Plant Physiologist Emeritus, California Agr. Expt. Sta., Davis, CA 95616.
Received for publication 22 Feb. 1988. New Jersey Agr. Expt. Sta. Pub. no. D-03232-6-88 supported by state and Hatch Act funds. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.