GROWTH SUBSTANCES FOR GEOTROPISM 195 



hypocotyls and epicotyls of Helianthus cannot be brought directly 

 into accord with the growth-substance explanation. In Taraxa- 

 cum, during the course of curvature, growth increased and often 

 was renewed in stalks that had previously ceased elongating. It 

 was shown that these organs may curve geotropically after growth 

 in length has stopped. A shortening of the upper side was 

 reported in HeHanthus. Beyer did not investigate growth- 

 substance content or distribution, so it is entirely possible that 

 formation of growth substance could have been renewed in these 

 horizontally placed organs and thus account for the observed 

 revivals of growth. 



The investigations of Gundel (1933) showed that in geotropi- 

 cally curv'ed parts of a plant, the acidity upon the convex side is 

 heightened while that of the concave side remains unchanged. 

 Metzner (1934) fomid increased acidity and plasticity of the cell 

 walls upon the convex side of Helianthus hypocotyls. In 

 accord with the investigations of Stnigger (see page 113), 

 this observation might be of significance for the comprehension 

 of geotropic curvatures in seedling stems of dicotyledonous 

 plants. It is not known what influence the accumulation of 



is not a conductor, hence by induction the internal polarity is positive on the 

 side toward the negative electrode. 



B, electrical polarity and longitudinal transport. Decapitated Avena coleop- 

 tiles with growth substance applied in agar blocks on the apical cut surface were 

 put in series ^-ith a small battery by attaching wet silk threads to the agar 

 blocks and closing the circuit at the other end where the roots dipped into the 

 water of the culture vessel. Transport of the auxin anion into the stump (as 

 determined by the amount of the ensuing curvature) was retarded by making 

 the base of the coleoptile negative to the tip, as shown in a, and promoted by 

 ha%'ing the base positive to the tip, as in h. The potential employed was 80 miUi- 

 volts and the current 0.0008 milliampere. 



C, effect of an electrical circuit upon geotropic curvature in roots. The main 

 root of Pisum. placed in a horizontal position normally curves downward, a. 

 When immersed in conducti\'ity water in an electrical circuit with the positive 

 pole above the root, as in h, the root conducts the current and bends toward 

 the positive pole, even away from the force of gravity, c. 



D, effect of applied potential upon erect Helianthus hypocotyls; they curve 

 away from the positive electrode. 



E and F, influence of applied potentials upon phototropism. E, when the 

 negative electrode is on the shaded side of a unilaterally illuminated Helianthus 

 hypocotyl, the electrotropic stimulus is stronger than the phototropic, and the 

 h>T>ocotyl bends away from the light. F. when the shaded side is made positive, 

 normal phototropic curvature is augmented. 



G and H, influence of applied potentials upon geotropism in hypocotyls of 

 Helianthus. G, in a horizontally placed hypocotyl, negatively geotropic curva- 

 ture is augmented by making the lower side positive. H, normal geotropic 

 curvature does not occur when the upper side is made positive. {B after Kogl, 

 19336; a^l others after Koch, 1934.) 



