'', the polka dot plant Many plants accumulate calcium oxalate crystals in response to surplus calcium, which is found throughout the natural environment. The crystals are produced in a variety of shapes. The crystal morphology depends on the taxonomic group of the plant. In one study of over 100 species, it was found that calcium oxalate accounted for 6.3% of plant dry weight. Crystal morphology and the distribution of raphides (in roots or leaves or tubers etc.) is similar in some taxa but different in others leaving possible opportunities for plant key characteristics and systematic identification;
mucilage in raphide containing cells makes light microscopy difficult, though. Little is known about the mechanisms of sequestration or indeed the reason for the accumulation of raphides but it is most likely as a defense mechanism against herbivory. It has also been suggested that in some cases raphides may help form plant skeletal structures. Raphides typically occur in
parenchyma cells in aerial organs, especially the leaves, and are generally confined to the mesophyll. As the leaf area increases, so does the number of raphides, the process starting in even young leaves. The first indication that the cell will contain crystals is shown when the cells enlarge with a larger nucleus. Raphides are found in specialized plant cells or crystal chambers called
idioblasts. Electron micrographs have shown that raphide needle crystals are normally four sided or H-shaped (with a groove down both sides) or with a hexagonal cross section and some are barbed. Wattendorf (1976) suggested that all circular sectioned raphides, as visible in a light microscope, are probably hexagonal in cross section The hexagonal crystals reported by Wattendorf in
Agave americana were apparently calcium oxalate. Microscopy using polarized light shows bright opalescence with raphides. Plants like
Tradescantia pallida also accumulate calcium oxalate crystals in response to heavy metals stress. == Harmful effects ==