Extraction and Measurement Techniques 37 



Reading 



Measure after 24 hours when approximately 90 per cent of growth has usually 

 occurred, or at 48 hours when growth is essentially finished. Again, if growth rate 

 is to be studied, readings will have to be made 6 hours after the test is started, 

 for the rate begins to decline after that length of time (Christiansen and Thimann, 

 1950). 



As in the previous tests, results are expressed as the mean per 

 treatment plus or minus the standard error. In general where serial 

 dilutions are used and a smooth curve is obtained repeatedly, the 

 standard error may be considered to be unnecessary. 



10 MG/L 



CONC. OF AUXIN 



Fig. 18. Results of an Avena straight-growth test and a pea straight-growth test 

 showing the approximate semi-logarithmic proportionality of growth to the concen- 

 tration of the auxin, indoleacetic acid (Thimann and Schneider, 1939). 



The growth obtained is approximately proportional to the log- 

 arithm of the concentration of auxin applied. In both the Avena 

 and the pea straight-growth tests the minimum amount of growtli 

 regulator detectable is approximately 0.01 mg./liter of indoleacetic 

 acid. Maximum growth is obtained at approximately 10 mg./liter of 

 indoleacetic acid. Sample curves for each test are shown in figure 18. 



Since straight-growth tests are not dependent as is the Avena test 

 upon polar transport of auxins, they can be used to test the growth 



