Light Growth Response and Auxin Curvatures of Avena

In an earlier paper in these PROCEEDINGS (Van Overbeek 1936) it was shown that decapitated Avena coleoptiles to which auxin-a was applied unilaterally, curved less if they were continuously exposed to light than if they were kept in the dark. This paper presents a closer study of the phenomenon; when the relation of time to the auxin curvatures is considered. This was possible by using a device designed by Went (in press) with which it has become possible to record the curvatures of a large number of decapitated coleoptiles at any time desired, without exposing them to light. The principal points of this method are the following. The primary leaves of etiolated seedlings were pulled out and removed and a straw from Bermuda grass (Cynodon Dactylon) about 10 cm. long, was stuck about 2-3 mm. into the lumen of the coleoptiles. A small part near the tip of these straws was photographed by a small horizontal beam of light on a revolving drum to which bromide paper was attached. In this way a record is obtained showing for each of the plants every 6 minutes the distance the straw has moved from its original position. For small angles this distance is practically proportional to the angle of curvature of the coleoptile. The curvatures of the exposed plants were recorded in a similar way. These plants were placed between two mirrors which reflected the light from a 10-watt lamp above them on two opposite sides of the coleoptiles. Between the lamp and mirrors was a water layer of about 10 cm. to check the heat radiation from the lamp. The intensity of the light striking the plants was about 500 M.C