206 GROWTH HORMONES IN PLANTS 



Cholodny attempted to prove that growth substance is formed 

 by the root. When Avena coleoptiles were decapitated, it was 

 found that the average growth rate and the capacity for photo- 

 tropic and geotropic curvature were decreased. In 4 to 5 hours 

 after decapitation, the tip is ''physiologically regenerated," and 

 the ability to grow and curve in response to stimuli is regained. 

 Knowing that by applying growth substance to the decapitated 

 coleoptile (reheadmg with the coleoptile tip) the ability to curve 

 can be produced sooner, Cholodny (1928a) then tried to show 

 that the same thing happens when root tips instead of coleoptile 

 tips are placed upon decapitated Avena coleoptiles. He also 

 (1929a) apphed root tips or root cylinders of Zea to decapitated 

 Avena coleoptiles and investigated the rate of growth. A typical 

 experiment consisted of three series of decapitated oat coleoptiles 

 (10 plants in each) selected for comparable length. To one 

 series were added Zea root tips, to another Zea root cylinders, and 

 the third remained as a control. In the first, the average increase 

 in growth was 9.3 per cent in 7 hours at 18°C.; in the second, 

 7.4 per cent; and in the third 7.2 per cent, under the same condi- 

 tions. The root tip seems to be more effective, but the amount of 

 increase over that of the controls is not great. It should be 

 pointed out that within the 7-hour period, a "regeneration" of 

 the tip in the Avena coleoptile would have taken place, and this 

 factor may have influenced the results. 



Keeble, Nelson, and Snow (19316) investigated the effects upon 

 growth and tropisms produced by placing excised tips of coleop- 

 tiles and roots upon decapitated roots. Their experiments 

 indicated that the same kind of growth regulator exists in both 

 roots and coleoptiles, though it inhibits the growth in length of 

 roots and promotes elongation in coleoptiles. 



In another series of experiments, Cholodny (1934) tested the 

 growth-promoting effect of excised tips from the roots of Zea mays 

 placed unilaterally upon decapitated Avena coleoptiles. In all 

 cases, negative curvatures resulted within 2 hours, and it was 

 concluded that a growth substance was present in the root tips. 

 In other experiments, a root tip was placed on one side and a 

 coleoptile tip on the other side of the Avena stump. Under these 

 conditions, no curvatures took place, or bending was toward the 

 side occupied by the root tip. From these results, he concluded 

 that an isolated tip of a Zea root gives off about the same 



