1. Barnes, W. C., Cornell Univ. Agr. Expt. Sta. Mem.186, (1936).
2. Curtis O. F. and Clark D. G., An Introduction to Plant Physiology. McGraw Hill Book Company Inc. New York (1950).
3. Leach L. D. and Davey A. E., J. Agric. Research64, 1?17 (1942).
4. Rishbeth J., Ann. Botany Lond.21, 215?245 (1957).
5. Turner J. W., Am. J. Botany9, 415?445 (1922).