The internodal cells of Nitella opaca L. have been used in earlier studies to assess the part which mechanical properties of the wall map play in the control of cell growth (Probine and Preston 1962). The wall is mechanically anisotropic in both its plastic and elastic properties, and it is shown in this paper by an approximate theoretical treatment that a mat of cellulose microfibrils, embedded in a plastic matrix and having a distribution in the plane of the wall like that observed in Nitella, would lead to longitudinal and transverse plastic extensions in the ratio observed in the growing cell. Factors which would affect cell shape are discussed. The pattern of growth of the Nitella internode is discussed, and it is shown that, if the wall is formed by the "multi-net" process, a steady-state microfibril distribution can be maintained, as would seem to be required by the observation that there is a constant ratio between increase in length and increase in diameter during extension growth. There is direct experimental evidence that the wall is formed by this process.