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There are over 56 genera of flowering plants estimated to contain at least 500 described species. The largest of these is currently the legume genus Astragalus (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 Plantarum was Euphorbia, with 56 species; Linnaeus believed that no genus should contain more than 100 species.[1]

Part of the disparity in genus sizes is attributable to historical factors. According to a hypothesis published by Max Walters in 1961, the size of plant genera is related to the age, not of the taxon itself, but of the concept of the taxon in the minds of taxonomists.[2] Plants which grew in Europe, where most of the early taxonomy was based, were therefore divided into relatively small genera, while those from the tropics were grouped into much larger and more heterogeneous genera.[2][3] 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 grasses, were split into many genera.[2] Where there were many classical names for groups of plants, such as in Apiaceae / Umbelliferae or Brassicaceae / Cruciferae, small genera were defined, whereas groups not subdivided by classical authors remained as larger genera, such as Carex.[2][3] A number of biological factors also influence the number of species. For instance, the occurrence of apomixis allows the recognition of large numbers of agamospecies, and such taxa have helped to bolster genera such as Ranunculus and Potentilla.[1]

The introduction of infrageneric taxa (such as the subgenus, section and series) 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. Corner believed that studying large genera might enable greater insights into evolutionary biology, and he concentrated his efforts on large tropical genera such as Ficus.[1]

Largest genera

According to a 2004 analysis by the botanical taxonomist David G. Frodin, a total of 57 genera of flowering plants contain at least 500 species.[1] It is clear that there are other genera with over 500 species, as the work of taxonomists continues. The actual numbers of species are imprecisely known, as many of the genera have not been the subject of recent monographs.[3] 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, Asplenium and Cyathea.[1]

Genera of flowering plants with at least 500 species[1]

Rank	Genus	Species	Family	Species list
1	Astragalus	3,270	Fabaceae	List of Astragalus species
2	Bulbophyllum	2,032	Orchidaceae	List of Bulbophyllum species
3	Psychotria	1,951	Rubiaceae	List of Psychotria species
4	Euphorbia	1,836	Euphorbiaceae	List of Euphorbia species
5	Carex	1,795	Cyperaceae	List of Carex species
6	Begonia	1,484	Begoniaceae	List of Begonia species
7	Dendrobium	1,371	Orchidaceae	List of Dendrobium species
8	Acacia	c. 1,353	Fabaceae	List of Acacia species
9	Solanum	c. 1,250	Solanaceae	List of Solanum species
10	Senecio	c. 1,250	Asteraceae	List of Senecio species
11	Croton	1,223	Euphorbiaceae	List of Croton species
12	Pleurothallis	1,120+	Orchidaceae	List of Pleurothallis species
13	Eugenia	1,113	Myrtaceae	List of Eugenia species
14	Piper	1,055	Piperaceae	List of Piper species
15	Ardisia	1,046	Primulaceae	List of Ardisia species
16	Syzygium	1,041	Myrtaceae	List of Syzygium species
17	Rhododendron	c. 1,000	Ericaceae	List of Rhododendron species
18	Miconia	1,000	Melastomataceae	List of Miconia species
19	Peperomia	1,000	Piperaceae	List of Peperomia species
20	Salvia	945	Lamiaceae	List of Salvia species
21	Erica	860	Ericaceae	List of Erica species
22	Impatiens	850	Balsaminaceae	List of Impatiens species
23	Cyperus	839	Cyperaceae	List of Cyperus species
24	Phyllanthus	833	Phyllanthaceae	List of Phyllanthus species
25	Allium	815	Amaryllidaceae	List of Allium species
26	Epidendrum	800	Orchidaceae	List of Epidendrum species
27	Vernonia	800–1,000	Asteraceae	List of Vernonia species
28	Lepanthes	c. 800	Orchidaceae	List of Lepanthes species
29	Anthurium	789	Araceae	List of Anthurium species
30	Diospyros	767	Ebenaceae	List of Diospyros species
31	Ficus	750	Moraceae	List of Ficus species
32	Indigofera	700+	Fabaceae
33	Justicia	c. 700[4]	Acanthaceae	List of Justicia species
34	Silene	700–900	Caryophyllaceae	List of Silene species
35	Oxalis	700	Oxalidaceae	List of Oxalis species
36	Crotalaria	699	Fabaceae	List of Crotalaria species
37	Centaurea	695	Asteraceae	List of Centaurea species
38	Cassia	692	Fabaceae
39	Eucalyptus	681	Myrtaceae	List of Eucalyptus species
40	Oncidium	680	Orchidaceae
41	Galium	661	Rubiaceae	List of Galium species
42	Cousinia	655	Asteraceae	List of Cousinia species
43	Ipomoea	650	Convolvulaceae	List of Ipomoea species
44	Dioscorea	631	Dioscoreaceae
45	Cyrtandra	622	Gesneriaceae	List of Cyrtandra species
46	Helichrysum	600	Asteraceae	List of Helichrysum species
47	Ranunculus	600	Ranunculaceae	List of Ranunculus species
48	Habenaria	600	Orchidaceae	List of Habenaria species
49	Schefflera	584	Araliaceae	List of Schefflera species
50	Ixora	561	Rubiaceae	List of Ixora species
51	Berberis	556	Berberidaceae	List of Berberis species
52	Quercus	531	Fagaceae	List of Quercus species
53	Pandanus	c. 520	Pandanaceae	List of Pandanus species
54	Panicum	500+	Poaceae	List of Panicum species
55	Polygala	500	Polygalaceae	List of Polygala species
56	Potentilla	500	Rosaceae	List of Potentilla species

References

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