1. Johansson, I., and Hansson, A., Ann. Agric. Coll. Sweden, 11, 145 (1943).
2. Whithurst, V. E., Grown, R. M., Phillips, R. W., and Spencer, P. A., Bull. Flor. Agric. Exp. Sta., No. 429, 34 (1947).
3. Hammond, jun., J., J. Agric. Sci., 34, 97 (1944).
4. Hammond, sen., J., U.N. Sci. Conf. on Conser. and Utiliz. Of Resources, 11, (b), 22 (1949).
5. Yeates, N. T. M., J. Agric. Sci., 39, 1 (1949).