CHAPTER III.

_Leaves._

Leaves are the lungs of plants. The food taken in by the roots has to pass through the stem to the leaves to be acted upon by the air, before it becomes sap and is fit to be used for the growth of the plant. No portion of a plant is more varied in parts, forms, surface, and duration than the leaf.

No one can become familiar with leaves, and appreciate their beauty and variety, who does not study them upon the plants themselves. This chapter therefore will be devoted mainly to the words needed for leaf description, together with their application.

THE LEAF.--In the axil of the whole leaf the bud forms for the growth of a new branch. So by noting the position of the buds, all the parts included in a single leaf can be determined. As a general thing the leaf has but one blade, as in the Chestnut, Apple, Elm, etc.; yet the Horse-chestnut has 7 blades, the Common Locust often has 21, and a single leaf of the Honey-locust occasionally has as many as 300. Figs. 17-58 (Chapter VII.) are all illustrations of single leaves, except Fig. 43, where there are two leaves on a twig. A number of them show the bud by which the fact is determined (Figs. 25, 26, 31, 33, 34, 36, 40, etc.); others show branches which grew from the axillary buds, many of them fruiting branches (Figs. 37, 42, 43, 50, and 54), one (Fig. 51) a thorny branch.

The cone-bearing plants (Figs. 59-67) have only simple leaves. Each piece, no matter how small and scale-like, may have a branch growing from its axil, and so may form a whole leaf. A study of these figures, together with the observation of trees, will soon teach the student what constitutes a leaf.

ARRANGEMENT.--There are several different ways in which leaves are arranged on trees; the most common plan is the _alternate_; [Illustration] in this only one leaf occurs at a joint or node on the stem. The next in frequency is the _opposite_, [Illustration] where two leaves opposite each other are found at the node. A very rare arrangement among trees, though common in other plants, is the _whorled_, [Illustration] where more than two leaves, regularly arranged around the stem, are found at the node. When a number of leaves are bundled together,--a plan not rare among evergreens,--they are said to be _fasciculated_ or in _fascicles_. [Illustration] The term _scattered_ is used where alternate leaves are crowded on the stem. This plan is also common among evergreens.

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.

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.

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).

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.

When there are several ribs starting together from the base of a blade, it is said to be _radiate-veined_ or _palmate-veined_.[Illustration] When the great veins all branch from the midrib, the leaf is _feather-veined_ or _pinnate-veined_. [Illustration] If these veins are straight, distinct, and regularly placed, the leaf is said to be _straight-veined_. The Chestnut is [Illustration] a good example. Leaves having veinlets joining each other like a net are said to be _netted-veined_. All the trees with broad leaves in the northern United States, with one exception, have netted-veined foliage. A leaf having its veinlets parallel to one another is said to be _parallel-veined_ or _-nerved_. [Illustration] The Ginkgo-tree, the Indian Corn, and the Calla Lily have parallel-veined leaves. The narrow leaves of the cone-bearing trees are also parallel-veined.

FORMS.--Leaves can readily be divided into the three following groups with regard to their general outline:

1. _Broadest at the middle._ _Orbicular_, [Illustration] about as broad as long and rounded. _Oval_, [Illustration] about twice as long as wide, and regularly curved. _Elliptical_, [Illustration] more than twice as long as wide, and evenly curved. _Oblong_, [Illustration] two or three times as long as wide, with the sides parallel. _Linear_, [Illustration] elongated oblong, more than three times as long as wide. _Acerose_, [Illustration] needle-shaped, like the leaf of the Pine-tree.

2. _Broadest near the base._ _Deltoid_, [Illustration] broad and triangular. _Ovate_, [Illustration] evenly curved, with a broad, rounded base. _Heart-shaped_ or _cordate_, [Illustration] similar to ovate, but with a notch at the base. _Lanceolate_, [Illustration] shaped like the head of a lance. _Awl-shaped_, [Illustration] shaped like the shoemaker's curved awl. _Scale-shaped_, [Illustration] short, rounded, and appressed to the stem. The Arbor-vitæ has both awl-shaped and scale-shaped leaves.

3. _Broadest near the apex._ _Obovate_, [Illustration] same as ovate, but with the stem at the narrow end. _Obcordate_, [Illustration] a reversed heart-shape. _Oblanceolate_, [Illustration] a reversed lanceolate. _Wedge-shaped_ or _cuneate_, [Illustration] having a somewhat square end and straight sides like a wedge.

