34 Fundamentals of Auxin Action 



is imu h greater. Some samj)lc data for dilution curves of several auxins 

 are shown in figure 16. By use of the stem-reference technique for 

 reading, the slit pea test can be used to measure quantitatively as little 

 as 0.01 mg./liter of indoleacetic acid. The maximum curvature is 

 usually obtained at approximately 10 mg./liter of indoleacetic acid, or 

 fifty times as great an auxin concentration as that producing the 

 maximum curvature in the Avena test. 



Very dilute or very weak auxins can sometimes produce negative 

 curvatures (Thimann and Schneider, 1938). 



The advantages of the slit pea test over the Avena test are: (1) 

 less exacting requirements for environmental controls, (2) the test 

 does n(K dej^end upon the ability of the plant to transport the auxin. 

 (.S) the manipulations are very easy and there is no exatting time 

 schedule, and (4) the measurable concentration range is miuh greater. 

 Along with these advantages, however, there are certain disadvantages: 

 (1) the slit ])ea test is quantitatively much less sensitive in low auxin 

 concentration ranges, (2) it requires relatively large quantities of the 

 auxin solution, and (3) it is sensitive to metallic impurities. These 

 characteristics make the slit })ea test highly desirable for the determina- 

 tion of growth regulator activity of synthetic substances but unusable 

 for the quantitative measurement of the small amounts of auxins in 

 plant extracts. 



The Straight-Growth Tests 



The physiological basis for straight-growth tests is the simple 

 stimulation of straight growth by auxins. There is no transport limita- 

 tion and no dependence upon differential growth to produce cinva- 

 ture. The presence of salts, sugars, and many other substances will alter 

 the results obtained. 



The straight-growth test, using short oat coleoptile cylinders, was 

 fust described by Bonner (1933), and that using pea stem sections was 

 described by Thimann and Schneider (1939). 



The environmental requirements are the same as in the slit pea 

 test. Usually the temperature is maintained at 25° C and only red 

 light is used. 



The materials needed for straight-growth tests are as follows: 



1. Seedlings of Alaska pea or Victory oats or other pure strain. 



2. Materials for culture of the seedlings. For peas there may be 

 porcelain trays and vermiculite or sterile wooden flats and hard maple 

 sawdust. For oats, large germinating dishes containing glass plates 

 wrapped with paper toweling are effective. 



3. A device for cutting sections of a standard length. Two razor 



