The Correlation of Different Aspects of Auxin Action

Abstract

an organic substance which promotes growth (i.e., irreversible increase in volume) along the longitudinal axis, when applied in low concentrations to shoots of plants freed as far as practical from their own in- herent growth-promoting substance. Auxins may, and generally do, have other properties, but this one is critical." 2 This definition emphasizes the aspect of auxin activity which was first studied by plant physi- ologists and which becomes particularly obvious when a stem or petiole responds with curvature to uni- lateral illumination. It was this curvature that first aroused the interest of Charles Darwin (2) and led to his classical studies on the power of movement in plants. This same curvature led Boysen-Jensen (1), thirty years later to his experiments with decapitated oat coleoptiles in which he demonstrated the capacity of the curvature stimulating influence to diffuse through a gelatin barrier and finally to the experi- ments of Went (12) who proved that agar on which An auxin has been defined by Thimann (10) as Physiol. 29: 307, 1954. This does not differ in any essential feature from the earlier definition given by Thimann. 1 Received October 9, 1954. 2 More recent definition of auxin was given inl Plant the coleoptile tips had been placed contained a sub- stance which caused curvature in decapitated cole- optiles. 20. MILLER, N. Oxidation of ferrous ions in aqueous solution by X- and -y-radiation. Nature 162: 448- 449. 1948. I. Oxidation of Jour. point of view to emphasize that aspect of auxin action which manifests as elongation growth. But, as Thi- mann points out in the same paper, the effects of auxin on plant cells are numerous and are by no means confined to stimulating growth in length. Stimulation of cell division in the cambium, initiation of adventitious roots, inhibition of growth of axillary buds, stimulation of parthenocarpic development in fruits are all aspects of the action of auxin. It is this multiplicity of effect which makes the action of an auxin so difficult to define and its mechanism so hard to explain. Nor is the problem made any simpler by the fact that there are many unrelated chemical com- pounds which function as auxins. In the present work the technique of cultivating It is therefore logical from a purely historical sterile