The structure of the wall of the green alga Valonia ventricosa

Abstract

Although for many years the study of cytology has tended to concentrate attention more and more on the protoplast as the fundamental unit of the plant, there can be no doubt that the membrane surrounding this unit plays a part of considerable importance in its life processes. The deposition of such a membrane, by a process which is as yet quite obscure, is obviously closely connected with protoplasmic activity, and a detailed investigation of its structure is bound to lead to a better understanding of this connexion. At the same time, the shape and size of a cell are clearly due in some degree to the action of forces external and internal on the membrane, so that a study of the structure of the plant cell wall should therefore also yield information of considerable importance in the solution of botanical problems concerned with cell elongation and growth. Comparatively recent investigations, carried out chiefly on plant fibres, have shown that the most important component of cell walls, from a structural point of view, is the polysaccharide cellulose. This substance is known to occur in varying proportions in the walls of almost all plant tissue and its structure has been worked out, chiefly by X-ray and chemical methods, with some degree of certainty. Although much remains to be discovered of the organization of cellulose in the wall, certain details are now quite clear. Celluloses obtained from many and varied plant sources have all proved to have essentially the same structure. They exist only in the form of chains of β -glucose residues, at least 500 A long (Hengstenberg and Mark 1928), bound together laterally by secondary valences to form a three-dimensional lattice. The conception of a definite micelle, in the sense of Nägeli, is no longer widely held, although the lattice is not uniformly regular throughout the wall. The chains of cellulose are more probably bound together into ill-defined bundles separated by regions in which they are not so perfectly oriented. This conception of the existence of cellulose in long molecular chains has arisen from the examination of the secondary walls of plants, but as yet no direct experimental determinations have been possible of its structure in primary walls where it is known to occur ( e. g . in Vicia faba , see TupperCarey and Priestley 1922). Recent work (Preston 1934) on the tracheids of the conifer, however, show that it is possible to carry over the idea of the long-chain structure of cellulose even to these delicate primary walls.