Epidermal crystals in Acalypha (Euphorbiaceae, Acalyphoideae) as a new taxonomic trait of the genus

The occurrence of epidermal crystals in the species of the genus Acalypha (Euphorbiaceae, Acalyphoideae) is described and analyzed. The epidermal surface of 190 Acalypha specimens, belonging to 110 species and subspecies, was studied with a scanning electron microscope (SEM), and the elemental composition of the found crystals was examined using energy-dispersive X-ray spectroscopy (EDS). The results show that 96.4 % of the studied species have epidermal crystals, mainly located on the leaf surface, but also on the bracts and flowers. The crystals exhibit significant variation, and four types of crystals are recognized: prismatic, styloids, stellate, and druses, which are, respectively, designated with Ac, Bc, Cc and Dc letters. We have not found any reference to stellate crystals in the angiosperm morphological or anatomical literature, and druses are observed and described under SEM in Acalypha for the first time. The elemental composition of the crystals shows that all of them exhibit typical peaks of calcium. We discuss the possible taxonomic importance of crystals, due to the association that some of these types of crystals have with groups of closely morphologically related species. We have not found a relation between the presence or the type of crystals and the habitat or distribution of the species that present them. We do not know other genera of Euphorbiaceae where the presence of crystals is so widespread, so we propose that they should be considered as a distinctive feature of this genus.