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.
Styloid-calcium-oxalate-crystal-containing idioblasts possess an interior cell-wall layer which has a lamellar ultrastructure. Idioblasts were isolated by centrifugation of an Agave americana leaf homogenate through 2M sucrose. The aliphatic monomers of the polymeric material from an idioblast fraction were primarily ω-hydroxy acids (32%) and dicarboxylic acids (35%), with C18:1 dicarboxylic acid being the most dominant monomer (25%). Nitrobenzene oxidation of the idioblasts yielded syringaldehyde and vanillin in a ratio of 0.46:1. The major class of wax associated with the idioblasts was free fatty acids (34%). A major homologue of both the fatty acid and fatty alcohol fractions of this wax was C22. The hydrocarbon fraction of the wax had a broad chainlength distribution with a large amount of even-numbered (47%) and shorter-chain homologues. The ultrastructure, the composition of the aliphatic and aromatic components of the polymeric material as well as the composition of the wax show that the idioblast cell wall is suberized. The wax and cutin polymer of the epidermis of A. americana leaves were chemically characterized for comparative purposes.
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