30 FORM OF LEAVES 



light. Especially instructive are the leaf arrangements of plants 

 growing in windows or creeping over trellises, etc., where compli- 

 cated twisting and elongation of stems and petioles are necessary 

 to adjust the leaves to the one-sided illumination. The angular 

 leaves of some begonias furnish excellent illustrations of this. 

 Perhaps so many plants bear angular leaves because they can be 

 better adjusted and fitted together without loss of space than 

 would be the case in rounded leaves, and smaller leaves can also 

 be more advantageously 'introduced between the larger ones 

 (Fig. 17). The lobing and branching of leaves has become a 

 characteristic of many plants because such variations permit the 



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FIG. 17. Leaf of Aralia. Note the angular shape of the leaflets and the 

 smaller ones filling the space between the larger ones. 



illumination of a larger leaf surface. This is particularly notice- 

 able in our oaks where the outer leaves are often deeply lobed. 

 thus permitting considerable light to pass through to the under- 

 lying leaves. Observe also that these lower leaves are larger and 

 less lobed, thus catching as much as possible of this rather feeble 

 light (Fig. 1 8). 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 develop- 

 ment of numerous leaves upon the branches without the danger 

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

 light catching arrangement of the leaves, try to substitute the 

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



