Raphide

Raphide refers to a needle-shaped crystal of calcium oxalate found in specialized plant cells known as idioblasts. The term derives from the Greek rhaphis, meaning needle. Raphides occur as bundles of slender crystals oriented within the idioblasts, and are most commonly composed of calcium oxalate, though variations in mineral content can occur. They are distributed across many plant groups, but are especially noted in the Araceae and in a wide range of dicot and monocot species. Raphide-containing tissues are often present in leaves, stems, and reproductive organs.

Biological function of raphides is primarily considered defensive. When plant tissue is damaged by chewing or wounding, raphide crystals can be released into the feeding site and may penetrate mucous membranes or tissue, causing mechanical irritation, pain, or swelling. In some cases, ingestion of raphide-bearing material can contribute to gastrointestinal discomfort. Raphides may act in concert with other plant compounds to deter herbivores, and some plants accumulate additional irritants or toxins in the same tissues.

Structure and formation details indicate that raphides are formed in crystal idioblasts and organized into bundles that maximize penetration potential if cells are disrupted. They are distinct from other crystal forms such as druses, which are irregular clusters of crystals.

In research and taxonomy, the presence and morphology of raphides can aid in understanding plant defense strategies and can serve as a histological feature for identifying certain plant groups. Overall, raphides represent a common, needle-like chemical defense mechanism in plants.