Acalypha genus was described in the Species Plantarum by Linné (1753), as belonging to the monoecia monadelphia class along with other Euphorbiaceae genera such as Croton, Jatropha and Ricinus. Previously it was also included in the Corollarium Generum Plantarum (Linné, 1737). Its definition has been changed very little eversince, and it is confirmed as a natural and well specified genus.
In its Species Plantarum, Linné sets forth the first three binominal names, i.e. A. virginica from North America, and A. indica and A. australis from Asia; in 1760 he describes a fourth species: A. virgata from Jamaica. In the same year, Nikolaus Joseph von Jacquin, as a result from his journey to Caribbean Sea, writes Enumeratio Systematica Plantarum, in which species A. villosa and A. carthagenensis from Colombia, and A. corensis from Venezuela are described.
The first Acalypha iconographies are also shown in the books by Jacquin; there coloured sheets of A. villosa (Jacquin, 1776), A. alopecuroides (Jacquin, 1792), A. cuspidata, A. diversifolia and A. macrostachya (Jacquin, 1797) can be found.
Until late 18th century the outstanding incorporations to the genus were those from Pehr Forsskål, a Linné 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.
19th century going on, 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.
Cavanilles also suggest a slight modification of the generic character defined by Linné as regards to dimension of seeds: Linné called “maxima” those seeds, character which should be suppressed as they are in most cases very small.
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, in which the first classification of genus is made. The species are grouped as to whether they are monoecious or dioecious, and to the position 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 eigth Acalypha species in his Systema Vegetabilium, seven of them brand new. He puts them together under a key with characters similar to those used by Willdenow, finally, the herbaceous species are separated from the 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 the French botanist Henri Baillon publication Étude générale du groupe des Euphorbiacées (1858), a large morphological description of genus is made, and a peculiar classification is presented: two sections are considered –“Sect. A” and “Sect. B”– depending on whether the inflorescence axis is simple or branched out, and the female flowers calyx consists of three or five sepals.
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, and specially from Brazil.
Johannes Müller Argoviensis, a Swiss botanist specialist in lichens and director of Geneva herbarium, was entrusted with making a good many Euphorbiaceae, Acalypha genus included, ready for De Candolle’s Prodromus (Müller Argoviensis, 1866).
As a preparation for that public book, he published in Flora journal a lot of descriptions of new species based on specimens of Hooker herbarium, at Kew Gardens (Müller Argoviensis, 1864).
In such a work, Müller for the first time uses a classification of Acalypha in two sections, i. e. “Linostachys” and “Euacalypha”, names which accompany each of the described species. In 1865, he publishes in Linnaea journal a first revision of genus, in which 164 species are gathered –67 of them first time described– and Acalypha is formally divided into two sections as above referred.
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. This latter section is in turn divided into “series”, “subseries”, and finally in groups designated by the symbol “§”, mainly according to the relative positions of male and female flowers in inflorescences and depending on whether these are axillary or terminal, unisexual or bisexual.
Finally, in De Candolle’s Prodromus (Müller Argoviensis, 1866) 30 new species are described, thus raising up to 215 the number of accepted species, which are ordered as per such a complex infrasectional classification. Nearly all names of subgeneric taxa from Müller are illegitimate according to the current rules of International Code of Botanic Nomenclature.
Müller Argoviensis was also entrusted with the preparation of Acalypha for the Flora Brasiliensis of Martius (Müller Argoviensis, 1874), in which 10 new species are described. There he maintains his two sections, but he does not use the infrasectional classification.
From 1867 to 1923 no new proposal is made concerning the classification of the high number of species which genus already consists of. This is, however, a plentiful period in discovery and description of new species: ca. 220 are published. We must highlight the work from John Hutchinson (1913) for Flora of Tropical Africa by Thiselton-Dyer, where he is dealing with 42 species in modern flower format and, for the first time, he introduces a dichotomous identification key.
In 1894 Ferdinand Pax, one of the most productive collaborators in the Engler German school, publishes 12 new African species of Acalypha. Pax, and synantherologist Karl August Otto Hoffmann, were entrusted with preparing the genus for the enormous Das Pflanzenreich, where all species known in the world so far are gathered and ordered (Pax & Hoffmann, 1924).
They deem ca. 390 species as valid ones, of which 81 are first time described. Such a work is considered, even now, as the most complete revision of genus worldwide, and the necessary reference point for any taxonomic or local flora study.
The infrageneric classification presented by Pax & Hoffmann is based on that from Müller Argoviensis, except in the rank of subgeneric taxa. Moreover, they use series and section categories in an opposite way to Muller’s: the series are described as higher ranked than sections are. So they divide Acalypha into three subgenera: Linostachys, Androcephala and Euacalypha. The Androcephala subgenus, single-typed, includes a species from Madagascar with pedicellate female flowers and capituliform male inflorescences.
They divide Linostachys subgenus into three sections, and Euacalypha subgenus into 8 series and 39 sections. Pax & Hoffmann consider as “sections” the lower category groups, which Müller designates by the symbol §. According to Grady Webster (1967), such “sections” from Pax & Hoffmann are comparable to subsections or series in other Euphorbiaceae genera.
The last input on Acalypha from Pax & Hoffmann is published in Die Natürlichen Pflanzenfamilien by Engler & Prantl (Pax & Hoffmann 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.
In a general paper on Euphorbiaceae systematics, Isao Hurusawa (1954) sets forth a new classification, where the rank of infrageneric taxa is even raised and a proposal is made to divide Acalypha into seven subgenera with 19 sections.
Grady Webster (1967), in a study of Euphorbiaceae genera from Southeastern USA, deems inadequate the treatments given by Pax & Hoffmann and Hurusawa; he thinks that by dividing genus into two sections with many infrasectional taxa, such as presented by Müller, the grade of kinship between Acalypha species seems to be better outlined.
Accordingly, Webster compiles Acalypha species from the United States into ten series, which are based on corresponding “§” groups from Müller.
Along the last decades just a few contributions to the genus have been made. Indeed, the floristic studies were still adding new species or combinations, but at a slower pace than in preceding times (ca. 55 new species described between 1960 and 2008). In the world checklist of Euphorbiaceae family, Rafaël Govaerts, David Frodin & Alan Radcliffe-Smith (2000) consider 462 species as valid ones.
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 & 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.
Finally, the natural classification of Acalypha species is still far from a wide recognition. In our opinion, independently of the rank to be assigned to the infrageneric taxa, those characters that are employed for their segregation should be carefully revised. A systematic and nomenclatural critical revision is needed, and their ranks of intraspecific variability must be established.
Likewise, the floristic studies are essential; they were significantly encouraged during the last decades, yet important knowledge gaps still found urge to be filled up.
José María Cardiel, Elena Dorda, Pablo Muñoz & Manuel Pardo de Santayana
Dpto. de Biología (Unidad de Botánica), Facultad de Ciencias. UAM
Ciudad Universitaria de Cantoblanco. 28049 Madrid. España