The essential elements Ca and P, taken up and used metabolically as Ca2+ and H2PO4−/HPO42− respectively, could precipitate as one or more of the insoluble forms calcium phosphate (mainly apatite) if the free ion concentrations and pH are high enough. In the cytosol, chloroplast stroma, and mitochondrial matrix, the very low free Ca2+ concentration avoids calcium phosphate precipitation, apart from occasionally in the mitochondrial matrix. The low free Ca2+ concentration in these compartments is commonly thought of in terms of the role of Ca2+ in signalling. However, it also helps avoids calcium phosphate precipitation, and this could be its earliest function in evolution. In vacuoles, cell walls, and xylem conduits, there can be relatively high concentrations of Ca2+ and inorganic orthophosphate, but pH and/or other ligands for Ca2+, suggests that calcium phosphate precipitates are rare. However, apatite is precipitated under metabolic control in shoot trichomes, and by evaporative water loss in hydathodes, in some terrestrial flowering plants. In aquatic macrophytes that deposit CaCO3 on their cell walls or in their environment as a result of pH increase or removal of inhibitors of nucleation or crystal growth, phosphate is sometimes incorporated in the CaCO3. Calcium phosphate precipitation also occurs in some stromatolites.