Optical and Electron Microscopic Studies of Cotton Fiber Structure

The appearance, growth, and structure of a cotton fiber are described, and photomicrographs and electron micrographs are used to illustrate both gross and fine features of the cell wall morphology. Heretofore, microscopy of the fiber has been limited to the use of dispersion, swelling and staining methods, in plain and polarized light. The development during the present study of methods for isolation of individual components of the cell wall for purposes of electron microscopy has permitted interesting observations with the light microscope. Quantitative measurements of shrinkage in the isolated primary wall of the cotton fiber during mercerization confirm its restrictive influence on the fiber during swelling in various agents. Electron microscope studies have been made on isolated primary wall fragments which have been purified by removal of noncellulosic constituents by extraction. These have revealed that the cellulosic portion of the membrane is a network of fibrils interlaced in a sort of fabric in which the general system of orientation is axial on the outer surface and transverse on the inner surface. The cellulose framework is impregnated with a complex of waxes, pectinaceous, and proteinaceous substances. The first layer of secondary thickening, called the "winding layer," has been isolated from 17-day old cotton fibers and photographed with both optical and electron microscopes. It is revealed to be a continuous sheet or layer made up of alternating bands of fibril bundles which spiral about the fiber at an angle. The wider bands have a closely packed parallel arrangement of fibrils while the narrower bands which connect them consist of two systems of fibrils at right angles to one another in an open-weave pattern. Electron micrographs of fragments of the main body of the secondary wall show an entirely different pattern of fibril arrangement from either the primary wall or the winding layer.