[Chap. Ill LEARNING TO NAME PLANTS 21 



recognized (Fig. 15). Leaves of oats and other grasses have parallel 

 venation, the veins extending more or less parallel from base to apex. 

 Many ferns and the curious ginkgo tree have leaves with forked or 

 dichotomous venation; that is, each vein divides at intervals into two 

 smaller veins of equal size. When the secondary veins extend from the 

 midrib like the divisions of a feather, as in the leaves of elm, the venation 

 is said to be pinnate. In the maple leaf, however, the principal veins 

 extend from the petiole near the base of the blade, roughly simulating 

 the bones in our hands. This type of venation is palmate. 



Simple and compound leaves. Every leaf consisting of one continuous 

 blade only is known as a simple leaf. A compoimd leaf such as that of 

 the rose consists of several leaflets. If the leaflets are joined to the end of 

 the petiole, as are those of the horse chestnut, the leaf is described as 

 palmately compound. When a compound leaf is composed of leaflets 

 joined to the sides of the central axis of the leaf (the rachis), it is termed 

 pinnately compound ( Fig. 16 ) . The leaflets in a compound leaf may be 

 odd or even in number. 



Compound and simple leaves are usually readily distinguished, but 

 some divided simple leaves closely resemble those commonly described 

 as compound. Such leaves as those of tomato and potato which have 

 deeply divided blades can scarcely be distinguished from compound 

 leaves. In fact, there is apparently every gradation between simple undi- 

 vided leaves and distinctly compound leaves. A bud may be found in 

 the axil of each leaf of broad-leaved trees and shrubs, but there are no 

 buds in the axils of leaflets. 



Leaf form. The shape of a leaf is usually rather characteristic of a 

 species, and the form of the apex and the base of the blade may be dis- 

 tinctive. Leaves may also have characteristic edges, or margins. Some of 

 the common teiTns used to describe leaves are illustrated in Fig. 17. 



Arrangement of leaves on stems. Leaves usually have definite arrange- 

 ments on the stem. On willow twigs the leaves are alternately and 

 spirally arranged. There is a single leaf at each node, and a line drawn 

 through the successive points of attachment forms a spiral about the 

 stem (Fig. 18). The leaves of maple occur two at a node on opposite 

 sides of the stem and exemplify the opposite arrangement. The plane of 

 attachment of each successive pair of leaves is at right angles to that of 

 the leaves immediately above and below. The catalpa twigs have three 

 leaves at a node arranged radially. This arrangement is termed whorled 

 or cyclic. 



