The kingdom is the largest, most inclusive classification in the Linnaean system. Most scientists recognize these six kingdoms: Animalia (animals), Plantae (plants), Fungi (fungal organisms, including mushrooms), Protista (simple organisms, such as protozoans and algae, whose cells have nuclei and organelles), Archaea (single-celled microorganisms with cells that do not have nuclei or organelles and that do not carry out chlorophyll-based photosynthesis), and Bacteria (single-celled organisms with cells that do not have nuclei or organelles; some species carry out chlorophyll-based photosynthesis).
Within the plant kingdom, organisms are sorted into angiosperms (plants with seeds enclosed in an ovary), gymnosperms (“naked seed” plants), ferns, fern allies, mosses, liverworts, and hornworts. Recent classification systems based on a more precise understanding of plant relationships and evolution refer to groupings as clades, signifying branches on the tree of life and, to a degree, they dispense with certain elements of the Linnaean system.
Groups of related genera make up plant families. Some plant families are quite large—the orchid, aster, and pea families are the three biggest—while others, such as the ginkgo family, contain only one species.
In an ongoing effort to standardize the classification and naming of plants, botanists in recent decades have modernized the naming of plant families. According to the current system, family names end with the suffix “-aceae” and are taken from the name of an included genus that typifies the family. (Genus and species names are italicized in print, but family names are not.) For example, Acorus calamus, sweet flag, is a member of the plant family Acoraceae. Plant families also are sometimes referred to by the common name of a genus associated with that family, such as “rose family” (for Rosaceae). So, while the English hawthorn (Crataegus laevigata) is not of the same genus as garden roses (Rosa spp.), as a member of the Rosaceae, it shares “rose family” traits, along with species of the genus Prunus, where plums, peaches, and apricots are found. However, there is no standardized list of common names for plant families, and some families are referred to using various names. For example, common names for the Marantaceae, a group of tropical species, include the prayer plant family and arrowroot family. And the Asteraceae is known variously as the daisy, sunflower, or aster family.
Botanists have agreed upon protocols for naming plants, known as the rules of nomenclature. For eight plant families, alternative family names are permitted under the current rules: These alternatives are (with modern equivalents in parentheses) Compositae (Asteraceae), Cruciferae (Brassicaceae), Gramineae (Poaceae), Guttiferae (Clusiaceae), Labiatae (Lamiaceae), Leguminosae (Fabaceae), Palmae (Arecaceae), and Umbelliferae (Apiaceae).
Some plant families are so large and complex that they are further divided into subfamilies and what botanists call “tribes.” The sunflower family (Asteraceae), one of the largest of all plant families, at one time was subdivided into two subfamilies. But now, after much more sophisticated analysis of the evolutionary relationships of this group, it is considered to have 12 subfamilies. Such distinctions are not always clear-cut, and botanists continue to study and refine our understanding of plant relationships and how species should be classified.
A genus consists of a group of species closely related to one another, determined primarily by the reproductive parts of their flowers. Some of the many genera in the mint family (Lamiaceae) are Lavandula (lavender), Mentha (mint), Thymus (thyme), Salvia (sage), Rosmarinus (rosemary), Monarda (bee balm), and Melissa (lemon balm). The rose family (Rosaceae) includes the genera Rosa (rose), Prunus (plum, almond, and cherry), Crataegus (hawthorn), Malus (apple), Rubus (raspberry and blackberry), Fragaria (strawberry), and many others.
The species is the most basic of all classifications. For example, Salvia officinalis (common garden sage) and S. sclarea (clary sage) are both highly aromatic plants, yet each has unique characteristics. In a publication, once the genus name has been established, it may be abbreviated thereafter in that same paragraph. When a species or species name is unspecified, it may be indicated with the abbreviation “sp.” For example, Salvia sp. indicates an unspecified species of sage. More than one species is indicated by the abbreviation “spp.”—for example, Salvia spp. refers to a group of Salvia species. These two abbreviations are not italicized.
Plant Subspecies and Varieties
A plant that differs genetically from some members of its species, but not enough to be classified as a species of its own, may be designated as a subspecies. Subspecies often result from interbreeding in geographically isolated populations. A subspecies is indicated by the abbreviation “ssp.” or “subsp.”
Plant subspecies can be further categorized into varieties. A variety, or variation in a species, is designated by the abbreviation “var.” followed by an italicized variety name, as in Achillea millefolium var. rubrum. This name indicates that the plant is a variety of common yarrow (Achillea millefolium) that has red (rubrum) flowers rather than white ones. Variety names are italicized; cultivar names are not.
Cultivars and Hybrids
Unlike subspecies and varieties, cultivars do not occur naturally, but rather have been developed and perpetuated by cultivation. The term “cultivar” was coined from the words “cultivated” and “variety.” Cultivars can be hybrids created by breeding members of different species, or they may simply be desirable selections made from one species, chosen for a specific shape, flower color, aroma, or tolerance to certain environmental conditions. Cultivar names can be trademarked and registered with an International Cultivar Registration Authority according to specific nomenclature rules. Names of cultivars appear in single quotation marks after species names. Wandering through your local nursery, you might see a tree labeled Ginkgo biloba cv. ‘Autumn Gold’, indicating that this is a cultivated variety that someone has selected, named, and registered. Compared to the usual variant of the species, the cultivar ‘Autumn Gold’ has particularly beautiful golden leaves in fall and a more compact shape.
Hybrids occur when two different species cross (either in nature or by the hand of humans) to produce a new plant, usually with traits from both of the parents. They are denoted with an “×” between the genus and species names, as in Mentha × piperita, a cross between watermint (Mentha aquatica) and spearmint (Mentha spicata) that produces a plant known to all gardeners as peppermint.