There are 57
generaIn biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of
flowering plantThe flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s estimated to contain at least 500
described speciesA species description or type description is a formal description of a newly discovered species, usually in the form of a scientific paper. Its purpose is to give a clear description of a new species of organism and explain how it differs from species which have been described previously, or are...
. The largest of these is currently the
legumeThe Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...
genus
AstragalusAstragalus is a large genus of about 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae, subfamily Faboideae. The genus is native to temperate regions of the Northern Hemisphere...
(milk-vetches), with over 3,000 species.
The sizes of plant genera vary widely from those containing a single species to genera containing thousands of species, and this disparity became clear early in the history of plant classification. The largest genus in Carl Linnaeus' seminal
Species PlantarumSpecies Plantarum was first published in 1753, as a two-volume work by Carl Linnaeus. Its prime importance is perhaps that it is the primary starting point of plant nomenclature as it exists today. This means that the first names to be considered validly published in botany are those that appear...
was
Euphorbia, with 56 species; Linnaeus believed that no genus should contain more than 100 species.
Part of the disparity in genus sizes is attributable to historical factors. According to a hypothesis published by
Max WaltersDr Max Walters was a British botanist and academic. As a conscientious objector in the Second World War, he worked as a hospital orderly in Sheffield and Bristol...
in 1961, the size of plant genera is related to the age, not of the
taxon|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
itself, but of the concept of the taxon in the minds of taxonomists. Plants which grew in Europe, where most of the early taxonomy was based, were therefore divided into relatively small genera, while those from the
tropicsThe tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately N and the Tropic of Capricorn in the southern hemisphere at S; these latitudes correspond to the axial tilt of the Earth...
were grouped into much larger and more heterogeneous genera. Likewise, plants which shared common medicinal properties, such as the many species of
Euphorbia, were united into a single genus, while plants of diverse uses, such as the
grassesThe Poaceae is a large and nearly ubiquitous family of flowering plants. Members of this family are commonly called grasses, although the term "grass" is also applied to plants that are not in the Poaceae lineage, including the rushes and sedges...
, were split into many genera. Where there were many classical names for groups of plants, such as in
Apiaceae / UmbelliferaeThe Apiaceae , commonly known as carrot or parsley family, is a group of mostly aromatic plants with hollow stems. The family is large, with more than 3,700 species spread across 434 genera, it is the sixteenth largest family of flowering plants...
or
Brassicaceae / CruciferaeBrassicaceae, a medium sized and economically important family of flowering plants , are informally known as the mustards, mustard flowers, the crucifers or the cabbage family....
, small genera were defined, whereas groups not subdivided by classical authors remained as larger genera, such as
CarexCarex is a genus of plants in the family Cyperaceae, commonly known as sedges. Other members of the Cyperaceae family are also called sedges, however those of genus Carex may be called "true" sedges, and it is the most species-rich genus in the family. The study of Carex is known as...
. A number of biological factors also influence the number of species. For instance, the occurrence of
apomixisIn botany, apomixis was defined by Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization. This definition notably does not mention meiosis...
allows the recognition of large numbers of agamospecies, and such taxa have helped to bolster genera such as
RanunculusRanunculus is a large genus of about 600 species of plants in the Ranunculaceae. Members of the genus include the buttercups, spearworts, water crowfoots and the lesser celandine....
and
PotentillaPotentilla is the genus of typical cinquefoils, containing about 500 species of annual, biennial and perennial herbs in the rose family Rosaceae. They are generally Holarctic in distribution, though some may even be found in montane biomes of the New Guinea Highlands...
.
The introduction of infrageneric taxa (such as the
subgenusIn biology, a subgenus is a taxonomic rank directly below genus.In zoology, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the generic name and the specific epithet: e.g. the Tiger Cowry of the Indo-Pacific, Cypraea tigris Linnaeus, which...
,
sectionIn botany, a section is a taxonomic rank below the genus, but above the species. The subgenus, if present, is higher than the section, and the rank of series, if present, is below the section. Sections are typically used to help organise very large genera, which may have hundreds of species...
and
seriesSeries is a low-level taxonomic rank below that of section but above that of species.In botany, a series is a subdivision of a genus...
) in the 19th century by botanists including Augustin Pyrame de Candolle allowed the retention of large genera that would otherwise have become unwieldy.
E. J. H. CornerEdred John Henry Corner FRS was a botanist who occupied the posts of assistant director at the Singapore Botanic Gardens and Professor of Tropical Botany at the University of Cambridge...
believed that studying large genera might enable greater insights into evolutionary biology, and he concentrated his efforts on large tropical genera such as
FicusFicus is a genus of about 850 species of woody trees, shrubs, vines, epiphytes, and hemiepiphyte in the family Moraceae. Collectively known as fig trees or figs, they are native throughout the tropics with a few species extending into the semi-warm temperate zone. The Common Fig Ficus is a genus of...
.
Largest genera
A total of 57 genera of flowering plants contain at least 500 species, according to a 2004 analysis by the botanical taxonomist David Frodin. The actual numbers of species are imprecisely known, as many of the genera have not been the subject of recent
monographA monograph is a work of writing upon a single subject, usually by a single author.It is often a scholarly essay or learned treatise, and may be released in the manner of a book or journal article. It is by definition a single document that forms a complete text in itself...
s. For instance, estimates of the number of species in the orchid genus
Pleurothallis range from 1,120 to 2,500. Genera from other groups of vascular plants, but which have similarly large numbers of species, include
Selaginella,
AspleniumAsplenium is a genus of about 700 species of ferns, often treated as the only genus in the family Aspleniaceae, though other authors consider Hymenasplenium separate, based on molecular phylogenetic analysis of DNA sequences, a different chromosome count, and structural differences in the rhizomes...
, and
CyatheaCyathea is a genus of tree ferns, the type genus of the fern order Cyatheales. They are mostly terrestrial ferns, usually with a single tall stem. Rarely, the trunk may be branched or creeping. Many species also develop a fibrous mass of roots at the base of the trunk. The genus has a pantropical...
.