GROWTH SUBSTANCES FOR NORMAL GROWTH 



91 



are no longer tenable in the light of the more recent discoveries 

 concerning the role of growth substance. 



A question of importance is whether growth takes place in the 

 Avena coleoptile when growth substance is completely absent. 

 As shown by Soding's experiments, some growth takes place 

 in the first 5 hours after decapitation, but Dolk (1930) showed 

 that this occurs only because of the growth substance still 



Interferometer 



Coleoptile 



Solution 



Vaseline 



Fig. 28. — Stimulating effect of growth hormone (acidulated pollen extract) 

 upon growth of a decapitated Avena coleoptile. A, diagram showing culture 

 chamber in which coleoptile is grown immersed in solution. B, graph showing 

 the increased rate of growth of a coleoptile treated with growth-hormone solution 

 at the time indicated by arrow, as compared with a control plant (lower curve) 

 which continues its growth in water. Growth was measured interferometrically. 

 (After Laibach and Kornmann, 1933a.) 



present in the coleoptile stump. If the coleoptile is decapitated 

 again 2 hours after the first decapitation, its growth ceases 

 almost completely but can be renewed by supplying growth 

 substance. From these experiments it may be concluded that 

 normally no gi'owth substance is formed below the tip of the 

 coleoptile and that the growth substance present in the stump at 

 the time of decapitation is gradually used up. In any case, 

 without growth substance there is no growth. 



