Flowering plants make up the largest grouping within the plant kingdom, containing more than 250,000 species that range from grasses to tulips to towering oak trees, according to the University of Arizona's Tree of Life project. When identifying flowering plants, most people's first instinct is to use the flower. However, when the plant isn't flowering or the species doesn't produce an obvious flower, you can fall back on the leaves to provide clues about the plant's identity.
Identifying plants requires gathering clues from your observation. When you look at a leaf, you probably notice its shape and any color deviations from basic green. In fact, there are many more details to observe and note, once you know what you're looking for. If you can't identify the plant in the field or collect a sample, take photos, make sketches and write careful notes about what you observe.
Begin by describing the leaf's shape. Botanists use a number of specialized terms to communicate with each other about the shapes of leaves. What's important for you, when collecting clues in the field, is to describe the shape in a way that you can easily remember exactly what it looked like. "Heart-shaped" works just as well as "cordate," and you can look up the precise terminology once you're home again. Also describe the leaf's margins. Are they jagged, wavy or smooth? Sketches will help you remember precise details later.
You will describe leaf vein arrangement with respect to the midrib, the large central vein that begins at the stem and usually extends the length of the leaf. If the veins occur parallel to the midrib, you know the plant is a monocot, a class of flowering plants that includes grasses and familiar garden flowers like lilies. If the veins originate at the midrib, note whether they originate from the same point or if they extend from different points along the midrib. These are palmate and pinnate leaves, respectively.
If you can, observe how the leaves emerge from the stem. Opposite leaves grow in pairs, as the name suggests, opposite one another on the stem. Alternate leaves occur singly on both sides of the stem. Whorled leaves occur singly as well but travel in a circular pattern around the stem. Leaf arrangement provides a very basic piece of information that can help with identification, if you can find access to a stem or branch rather than a single leaf.
Once you have your information collected, you will use it to identify possible matches in a field guide. You can find field guides at your local library, in addition to online versions maintained by universities and nature clubs. To use a field guide, use the photos and illustrations to find possible matches for your plant. Consult the detailed descriptions provided to ensure that the observations you've collected match the attributes of that plant.