New Phytologist (1968), 67, 1-13
by Rupert Sheldrake

The formation of callus at the basal end of tobacco internode tissues cultured on a basic medium has been used as an indication of the presence of auxin within the tissues. It has been shown in this way that sections of internode are capable of producing auxin. This production of auxin is related to the continued activity of the vascular cambium. If cambial activity and vascular differentiation are eliminated, auxin is no longer produced. When tissues in which cambial activity and vascular differentiation are taking place are cultured on a medium containing an inhibitor of polar auxin transport, tri-iodo benzoic acid, serried ranks of xylem tracheids are formed. It is suggested that auxin is produced as a consequence of xylem differentiation and the observations reported in this paper are interpreted in the light of this hypothesis. It is also suggested that kinins may be produced as a result of xylem and phloem differentiation, and the possibility that autolysing cells are a major source of both auxins and kinins in the plant is discussed.