The Cytokinins of Cultured Sycamore Cells

S ummary Two cytokinin fractions, active in the tobacco and soybean callus bio‐assays, have been obtained from sycamore cells cultured in a cytokinin‐free medium. One of these (Cyt 1) corresponds in its properties to an uncharacterized cytokinin detected in water melon and cucumber seeds. The second fraction (Cyt 2) is coincident with zeatin riboside in chromatographic properties. No evidence was obtained for the presence of a ‘nicotinamide‐like’ cytokinin. The ‘staled’ culture medium which had supported the growth of sycamore cells contained Cyt 1 activity, low Cyt 2 activity and a cytokinin coincident with zeatin in chromatographic mobility. There is a rapid rise in the cytokinin activity in the cells during the lag phase which precedes active division in batch culture. Later the cytokinin activity per cell and per unit volume of culture progressively declines.