The leaf is an essential organ of all plants which live independently, that is, are not parasitic upon other plants (like the Dodder) or saprophytic upon dead plant remains (like the Indian Pipe). The leaf manufactures food for the plant, gives off excess water (transpiration) and is the breathing organ of the plant. To accomplish these functions the leaf is built up by a complex arrangement of cells and is variously modified in different groups of plants to meet the external conditions of environment and competition by other plants.
'The cuts in this section are adapted from Gray's Lessons in Botany. Copyright by Asa Gray. Reproduced by permission of the American Book Company, publishers.
The parts of a leaf are designated as blade, petiole and stipules. The leaf blade (figure 1A) is the broadly expanded portion, although in some species the leaf blade is very narrow or even threadlike. The petiole (figure 1 B) is the stalk which supports the blade, and may be lacking in some cases, when the leaf is said to be sessile. The stipules (figure 1C) are small, leaflike organs at the base of the petiole, and are best typified by the rose leaf. Frequently the stipules encircle the stem at the base of the petiole and often they are entirely lacking or fall away so soon after the leaves expand that they are not found when the plant is in bloom.
Terms of leaf outline: The various shapes of leaf blades may be expressed by the following terms:
Subulate; awl-shaped, without visible expansion of blade, and usually tapering to the apex (figure 3).
Linear, or ribbon-shaped; elongated and several times longer than wide (figure 4).
Lanceolate; in which the leaf blade is three times as long as wide, or longer, and broadest at or below the middle (figure 5).
Oblong; in which the blade is somewhat longer than wide, broadest in the middle or with sides almost parallel (figure 6).
Ovate; shaped like an egg; that is, broadest below the middle or near the base (figure 7).
Elliptical; rounded at both ends, somewhat longer than wide (figure 8). Orbicular or rotund; in which the blade is nearly or quite circular in outline (figure 9).
Reniform; in which the blade is broader than long, with a heart-shaped base (figure 10).
Deltoid; triangle-shaped, similar to ovate but conspicuously broadened at the base and pointed at the apex (figure 11).
Consideration of a few leaf blades shows immediately that these terms are not always sufficient to express accurately the shape and we may have recourse to combinations of terms, such as oblong-lanceolate, ovate-lanceolate (figure 13), etc.
The shape of leaf blades which are broadest above the middle may be expressed by the following terms:
Obovate; ovate in shape, but broadest near the apex or above the middle (figure 14).
Oblanceolate; lanceolate in shape but broadest above the middle or near the apex (figure 15).
Spatulate; in which the blade is oblanceolate or obovate in shape with the base conspicuously elongated (figure 12).
Terms applied to the apex of the leaf:
Obcordate; broad and heart-shaped at the apex (figure 16).
Emarginate; with a slight depression at the somewhat narrowed apex (figure 17).
Retuse; terminating in a semicircular end, the center of which is somewhat indented (figure 18).
Truncate; with a flat or abrupt apex (figure 19).
Acuminate; when the apex of the blade is longer than broad (figure 20). Acute; when the apex of the blade is about as broad as long (figure 21). Obtuse or blunt; when the apex is much broader than long (figure 22).
Mucronate; when the apex is terminated by a short blunt tip (figure 23)
Cuspidate; when the tip of the blade is hard and stiff (figure 24).
Terms applied to the base of the leaf:
The terms truncate, acuminate, acute, obtuse (defined above) may also be applied to the shape of the base of the leaf blade, in addition to the following:
Cordate; heart-shaped (figure 25).
Cuneate, or wedge-shaped; when the sides of the leaf blade taper to an acute angle at the base (figure 27).
Auriculate, when the depression at the base of the blade is deep and produces on either side conspicuous basal lobes (figures 28 and 32).
Sagittate; when the basal lobes point downward like the head of an arrow (figure 30).
Hastate; when the basal lobes are turned outward (figure 29).
Peltate; a rounded leaf blade with the petiole attached at or near the middle of the lower surface (figure 31).
Perfoliate; when the bases of leaf blades meet and join around the stem of the plant (figure 26).
Terms applied to the marginal segmentation of leaf blades:
Sinuate; when the marginal lobes of the leaf blade present a wavy outline (figure 33).
Pinnately lobed; when the tissue between the veinlets is cut out nearly to the midrib of the leaf and the divisions are arranged like the pinnae of a feather (figure 34).
Palmate; when the blade is deeply divided nearly or quite to a common base (figure 38).
Serrate (figure 35); when the margin is sharply toothed with coarse teeth, like a saw. When the teeth are rounded inward or are convex, the margin is said to be dentate (figure 37). When the margin is formed of rounded teeth it is said to be crenate (figure 36). If the teeth are very small, the diminutives of the above terms are used, namely, serrulate (figure 39), denticulate (figure 40) and crenulate (figure 41).
When a leaf possesses several divisions or segments upon a common petiole or rachis, it is said to be compound. The distinction between a simple leaf, which is deeply divided, and a compound leaf, rests upon the presence of distinct articulation between the leaf segment of the compound leaf and the petiole. Compound leaves may be pinnate (figure 42), when the leaflets are arranged on either side of a common petiole (Ash, Rose, Walnut etc.) or palmately compound (figure 43), when the leaflets all join the petiole at its summit (Horse-chestnut).
Alternate, when the leaves are arranged one at a node and each leaf is opposite and above the preceding leaf; spirally arranged, when the nodes are not opposite; and opposite when the leaves are in pairs opposite each other on the same node. When several leaves are inserted on the same node they are said to be whorled or verticillate.