CELL WALL growth usually involves surface expansion and an increase in total wall mass. Localization of the growth mechanism concerns both processes. With reference to surface expansion, the restriction of elongation to the cell tip has been found in fungal hyphae, root hairs, latex tubes, pollen tubes, and in the bast fibers of ramie (FreyWyssling, 1952). A generally even distribution of growth along the cell axis has been revealed, by marking experiments, in the epidermal cells of the Avena coleoptile (Castle, 1955) and in the internodal cells of Nitella (Green, 1954). In cells not accessible to direct marking procedures, the distribution of pit fields has been the basis for proposals of growth distribution along the cell axis. Wardrop (1955) found an even distribution of elongation in Avena coleoptile parenchyma and Wilson (1957) came to a similar conclusion for the cortical parenchyma of Elodea. It is more difficult to determine the site of the addition of new wall substances because here the position within the cross-section of the wall must be specified. Proposals as to the site of formiation of new wall are often based on the appearance of the cellulose framework of the wall in question. Houwink and Roelofsen (1954) present structural evidence that the cell walls of cotton hairs, stellate pith cells of Juncus, and corn root hairs grow through the addition of cellulose microfibrils, generally in the transverse direction, to the inner surface of the elongating wall. The elongation of the cell has the effect of passively reorienting previously deposited meshes of microfibrils to an isotropic or longitudinal arrangement which is observed on the outside of the cell wall. This type of growth is termed multi-net growth. The large vesicular cells of the alga Valonia have been studied in the electron microscope and here also microfibrillar patterns suggest that new microfibrils are deposited at or near the inner surface of the wall (Steward and Miihlethaler, 1953; Preston, et al., 1953). Wilson (1951) has expressed the view that some component of the