THE MOLECULAR CHAIN STRUCTURE OF CELLULOSE AND ITS BOTANICAL SIGNIFICANCE1

Abstract

SummaryThe impetus which modern work on growth has given to wall studies promises to combine these into a branch of plant science of considerable importance to botanists in many fields. The present review represents an attempt to discuss, in relatively simple terms, the work which has led to present views on the structure of cellulose, the chief structural polysaccharide of the plant cell wall. To this end, the results of organic chemistry, X‐ray analysis, and of polarization optics, are mentioned, and data from other sources are summarized where necessary.Cellulose is shown to consist of long molecular chains, of which the links are β‐glucose residues bound together, in the chain, by primary valences. The linkage is universally of the 1:4 type. The chains themselves are in turn bound together by secondary valences, or van der Waals forces, into ill‐defined bundles which correspond to the classical micelle of Nageli. Incrusting substances like pectin and lignin are certainly deposited in the intermicellar spaces, as against hemicelluloses like xylan, which are equally certainly intramicellar. The evidence for the existence of micellar aggregates, both in the wall itself and in the cytoplasm, is discussed in some detail.In any one layer of the secondary walls in the majority of plant cells, the cellulose chains form a single spiral round the cell. With many fibres and tracheids this spiral retains the same sign, and approximately the same pitch, from layer to layer of any one wall. On the other hand, both the sign and th? pitch of the spiral varies widely in some cell types. This has been demonstrated most clearly in the algae (e.g. Valonia, Cladophora, Chaetomorpha), where the cellulose chains of odd layers, say, point in exactly the same direction, but at a considerable angle (some 83° in Valonia) to the direction of those in the even layers.In spite of the increasing prevalence of the conception that the cellulose chains in the primary wall lie transversely, it is quite clear that in those cases critically examined they form a spiral resembling that in the secondary wall. It would seem that the development of a spiral in the secondary wall is not unconnected with its existence in the primary wall.The various theories of wall growth which are based on a transverse orientation of the cellulose chains, in the primary wall, must clearly be abandoned. Such evidence as there is points to a change, during growth, of the inclination of an original spiral.The nature, both of the cell wall and of the cytoplasm, is probably involved in wall deposition. Thus, a new layer deposited on a wall may be so influenced by the existing layers that the cellulose chains composing it lie parallel to those of the old wall. Yet the fundamental orienting mechanism must lie in the cytoplasm, for each wall originates as a new layer at cytokinesis. The evidence pointing to protoplasmic streaming as the mechanism involved is hardly convincing, and it seems not unreasonable to suggest that the configuration of the protein molecules at the cytoplasm‐wall interface may be involved.