¹ From the Laboratory of General Physiology, Harvard University, Cambridge

Applying a photographic recording method, and working on enlargements of the plates so obtained, the shape of the geotropically curving coleoptile of Avena was studied. This shape is expressed in terms of curvature of the neutral axis of the coleoptile; for the true geotropic response of the strain of Avena used, the curvature of the neutral axis is an arc of a circle. The "rate of curvature" is taken as the derivative of the curve relating time with the angle with the horizontal made by the tangent to the neutral axis at the tip. This rate increases up to a maximum and then decreases gradually. No "geogrowth" of the whole coleoptile is found. It is shown that the curvature is due to an increase in the elongation of the lower side of the horizontally placed coleoptile with a concomitant decrease of the rate of elongation of the upper side. This is correlated with a shift in distribution of "growth substance" in the tip as affected by change of position of the coleoptile.

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