Taxonomy is the science of classification. Revolutionized in the mid-1700s by Carolus Linnaeus, modern taxonomy classifies organisms based on the relationships among them. The broadest classification occurs at the kingdom level, and organisms in different kingdoms may be separated from each other by millions of years of evolution. One of five kingdoms, the Plant Kingdom includes a diverse array of species that produce energy from sunlight using a metabolic process called photosynthesis.
Within the Plant Kingdom, plants are organized into groupings, called taxa. As taxa descend from the kingdom level, organisms tend to be more alike. Traditionally, taxa classified species based on their evolutionary relationships, so plants classified together in lower taxa from the kingdom level had diverged more recently in evolution than those who share only a kingdom or other higher-level taxon. Today, however, "Taxonomy is a synthetic science, drawing on data from such diverse fields as morphology, anatomy, cytology, genetics, cytogenetics, chemistry and molecular biology," according to Tod F. Stuessey in his book "Plant Taxonomy." Taxonomy, then, is a complicated science that seeks to bring many details together in an attempt to uncover relationships among species.
The taxon below the kingdom level is the phylum. There are 10 plant phyla that classify plants based in a large part on evolutionary advancement. The most primitive plants--the mosses--belong to the most primitive phylum, and plants classified here lack vascular tissue and reproduce with spores, not seeds. Ferns developed vascular tissue but not seeds and belong to four phyla. Next, there are four phyla of gymnosperms, the first plants to develop seeds. Finally, the last, largest and most advanced phylum includes the flowering plants, which developed seeds protected by an ovary and flowers for attracting pollinators.
From the phylum, classifications branch into increasingly narrower taxa that include fewer organisms that are more closely related. Each taxon is descriptive: It includes only plants that share a specific trait in common. For example, the next lower taxon below phylum is class, and the flowering plant phylum is dividing into monocots and dicots, a classification based on the number of leaves on the embryo in the seed. Further taxa include the order, family, genus and species.
Before Linnaeus' revolutionary work in taxonomy, new species were given lengthy, descriptive names in Latin. Sometimes six or seven words long, these names provided a glut of information and were difficult to remember and use. Under Linnaeus' system, each species was given a two-part name in Latin called the binomial name. Today, this name is also called the scientific or Latin name. The first part of the name--always written with a capital letter--designates the genus to which the plant belongs. The genus is a grouping of closely related species. For example, the genus Rosa contains all species of roses. The second name identifies the species, a taxon that includes only organisms that can reproduce with each other and create fertile offspring. Using scientific names allows horticulturalists and gardeners to distinguish among plant species that otherwise look much alike, even identical.
Not surprisingly, given the complexity of the organisms it seeks to classify, the current system of classification is subject to debate among botanists. Not all botanists agree which organisms belong in the Plant Kingdom, with organisms like algae and lichen sharing characteristics with plants and organisms in other kingdoms. Determining when a plant diverges into a new species becomes contentious as well. Plant classifications are in no way fixed, and taxonomists are constantly shuffling species in and out of taxa as they learn more about the plant's evolutionary history and genetic makeup.