Comprehensive Guide to Botanical Taxonomy and Nomenclature

Principles of Plant Taxonomy and the Population Concept

Plant taxonomy is governed by the fundamental concept of the species or population. It must be recognized that in many cases, the definition of a "species" escapes a strictly objective and general definition that can be applied universally without the need for personal or subjective judgment. It is often noted that specific criteria or standards are applied in appropriate cases to identify and distinguish a species. One such criterion is the similarity of the constituent individuals, which allows for their collective recognition. Furthermore, the range of variation within a species must be distinct from other species through clear and continuous gaps.

A species is generally defined as a group of individuals (not necessarily exclusively) that share a natural and defined geographical distribution area. It is important to note that most species do not occupy every single area that is ecologically suitable for their distribution. Within a species, individuals are capable of interbreeding with one another without losing their fertility or significantly decreasing the fertility rate of their offspring. Conversely, if individuals from different species interbreed, the resulting fertility units (offspring) typically exhibit reduced or no fertility.

The Taxonomic Hierarchy: Units Above the Species Level

There are several taxonomic ranks higher than the species level. The first is the Genus ( $Genus$ ), which comprises a group of species that resemble each other more closely than they resemble species in other genera. The concept of the genus has a mathematical nature based on the characteristics that distinguish species, expressing the existence of close natural relationships. An example given is the genus of roses, $Rosa$ .

Above the genus is the Family ( $Family$ ). Families consist of genera that are closely related to one another. A family is named after the genus that serves as its type, typically by adding the suffix $-aceae$ . An example provided is the rose family, $Rosaceae$ .

Next is the Order ( $Order$ ), which groups together closely related families. The name of the order is derived from the most representative family within it, characterized by the suffix $-ales$ . For example, the order $Rosales$ belongs to the class of dicotyledons within the division of flowering plants. Specifically, the order is linked to the family $Rosaceae$ .

Higher Classification Groups: Classes and Divisions

The Class ( $Class$ ) is a group of orders that share certain characteristics. The name of the class is often distinguished by the suffix $-opsida$ or $-idae$ . Examples include the class of flowering plants, $Magnoliopsida$ , or the dicotyledons, $Magnoliidae$ .

The Division or Phylum ( $Phylum$ ) aggregates all similar classes into a single large group. The suffix for a division is typically $-phyta$ . Collectively, these divisions make up the Kingdoms, which are the largest taxonomic groups. In summary, the formal sequence follows this order: Kingdom, Division ( $-phyta$ ), Class ( $-opsida$ ), Order ( $-ales$ ), Family ( $-aceae$ ), Genus, and Species ( $sp.$ ).

Taxonomic Units Below the Species Level

Classification also extends below the species level to account for finer variations. The Subspecies ( $Subspecies$ or $subsp.$ / $ssp.$ ) consists of groups or populations within a species that are distinct from one another. While they remain fertile and capable of interbreeding, they are distinguishable by a few traits. These populations are often geographically or ecologically isolated, leading them to be considered a subspecies.

Another lower rank is the Variety ( $variety$ or $var.$ ). Varieties are populations that may grow together under natural conditions yet maintain distinct characteristics. Typically, the difference between two varieties is less pronounced than the difference between two subspecies. While the distinguishing characteristics in both cases may be few, geographic isolation is an additional factor that favors the designation of a subspecies over a variety.

Scientific Versus Common Nomenclature

Common or colloquial names ( $Vernacular names$ ) often pose scientific challenges because the same common name might be applied to several different but similar-looking species. For instance, many types of clover are referred to as "trifolium" or "three-leaf" in common parlance. Conversely, different species that share the same geographical or medicinal use might be referred to by a single common name based on their medical properties or the diseases they treat.

In the 18th century, when Latin was the language of medical and clinical sciences, plants were named using long descriptive phrases known as polynomials. For example, what is known today as $Trifolium repens$ was previously described by a cumbersome phrase: $Trifolium$ $caule$ $repente$ , $spicis$ $depressis$ , $siliquis$ $tetraspermis$ . This naming method proved far too long and impractical for effective scientific communication.

The Advent of Binomial Nomenclature

In the middle of the 18th century, the need for a standardized naming system led to the proposal that every species should be named using only two words. Carolus Linnaeus ( $Linnaeus$ ) suggested the Binomial Nomenclature system. In this system, the first word represents the Genus ( $Genus$ ) and is treated as a noun, written with an initial capital letter. The second word represents the species epithet ( $Species$ ), written in lowercase. Together, these two words form the scientific name of the plant.

