Though some scientific studies on the abscission of plant organs had started as early as 1860 (10), the role of the growth hormone in this process was first suggested by the work of Laibach in 1933 (6). He observed that orchid pollinia inserted in the split ends of petioles of debladed Coleus delayed abscission 11 to 20 days beyond that of untreated petioles. Soon after, La Rue (7) found that indoleacetic acid, pollen extracts or urine delayed petiole fall for a few days when they were applied to debladed petioles. He suggested that abscission resulted when the development of the abscission layer was not inhibited by growth substances produced in the leaf blades. Since then many practical uses have been made of the delaying action of auxin on abscission. Myers (11, 12, 13) and Gardner and Cooper (4) by their deblading experiments in Coleus have essentially proved that auxin is responsible for the prevention of leaf drop. They concluded that the leaf blade was the source of the phytohormone naturally serving this function. A new contribution was made when Shoji et al (15) proposed the auxin gradient theory of abscission. They made auxin determinations in bean leaves at various intervals preceding abscission, and found that although the auxin concentration in the blade started falling just before abscission, the concentration in the petiole itself remained unchanged. From this they deduced that a critical auxin concentration gradient across the abscission zone may be necessary to prevent the leaf from abscissing rather than the auxin concentration itself. Addicott and Lynch (1) further observed that whereas distal applications of auxin to debladed petioles prevented abscission, proximal ones induced it. This finding corroborated the gradient theory. Auxin is generally considered to be an abscission inhibiting material, although no convincing quantitative studies to this effect have been carried out. In view of the fact that most growth functions, inhibited by higher concentrations, are also promoted by lower ones, it would seem worthwhile to investigate the quantitative aspects of auxin effects on abscission.