Leaf Coloration and the Fall of Leaves 69 



Self-pruning. A large num])cr of our common trees, like 

 the Cottonwood, maple, and elm, develop abscission layers 

 which cut off twigs and some- 

 times branches an inch in thick- 

 ness. In these trees we have twig 

 fall as well as leaf fall. The 

 falling of flowers, and of fruits 

 like apples and nuts, is due to 

 the abscission layers formed in 

 the stems. 



Evergreen and deciduous trees. 

 In the Northern states many 

 persons have come to think that 

 the evergreen habit is associated 

 only with needle leaves, because 

 in the North the evergreens are 



, . , 11 1 r 1 ^^^- 43. Abscission of branches 



mostly of the needle-leafed type, of cottonwood. Twigs and small 



But in the Southern states there branches as well as leaves and fruits 

 1 11 c 1 J T1 are cut off by the formation of abscis- 



are many broad-leafed trees, like ^^^^ jj^y^^g 

 the magnolia, rhododendron, and 



holly, that are also evergreen. Moreover, the tamaracks of 

 the North and the bald cypress of the South furnish examples 

 of needle-leafed trees that are deciduous. If we include the 

 shrubs, there are many broad-leafed plants, both in the North 

 and in the South, that have the evergreen habit. In the tropics 

 most of the trees are evergreen, and almost all have broad leaves. 

 It must be noted that even in the case of evergreens individual 

 leaves remain on the trees for only a limited number of years. 

 The leaves of the evergreens are quite different structurally 

 from the leaves of deciduous trees. The evergreens must be 

 able to withstand freezing and thawing, and also the dry 



