Received for publication 8 Sept. 1983. This work was supported by Elvenia J. Slosson Endowment Fund and the Agr. Expt. Station project Univ. of Calif., Davis, CA-D*-EHT4063-H. 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.