The light effect on the growth inhibitors produced by the root cap.

Under light, some growth inhibiting substances were produced in the root cap of maize; they moved basipetally from the tip to the extending zone. An asymmetrical uptake by the root stumps of these inhibitors induced a curvature of the root segments. Evidence was given that these growth regulators, formed in the root cap on exposure to light, can cause root curvature in darkness. Assays with two varieties of maize (Anjou which is georeactive both in dark and in light, and Kelvedon georeacting in light only) and with IAA-applied on the basal cut end of the root segments-were discussed in relation to the light effect on the formation of the cap growth factors. Experiments involving use of ABA-which has some growth properties identical to those of these inhibitors-lead to the conclusion that the light was only acting on the formation, in the root, of the growth-inhibiting substances. But light seems not to have an effect on the transport of these inhibitors from the cap to the stump or on their action on the elongating part of the roots.