Taxonomy and Nomenclature

Why Naming a Plant and Classifying a Plant Are Not the Same Thing

Plant taxonomy and botanical nomenclature are closely connected, but they solve different problems.

Taxonomy asks what biological entities should be recognized and how they are related. Are two populations separate species? Should several species remain together in one genus? Does a lineage deserve family rank?

Nomenclature asks what scientific name can correctly be applied once that taxonomic decision has been made.

This distinction explains much apparent disagreement among floras, databases, papers, and Tree TSAR. Two sources can follow the same international nomenclatural rules while accepting different taxonomic classifications.

The Rules for Botanical Names

Scientific names of algae, fungi, and plants are governed by the International Code of Nomenclature for algae, fungi, and plants (ICN), commonly called the Code. The current edition is the Madrid Code, published in 2025 and reflecting decisions of the Twentieth International Botanical Congress held in Madrid in July 2024 (Turland et al. 2025).

The Code addresses publication, priority, typification, legitimacy, conservation, rejection, author citation, hybrid nomenclature, and the formation of names. It does not provide a biological formula for deciding what constitutes a species or how broadly a genus should be circumscribed.

Names Need Anchors

Botanical names are linked to types. The type fixes the application of a name; it is not an idealized or average example of everything included in the taxon.

A species name is normally typified by a specimen or other type permitted by the Code. A generic name is typified through a species name, and family names are ultimately tied to a type genus. This becomes crucial when classifications change. If a genus is divided, the lineage containing the nomenclatural type retains the generic name; other lineages must use other available names or receive properly published new ones.

The International Plant Names Index (IPNI) is particularly useful for this kind of information. IPNI describes itself as a nomenclatural index and provides data such as spelling, authorship, types, and first place and date of publication; it explicitly directs users elsewhere for taxonomic opinion.

Priority, Conservation, and Rejection

Botanical nomenclature generally follows priority: among competing legitimate names at a rank, the earliest applicable name normally has precedence. But “the oldest name always wins” is too simple. The Code provides mechanisms to conserve names whose continued use promotes stability and to reject names whose application would be seriously disruptive or otherwise problematic under its provisions.

This balance is one reason botanical nomenclature can preserve historical continuity without allowing priority to destabilize every familiar name mechanically.

Published, Legitimate, Accepted, and Synonymous

These terms answer different questions:

Validly Published - The name satisfied the applicable requirements for valid publication.
Legitimate - The name is not rendered illegitimate under provisions such as those concerning certain later homonyms or superfluous names.
Accepted - A particular taxonomic treatment chooses to use that name for the taxon it recognizes.
Synonym - A name that, in that treatment, applies to a taxon accepted under another name.

The first two are principally nomenclatural. The last two depend heavily on taxonomic judgment. IPNI makes the distinction explicit: it is a nomenclatural database, whereas Plants of the World Online provides accepted-name and synonym treatments.

Why Databases Can Disagree

Different botanical resources are designed for different jobs. IPNI records published names and nomenclatural details. Plants of the World Online presents a taxonomic backbone. Regional floras may follow regional traditions, and specialist revisions may adopt newly proposed circumscriptions before broader databases do.

Tree TSAR therefore does not resolve disagreement by counting databases. It asks which source is authoritative for the question at hand and, when classification itself is disputed, returns to the underlying evidence.

What Does the Author’s Name Mean?

The authorship following a scientific name identifies the nomenclatural author or authors associated with publication of that name or combination. In Ginkgo biloba L., L. refers to Linnaeus.

Parentheses often preserve part of a name’s history. When a species is moved to another genus, the author of the basionym normally appears in parentheses, followed by the author of the new combination. Scientific names are italicized where appropriate; authorships and rank indicators remain roman.

Scientific Names Can Change Without the Plant Changing

A species may receive a new combination because phylogenetic evidence changes its generic placement. A different name may become necessary because an older legitimate name has priority. A type may be reexamined and show that a familiar name has been applied incorrectly. These are different pathways to a changed label and should be explained as such.

This is why Tree TSAR separates the taxonomic question “What is this lineage?” from the nomenclatural question “What name correctly applies to it?”

Names Below Species

The Code provides formal ranks below species. Tree TSAR uses ssp. for subspecies, var. for variety, and f. for forma in public display, with rank abbreviations in roman type.

The Code governs how such names are formed and applied nomenclaturally; it does not dictate how much biological differentiation qualifies as a subspecies or variety. Tree TSAR addresses those decisions through its species-delimitation methodology.

Hybrids

The Madrid Code contains specific provisions for hybrid nomenclature. The multiplication sign × can indicate hybrid nature, and notho- is used in the terminology of nothotaxa. Hybrid nomenclature does not imply that parent species or parent genera cease to be valid simply because they can hybridize.

Graft chimeras and highly synthetic engineered entities are biologically different from sexual hybrids and are treated separately in Hybrids, Chimeras, and Synthetic Plants.

When the Biology Is Clear but the Name Does Not Yet Exist

Scientific interpretation can move faster than formal nomenclature. A phylogenetic analysis may strongly support transferring a species to another genus before anyone has validly published the required combination.

Tree TSAR may mark such a placement with [TSAR] when a provisional editorial expression is needed and no suitable validly published name is available. [TSAR] is not botanical authorship, does not create a validly published name, and does not replace the requirements of the Code. Its purpose is to prevent a strongly supported biological placement from being confused with a completed nomenclatural act.

Where a validly published applicable name already exists, Tree TSAR uses it rather than creating a provisional substitute.

Tree TSAR Can Disagree with a Published Treatment

Peer-reviewed publication makes a proposed classification part of the scientific literature; it does not make the rank treatment compulsory. A study can recover a strong phylogeny while leaving several defensible ways to delimit genera or families.

Tree TSAR synthesizes the evidence and may adopt a different circumscription when it better satisfies evolutionary coherence, diagnosability, nomenclatural feasibility, stability, and usability. That editorial judgment does not alter the nomenclatural rules themselves.

Tree TSAR Is Not a Nomenclatural Authority

Tree TSAR can interpret names and classifications, identify apparent nomenclatural problems, and explain why it accepts one treatment over another. A webpage cannot, merely by displaying a new combination, transform it into valid publication under the Code.

That boundary lets Tree TSAR be interpretive without confusing editorial synthesis with formal nomenclatural action.

Explore Further

Scientific Names and Common Names - How formal and vernacular names serve different purposes.
Synonyms, Hybrids, and Search - How Tree TSAR preserves alternative names while maintaining one accepted page.
How Tree TSAR Delimits Species - The biological evidence used before an infraspecific or species name is selected.
How Tree TSAR Delimits Genera - How phylogeny and nomenclature interact when generic boundaries change.

References and Further Reading

International Plant Names Index (n.d.) International Plant Names Index. Royal Botanic Gardens, Kew; Harvard University Herbaria; Australian National Herbarium. Accessed 17 August 2026. https://www.ipni.org/

Turland NJ, Wiersema JH, Barrie FR, Gandhi KG, Gravendyck J, Greuter W, Hawksworth DL, Herendeen PS, Klopper RR, Knapp S, et al. (eds.) (2025) International Code of Nomenclature for algae, fungi, and plants (Madrid Code). Regnum Vegetabile 162. University of Chicago Press. https://www.iaptglobal.org/madrid-code-online