Received for publication October 5, 1981. Journal Series No. 2630 of the Hawaii Institute of Tropical Agriculture and Human Resources from a dissertation submitted by the senior author in partial fulfillment of the requirements of the PhD degree at the University of Hawaii.
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Present Address: CRECIDATH. Colegio de Postgraduados de Chapingo Apdo. Postal No. 421., Veracruz, Ver., Mexico. The senior author extends his appreciation to CONACYT, INIA and Banco de México, agencies of the Mexican government, for support while at the University of Hawaii.