1. AGRIC RES 12 9: 1 (1964).
2. Darwin, F., Philosophical Transactions of the Royal Society B 190: 531 (1898).
3. FREUDENBERGER, H, PROTOPLASMA 35: 15 (1940).
4. GAASTRA, P, MEDEDELINGEN LANDBOUWHOGESCHOOL WAGENINGEN 51: 1 (1959).
5. Heath, O. V. S., Encyclopedia of Plant Physiology 17: 415 (1959).