A third type of raphide crystal in the plant kingdom: Six-sided raphides with laminated sheaths in Agave americana L.

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Abstract
Raphides in leaves of Agave americana L. have six-sided cross sections. Each crystal tapers off to a point at both ends. It is enveloped in a 100 nm thick sheath which, in cross section, shows lamellae with periods of 6–9 nm. No polysaccharides could be detected in the sheaths with the Thiéry reaction. Dissolution of a raphide in acid occurs slowly from both ends leaving the crystal sheath visible in the light microscope. The raphide cell walls contain a layer that in glutaraldehyde-fixed tissue reacts neither with the Thiéry stain nor with potassium permanagnate. Its morphology resembles the “isotropic layer” of Chafe and Chauret (Protoplasma 80, 129–147, 1974) but no lignification could be shown as yet. Though up to now only raphides with four-sided or H-shaped cross sections have been observed by electron microscopy, we suggest that many raphides described as “rounded” in light microscopy might in fact be six-sided.