The cellulose system of the cell wall of Micrasterias denticulata and Micrasterias rotata was analyzed by diffraction contrast transmission electron microscopy; electron diffraction, and X-ray analysis. The studies, achieved on disencrusted cell ghosts, confirmed that the cellulose microfibrils occurred in crisscrossed bands consisting of a number of parallel ribbon-like microfibrils, The individual microfibrils had thicknesses of 5 nm for a width of around 20 nm, but in some instances, two or three microfibrils merged into one another to yield larger monocrystalline domains reaching up to 60 nn in lateral size. The orientation of the cellulose of Micrasterias is very unusual, as it was found that in the cell wall, the equatorial crystallographic planes of cellulose having a d-spacing of 0.60 nm [<(1(1)over bar 0)> in the I beta cellulose unit cell defined by Sugiyama ct at, 1991, Macromolecules 24, 4168-4175] were oriented perpendicular to the cell wall surface. Up to now such orientation has been found only in Spirogyra, another member of the Zygnemataceae group, The unusual structure of the secondary wall cellulose of Micrasterias may be tentatively correlated with the unique organization of the terminal complexes, which in this alga occur as hexagonal arrays of rosettes. (C) 1996 Academic Press