Mature Raphid and Raphid Idioblast Structure in Plants of the Edible Aroid Genera Colocasia, Alocasia, and Xanthosoma

Mature raphid idioblasts examined in this study appear to consistof a bundle of raphid crystals contained within a polysaccharidematrix. The cytoplasm consists of a thin parietal layer withfew discernible organelles. These cells appear to be true idioblasts,and no intercellular connections were observed between thesecells and adjacent parenchyma cells. Dissolution of the middlelamella occurs in Colocasia and to some extent in Xanthosoma.The mechanism of crystal release in the five species studiedis the same, e.g. a swelling of a polysaccharide material whichbreaks the idioblast wall and forces the release of the raphides.In Colocasia this results in a forceful ejection with releaseof a relatively few number of crystals at any one time. Xanthosomaand Alocasia do not exhibit a forceful ejection of their crystals,but a less forceful release of a crystal mass which then disassociatesto free the individual crystals. The raphides differ in structureand size, however, they have three structural features in common.First, the crystals have two distinct points, one tapering toan elongate point, the other abruptly pointed. Second, the crystalshave surface barbs with tips oriented away from the taperingpoint. And third, deep grooves are present along the lengthof the crystals. The possible relationship between raphid structureand the acrid nature of the idioblasts is discussed.