The first major scientific work to employ this system was "Species Plantarum," published in $1753$ . This book is regarded as the starting point for modern botanical nomenclature; names published before this date are generally not recognized by the scientific community. The genus name describes the organism, and the species epithet provides a specific description, often appearing only once in combination with that genus.

The International Code of Botanical Nomenclature (ICBN)

The rules for Latin naming were first formalized in a meeting at the end of the 19th century, which led to the publication of the "International Code of Botanical Nomenclature," abbreviated as $ICBN$ . This code is reviewed periodically in international conferences held across the 20th century, such as those in Geneva. The code establishes seven primary taxonomic units, as well as secondary units, each with a specific required suffix.

While the suffix $-aceae$ is the standard for families, eight specific families have traditional names that are still accepted alongside their modern, systematic names. The code suggests using the modern equivalents:

Gramineae is now $Poaceae$
Cruciferae is now $Brassicaceae$
Leguminosae is now $Fabaceae$
Labiatae is now $Lamiaceae$

Formatting and Validation of Scientific Names

A full scientific name includes the Genus, species, and the author citation. For example: $Vicia$ $sativa$ $L.$ $Subsp.$ $amphicapa$ ( $Dorth$ ) $Aschers$ $et$ $Graebn$ . Another example is $Brassica$ $oleraceae$ $var.$ $capitata$ . The genus must always start with a capital letter, while the species and variety epithets are written in lowercase.

Author names are often abbreviated, such as $L.$ for Linnaeus or $Boiss.$ for Boissier. For instance, $Malva$ $sylvestris$ $L.$ or $Malvella$ $oxyloba$ $Boiss$ . If a species is moved to a different genus or its rank is changed, the name of the original author is placed in parentheses, followed by the name of the scholar who made the change.

For a new plant name to be legally recognized, it must be published in a globally available journal and must include a description of the plant in Latin. Furthermore, the researcher must designate a "Type" specimen, which is a dried or preserved specimen (often in alcohol) kept in a recognized herbarium. Hybrid species are indicated by a multiplication sign ( $\times$ ) between the parent species, such as $Calystegia$ $sepium$ $\times$ $Calystegia$ $silvatica$ . Alternatively, a hybrid can be given a binomial name preceded by the sign, such as $Crataegus$ $\times$ $shumiana$ .

Usage of External Authors and Citations

If a researcher uses a name proposed by another scientist that was not legally published, both names are cited, separated by "ex." For example: $Ramatuella$ $virens$ $Spruce$ $ex$ $Eichle$ . This indicates that Spruce provided the name on herbarium specimens, but Eichle was the one who published it legally. Full citations should also include the publication source and page number, such as: $Ranunculus$ $L.$ , $acris$ $L.$ , $Species$ $Plantarum$ $p.$ ( $554$ ). If the work is part of a larger volume edited by others, the word "in" is used: $Ramatuella$ $virens$ $Spruce$ $ex$ $Eichle$ $in$ $Martius$ , $Flora$ $Brasiliensis$ .

Cultivars and the Concept of Synonyms

Cultivated plants, or "Cultivars," are given common names in living languages rather than Latin. These follow the International Code of Nomenclature for Cultivated Plants. A cultivar name is written after the species name, enclosed in single quotes and not in italics. Example: $Rubus$ ( $idaeus$ ) ' $Malling$ $Wonder$ '.

Synonyms ( $Syn.$ ) occur when a single species is given multiple names by different taxonomists working in isolation or at different times. The Rule of Priority states that the oldest legally published name is the correct one. For example, a single species was named differently over time:

$1830$ : $Rotala$ $mexicana$
$1856$ : $R.$ $pusilla$ (from Madagascar)
$1862$ : $R.$ $leptopetala$ (from Ethiopia)
$1867$ : $R.$ $sphuerocarpa$ (from Brazil) Because $Rotala$ $mexicana$ was the earliest name ( $1830$ ), it is the valid name, and the others are considered synonyms. Taxonomic perspectives may also differ; some scientists (lumpers) group many variations into a single large species, while others (splitters) divide them into numerous small species. Notations for synonyms often look like: $Erodium$ $cicutarium$ ( $L.$ ) $L'Her$ $=$ $Geranium$ $cicutarium$ $L.$ .