The mesophyll structure of rice leaves inserted at different levels of the shoot were observed. The individual cells of the mesophyll are tabular in shape, having several lateral protuberances or arm-like processes. In transverse section, these look like folds inwardly directed from the cell wall. These cells are so-called the arm-palisade cells. The protuberances are columnar or hemispheric in shape and their diameter (r) is nearly the same as the cell thickness (T). On the adaxial surface of the leaf, the protuberances are conspicuously elongated at right angles to the leaf surface. The upper leaves have thicker mesophyll than the lower leaves. The ribs and furrows are more developed on the adaxial than the abaxial surface of the leaf. The ratio of the adaxial mesophyll surface area to the leaf area (MS/LA) increases in the upper leaves. The number of protuberances in a mesophyll cell increases from the lower to the upper leaves. The cell thickness is smaller in the upper leaves than that in the lower. Therefore, the ratio of the cell surface to the cell volume (S/V) is increased in the upper leaves. The total cell surface per unit leaf area is enlarged in the upper leaves, as a result of increase in the number of cells.