Diversity of calcium oxalate crystals in Cactaceae

Abstract

The occurrence of various types of calcium oxalate crystals was studied in 251 species and subspecies of Cactaceae to determine whether they are useful characters for Cactaceae systematics. Crystal hydration states were identified by X-ray powder diffraction and polarizing microscopy as monoclinic calcium oxalate monohydrate (COM) and tetragonal calcium oxalate dihydrate (COD). Ninety-eight percent of taxa studied contained either COM or COD crystals, or both. Different morphologies of crystals were further defined by light microscopy and scanning electron microscopy as druses, raphides, styloids (prisms), and crystal sand. In particular, the preponderance of one of the hydration states (COM or COD) was characteristic for certain Cactus subfamilies. Data showed that in Pereskioideae, Maihuenioideae, and Opuntioideae COM is predominant, while in Cactoideae COD prevails. In the remainder of Cactoideae, the crystals were quite variable. In tribe Hylocereeae, many species form both COM and COD as well. In the genera Hylocereus , Epiphyllum , Selenicereus , and Weberocereus , COM forms were almost exclusively represented by raphides together with different crystal forms in their epidermal cells. In the remainder of the Cactoideae, crystals did not follow any observable patterns. Development of crystallographic standards for identifying crystal forms microscopically is proposed for future crystal studies.