Heterosis in plant breeding : proceedings of the seventh congress of Eucarpia the European Association for Research in plant breeding L'Association Europeenne Pour 1'Amelioration des plantes Die Europaische gesellschaft fur Zuchtunges forschung Budapest 24-29 June, 1974 - Janossy, A., ed.; Lupton, F.G.H., ed. (Elsevier Science Publishers Amsterdam, 1974)