Plants, as rooted, sessile life forms have evolved intricate mechanisms for avoiding predation (herbivory), such as thorns, toxic and repellant chemicals, and even guardian commensal organisms such as valiant ant colonies. Many plant species produce crystal inclusions as a defense mechanism against herbivory.

Most crystalline aggregations, or druse crystals, found in plants are made of calcium oxalate, which is the compound that most frequently forms kidney stones in humans. If ingested, the material is poisonous to humans as well as to most animals. Druse crystals are usually globular or star-like in shape, but calcium oxalate may form as needles or spear-shaped crystals (raphides). Any of these forms is thought to deter herbivory, and a number of species of plants are considered poisonous due to the high calcium oxalate content. The crystals, whether druse or raphides in form, also allow the plants to store excess calcium in the form of calcium oxalate so that the calcium can be remobilized if needed in the process of calcium regulation.