The Nature of Leaves

A typical leaf consists of a flat blade and a petiole (stalk), which attaches the blade to the stem. Leaves often vary greatly in shape and size among different species. The simplest shape of leaves is the undivided, oval form. The oval-shaped leaf shown in the left photo, for example, is from a sassafras tree. In contrast, many simple leaves are lobed. Not all sassafras leaves are oval, for example; some are divided into a "mitten shape" or a three-lobed shape (as seen in the bottom photo).

Young mulberry leaves frequently have deep indentations, which make the leaf three to five-lobed. Notice also the serrated, or "saw-toothed," borders of the leaves.

This leaf from a silver maple has three major lobes and even more prominent indentations along the outer borders.

A leaf may have more than one blade, resulting in a compound leaf . Each blade of a compound leaf is called a leaflet. The poison ivy leaf pictured in the above photo is so subdivided that it is in the form of three leaflets, rather than three lobes.

Compound leaves occur in a variety of forms. The leaf in the top photo is from a Virginia Creeper vine and has five leaflets that originate from a central point, which is called palmately compound. Another plant with palmately compound leaves is the Red Buckeye, a woody shrub (bottom photo).

The compound leaves of some plants have leaflets spread along an axis called the rachis. Leaves with this form are called pinnately compound. Several leaflets are visible in the photos of these pinnately compound leaves from winged sumac (top) and hickory (bottom).

The young Kentucky Coffeetree leaf in the top photo exhibits a type of leaf division referred to as bipinnately compound, meaning that the leaves are divided into leaflets which are themselves divided. At least ten leaflets are visible, resembling a pinnately compound leaf. Where two leaflets would have been, however, there are other axes which have more leaflets. One of the largest leaves - although it resembles a branch - found on a plant native to Arkansas, the Devil's Walking Stick, is pictured in the photo on the bottom.

The leaves of young Eastern Red Cedars are very pointed (left photo) but are represented as triangular-shaped leaf scales on older plants (right photo).

Submergent vegetation - plants that grow completely submerged in water- typically have very dissected leaves (shown in top photo of Cabomba), which is presumed to provide greater surface area to absorb the carbon dioxide necessary for photosynthesis. Notice the dissected leaves on the lower portion on the water milfoil plant in the bottom photo. This plant grows under the water but can emerge above the surface. The upper part of the plant that grows above the surface no longer shows the dissected leaves.

When sunlight reaches the forest floor in the winter, an orchid called Tipularia begins to grow leaves. The undersides of the leaves are a purplish color, indicating the presence of pigments known as anthocyanins. Anthocyanins absorb blue, blue-green, and green wavelengths of light, allowing the plant to get the most out of the limited sunlight available in the winter. These pigments reflect red light, which causes them to appear reddish to purplish (depending on the pH of the cell sap) to the human eye.

The new growth of many plants is often reddish before later changing to green as chlorophyll and chloroplasts are synthesized. The red color is due to anthocyanins, which provide some protection for the newly developing chloroplasts against potentially damaging ultraviolet (UV) radiation. In the photographs to the right, the new, reddish growth on a blackberry (top) and pepper vine (bottom) is visible.

Trees also produce new leaves that are red in color. In the photograph of the oak branch on the top, the new growth is easily distinguished from the older, green leaves. Perhaps even more obvious is the new growth on the red maple branch (pictured bottom).

A small, carnivorous plant, known as the short-leaved sundew (Drosera brevifolia), is found in Arkansas. The short-leaved sundew is about 2.5 cm (1 inch) in diameter with a rosette of leaves that lie flat on the ground, spread in a circle. Hairs tipped with a sticky substance cover the surface of the leaves (bottom photo). Sometimes small insects get stuck to these glandular hairs, and the sundew digests the insects to get necessary minerals such as nitrogen.

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