fastens the leaf to the 

 stem. 



