Abstract
T.nigrescens (2n = 16) crosses with T .isthomocarpum (2n = 16) reciprocally. The viability of hybrid seedlings depend from the direction of the cross. At the time of diakinesis and metaphase I the average chromosome figures per PMC's was 7.014II, 0.005III, 0.435IV and 0.209I. For one half of the PMC's in the metaphase I the typical chromosome arrangement was 8II. The F1 plants was almost completly sterile. The causes of viability of hybrid seedlings depending on the direction of the cross, and the sterility of hybrid plants, are discussed.