Electron microscopic takings. At the top: Cross-section. The convex central torus has a larger extension than the opening of the bordered pit that is enclosed by the secondary wall. The torus is kept in its position by the primary wall material. Below: Torus in top view. The torus is located within a net of radial fibrils partly crossing each other produced by the primary wall material. The net is permeable for water and solutes. (J. BAUCH, W. LIESE, R. SCHULTZE, 1972).