The auxin production by the tip of the Avena coleoptile seems too high for a complete geotropic response (a curvature of 90 °) under the present experimental conditions (weak red light). It is sufficient for a maximum rate of geotropic curvature, but it is too low for a maximum rate of elongation. The entrance of indoleacetic acid through the cuticle is 2500-3000 times as difficult as that via the cut surface of a decapitated Avena coleoptile.