O PLANT GROWTH SUBSTANCES 



an agar block containing the active material is placed on one side of the 

 cut surface of the coleoptile, the primary leaf thereby serving as a 

 support. After no minutes the curvature caused by the growth dif- 

 ference between the two sides is measured. The curvatures are, up to 

 a certain concentration, proportional to the amount of growth substances 

 applied. While Went used this test primarily to measure the growth 

 substance diffusing from tips into gelatin or agar, this method has been 

 extended to the quantitative assay of extracts obtained by chemical 

 procedures. This introduces complications which are often underesti- 

 mated. Salt concentration, pH, and the preparation of the agar blocks 

 have been shown to affect the curvatures. A more thorough investiga- 

 tion of these factors will undoubtedly increase the reliability of the 

 results obtained. The Avena test has a high degree of sensitivity; 25 

 gammas of indoleacetic acid per Hter gives a curvature of about 10°. 

 This means that each plant receives 1/20,000,000 milligram of growth 

 substance. A three- to fivefold increase in the sensitivity of this test 

 was obtained by removing the seed without damaging the embryo, 

 twelve to eighteen hours before the test (53). Straight growth measure- 

 ments have also been used for the determination of the auxins and 

 resulted in the development of the coleoptile cylinder method, whereby 

 short pieces of the coleoptiles are immersed in the test solution. The in- 

 crease in length is proportional to the logarithm of the concentration (4). 



A test which has found wide application was devised by F. W. Went 

 (66) who observed that the internodes of pea stems, when spUt 

 lengthwise, curved inward when in contact with auxin solutions. The 

 inward curvature is due to the difference in growth between the two 

 sides of the organ, the outer side being more sensitive to auxin stimulation 

 than the inner side. The pea test has been of great value in the testing 

 of a number of synthetic substances, which, because of their lack of 

 transportability, do not react in the Avena test. In comparing the 

 activity of these substances it would be beneficial to apply the same 

 rigid standardization to the pea test as Went introduced for the Avena 

 test. 



Many plants, among them the Avena, show this behavior, and it 

 might be of advantage to use this more uniform material. Using Avena 

 a greater sensitivity was obtained by splitting the coleoptile into quarters 

 instead of halves (60). A further refinement of this method consists 

 of removing the quarters containing the vascular bundles, which have 



