Received for publication 30 June 1986. Contribution of the G.W. Carver Agricultural Experiment Station, CSRS/USDA, Project no. ALX-5P- 1, Tuskegee Univ., Tuskegee, AL 36088. 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.