AUXIN AND GROWTH 87 



less to applied auxin. Since their relative auxin production 

 and their sensitivity to applied auxin were the same, namely 

 55 per cent of the normal, van Overbeek concluded that 

 both these effects were due to an increased power of inacti- 

 vating auxin. This was proved directly by measurements 

 of the auxin inactivation; when mesocotyl sections were 

 placed on agar containing auxin, the nana inactivated about 

 twice as much auxin as the normal. This power affects 

 particularly the growing zones distant from the auxin- 

 producing center, so that although the coleoptile may reach 

 normal size, the mesocotyl scarcely elongates at all. If 

 the normal plants were warmed to 48° for 30 minutes, the 

 rate of auxin inactivation was increased, and correspond- 

 ingly the growth rate decreased. In dwarf races more 

 extreme than nana, having both coleoptile and mesocotyl 

 greatly reduced in length, a correspondingly still smaller 

 production of auxin has been found (van Overbeek, u). 



Lehmann (1936), Hinderer (1936), and Graze and Schlen- 

 ker (1936) have investigated auxin production in the stem 

 tips, and auxin content in the ripe anthers, of a number of 

 Epilobium strains and their hybrids. That the small Epilo- 

 bium hybrids are prevented from growing simply by lack of 

 auxin was directly shown by applying auxin to them, when 

 very good growth resulted (Schlenker and Mittmann, 1936). 

 The aim of this group of workers was to account for the 

 differences in length of the reciprocal hybrids in terms of 

 auxin production by them and by their parents. While 

 they find a gross correspondence, the exceptions make it 

 clear that other factors are also involved. Of these, we sug- 

 gest that the sensitivity to auxin is the most important. If 

 sensitivity be assumed to be determined by the genes, and 

 auxin production by the cytoplasm, then a qualitative 

 agreement between the observed and predicted values is 

 obtained. 



These first attempts are doubtless the beginning of the 

 application of our knowledge about auxins to problems in 

 plant genetics. Auxin is one of the principal internal factors 



