Cellulose from the cell wall of the green alga Microdictyon tenuius was studied by electron diffraction. The diffractograms disclose two distinct crystalline phases. The major phase has a one-chain, triclinic (PI) structure with unit cell parameters of a = 0.674 nm, b = 0.593 nm, c (chain axis) = 1.036 nm, a = 117°, 0 = 113°, and y = 81°. The crystal unit cell of the minor component has two chains and is monoclinic (P2i), with a -0.801 nm, b = 0.817 nm, c (chain axis) = 1.036 nm, and y the monoclinic angle = 97.3°. The triclinic phase is metastable, and annealing it in dilute alkali at 260 °C converts it into the monoclinic form. The presence of two phases in Microdictyon can be extended to other algal celluloses and is consistent with the biphasic character deduced from 13C CP/MAS NMR spectroscopy. The triclinic and the monoclinic structures correspond to the la and Id spectra, respectively.

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