120 



GROWTH HORMONES IN PLANTS 



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Fig. 37. — Various phenomena brought about by the application of growth 

 hormone to plants. The paste containing the hormone is indicated by stippling. 



A, unilateral application of 3-indole acetic acid paste, as shown in a, induces 

 curvature in the Avena coleoptile {b and c). (After Laibach, 19336, 1935.) 



B, growth curvature in roots may be regulated by the unilateral application of 

 growth hormone: a, when applied to a vertical root, lateral bending occurs, as 

 shown in b; c, primary roots in the horizontal position normally curve down- 

 ward, but growth hormone applied on the upper side causes negative geo tropic 

 curvature, d; applied on the lower side of a horizontal root it promotes positive 

 geotropic curvature, e. (After Koch, 1934.) C, young leaves of Nicotiana and 

 other plants exhibit hyponasty, a, and epinasty, &, as a result of the addition of 

 3-indole acetic acid paste. (After Avery, 1935.) D, Mimosa petioles bend 

 upward, a, or downward, b, upon application of 0.01 per cent 3-indole acetic 

 acid paste to the lower or upper sides of the primary pulvini. (Burkholder and 

 Pratt, 1936.) E, application of growth-hormone paste around the hypocotyl 

 of a Helianthus seedling nullifies polarized growth in the long axis: a, normal 

 seedling; b, with ring of hormone paste; c, untreated seedling; d, treated seedling, 

 after 7 daiys. (After Czaja, 1935a.) F, if growth hormone is applied unilaterally 

 to a young stem, as shown in a, bending occurs away from the side of application, b. 

 (After Zimmerman and Wilcoxon, 1935.) 



