TECHNIQUE OF AUXIN DETERMINATIONS 37 



found that curvature is produced. Apparently higher con- 

 centrations are needed than in the normal Avena test but 

 the experiment provides an interesting demonstration that 

 auxin may enter through the epidermis. The same fact 

 has been utilized in a still further modification (Laibach, 

 1933a, 1935; Brecht, 1936) in which auxin is dissolved in 



1 



\ 



R n 



Fig. 16. De-seeding of Avena. A, normal seedling in glass holder; B, same 

 with seed removed and coleoptile held in guide with cotton wool; C, decapita- 

 tion; D, curvature. (From Skoog, J. gen. Physiol. 20: 311-334, 1937.) 



lanoline (Laibach's method), and this paste applied exter- 

 nally to intact or decapitated coleoptiles. 



If about 4 mg. of such paste be smeared over a length 

 of 1 cm. of one side of the coleoptile, a curvature results 

 which increases up to 24 hours, and is within limits propor- 

 tional to the concentration of active substance in the paste. 

 However, the lowest concentration which produces curva- 

 ture appears to be about 10 times that which produces the 

 same curvature when agar is used (see also Jost and Reiss, 

 1936). The method might be useful for the standardization 

 of such pastes, but unfortunately it is highly non-specific, 

 because acetic acid and other acids incorporated in the 

 paste also give rise to curvatures. In the application of 

 lanoline-auxin-paste to plants in general great care should 



