NO. II ASSAY METHOD FOR GROWTH SUBSTANCES WEINTRAUB 7 



in darkness thereafter. It has not been determined whether this is 

 true also of the coleoptile stumps attached to the seeds as employed in 

 the present technique. As has been mentioned previously, constant 

 illumination with red light has been used, since this permits absolute 

 reproducibility in successive lots of plants and is much more con- 

 venient when successive lots are grown concurrently in a single dark- 

 room. It has been found, however, that the sensitivity is not ap])reci- 

 ably different whether the plants are kept in darkness or given red 

 light during the test period itself. 



With an adequate water supply to the roots of the plants, consid- 

 erable differences in atmospheric humidity do not influence the sensi- 

 tivity. No significant difference in growth rate was found between 

 plants at lOO percent and at 75 percent relative humidity even though 

 the test blocks shrink very considerably at the lower humidity. 



MEASUREMENT OF GROWTH 



For measurement of the shadowgraphs a dissecting microscope 

 equipped with a 14 X ocular and a 2 X objective has been used. The 

 ocular is provided with a i-cm scale subdivided into 100 divisions. 

 One mm on the scale (10 divisions) corresponds to 0.5 mm on the 

 shadowgraph so that the length can be read directly to 0.05 mm. 

 The uncertainty in measuring is of the order of one scale division. 

 Since the growth of the control plants in four hours is about 0.7 mm, 

 this corresponds to an error of about 7 percent ; in the test plants, which 

 make more growth, the error of measurement is correspondingly re- 

 duced. Furthermore, the error tends to be minimized when the aver- 

 age of a number of plants is taken. 



RESULTS 



The usefulness of the Avcna coleoptile as a test object rests upon 

 the fact that the induced growth is proportional, within certain limits, 

 to the concentration of applied growth-promoting substance. That 

 such a proportionality does exist was demonstrated by Thimann and 

 Bonner (1933), and has been confirmed repeatedly in the present 

 study. Figure i represents the relationship between growth and 

 growth substance concentration which has been found with the pro- 

 cedure here employed. It will be seen that the curve is a typical Black- 

 man curve with a very short transition region, very similar to that 

 obtained originally by Went (1928) for the curvature test. The work 

 of Thimann and Schneider (1938) indicates that, in the curvature test 



