1. With an automatic photokymograph, the procedure of curvature in the Avena test was investigated. In general, when auxin was applied unilaterally there was little or no effect for about 20 minutes, after which curvature set in and continued at a constant rate for about an hour, then diminished or reversed, depending on the treatment. 2. It was found that increasing the auxin concentration inside the plant decreased the sensitivity to unilaterally applied auxin. This explains the increase in sensitivity after decapitation, since the auxin content decreases after removal of the tip. 3. To study this process in detail, the effect of the i.d.p. (length of interval between decapitation and putting on of the agar block) and the number of decapitations was followed for different concentrations of auxin. 4. For all concentrations up to maximum angle concentration, the sensitivity increased rapidly for the first 30 minutes i.d.p., increased gradually until 200-240 minutes i.d.p., then gradually decreased. 5. But...