TECHNIQUE OF AUXIN DETERMINATIONS 27 



be "trapped" in the gelatin. He cut off a number of tips of 

 coleoptiles, placed them for some time on gelatin, removed 

 them, and placed the gelatin one-sidedly on the cut surface 

 of freshly decapitated coleoptiles. For the preparation of 

 the test coleoptiles he used essentially Stark's technique. 

 However, as measure of the growth-promoting activity, he 

 used, not the percentage of plants which curved, but the 

 actual degree of curvature. The reason for this is that in 

 any group of plants one or two will fail to curve due to 

 experimental faults, and these should not be included in 

 the mean. The percentage of plants which curve thus meas- 

 ures the efficiency of the technique rather than the growth- 

 promoting activity. With this method he was able to show, 

 amongst other things, that heating the gelatin for a short 

 time at 90° C did not inactivate the substance, which cannot 

 therefore be an enzyme. These fundamental experiments 

 provided final proof of the material nature and stability 

 of the growth-promoting substance of the coleoptile, and 

 laid the foundation for further work. In a more detailed 

 publication (Went, 1928), the Avena test method for deter- 

 mination of the growth hormone was worked out quanti- 

 tatively. Since this method has been so extensively used as 

 the basis of all phytohormone work it will be well to treat 

 it here in detail. 



C. The Avena Method in Its Present Form 

 1. Dark Room and Equipment 



Although it is not strictly necessary to maintain darkness 

 and constant conditions for carrying out qualitative work 

 with Avena curvature, nevertheless for quantitative study 

 the conditions outhned below must be strictly adhered to. 

 Soding (1935) has used the Avena test in diffuse daylight 

 with uncontrolled temperature, but under these conditions 

 the sensitivity of the plants is very much less, and there is 

 no strict proportionality between concentration of the 

 active substances and curvature produced (see van Over- 

 beek, 1933, 1936a). 



