In both plant, animal and bacterial cells the active protoplasm is bounded by a lipid membrane known as the plasmalemma. This performs a large variety of functions but is basically concerned with transport of materials in and out of the protoplast. In plants and bacteria however the membrane is itself surrounded by another structure known as the cell wall which imparts protection and firmness to the cell. The osmotic pressure exerted by the protoplast, by uptake of water from a hypotonic environment, on the cell wall can be considerable and thus a fundamental property of the wall must be a high tensile strength to withstand this pressure from within. Polysaccharides account for by far the majority of wall material in young plant tissue and these polymers must be arranged so as to surround the protoplast with a mesh of the necessary properties. In the polarising light microscope the cell wall shows birefringence whereas the layer between adjacent cells (the middle lamella) appears to be amorphous. In the electron microscope the cell wall has been shown to be composed of microfibrils of indefinite length embedded in an apparently structureless matrix (17). In higher plants the microfibrils are made up of bundles of highly orientated cellulose molecules. The staining properties of the wall and middle lamella with the basic dyes methylene blue and ruthenium red (18) indicate that the matrix is composed, in part, of the acidic pectic substances, although hemicellulosic polysaccharides and in older cells the non -carbohydrate polymer, lignin, are also present.