26 Fundamentals of Auxin Action 



which obviously have been altered by poor placing of the block or 

 by subsequent lifting of the block from the tip by extension of the 

 primary leaf may also be discarded. Usually 12 readings are taken 

 for each treatment and these are expressed as the mean. The variability 

 of the data is usually expressed as the standard deviation of the mean, 



or standard error: SE = ^ f.,,.. — 77 where A is the difference 



\1N(N — 1) 



of each reading from the mean, and N is the number of readings. A 

 variability or standard error of 10% of the maximum curvature is 

 generally considered to be approximately the greatest permissible in 

 a good test. 



Early workers using the Avena test have showed that the curva- 

 ture obtained was in linear proportion to the auxin concentration. 

 This is true generally for concentrations ranging from about 0.02 mg./ 

 liter to 0.2 mg./liter of indoleacetic acid. Lower concentrations gen- 

 erally show a skewness and concentrations above about 0.2 mg./liter 

 give curvatures which are not proportionally greater. The linear range 

 and the non-linear range are shown in figure 11. 



It should be understood that the straight linear curve is not al- 

 ways obtained even in the most careful tests. Linearity is altered by 

 agar concentration, by the time interval between application of the 

 agar and reading, by range of auxin concentrations being tested, and 

 by the presence of inhibiting substances in the agar with the auxin. 

 By using a high concentration of agar (4%) Pilet (1951) has been able 

 to convert the curvature response from a linear one to a semi-log 

 response. 



Althougii the degree of dissociation of the acid auxin has a very 

 large effect on its activity in other tests, in the Avena test the degree 

 of dissociation and pH apparently have no bearing on the curvature 

 obtained. Nielson (1930) found that unbuffered auxin solutions rang- 

 ing from pH 2.4 to 9.6 produced no differences in curvature. 



A certain inherent variability is generally found in the amount 

 of curvature obtained by a given auxin concentration from day to 

 day and from season to season. This variability has been noted by 

 most research workers using the Avena test. A notable exception to 

 this was the work of van Overbeek (1943-1947) under tropical con- 

 ditions in Puerto Rico. In most assay work a known indoleacetic acid 

 control needs to be run with each Avena test in order to eliminate the 

 source of error due to the inherent variance from one test to the 

 next. 



The presence of some inhibitors along with auxin in the agar 

 block may have a strong effect on curvature obtained. Not only is 



