Acalypha

The genus Acalypha was first mentioned by Carl Linnaeus in his Corollarium Generum Plantarum (1737) and formally described in Species Plantarum (1753). Linnaeus placed it in a class (monoecia monadelphia) together with other Euphorbiaceae such as Croton, Jatropha and Ricinus. Since then, the circumscription of the genus has remained largely stable, and Acalypha is considered a well-defined natural group.

In Species Plantarum, Linnaeus established the first three binomials: Acalypha virginica from North America, and A. indica and A. australis from Asia. In 1760, Linnaeus added A. virgata from Jamaica and Nikolaus Joseph von Jacquin described three more species (A. villosa, A. carthagenensis, and A. corensis) from the Caribbean region in his Enumeratio Systematica Plantarum. The first Acalypha iconographies are also shown in the books by Jacquin; there coloured plates of A. villosa (Jacquin, 1776), A. alopecuroides (Jacquin, 1792), A. cuspidata, A. diversifolia and A. macrostachya (Jacquin, 1797) can be found.

Until late 18^th century the outstanding incorporations to the genus were those from Pehr Forsskål, a Linnaeus student, who sets forth six new species in his Flora Aegyptiaco-Arabica in 1775; and also those from Swedish botanist Olof Swartz, author of Nova Genera et Species Plantarum seu Prodromus, in 1788, where eight new species are published as a result of a journey around Western Indies. In 1789 Antoine de Jussieu also incorporates Acalypha in his Genera Plantarum.

The 19^th century marked a major expansion in Acalypha taxonomy. In 1800 Antonio José Cavanilles describes six species from Mexico: these were collected by Luis Née, a botanist from Malaspina Expedition (Cavanilles, 1800). One year later, such species are again included in his book Icones et Descriptiones Plantarum together with some excellent monochrome drawings.

In 1804 and 1816, Jean Louis Marie Poiret makes the first compilation of all Acalypha known species in the Encyclopédie Méthodique, Botanique by Jean Baptiste Lamark (vol. VI and suppl. IV). There he describes forty species, thirteen of them for the first time. In 1805, Carl Ludwig Willdenow compiles and describes thirty-nine Acalypha species in his Species Plantarum and proposed the first infrageneric classification based on the position and sexuality of flowers and inflorescences.

As an offspring from the American journey made by Alexander von Humboldt and Aimé Bonpland, twelve new Acalypha species, from Mexico and Colombia, are described by Sigmund Kunth in Nova Genera et Species Plantarum (1817, 1825). In 1826, Kurt Sprengel compiles fifty-eight Acalypha species in his Systema Vegetabilium, seven of them brand new, and refined Willdenow’s classification by separating herbaceous from woody species.

Along the first half of 19th century, many new species are published all around the world, with emphasis on the works by Eduard Poeppig (1841) and George Bentham (1839, 1844) about American species.

In 1858 Henri Ernest Baillon, in his Étude générale du groupe des Euphorbiacées, provided a detailed morphological account and introduced two informal sections based on inflorescence and calyx morphology. Between 1860 and 1864, Baillon described over 50 new species, mainly from South America, especially from Brazil. Baillon continued to publish a series of works under the title Euphorbiacées américaines, to be issued in the first volumes of Adansonia journal, between 1860 and 1864. More than fifty new species of Acalypha are described thereto, most of them from South America.

Around the same time, Johannes Müller Argoviensis, commissioned for De Candolle’s Prodromus, undertook a critical revision of Acalypha. In 1864, he published preparatory work in Flora journal, introduce the first formal infrageneric classification: two sections, Linostachys and Euacalypha (Müller Argoviensis, 1864). The Linostachys section is based on the homonymous genus from Klotzsch (1846), and includes seven species with pedicellate male flowers and bracts non-increasing in the fruit. The Euacalypha section incorporates all 157 remaining species, which have sessile female flowers and bracts increasing in the fruit. In 1865, his revision in Linnaea journal recognised 164 species, 67 of them newly described. Euacalypha was further divided into series, subseries, and informal groups (designated by the symbol “§”) based on inflorescence sexuality and arrangements. Although most of Müller’s infrageneric names are illegitimate under current nomenclatural rules, this was the first attempt to classify address the genus’ diversity.

