28 



LOSING OF LEAVES 



lobing and branching of leaves has become a characteristic of 

 many plants because such variations permit the illumination of 

 a larger leaf surface. This is particularly noticeable in our oaks 

 where the outer leaves are often deeply lobed, thus permitting 

 considerable light to pass through to the underlying leaves. Ob- 

 serve also that these lower leaves are larger and less lobed, thus 

 catching as much as possible of this rather feeble light (Fig. 18). 



FIG. 18. Leaves of red oak: A, sunned leaf. B, shaded leaf. 

 FIG. 19. Compound leaves: A, red ash. B, horse chestnut. 



In many plants the lobing extends quite to the middle of the leaf 

 and the lobes are often attached to the midvein or midrib by a 

 petiole. In this latter case the leaf is said to be compound 

 (Fig. 19). All such modifications permit the development of 

 numerous leaves upon the branches without the danger of shad- 

 ing. If there is still any doubt as to the perfection of this light 

 catching arrangement of the leaves, try to substitute the some- 

 what similar leaves of two different trees as the birch and elm 

 or the water beech (Carpinus) and the beech, noting how the 

 leaves of one fit when placed upon the branch of the other. 



15. The Cause of Leaf Arrangements. How is this perfec- 

 tion in the arrangement of leaves accomplished? The leaf or 



