On educational leave from Agr. Canada, Res. Sta., Harrow, Ontario, Canada, NOR 1G0 and to where correspondence should be sent.
Received for publication 24 Jan. 1985. This research was supported in part by Operating Grant No. A6697 of the Natural Sci. and Eng. Res. Council of Canada held by J.T.A. Proctor. The authors gratefully acknowledge the comments related to the statistical analysis by V.A. Dirks, Agr. Canada, Res. Sta., Harrow, Ontario, Canada, NOR 1G0. 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.