These words are often united to form compound ones when the form of the leaf is somewhat intermediate. The term which most nearly suits the general form is placed at the end; thus _lance-ovate_ indicates a leaf between lanceolate and ovate, but nearer ovate than lanceolate; while _ovate-lanceolate_ indicates one nearer lanceolate.

BASES.--Oftentimes leaves are of some general form, but have a peculiar base, one that would not be expected from the statement of shape. An ovate leaf which should have a rounded base might have a tapering one; it would then be described as ovate with a _tapering base_. [Illustration] A lanceolate leaf should naturally have a tapering base, but might have an _abrupt_ one. [Illustration] Many leaves, no matter what their general form may be, have more or less notched bases; such bases are called _cordate_, [Illustration] _deeply_ or _slightly_, as the case may be; and if the lobes at base are elongated, _auriculate_. [Illustration] If the basal lobes project outward, the term _halberd-shaped_ [Illustration] is used. Any form of leaf may have a base more or less _oblique_. [Illustration]

POINTS.--The points as well as the bases of leaves are often peculiar, and need to be described by appropriate terms. _Truncate_ [Illustration] indicates an end that is square; _retuse_, [Illustration] one with a slight notch; _emarginate_, one with a decided notch; _obcordate_, with a still deeper notch; _obtuse_, [Illustration] angular but abrupt; _acute_, [Illustration] somewhat sharpened; _acuminate_, [Illustration] decidedly sharp-pointed; _bristle-pointed_ and _awned_, [Illustration] with a bristle-like tip; _spiny-pointed_, with the point sharp and stiff (Holly); _mucronate_, [Illustration] with a short, abrupt point.

MARGINS.--_Entire_, [Illustration] edge without notches; _repand_, [Illustration] slightly wavy; _sinuate_, [Illustration] decidedly wavy; _dentate_, [Illustration] with tooth-like notches; _serrate_, [Illustration] with notches like those of a saw; _crenate_, [Illustration] with the teeth rounded; _twice serrate_, [Illustration] when there are coarse serrations finely serrated, as on most Birch leaves; _serrulate_, with minute serrations; _crenulate_, with minute crenations. Leaves can be _twice crenate_ or _sinuate-crenate_. _Revolute_ indicates that the edges are rolled over.

When a leaf has a few great teeth, the projecting parts are called _lobes_, and the general form of the leaf is what it would be with the notches filled in. In the description of such leaves, certain terms are needed in describing the plan of the notches, and their depth and form.

Leaves with palmate veining are _palmately lobed_ [Illustration] or _notched_; those with pinnate veining are _pinnately lobed_ [Illustration] or _notched_. While the term _lobe_ is applied to all great teeth of a leaf, whether rounded or pointed, long or short, still there are four terms sometimes used having special signification with reference to the depth of the notches. _Lobed_ indicates that the notches extend about one fourth the distance to the base or midrib; _cleft_, that they extend one half the way; _parted_, about three fourths of the way; and _divided_, that the notches are nearly deep enough to make a compound leaf of separate leaflets.

So leaves may be palmately lobed, cleft, parted or divided, and pinnately lobed, cleft, parted or divided. The term _pinnatifid_ [Illustration] is often applied to pinnately cleft leaves. The terms _entire_, _serrate_, _crenate_, _acute-pointed_, etc., are applied to the lobes as well as to the general margins of leaves.

SURFACE.--The following terms are needed in describing the surface of leaves and fruit.

_Glabrous_, smooth; _glaucous_, covered with a whitish bloom which can be rubbed off (Plum); _rugous_, wrinkled; _canescent_, so covered with minute hairs as to appear silvery; _pubescent_, covered with fine, soft, plainly seen hairs; _tomentose_, densely covered with matted hairs; _hairy_, having longer hairs; _scabrous_, covered with stiff, scratching points; _spiny_, having stiff, sharp spines; _glandular-hairy_, having the hairs ending in glands (usually needing a magnifying glass to be seen).

TEXTURE.--_Succulent_, fleshy; _scarious_, dry and chaffy; _punctate_, having translucent glands, so that the leaf appears, when held toward the light, as though full of holes; _membranous_, thin, soft, and rather translucent; _thick_, _thin_, etc.

DURATION.--_Evergreen_, hanging on the tree from year to year. By noticing the color of the different leaves and their position on the twigs, all evergreen foliage can readily be determined at any time during the year. _Deciduous_, falling off at the end of the season. _Fugacious_, falling early, as the stipules of many leaves.