Abstract:Micro-cross sections (MCS) of midveins of leaves from hybrid Populus ( P. nigra var. betulifolia x P. trichocarpe) NE 299 were used for studying the adventitious bud formation in vitro. MCS of 400 pm or 800 fan in thickness were cut with a vibrating microstome (Lancer Vibratome, model 1000) and cultured in woody plant medium (WPM) supplemented with 0.2 mg/L BA and 0.01 mg/L NAA. Then the division of some parenchyma cells located on each side of the vascular cylinder and connected with the palisade tissue (i. e. adaxial side) were initiated. At the same time the palisade tissue connected with vascular bundles also began to divide and then formed the embryonic meristematic packed cells. The origin of the midvein calli developed mainly from the vascular cylinder, the palisade tissues and the phloem parenchyma cells. Adventitious buds were generally formed on the peripheral parts of the calli, also originated from the cambium-like cells surrounded by the vascular nodules. The parenchymatous bundle sheath of the lateral veins had much higher capacity of division and grew directly into the adventitious buds with no callus phase. Observing the parenchyma cells of the vascular cylinder on the midvein, it was shown that the cells adjacent to the mesophyll cells usually contained little smaller chloroplasts while those on both abaxial and adaxial side contained stored starch granules. The defined sites and the specific cell potential in morphogenesis are discussed.