The Significance of the root cap for geotropism

The geotropic curvature of pea roots decapitated at 0.3 or 0.4 mm was retarded and no positive geotropic response occurred when the roots were decapitated at 0.5 mm or more. The elongation of the roots was not affected by these decapitations. Calfeic acid and 2,4-dichlorophenol, when applied to the root cap, inhibited the geotropic curvature initially, probably because these compounds decreased the lateral distribution of the auxin in the root cap. 2,3,5-Triiodobenzoic acid, when applied to the root cap prevented the geotropic curvature almost completely. None of these treatments altered the elongation of the roots. It is concluded from the results presented in this and a previous paper (Koninos 1967) that the part of the root cap between 0.2 and 0.5 mm from the apex, where the starch-containing columella cells are localized, controls both geotropism and the lateral distribution of auxin.