2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



promoting substance across the coleoptile, producing a growth ac- 

 celeration on the far side of the plant. The influence of the last-named 

 factor is very marked in the case of a number of substances which 

 show relatively little or no activity by the curvature method but have 

 considerable effect on straight growth (cf. table XII, pp. 137-139, 

 Went and Thimann, 1937). This lateral transport is greater the 

 higher the concentration of growth-promoting substance applied ; the 

 net result is a decrease in the sensitivity of the test. 



Methods employing straight growth have been employed occasion- 

 ally but have not come into routine usage, largely because of the 

 inconvenience in measuring the growth. The present report describes 

 an assay procedure utilizing straight growth of the coleoptile of Avena 

 sativa!' in which the sensitivity of the response and the ease and ac- 

 curacy of measurement are at least as great as in the commonly used 

 curvature test. In addition, the method offers a number of other tech- 

 nical advantages. 



The procedure will be outlined briefly first, and then each step will 

 be discussed in greater detail. 



SUMMARY OF METHOD 



Seeds are planted on agar slants in small test tubes at a determined 

 distance below the rim of the tube. The seedlings are germinated and 

 grown under controlled conditions. When the coleoptiles have at- 

 tained a given length they are decapitated level with the rim of the 

 tube and the leaf is withdrawn completely. Blocks of agar, containing 

 the growth-promoting substance to be tested, are placed terminally 

 upon the entire cut surface of the stump. After some time a shadow- 

 graph is made in the usual manner. The length of the coleoptile which 

 extends above the rim of the tube represents the growth increment 

 during the test period. It can be measured very easily with a dissect- 

 ing microscope equipped with an ocular micrometer. 



TEST TUBES AND RACKS 



Soft glass or Pyrex tubes having an inner diameter of about 15 

 mm and a length of 7 cm have been found satisfactory. Trials with 

 larger tubes indicate that within reasonable limits the size is imma- 

 terial. A simple rack for the tubes may be constructed by boring a row 

 of holes in a wood block. The holes should be of such depth and 



1 Avcna- satiiHi var. Markton has been used exclusively. The seeds were ob- 

 tained through the courtesy of Mr. T. Ray Stanton, of the U. S. Department 

 of Agriculture. 



