Received for publication April 8, 1978. Published with the approval of the Director of the Delaware Agricultural Experiment Station as Miscellaneous Paper No. 817. Contribution No. 82 of the Department of Plant Science. Financed by funds from McIntire-Stennis Forrestry Research Program and the Delaware Agricultural Experiment Station. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper must therefore be hereby marked advertisement solely to indicate this fact.
Graduate Assistant, present address: Extension Service, University of Maryland.
Present address: Associate Professor, Department of Agronomy University of Nebraska, Lincoln.