Original paper

Dimorphism of calcium oxalate crystals in stem tissues of Rhipsalideae (Cactaceae) - a contribution to the systematics and taxonomy of the tribe

Hartl, Walter P.; Barbier, Bruno; Klapper, Helmut Müller; Barthlott, Wilhelm

Abstract

The generation of calcium oxalate crystals in stem tissues of all 58 species of the tribe Rhipsalideae is established and documented by conventional light microscopy and polarizing optics. By X-ray diffraction it is shown, that within the tribe a dimorphism of the calcium oxalate crystal type exists: The genus Rhipsalis is characterized by crystals of the monoclinic calcium monohydrate (whewellite) exclusively, whereas the genera Lepismium, Hatiora and Schlumbergera only form crystals of the tetragonal calcium oxalate dihydrate (weddelite). Not in one species a co-occurrence of both crystal types is observed. Dimorphism is evident also morphologically in so far as in stem tissues of the genus Rhipsalis, druses consisting of calcium oxalate the monohydrate (whewellite) is the dominating crystallographic finding, whereas in sections of the genera Lepismium, Hatiora and Schlumbergera, druses and styloids of calcium oxalate dihydrate (weddelite) prevail. Morphological dimorphism is documented also by scanning electron microscopy, demonstrating the monoclinic or tetragonal character of the corresponding crystals and thus permitting an exact differentiation and assignment of druses and other crystal forms in the stem tissue of Rhipsalideae. The structural dimorphism of calcium oxalate in tissues of Rhipsalids is a strict discontinuous character of the tribe on the genus level and can be used for systematical and taxonomical studies of Rhipsalideae. The exclusive occurrence of monoclinic calcium oxalate monohydrate in the genus Rhipsalis within the tribe could indicate a phylogenetically distinct position in comparison to the genera Lepismium, Hatiora and Schlumbergera.

Keywords

calcium oxalatedrusesrhipsalideaesystematics