H. W. B. BARLOW 

 C. R. HANCOCK 

 H. J. LACEY 



East Mailing Research Station 

 England 



Some Biological Characteristics of an Inhibitor 



Extracted From. Woody Shoots 



Since the adoption of chromatography for the separation of natural 

 plant growth regulators, many workers have reported the presence of 

 substances which inhibit the extension of coleoptile sections. Simi- 

 larity in position on many chromatograms of diverse plant extracts 

 has tended to give the impression that all these inhibitory zones 

 exert a physiologically similar effect on the test object by which they 

 are located in the first place. There are, however, certainly two 

 strongly contrasting types: those in which the inhibition is readily 

 reversible, as described by the present authors (2), and more recently 

 by Van Steveninck (16), and those which are merely toxic, such as 

 the substances isolated by Housley and Taylor (8). Within these two 

 types — and perhaps only the nontoxic one commands much inter- 

 est — seA'eral modes of action may be involved, not all of which will 

 necessarily produce inhibition in all tests. This paper presents the 

 results of several biological tests carried out with a material separat- 

 ed from extracts of plum shoots, and foiuid to inhibit coleoptile 

 section extension, and the variation in behavior in different assays 

 suggests that it is rash to make generalizations about the growth 

 regulatory properties of a particular substance on the basis of any 

 one biological test. 



MATERIALS AND METHODS 



The active material used in the tests to be described, and sub- 

 sequently referred to simply as "the inhibitor," is a substance, or 

 mixture of substances, found on chromatograms as detailed below, 

 which causes a reduction in the extension of wheat coleoptile sec- 

 tions compared with their growth in water; at no concentration does 

 it promote extension. The action of the inhibitor is largely reversible, 



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