Abstract: Pollens of Tripsacum L. could germinate on the stigmas of maize (Zea mays L.). After extending in maize style, the pollen tubes reached the top of ovary and released androgamete into embryo sac. But the normal hybrids of maize × Tripsacum L. were not obtained because of embryo sac incompatibility or hybrids abortion. Pollens of Coix L. could also germinate on the stigmas of maize, and the pollen tubes of C. lacryma-jobi L. var. frumentacea could reach the base of maize style, but the pollen tubes of Coix lacryma-jobi L. stopped growing at maize pistil. This indicated that there was a strict reproductive isolation between maize and Coix L. and style or embryo sac incompatibility was the crossing barrier between maize and Coix L. The ratio of the abnormal pollen tubes in the cross between maize and Coix L. was higher than that between maize and Tripsacum L., suggesting that maize has a closer relationship to Tripsacum L. than to Coix L.

Key words: Pollen tube, Distant crossing, Sexual barrier, Maize, Tripsacum L., Coix L., Fluorescence microscopy