66 GROWTH HORMONES IN PLANTS 



meter-candle seconds as compared with plants kept in darkness. 

 Although these various observations disagree in certain points, 

 they may be brought into harmony later by further investiga- 

 tions on the problem under controlled conditions. 



Mature grass nodes which contain no growth substance have 

 been reported to regain their ability to form it when stimulated 

 by gravity (Schmitz, 1933). This interesting phenomenon has 

 not yet been explained satisfactorily. 



That the formation of growth substance is influenced by 

 temperature has been shown by the experiments of Hawker 

 (1933). Root tips of Lathyrus odoratus grown at 20°C. produced 

 more of the hormone controlling root growth than similar tips 

 grown at 5°C. (Fig. 62). The influence of temperature has not 

 yet been studied carefully enough to permit any statement with 

 respect to a "temperature coefficient" for the process. 



As a result of certain toxic effects, the ability to form growth 

 substance may be diminished or entirely lost. Roots of Vicia 

 Faha and Pisum sativum that have been treated with erythrosin 

 react ageotropically (see Boas and Merkenschlager, 1925) and 

 contain much less growth substance than normal roots (Boysen 

 Jensen, 1934). Van der Laan (1934) showed that ethylene 

 decreases the production of growth substance in Avena and in 

 Vicia Faha but not its utilization, for, when growth substance is 

 directly applied, growth is not affected. 



Studies by Skoog (1935) have shown that X rays inactivate 

 both auxin a and 3-indole acetic acid in aqueous solutions. 

 The growth hormone normally present in plants is partially 

 inactivated also by moderate treatments. The formation 

 of auxin in green plants (Pisum and Vicia) is inhibited by 

 irradiations of 300 to 1200 R. When Avena coleoptiles were 

 irradiated with dosages of the same magnitude, no effect upon 

 transport or formation of growth hormone could be found. The 

 treated coleoptiles grew only slightly less than the controls. 

 Applications of growth hormone to the irradiated coleoptiles 

 showed that the capacity for growth was not affected by the 

 X-ray treatment. Failure of irradiation to alter the growth 

 rate markedly in Avena may find its explanation in the recent 

 observation of Pohl (1935) that the growth hormone is not formed 

 in the coleoptile but moves from the endosperm up to the 

 coleoptile tip, where it is activated and distributed downward. 



