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

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

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