NO. II ASSAY METHOD FOR GROWTH SUBSTANCES WEINTRAUB 5 



DECAPITATION 



It is essential that the cut surface of the coleoptile be exactly at the 

 level of the test tube rim and that the cut be clean and horizontal (at 

 right angles to the long axis of the coleoptile). Otherwise the plants 

 may bend and measurement will be difficult. Decapitation can be per- 

 formed quite rapidly by making a small cut partially through one (or 

 two opposite) sides of the coleoptile with a thin safety razor blade held 

 flat against the rim of the tube and bending the coleoptile toward the 

 cut with the fingers or forceps until it breaks. The leaf is pulled out 

 completely. It is often possible to break off the coleoptile and withdraw 

 the leaf in a single motion. One hundred coleoptiles can be decapitated 

 in 30 minutes or less. 



AGAR TEST BLOCKS 



In the development of the method, weighed amounts of dehydrated 

 agar were mixed with aqueous solutions of known concentrations 

 of indole-3-acetic acid, or of auxin-a.^ Similar results have been 

 obtained with both of these growth-promoting substances. The test 

 blocks were priepared with an apparatus similar to that described by 

 DuBuy (1938). 



Thimann and Schneider (1938) have reported that the concentra- 

 tion of agar in the test blocks is of considerable importance in the 

 Aveua curvature test. In general they found that a given concentra- 

 tion of indole-3-acetic acid produced larger curvatures the lower the 

 agar concentration. Similar, although less marked, differences have 

 been found in the present study of straight growth. The use of 1.5 

 percent agar has been adopted as ^l general procedure. 



SIZE OF TEST BLOCKS 



Went (1928) concluded that with 0.9 mm^ blocks the curvatures 

 are proportional to the absolute amount of growth substance in the 

 blocks. Van der Weij (1931) and Thimann and Bonner (1932) con- 

 cluded that the curvatures are proportional to the concentration of 

 growth substance in the block. The data of Thimann and Bonner indi- 

 cate that the rate at which the growth substance passes from the 

 block to the plant is proportional to its concentration in the block at 

 any moment. The change in concentration of growth substance in the 

 block during any given period will be less the greater the volume of 



2 A solution of pure crystalline auxin-a was very generously supplied by 

 Prof. F. Kogl, of the University of Utrecht. 



