People often speak of their family characteristics--certain traits shared by members of the family that show they are related to each other. Just like people, plants belong to families, and those families also have key characteristics in common. When you're learning to identify plants, becoming familiar with the major plant families makes finding the exact species an easier task.
Taxonomy is the science of classification. Carolus Linnaeus revolutionized plant taxonomy in the 18th century, sorting plants into groups based on their relationships to each other. The largest grouping occurs at the kingdom level--all plants belong to the plant kingdom--and the smallest at the species level, where individuals share enough in common to be able to reproduce and bear fertile offspring. Genera include groups of species, while families include groups of genera.
The kingdom and higher levels of classification often don't provide enough specific information about plants to be meaningful, while species and genera can be too specific to understand how plants relate to each other. According to botanist William A. Niering, learning plant family characteristics is one of the best ways to become skilled at identifying plants.
Botanists sort plants into families based on structure, according to the University of Cincinnati. Looking at flowering plants, characteristics used to group plants into families include the number of petals and sepals, the number of pistils and stamens, the arrangement of flower parts in relation to each other, the shape of different flower parts and whether parts are separate or fused. For example, members of the mustard family (Cruciferae) have petals in multiples of four and six stamens occurring in groups of two.
When trying to classify a plant into a family, you will rely primarily on your observational abilities. Record details about leaves, stems and branches, buds and flowers. Note the number, arrangement, shape, size and color. Writing careful notes, making sketches and taking photographs will help you to remember what you've observed when you try to identify the plant family later.
Learning more about plant families doesn't require any complicated or expensive resources. Naturalist Jim Conrad recommends field guides and identification keys as the best tools for the budding naturalist. Many field guide series, including the Audubon Society guides, include information about plant families and are available from the public library. Identification keys are harder to find, but universities and local wildlife clubs often host websites where you can identify plant families by selecting from a list of traits.