A re-examination of the fine structure of the walls of vesicles of the green alga Valonia

Abstract

The structure of the walls of vesicles of Valonia ventricosa and, to a less extent of other species, has been re-examined by the methods of X-ray analysis and electronmicroscopy, with particular reference to the criticism by Steward and Muhlethaler of the earlier statements of Preston and Astbury. It is shown that the cellulose microfibrils are present in three orientations, in separate lamellae. The third orientation, noted occasionally by Preston and Astbury but not recorded in their model, corresponds to microfibrils which are much less abundant than are those of the other two orientations both because the lamellae with this orientation are less frequent and because the micrafibrils are more loosely packed in each lamella. The two "major" directions (A and B) lie on an average at rather less than at right-angles to each other; the third direction (x) forms a bisector of this angle. The repeat from one lamella to the next can be ...ABAB... or ... AxB..., i.e. an interrupted two-lamella repeat, and not a three-lamella repeat as proposed by Steward and Muhlethaler. The structures of the walls of two whole vesicles have been worked out and give identical models. These are strictly equivalent to the model proposed by Preston and Astbury except that the third microfibril direction is present, making a rather steep right-hand spiral around the vesicle.