28 



LOBIXG 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 arc 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) 



Fie. 18. Leaves of red oak: .1, sunned leaf. B, shaded leal 

 Fig. 19. Compound leaves: ./. 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 bj 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 tin- branches without the danger of shad 

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

 catching arrangemenl of the leaves, try to substitute the some 



what similar leaves of two different trees as the lurch and dm 



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



on.- fit when placed upon the branch of the other. 

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

 tion in the arrangemenl of leaves accomplished? The leaf 01 



