128 Barlow, Hancock, and Lacey 



i. e., if sections have not been too drastically inhibited, they will re- 

 cover after washing with w^ater. 



The inhibitor was obtained in the following way: current year's 

 shoots of the plum rootstock 'Myrobalan B,' grown on a layerbed, 

 were cut into short lengths and twice extracted for 24 hrs. at 3° to 

 5° C. with peroxide-free diethyl ether. The inhibitor was originally 

 found in the acid fraction (9) on descending chromatograms run in 

 butanol-propanol-ammonia-water, 5:5:1:1, at Rf 0.6 to 0.7, but was 

 obtained in larger amounts by a method which cleared the extract 

 of fatty material, allowing heavier loading of the chromatograms. 

 The ether extract was evaporated down in the presence of water, 

 precipitating the chlorophyll and fats; the aqueous filtrate {ca. pH 4) 

 was shaken out three times with ether, and after concentration, this 

 was applied as a band to AMiatman 3 MM paper, and run in 

 plain water. The inhibitor occurred at Rf 0.85 to 0.95, and after 

 elution and re-running in either water or the organic solvent mix- 

 ture, a reasonably clean product was obtained. The concentration of 

 the solutions used in the tests is given as "internodes per ml." 

 (ints/ml); one internode was on the average about 1 g. fresh weight. 



The auxin used throughout was the sodium salt of indole-3- 

 acetic acid (NalAA), diluted from a stock solution of 1,000 p. p.m. 

 adjusted to pH 7.0. Upon dilution the pH falls to that of the water 

 used (about pH 5.6). The inhibitor solution at 100 ints/ml was of 

 pH 5.0. 



Test methods will be briefly described along with the results in 

 the next section. 



BIOLOGICAL CHARACTERISTICS 



Cell Extension in Colcoptile Sections 



In the usual section test in this laboratory, five 10-mm. sections 

 of wheat coleoptiles are used in 0.5 ml. of solution in a small vial 

 which is rotated horizontally about its axis for the growth period of 

 about 20 hrs. (3). Neither a buffer nor sucrose is added. By definition 

 the inhibitor reduces the extension of sections imder these condi- 

 tions, but does so similarly when the test is carried out in KH2PO4 

 buffer at pH 4.6 or in 2 per cent sucrose. 



Extension due to an exogenous growth promoter (NalAA) is also 

 reduced by the inhibitor, as shown by Figure 1, which represents 

 the growth of coleoptile sections in a range of concentrations of 

 NalAA and inhibitor; the left rear edge of the diagram shows the 

 typical concentration-response curve for NalAA of sections under 

 our conditions, with concentration on a logarithmic scale increasing 

 from left to right; (wheat coleoptile sections arc remarkably tolerant 

 of high concentrations of NalA.V (6), and 500 p.jxm. in this experi- 



