1980b). They may represent storage forms of calcium and
oxalic acid, and there has been some evidence of calcium
oxalate resorption in times of calcium depletion (Arnott and
Pautard, 1970; Sunell and Healey, 1979). They could also
act as simple depositories for metabolic wastes which would
otherwise be toxic to the cell or tissue. In some plants they
have more specialist functions, such as to promote air space
formation in aquatic plants (see below), or help prevent
herbivory, although many plants containing calcium oxalate
crystals are eaten by birds and animals. The barbed and
grooved raphides of some Araceae (e.g. Xanthosoma
sagittifolium) are particularly irritating to mouth and throat
tissues when eaten. Grooves in crystals which have
embedded themselves in animal tissues may allow the
entrance of a chemical irritant such as a toxic proteolytic
enzyme (Walter and Khanna, 1972) or a glucoside (Saha
and Hussain, 1983) into the wound (Sakai, Hanson and
Jones, 1972).