no 



GROWTH HORMONES IN PLANTS 



Although Soding did not state that plastic extensibility cannot 

 assist in growth, he concluded that this is neither the only cause 

 of growth nor the main one. He found that the plasticity of 

 flower stalks is decreased only sHghtly by topping, although the 

 growth of the stalks suffers a great decrease. Moreover, the dif- 

 ferences in the plasticity of individual stalks are considerable. 

 He concluded it improbable "that such a variable and continually 

 changing property as plasticity is the single cause of a regularly 

 occurring process." This implies the assumption that the Avena 



J 



<*■ b O] 61 02 62 



Fig. 36. — Method of determining elastic and plastic extensibility in portions 

 of Lupinus hypocotyls. An agar block without growth hormone was applied 

 to a segment from the hypocotyl as shown in a. A similar agar block containing 

 growth hormone (indicated by stippling) was applied to another hypocotyl 

 segment, h. After 23^2 hours the hypocotyl segments were bent by mechanical 

 force to an angle of 45° where they were held for 5 minutes (ai and ?)i). Upon 

 removal of the force, the hypocotyls returned part way to their original vertical 

 position; the segment treated with growth hormone retained an angle of curvature 

 of 22.9° (62), while the control retained an angle of only 12.5° (02). Hence, plas- 

 tic extensibility is nearly twice as great in the hypocotyl treated with growth 

 hormone. (Adapted after Heyn, 1934a.) 



coleoptile behaves in a fashion similar to the inflorescence stalks. 

 In a growth-substance curvature the plasticity of the convex 

 side is greater than that of the concave side, and this difference 

 still remains after the curvature has reached its maximum point, 

 i.e., when no further growth is taking place. 



The fact that extensibility in nongrowing coleoptiles is less 

 than in growing coleoptiles is yet to be explained. It is influenced 

 neither by growth substance nor by low temperature (0°C.). 

 Heyn originally concluded that it was brought about by an 

 increase of the cell-wall matter, perhaps by intussusception. 

 Later he showed that the decrease in extensibility is due to a 

 reduction of the extension capacity of the elastically extended 

 outer layers of the wall. Soding considered that the decrease in 

 extensibihty may be conditioned by a hardening of the wall and, 

 conversely, that the increase in elastic extensibility of the wall 



