Acalypha herzogiana (Euphorbiaceae), the correct name for an intriguing and commonly cultivated species

Abstract

The genus Acalypha (Euphorbiaceae) contains three commonly cultivated ornamental species. Two of these have been propagated for centuries and are not known in the wild: Acalypha hispida and A. wilkesiana. A third species has only recently appeared in the horticultural trade, with the earliest evidence of its cultivation from the mid-1980s. This taxon is often grown in hanging baskets or as groundcover and owes its appeal to the possession of attractive, terminal pistillate inflorescences. Although it is commonly misidentified as A. hispaniolae, A. pendula, A. reptans, or A. repens, we present evidence that it is instead A. herzogiana, a native of Argentina, Bolivia, and Paraguay. Careful examination of the cultivated plants reveals that they differ from plants in the wild by possessing dimorphic pistillate inflorescences, a feature otherwise unknown in Acalypha. One of these inflorescence types is relatively inconspicuous and presents the typical structure of the majority of Acalypha in that the pistillate flowers are closely enveloped by an accrescent, foliaceous bract and not densely clustered. By contrast, the other inflorescence type is showy and highly anomalous. It consists of hundreds of densely clustered, ebracteate, abortive pistillate flowers. These unusual pistillate flowers are born in glomerules like the staminate flower and have no ovaries, but instead produce five to eight styles that arise directly from the receptacle in the position normally occupied by stamens in staminate flowers. We conclude that these unusual flowers are homologous to the staminate flowers of the wild plants and are the result of a homeotic mutation. Furthermore, we consider it unlikely that the homeotic mutation resulting in the development of the anomalous, showy pistillate inflorescences occurred in cultivation because without such inflorescences the plants would be non-descript and of little horticultural interest. The plant is a worthy subject for future research on hometic mutations, floral development, and gene expression.