Calcium oxalate is an insoluble compound formed when calcium binds with oxalic acid in plant tissues. Plants commonly store it as crystals inside specialized cells, where it can help regulate calcium levels and provide a constitutive defence against herbivores.
Calcium oxalate crystals occur in several forms, including needle-shaped raphides and other crystal shapes. When plant tissue containing raphides is chewed, the crystals can mechanically irritate the mouth and other tissues.
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