A study of raphide microstructure in relation to irritation.
Raphides from fourteen genera in seven families of flowering plants were examined using light and scanning electron microscopy. Raphides from an additional fifteen genera were described from literature. Of these thirty-four plants, fourteen were known to cause irritation. Barbs are associated with irritation in the families Araceae and Leeaceae, but not Vitaceae. Of the seventeen taxa described that contain raphides with barbs, six also have grooves on their crystals. The relationship between grooves and irritation is inconsistent. In seven examples in the family Araceae, raphides with grooves are associated with irritation. In three examples, one in the family Araceae and two in the Lemnaceae, raphides with grooves are present with little or no associated irritation. Plants that contain raphides without barbs or grooves are associated with irritation only if the raphides are greater than 180 microns in length. Four taxa that contain raphides longer than 180 microns without barbs or grooves are associated with irritation. However, nine taxa that contain raphides shorter than 180 microns without barbs or grooves do not cause irritation. It appears that raphide microstructure and size both contribute to irritation. However, the lack of a consistent association of barbs and grooves with irritation does not rule out the additional presence of a chemical toxin.