Formation of calcium oxalate crystals is common in the plant kingdom, but biogenic formation of calcium sulfate crystals in plants is rare. By means of microscopy, He et al. investigate the occurrence of calcium crystals in phyllodes (a type of foliar organ) and branchlets of Acacia robeorum (Leguminosae). Crystals are observed in almost all tissues and are of various morphologies and composition. It is concluded that they may have functions in removing excess calcium, magnesium and sulfur, in protecting the plants against herbivory, and in detoxifying aluminium and heavy metals.