THE FORMATION OF ADVENTITIOUS ORGANS. I. CYTOKININ-INDUCED FORMATION OF LEAVES AND SHOOTS IN CALLUS CULTURES OF POPULUS NIGRA L. ‘ITALICA‘*

One- to four-year-old callus cultures of the Lombardy poplar (Populus nigra L. ‘Italica‘) are able to form roots and leaves or shoots. There may be single leaves, small buds, elongating shoots, or a combination of these organs. Of the four cytokinins used to stimulate organ formation, zeatin was the most active, stimulating shoot formation at a concentration as low as 10–8 M. The highest zeatin and BA concentrations (10–5 M) stimulated leaf formation, but fewer elongated shoots were formed than at a lower cytokinin concentration. 2,4-D was inhibitory to organ formation, even in low concentrations that stimulated callus growth (0.05–0.1 mg/1). This 2,4-D-inhibition is discussed in relation to the suppression of tissue differentiation.