28 Fundamentals of Auxin Action 



salts in the agar can strongly afiect the diffusion of auxin from plant 

 material into the agar. Potassium ions are particularly active in pro- 

 moting auxin diffusion into the agar. A concentration of 10-^ molar 

 KCl has been found to be optimal in this regard (Thimann and 

 Schneider, 1938). 



Occasional positive curvatures are obtained in the Avejia test. In 

 such cases the coleoptile bends toward the agar block instead of away 

 from it, as one would expect when a growth promoting substance has 

 been applied. Small positive curvatures may be caused by: (1) exces- 

 sively low auxin concentrations, (2) the presence of growth inhibitors 

 in the agar, or (3) too long or too short an interval between the time 

 of applying the agar block and taking the shadowgraph reading. 



Before the recognition of indoleacetic acid as an auxin, a method 

 of expressing auxin concentration was very much needed. Almost 

 simultaneously three different methods were worked out and one 

 occasionally finds them in the literature today. These units of auxin 

 activity expressed as the amount of curvature per unit of agar are: 



(1) AE (Avena Einheit) = 10° curvature from each 2 mm.^ block, 



(2) p. u. (plant unit) =1° curvature from a 10 mm.^ block, (3) WAE 

 (Wuchsstoff Avena Einheit) = 50° total curvature from a 100 mm.^ 

 block. A detailed comparison is given by Boysen-Jensen (1936, p. 33). 

 The use of these auxin units has been generally discarded now and, 

 instead, with each test the curvature from a known quantity of 

 indoleacetic acid is measured and the auxin is expressed as indoleacetic 

 acid equivalents. 



As we have stated before, the sensitivity of any given test is subject 

 to considerable variability both in relation to time of day and season 

 of the year. The maximum angle of curvature obtainable will gen- 

 erally vary from 15° to 35°, but under any given set of conditions, the 

 concentration of auxin required to give the maximum angle will 

 remain the same. Concentrations greater than that required to pro- 

 duce the maximum angle cannot be measured quantitatively by the 

 Avena test. 



The Slit Pea Test 



The slit pea test, originated by Went (1934), is described in 

 "Phytohormones" (Went and Thimann, 1937, p. 54) and in further 

 detail by van Overbeek and Went (1937) and Went (1939). This test 

 has had extensive use in the subsequent years and few changes have 

 been made. 



The physiological basis for the slit pea test lies in the differential 

 growth of the epidermal cells of etiolated pea stems in response to 



