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

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.