1. Hewitt, E. J., and Jones, E. W., J. Pomol. and Hort. Sci., 23, 254 (1947).
2. Mulder, E. G., Plant and Soil, 1, 94 (1948).
3. Lemoigne, M., Monguillon, P., and Desvaux, R., C.R. Acad. Sci., Paris, 204, 1841 (1937).
4. Virtanen, A. I., Experientia, 8, 313 (1949).
5. Virtanen, A. I., and Von Hausen, S. S., Nature, 163, 482 (1949); Z. Pfl. Ernähr., 45, 11 (1949).