DETECTION AND QUANTITATIVE DETERMINATION 37 



instances, but in certain cases it has failed to show a response. 

 Stark (1921a) found that segments of the shoot of Brassica 

 napus placed on one side of a decapitated Avena coleoptile 

 caused positive curvature, toward the side with the applied 

 object, thus indicating inhibition of growth. Gradmann (1928) 

 obtained similar results using tips of Convolvulus. In a more 

 extensive study Soding (19356) found that certain genera, such as 

 Symphoricarpos, Rheum, and Cephalaria, do not give the Avena 

 coleoptile test for growth substance. However, when decapitated 

 seedlings of the hybrid Cephalaria tatarica X C. alpina were 

 employed as test objects, positive indication of growth-promoting 

 substance was obtained by unilateral application of agar blocks 

 containing exudate from seedlings or parts of mature plants of 

 C. tatarica. This suggested that there might be other types of 

 growth hormone in plants distinct from those in Avena. Also, 

 there is the possibility of the existence of toxic substances or 

 destructive enzymes in the growth-substance test preparations 

 which nullify the growth-promoting effects. Moreover, the 

 failure of growth hormone from one plant source to penetrate 

 the tissue of another plant used as the test object could explain 

 the failure to get response. Recently, however, Soding has 

 shown that blocks containing hormone from Avena give test 

 curvatures with Cephalaria (1935c). Soding (1936) has pointed 

 out, furthermore, that in many instances Cephalaria is more 

 sensitive than Avena. For example, in testing the growth- 

 substance content of Taxus, about 15 times greater curvatures 

 were obtained with Cephalaria. Soding concluded that the 

 difference in response of test plants is due, not to the existence of 

 chemically different growth substances in different plants, but 

 rather to the difference in sensitivity to the presence of low 

 concentrations of the same hormone. Experimental evidence 

 was presented to show that Avena is relatively unresponsive to 

 extremely dilute preparations of heteroauxin while Cephalaria 

 exhibits marked curvatures to the same low concentrations of 

 the hormone; on the other hand, the amount of curvature shown 

 by Avena over the range in which growth is proportional to 

 concentration of hormone indicates its superiority for general 

 quantitative test purposes (Fig. 185). In order to increase 

 sensitivity in Avena as a test object, Heyn (1935) has resorted 

 to three decapitations, allowing two hours between each, and 



