Received for publication on 21 Jan. 1986. I would like to thank A.D. Draper for supplying the plant material and also G.J. Galetta for their ideas and criticisms over the course of the study. Mention of a trademark, proprietary product, or vendor 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 may also 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.