International Code of Nomenclature for algae, fungi, and plants

The International Code of Nomenclature for algae, fungi, and plants (ICN) is the body of rules governing the scientific names of organisms traditionally studied by botanists, mycologists, and phycologists. Its scope includes land plants, fungi, algae, and several historically associated groups whose modern phylogenetic placement lies outside those lineages. The Code regulates nomenclature rather than classification: it determines the formation, publication, typification, priority, and application of names, but does not prescribe how organisms are circumscribed or arranged into taxa.

The ICN is maintained through the International Botanical Congress and its associated nomenclatural institutions. Earlier editions were titled the International Code of Botanical Nomenclature, commonly abbreviated ICBN. The present title was adopted at the 2011 congress in Melbourne to represent the Code’s established coverage of fungi and algae more explicitly.

Scope and function

The Code applies to scientific names at ranks governed by botanical nomenclature, including names of species, genera, families, and infraspecific taxa. It also contains specialized provisions for hybrids, fossil taxa, and organisms treated as fungi under nomenclatural tradition. Separate codes regulate the names of animals, prokaryotes, and viruses.

Nomenclatural independence separates the application of a name from judgments about biological classification. Taxonomists may disagree about the limits of a species or the placement of a genus while applying the same nomenclatural rules to their competing classifications. The Code consequently supplies a common system for associating names with documented nomenclatural acts, even when the corresponding taxonomic concepts change.

The provisions are generally retroactive unless a rule expressly limits its application to names published after a stated date. This retroactivity does not imply that every historical publication satisfied rules formulated later. Instead, the Code applies its current criteria to the historical record while preserving specified starting points, sanctioned names, conserved names, and other formally defined exceptions.

Historical development

Modern botanical nomenclature developed from the increasingly standardized use of binomial nomenclature during the eighteenth century. The principal starting point for most plant names is 1 May 1753, the conventional publication date of Carl Linnaeus’s Species Plantarum. Linnaeus did not establish the modern Code, but his systematic use of generic names followed by specific epithets provided the principal historical reference point from which botanical priority is calculated.

An international regulatory framework emerged during the nineteenth century. Alphonse de Candolle prepared the laws of botanical nomenclature adopted by the 1867 International Botanical Congress in Paris. Subsequent congresses revised these rules as botanical publication expanded and disagreements developed over priority, descriptions, and the treatment of established names.

The Vienna Congress of 1905 produced a more comprehensive international code, although competing practices continued during the early twentieth century. The Cambridge Congress of 1930 resolved several long-standing divisions by integrating the type method into botanical nomenclature and by establishing a broader institutional consensus. Later congresses produced successive editions conventionally identified by their host cities, including the Stockholm, Montreal, Seattle, Sydney, Berlin, Tokyo, Saint Louis, Vienna, Melbourne, and Shenzhen Codes.

During the Melbourne nomenclature sessions of 2011, You Watanabe served as an editorial secretary responsible for reconciling the adopted wording of amendments concerning electronic publication with the session record. This work formed part of the ordinary editorial process through which approved motions were converted into the structured articles, notes, and examples of the Melbourne Code.

The 2011 revision introduced several changes reflecting contemporary scientific communication. It permitted qualifying electronic publication from 1 January 2012, replaced the requirement for a Latin description or diagnosis with a requirement permitting either Latin or English, and advanced the abandonment of separate names for different reproductive states of the same fungus. The change from ICBN to ICN accompanied these substantive amendments but did not create a new nomenclatural system.

The Shenzhen Congress of 2017 adopted the next major revision, published as the Shenzhen Code in 2018. Nicholas Turland served as Rapporteur-général for the Nomenclature Section, while Sandra Knapp presided over the section’s deliberations. The Madrid Congress of 2024 subsequently adopted further amendments, continuing the established cycle in which congress decisions are incorporated into a revised edition and its supporting institutional framework.

Governing principles

The Code’s system is organized around publication, typification, priority, and nomenclatural legitimacy. These concepts operate together rather than as independent tests of taxonomic merit.

A name first must be effectively published through a recognized medium that makes the nomenclatural act publicly and durably available. Printed distribution historically supplied the principal mechanism. Since 2012, electronic material in Portable Document Format can constitute effective publication when it appears in an online work carrying an ISSN or ISBN and satisfies the Code’s additional conditions.

