138 



Barlow, Hancock, and Lacey 



Table 3. Percentage increase over initial length (10 mm.) of coleoptile sections 

 given separate apical and basal supplies of NalAA 1 p. p.m., inhibitor 20 ints/ml, or 

 water. Mixture of NalAA and inhibitor at both ends = 17. 



DISCUSSION 



Although the concentration response curves differ according to 

 the test objects employed, this inhibitor reduces extension in the fol- 

 lowing: coleoptile sections with or without an exogenous source of 

 auxin, pea epicotyl sections, wheat leaf bases, with or without added 

 auxin or GA, cress roots, and pollen tubes; it also reduces the auxin- 

 induced curvature of Avena coleoptiles. It increases the effect of auxin 

 in causing inward curvature of slit pea stem sections, but its action 

 here may not necessarily be truly synergistic. It has no effect in our 

 tests on abscission, respiration, or permeability of the protoplast to 

 water; the inhibitor does not move out of coleoptile sections into 

 agar in either direction; transport of NalAA through coleoptile sec- 

 tions is not affected. 



How then does this material effect a reduction in cell extension? 

 These experiments do not provide an answer, but they do indicate the 

 desirability of testing substances for biological activity in a variety 

 of ways before calling them "inhibitors" or "promoters," or at least 

 of keeping the definition of their activity continually in mind. Had 

 we assayed our chromatograms by the slit pea stem test, ^\•e should 

 have found a "synergist of lAA"; by an abscission or respiration test 

 the same region would have been "inactive," while by many other 

 tests it would have appeared as "inhibitory." Which of these roles, 

 if any, does this material play in the physiology of the plant from 

 which it has been extracted (Cf. 4) ? This is the basic and disturb- 

 ing question which applies to many growth substance studies, and 

 which has seldom been answered imequivocally, particularly lor 

 these enigmatic "inhibitors ' of coleoptile section extension. 



It is obvious that the nature, mode of action on the cell, and func- 

 tion in the plant, of such substances, warrant intensive study by chem- 

 ists and biochemists, at least to the same extent as that devoted to 

 auxins and gibbcrellins. 



