Calcium oxalate crystal macropattern and its usefulness in the taxonomy of Baccharis (Asteraceae)

This study provides a comprehensive account of the various types of calcium oxalate crystals found in the genus Baccharis and assesses the exceptional value of crystal macropatterns for the taxonomy of the genus. The morphotype, occurrence, and chemical composition of the crystals found in the stems and leaves are studied. The 44 species included in this study were selected based on a broad phylogeny-based sampling covering seven subgenera and 31 sections. These species were chosen to represent all the main phylogenetic lineages of Baccharis; thus, the sampling also represents a comprehensive coverage concerning evolutionary significance for such a large and environmentally and economically important plant group. The samples were analyzed by light microscopy, scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDS). Several morphotypes of crystals, including druses, crystal sand, styloids and prisms, were present. Based on their chemical composition, the crystals were classified as pure calcium oxalate, mixtures of oxalates and sulfates, and mixtures of oxalates, sulfates, and silica. The crystal macropatterns observed in this study aid in species identification and provide novel data for the taxonomy of Baccharis. RESEARCH HIGHLIGHTS: Most species of Baccharis have a specific crystalline pattern. Each species produces a crystal morphotype or a set of morphotypes specific to it. The crystals observed are formed by calcium oxalate.