CAUTION.--In some plants the leaves on the side shoots or spurs of a twig are so close together, the internodes being so short, that at first sight they seem opposite. In such cases, the leaf-scars of the preceding years, or the arrangement of the branches, is a better test of the true arrangement of the leaves. The twig of Birch shown in Fig. 5 has alternate leaves. [Illustration: Fig. 5.] [Illustration: Fig. 6.] There is one variety of alternation, called _two-ranked_, which is quite characteristic of certain trees; that is, the leaves are so flattened out as to be in one plane on the opposite sides of the twig (Fig. 6). The Elm-trees form good examples of two-ranked alternate leaves, while the Apple leaves are alternate without being two-ranked. Most leaves spread from the stem, but some are _appressed_, as in the Arbor-vitæ (Fig. 7). In this species the _branches_ are _two-ranked_. PARTS OF LEAVES.--A _complete leaf_ [Illustration] consists of three parts: the _blade_, the thin expanded portion; the _petiole_, the leafstalk; and the _stipules_, a pair of small blades at the base of the petiole. The petiole is often very short and sometimes wanting. The stipules are often absent, and, even when present, they frequently fall off as soon as the leaves expand; sometimes they are conspicuous. Most Willows show the stipules on the young luxuriant growths. [Illustration: Fig. 7.] VEINING.--The leaves of most trees have a distinct framework, the central line of which is called a _midrib_; sometimes the leaf has several other lines about as thick as the midrib, which are called _ribs_; the lines next in size, including all that are especially distinct, are called _veins_, the most minute ones being called _veinlets_ (Fig. 8). [Illustration: Fig. 8.] KINDS.--Leaves are _simple_ when they have but one blade; [Illustration] _compound_ when they have more than one. Compound leaves are _palmate_ when all the blades come from one point, as in the Horse-chestnut; [Illustration] _pinnate_ when they are arranged along the sides, as in the Hickory. Pinnate leaves are of two kinds: _odd-pinnate_, [Illustration] when there is an odd leaflet at the end, as in the Ash, and _abruptly pinnate_ [Illustration] when there is no end leaflet. Many trees have the leaves _twice pinnate_; they are either _twice odd-pinnate_ [Illustration] or _twice abruptly pinnate_. [Illustration] The separate blades of a compound leaf are called _leaflets_. Leaves or leaflets are _sessile_ when they have no stems, and _petiolate_ when they have stems.