Leaf this to me!
Just as with flowers, leaves come in a staggering variety of sizes, shapes, textures, margins and arrangements. The function of all leaves is to act as the organ of photosynthesis. Photosynthesis is the process whereby the plant produces sugars and oxygen from sunlight, carbon dioxide and water. However, in this article I am focusing on the morphology, or physical characteristics, of leaves so you can understand and describe your own plants more precisely.
Leaves are composed of two main parts, the leaf stem, or petiole, and the flat part of the leaf, or lamina. The lamina is also known as the leaf blade. A few other terms apply to petioles. If the petiole has a swollen area at the base, where it joins to the stem, the swollen area is known as a pulvinus (plural, pulvini). The sensitive plant, Mimosa pudica, has leaves that close or move when touched. The swollen joints of the leaves where the movement occurs are pulvini. Some plants have petioles that are flattened, and some are flattened to such a degree that they assume the function of the leaf blade, which does not develop. Some trees in the genus Acacia produce petioles like this; these specialized structures are known as phyllodes. Acacias like this actually start out with "normal" leaves, but the petioles gradually lose the leaf blades and turn into phyllodes in the more mature plant. In plants such as grasses, the petiole base is flattened and surrounds the stem; these specialized bases are called sheaths.
The simplest leaf consists of a petiole and a roundish or elongated lamina. But many leaves are not simple, and even among the simple ones can be found a bewildering array of shapes and sizes. Leaves with just one petiole and one lamina are considered simple, while those with more than one lamina (called leaflets) are known as compound. Leaflets look just like simple leaves, and a quick glance might have you thinking that you are looking at simple leaves. The giveaway is that a bud is usually found at the stem where the petiole joins it. Such buds are absent if the "leaf" in question is really a leaflet.
Compound leaves can be seen in a variety of types. The following are the major ones you are likely to encounter in your garden. The palmate, leaves (see photo at left) are those in which all the leaflets are attached at a common point, like the fingers attached to your palm. Pinnate leaves have the leaflets lined up along a common central stalk, like the tines on a comb. This central stalk has a special name; it is known as a rachis. Pinnately compound leaves may have a secondary set of branches coming off the main rachis; such leaves are common in leguminous trees like Acacia. These leaves are known as bipinnate. Even more complex iterations of this pattern are found in nature. Of note is that some pinnately compound leaves have a terminal leaflet, and some don't. Those with the terminal leaflet are known as imparipinnate, or odd pinnate, while the ones without are paripinnate, or even pinnate.
A few derivations of palmate and pinnate leaves are the ternate, which are essentially palmate leaves with only three leaflets, and trifoliate, which are pinnate leaves with only three leaflets.
A multitude of other terms apply to leaves and can help you distinguish similar plants one from another. These terms describe the overall leaf shape, the type of leaf margin, the type of lobes, the leaf tips, the leaf bases, leaf arrangements on the stem, and leaf-related structures. In my upcoming Botany for Gardeners articles, I will explain these terms in greater detail.
Photo credits: PDPhoto.org and LariAnn Garner