In 1866, De Candolle’s Prodromus appeared, with Müller adding 30 more species, bringing the total to 215, which are ordered as per such a complex infrasectional classification. He maintained the two-section framework in Flora Brasiliensis (1874), where 10 additional species were described, though the infrasectional classification was not retained.

From 1867 to 1923, no major taxonomic reorganisation was proposed, but ca. 220 new species were published. Significant among these was John Hutchinson’s contribution (1913) to the Flora of Tropical Africa, where he treated 42 species using a dichotomous key – the first of its kind for the genus.

At the end of the 19^th century, Ferdinand Pax, one of the most productive collaborators in the Engler German school, described 12 African species. He and Käte Hoffmann went on to produce the global revision of Acalypha in Das Pflanzenreich (1924), recognising ca. 390 species, 81 of them new.

Their infrageneric classification retained Müller’s basic framework but modified the ranks. They proposed three subgenera: Linostachys, Androcephala (a single Malagasy species with pedicellate female flowers), and Euacalypha. They divide Linostachys into three sections, and Euacalypha into 8 series and 39 sections. Pax and Hoffmann consider as “sections” the lower category groups, which Müller designates by the symbol §. Unlike Müller, they treated series as higher in rank than sections. Their treatment remains the most comprehensive revision of the genus to date and is still a major reference.

The last input on Acalypha from Pax and Hoffmann is published in Die Natürlichen Pflanzenfamilien (1931). It is just a synthesis of the treatment given in Das Pflanzenreich, where all species are classified and listed without any description or key.

Subsequent proposals, including Isao Hurusawa’s (1954) division of Acalypha into seven subgenera and 19 sections, and Grady Webster’s (1967) reassertion of Müller’s two-section system, had limited impact on prevailing classifications. The framework proposed by Pax and Hoffmann prevailed, and their three subgenera are still widely recognised.

Taxonomic activity declined significantly for several decades after Pax and Hoffman’s contributions. However, between 1960 and 2008, 55 new species were described, marking a modest resurgence.

Ole Seberg (1984) undertakes a phylogenetic analysis, through a cladistic analysis, for a group of neotropical species but focused in fact on Galapagos Islands. Geoffrey Levin, Victor Steinmann and Vernie Sagun (2005) perform a preliminary phylogenetic study, by using molecular data from 100 species. The obtained results support the monophyly of genus as well as the monophyly of Acalypha and Linostachys subgenera, and also support that most other previously recognized taxa are not monophyletic. In 2022, Levin and collaborators published a more extensive molecular phylogenetic study including c. 160 species. Their results confirmed again the monophyly of the genus and supported a revised classification into four monophyletic subgenera: the three already described, Acalypha, Linostachys and Androcephala, plus a newly proposed Hypandrae, separated from Linostachys.

As with many other tropical plant groups, knowledge of the genus remains uneven in the 21^st century. Extensive taxonomic studies have been conducted in South America —where we have worked over the past three decades — as well as in the Western Indian Ocean Region, including Madagascar, and in Southeast Asia. Up to 42 new species or combinations have been published between 2009 and 2026.

In contrast, Africa remains one of the least well-documented regions for the genus. A comprehensive taxonomic revision of African Acalypha is thus currently underway as part of our project, with an extensive nomenclatural review and several species new to science recently described, alongside broader efforts to assess species diversity and improve understanding of evolutionary patterns globally. Continued floristic exploration remains essential, as significant knowledge gaps persist in other regions. Today, nearly 470 species are formally recognised in the genus, and new ones continue to be described every year.