26 PHYTOHORMONES 



The above experiments provided evidence for the existence 

 of the growth hormone in the coleoptile, and experiments of 

 Beyer (1925) gave similar evidence in the case of Helianthus 

 hypocotyls. Attempts were therefore made to prove the 

 existence of this substance directly by extraction. Stark 

 (1921) crushed up coleoptiles, mixed the extract with warm 

 5 per cent agar, and when the mixture had solidified, he 

 divided it into small blocks and placed them one-sidedly 

 upon the cut surface of decapitated coleoptiles. The decap- 

 itation was done by removing the tip without cutting it 

 completely through, so that the primary leaf remained intact 

 and could be used as a support for one side of the agar block. 

 However, in all his experiments, embracing several hun- 

 dred plants, only positive curvatures were obtained, that is, 

 curvatures towards the applied block of agar. Extracts of 

 stems or hypocotyls were similarly inactive. Hence no 

 growth-promoting substance was extracted in this way; 

 the explanation for the positive curvatures will be given 

 later (III C 6). An important technical improvement was 

 then introduced by Stark and Drechsel (1922), who pulled 

 the primary leaf loose, breaking it at the base so that it 

 did not grow further, and thus did not lift off the agar 

 block. The differential growth rate of the leaf and coleoptile 

 is caused by the decapitation, which only reduces the 

 growth rate of the coleoptile itself. 



Nielsen (1924) and Seubert (1925) modified the experi- 

 ment of Stark by crushing only the extreme tips of the 

 coleoptiles, but, except for one experiment by Seubert, 

 failed to obtain any growth-promoting extract. Seubert, 

 however, was able to obtain growth promotion — that 

 is, negative curvature — with agar blocks containing diastase, 

 pepsin, malt extract, or saliva, and, in one experiment 

 only, with concentrated press juice from coleoptiles. 



Starting from the facts that the growth hormone is able 

 to diffuse through gelatin, and that only the tip actually 

 produces it. Went (1926) ^ showed that the substance could 



1 See footnote to page 5. 



