Cells are flattened radially, are short, and are square, hexagonal, or even sprocket shaped (鏈輪型) tangentially. Walls are thin to very thick, and it is reported that the thin-walled phellem cells tend to be heavily suberized (木栓化) (wax impregnated), whereas the thicker walled elements lignified but lack suberin(木栓質 or 軟木脂).This lignified and suberized phellem: protecting the stem against moisture loss