Study on Organogenesis of Liriodendron chinense×L.tulipifera Effects of Phytosulfokine on the Somatic Embryogenesis of Liriodendron hybrids(L. chinense×L. tulipifera) Ultra-cytochemical Localization of ATPase Activity in Somatic Embryogenesis of Liriodendron Hybrid (Liriodendron chinense × L. tulipifera ) STUDIES ON THE SOMATIC EMBRYOGENESIS OF LIRIODENDRON HYBRIDS(L.CHINENSE×L. TULIPIFERA)