82 



TEE LEAF 



68 



A young bud of Lirio- 

 dendron, much en- 

 larged, showing the 

 manner in which the 

 blade of a young 

 leaf is shaped in its 

 growth by the con- 

 figuration of the 

 parts upon which it 

 lies folded. — Lub- 

 bock. 



of many leaf shapes. Leaves standing side by side on the 

 same bough or around the same stem are thus shaped 



so that they fit well together with 

 little overshading. Divided and com- 

 pound blades (see § 177) seem to be 

 better than entire forms in the matter 

 of allowing sunlight to filter through 

 to foliage on lower parts of the stem. 



139. Perhaps enough cases have 

 been given to make it clear that the 

 philosophy of leaf forms is to be 

 sought in the circumstances of life 

 of the different sorts of plants. 



140. Division of the blade: the 

 margin. — The margin of the blade 

 may be even, or entire^ through- 

 out. Oftener it is more or less in- 

 dented. If slightly irregular, and 

 the projections are pretty sharp, the 



margin is toothed, or dentate (Fig. Ill) ; or, if the teeth 

 point forward like those of a ripsaw, the margin is serrate 

 (Fig. 110). If the depressions are rather deep and sharp, 

 like cuts, the margin is incised (Fig. 115). Large projec- 

 tions, especially if somewhat rounded, are termed lobes. 

 All degrees and kinds of marginal irregularity are similarly 

 designated by proper terms for the ready description and 

 recognition of the various species of plants : in two or 

 three words the botanist may describe any one of the 

 almost endlessly diversified shapes of leaves so as to give 

 a definite idea of it. 



141. Compound leaves. — The blade is often so deeply 

 divided that it consists of quite separated parts. The blade 

 (and the leaf) is then co7n2:)ound (Figs. 59, 124). Each 

 part often has a stalklet of its own, and the stalklet (or 

 petiolule) is often jointed with the main leaf stalk just as 

 this is jointed with the stem. 



142. Leaves with no distinction of petiole and blade. — The leaves 

 of Iris show one form of this. The flat but narrow leaves of 



