Extraction and Measurement Techniques 21 



auxin sensitivity. Consequently, a temperature of 25° C is generally 

 recognized as a good compromise. Third, humidity must be controlled 

 in order to maintain good conditions for the quantitative diffusion of 

 auxin from the agar block into the severed coleoptile. If humidities 

 below 85 per cent are used, the agar block will tend to dry out. If 

 humidities above 90 per cent are used, the seedlings tend to guttate. 

 In either case, the quantitative diffusion of the auxin out of the agar 

 into the coleoptile is seriously restricted. A relative humidity of 85 

 to 90 per cent must be strictly maintained. In some laboratories the 

 oat seedlings are cultured in sand instead of in the usual glass holders 

 and in these cases, the optimal relative humidity is found to be 80 

 per cent (van Overbeek et al, 1945). 



Fig. 7. Device for holding seedlings for Avena test. 



The materials required for the Avena test are fairly simple and, 

 again, have been described in detail by Went and Thimann (1937). 

 In brief, these may be listed as follows: 



1. Seeds of Victory oats. Because the details of the test have been 

 worked out almost exclusively using this variety, it is best to use it in 

 the Avena test. However, van Overbeek et al (1945), have found that 

 Kanota oats give essentially the same results. 



2. Glass holders into which the seedlings fit, brass clips and 

 wooden racks for support of the glass holders, and metal or plastic 

 water troughs into which the seedling roots are suspended. These 

 items are illustrated in figure 7. They are very convenient in that in- 

 dividual plants may be sorted for uniformity and grouped together in 



