There are two types of names that are applied to every useful plant: common names and scientific names. Common names are those that are used in ordinary conversation, and are in a vernacular language (e.g., wheat, apple, dandelion). These are convenient enough for most purposes, but can create confusion. Many plants have multiple common names, e.g., Chenopodium album can be called goosefoot, fat-hen, lamb’s quarters, or pigweed in different parts of North America. Moreover, the same common name may be used for different plants in different places: pigweed sometimes refers to species of Amaranthus, another edible weed. To minimize this problem, American commercial products are required to use “standardized common names.” The American Herbal Product Association’s book Herbs of Commerce, 2nd ed. (McGuffin et al., 2000) provides an extensive list of marketed species and specifies for each the preferred common name (which in some cases is the scientific name or a portion of it). However, many of their choices are not exclusively used in ordinary English. For example, although the name “Siberian ginseng” for Eleutherococcus senticosus has been specifically outlawed in commercial speech, it is still the common name most used by laymen. Then there is the question of scientific or commercial communication with people who speak different languages. And, of course, there are many foreign plants that have no common names in English. A scientific or Latin name is intended to provide a name for a plant that is “always correct” and can be used to communicate about that species among botanists worldwide, no matter what language they speak.
A full scientific name has three parts, in the form Genus epithet Auth.: the genus in which the plant is placed, the specific epithet, and an abbreviation of the name of the “authority” who first published that species in scientific literature. Binomial or Linnaean nomenclature requires that each species belong to a genus (plural: genera), a group of closely related plants, such as Echinacea or Panax. The species within each genus are denoted by specific epithets, which often refer to the plant’s appearance, origins or uses, e.g., angustifolia, narrow-leaved; africana, from Africa; officinale, of the shops (often used for medicinal plants). The species name includes both the genus and specific epithet, such as Echinacea angustifolia, and is printed in italics because it is in Latin or Latinized. The ending of the specific epithet is in the form of a Latin adjective, and changes depending upon the Latin gender of the genus. (In an homage to classical tradition, the name of a tree is traditionally treated as feminine no matter what its apparent gender.) After the first mention in a text, the genus is often abbreviated (except at the beginning of a sentence, where it should be spelled out), and the authority is always omitted, e.g., E. angustifolia. The reason for including the authority at first mention is that sometimes the same name has been used by two authors to refer to two different species, and providing the authority leaves no doubt which one is meant. The names of older authorities who published thousands of names are sometimes highly abbreviated, e.g. “L.” for Linnaeus. Sometimes the authority is of the form “Smith ex Jones,” which indicates that Jones actually published the name, but gave Smith credit as the authority responsible for the new species.
Most hybrid plants are not given formal names, but are described by their parentage, e.g. “Salix alba L. ×S. nigra Marsh.” If a particular hybrid is common and morphologically consistent, especially if it is reproductively isolated from its parents, it may be given a species name with an × before the specific epithet to indicate that it originated as a cross between two other species. This symbol may be added after a species has been described if it is later determined to be of hybrid origin, e.g., Mentha ×piperita L.
In the early days of botany, it was common for multiple names to be published for a single species, especially one that is found in many places or that is variable enough to be mistaken for a group of several species. These excess names are called synonyms. To reduce confusion, there should be only one correct name for a species under a given taxonomic treatment. The International Code of Botanical Nomenclature provides rules to determine what that correct name is. These rules are too complicated to recount in detail, but the principle of priority usually applies. That is, the oldest name that was “validly and legitimately” published for a species provides the specific epithet that should be used if possible. Certain errors in the publication of a name can make it invalid or illegitimate, meaning that it should not be used no matter what its age. (For example, if you publish a new name that had already been used by someone else, your name will be illegitimate.) If the strict application of these rules would lead to great nomenclatural confusion, there are procedures by which younger names can be conserved or older names rejected at the meetings of the International Botanical Congress, which are held every six years.
A particular issue to note is that the determination of the “correct name” only applies to a specified taxonomic treatment, and that under different treatments, a different name may be correct. Outside the botanical community, people often assume that species and even genera are groups that have a definite, objective biological meaning. This, perhaps, is because the mammals with which they are most familiar are easily distinguished and genetically separated from related species. In variable or freely hybridizing groups of plants, it is not always clear how many species should be recognized. The boundaries of species described in the literature are often someone’s taxonomic opinion rather than a hard fact, and other authors may choose to recognize more or fewer species in a group. The term “genus” has no real biological meaning at all. It is simply a group of closely related species that are recognized as such by being given the same name, often because they are similar enough to be recognized by their shared features. In modern taxonomic practice, it is desirable for genera to be natural groups, which comprise all of the descendants of some common ancestor, but the size of these groups is arbitrary. Where one taxonomist chooses to recognize one large genus, another might break that genus into half a dozen smaller genera. The same applies to the grouping of genera into families.
As for nomenclature, when a species is transferred from one genus to another, a new species name must be created for it. This name ordinarily uses the original specific epithet, with the original authority’s name or abbreviation in parentheses before that of the new authority. For example, black cohosh was described by Linnaeus as Actaea racemosa L.; later taxonomists chose to split Actaea into two genera, and the name Cimicifuga racemosa (L.) Nutt. was published by Nuttall for the same species. Each of these names is a synonym for the other, and A. racemosa is also the “basionym” of C. racemosa, or the original name from which its epithet is derived. Depending upon whether you recognize two genera or just one (which more accurately reflects relationships, according to the latest studies), the use of either name could be correct. Neither name becomes invalid or illegitimate. Occasionally the specific epithet cannot be transferred in that way because another species in the second genus has already been given that epithet. In this case, the epithet of a more recent synonym is used, or if necessary a new epithet is published for the species.