Plant specialists organize plants into classification groups based on shared traits. As a result, when learning to identify plants, a good starting point entails finding out what traits to look for and what they tell you about a plant. Leaves provide many clues to a plant's identity. The key is learning what to look for and how to use the clues you find to identify your plant.
Learning to identify plants is an inexpensive hobby. If you only have a leaf to work with, you'll want to start with a field guide, a book that collects pictures and descriptions of plants. Most public libraries carry a selection of field guides, usually focusing on your local region. George A. Petrides, author of the field guide "Eastern Trees," recommends procuring a hand lens as well for identifying small details on your samples.
According to Petrides' field guide, leaves can be classified by type and arrangement, which help you begin to know where to look. Needle-like and succulent leaves are obvious types. Broad leaves are classified by their arrangement on the stem or branch, so if you have a single leaf, you may not have this information. Opposite leaves occur in pairs growing opposite each other on the stem, while alternate leaves occur singly, switching sides of the stem as they progress down the branch. Whorled leaves grow in a ring around the branch. Finally, simple or compound types refer to whether one leaf or multiple leaflets grows from a single bud. If you find a bud at the base of a single leaf, it is a simple leaf. A bud at the base of several leaves indicates a compound leaf.
Basic leaf shapes also provide clues. Although botanists use special terms to describe leaves, use language comfortable for you, make a sketch or bring home a sample to help you match your leaf with the correct term. Some descriptions also include information about the shape of the base and tip of the leaf. For example, a cordate base means that the part of the leaf that connects to the stem is heart-shaped.
Also note the size and color of the leaf on its top and underside. Describe the texture: Is the leaf shiny, waxy or hairy? Pay attention to the shape of the leaf margins, which may be jagged, wavy, lobed or smooth. Finally, note the arrangement of the leaf veins. Palmate veins originate from a single point at the base of the leaf, while pinnate veins radiate out from the midrib that runs down the middle of the leaf.
Once you've gathered your information, made notes and sketches or collected samples, turn to your field guide to find leaves that resemble yours. If you can't find an exact match, find plants that share many of the same characteristics. If they belong to a single family, you may have a starting point for identifying your sample.