Taxonomy, the science of classification, is an ancient study, one of the most obvious uses of which is the classification of plants. Classifying plants helps people to understand the history and evolution of a species or the relationships between different types of plants. Botanists, horticulturalists and master gardeners use the language of taxonomy to communicate about specific plants or types of plants without the confusion of common names that can differ greatly between regions.
Even early human societies engaged in plant classification, identifying and sorting plants according to usefulness for food, medicine, clothing and shelter. Aristotle developed one of the first formal classification systems, or taxonomies, organizing species in a hierarchical fashion depending on each species' complexity when compared to others. In this so-called "great chain," each species occupied one link, and no two species could occupy a single link. Today's taxonomic system was coined by Carolus Linnaeus, a Swedish physician and botanist who arranged species according to their relationships, not perceived position higher or lower than another.
Linnaeus' system sorted plant species into increasingly complex categories, or taxa. The highest taxon is the kingdom, and all plants belong to the plant kingdom, placed there because they produce their own food via photosynthesis. In lower taxa, plants have closer relationships to each other. The final taxon is the species, which is commonly defined as organisms closely related enough to reproduce and produce fertile offspring.
The plant kingdom further divides into divisions or phyla, each of which contains fewer and more closely related species. Although there are 10 plant phyla, they can be further grouped into four broad divisions based on plant structure and reproduction. The simplest plants are the bryophytes or mosses, which reproduce using spores and contain no vascular tissue. Ferns also reproduce with spores but developed vascular tissue for transporting water and nutrients throughout the plant. Spores gave way to seeds in gymnosperms, and flowering plants, or angiosperms, further protect their seeds inside of ovaries and contain more advanced vascular systems.
Botany Professor William A. Niering, author of "The Audubon Society Field Guide to North American Wildflowers," identifies the family as the first taxonomic level where most laypeople can see relationships between the species included. For example, the orchid, daisy and lily families all include species that resemble each other physically and have similar habits.
Genus and Species
Beneath the family level, plants are classified into genera that contain very closely related species. Species in a single genus often resemble each other so closely, in fact, that they are difficult for nonexperts to tell apart. The final taxon is the species level. While some genera contain dozens of species, others have only one.