Received for publication 4 Oct. 1984. We gratefully acknowledge the assistance of M.M. Finney and E. Mitcham. Mention of a trademark or proprietary products does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that also may be suitable. 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.