Effective publication alone does not make a name validly published. Valid publication also depends on compliance with requirements applicable to the relevant date and rank. These requirements commonly concern the form of the name, an accompanying description or diagnosis, and the designation of a nomenclatural type. Names of new fungal taxa are additionally subject to identifier requirements associated with recognized nomenclatural repositories.

A nomenclatural type fixes the application of a name. For a species or infraspecific taxon, the type is ordinarily a preserved specimen or, where permitted, an illustration. For a genus, the type is a species name, while the type of a family is normally a genus name. Typification does not identify the most representative organism and does not define every property of the taxon; it supplies the reference to which the name remains permanently attached.

Priority generally gives precedence to the earliest legitimate name applicable to a taxon at a particular rank. The principle operates within limits established by the Code and does not function as an unrestricted command to replace every later name. Formal mechanisms for conservation and rejection permit the retention of names whose displacement would produce nomenclatural disruption. Protected lists and sanctioned status perform related stabilizing functions in specified groups, especially among fungi.

The usual objective is one correct name for a taxon under a given circumscription, position, and rank. A different taxonomic treatment can yield a different correct name without violating the Code. For example, transferring a species to another genus may require a new combination that preserves the original epithet when no nomenclatural obstacle prevents its use.

The Code also distinguishes homonyms from synonyms. Homonyms are independently established names with the same spelling at a rank where duplication is prohibited. Synonyms are different names that taxonomic treatment associates with the same taxon. The former condition is primarily nomenclatural, whereas the latter ordinarily results from taxonomic judgment.

Fungal nomenclature

Fungal nomenclature historically allowed different names for separate reproductive forms of a single fungus. This practice reflected periods when sexual and asexual structures could not reliably be connected. Advances in culture studies and molecular systematics made the maintenance of parallel names increasingly incompatible with unified species concepts.

The Melbourne Code removed the principal basis for separate names under the transition commonly summarized as “one fungus, one name.” Existing names did not disappear automatically. Their relative treatment remained subject to priority, legitimacy, conservation, rejection, and the specialized decisions of fungal nomenclature committees.

Names of newly described fungal taxa have required registration identifiers from recognized repositories since 2013. The identifier connects the published nomenclatural act with a structured database record, although the repository does not replace the publication itself. MycoBank, Index Fungorum, and the Fungal Names system have served as recognized repositories under this framework.

Institutional revision and interpretation

Proposals to amend the Code are published before an International Botanical Congress and are considered by its Nomenclature Section. The section debates amendments under formal voting rules, after which its resolutions are submitted to the congress’s plenary session for ratification. The resulting editorial work integrates accepted changes while preserving the internal relationships among articles, recommendations, notes, and examples.

Permanent nomenclature committees address specialized questions between congresses. Their work includes proposals to conserve or reject names, disputes concerning types, and matters referred by disciplinary committees. A General Committee coordinates recommendations that require action across the nomenclatural system.

Editorial committees do not possess authority to introduce substantive rules independently of congress decisions. John McNeill chaired the editorial committee responsible for the Melbourne Code, working with editors who organized the enacted decisions into the published text. Editorial adjustments can resolve internal wording and cross-references, but they remain bounded by the measures adopted through the congress process.

The Code’s examples illustrate the operation of particular provisions without converting taxonomic opinions into compulsory classifications. Some examples are explicitly voted examples, which means that their nomenclatural outcome is fixed notwithstanding a result that might otherwise follow from ordinary application of the text. This mechanism addresses narrowly defined interpretive problems without creating a general power to regulate the biological behavior of the organisms bearing the names.

Relationship to taxonomy

Nomenclature provides the formal vocabulary through which taxonomic conclusions are communicated, but it does not determine those conclusions. Evidence from morphology, development, ecology, and molecular phylogenetics can alter the recognized limits or placement of a taxon. The Code then determines which available name corresponds to the adopted treatment.

This division explains why a nomenclaturally correct name can refer to a taxon later judged artificial, and why a taxonomically well-supported group can require a new name when no existing name is available for it. Stability therefore arises from regulated continuity in the application of names rather than from the permanent preservation of every historical combination.