THE ROLE OF AUXINS IN CELL ELONGATION 579 



Not all translocation of growth substance in plants occurs basipetally, 

 however. Hitchcock and Zimmerman (1935, 1938) have shown that upward 

 transport of certain growth substances occurs when they are supplied to the 

 plant from an external source. For example, when relatively high concentra- 

 tions of certain growth substances were added to the soil, they were absorbed 

 and translocated in an upward direction through the plants rooted therein. 

 It is not certain at the present time, however, to what extent naturally occur- 

 ring auxins move in an upward direction through plants. 



The Role of Auxins in Cell Elongation. — The influence of auxins on 

 cell elongation in the oat coleoptile has already been described. Auxins play 

 a similar role in the elongation phase of growth in many other plant organs, 

 and it is now generally considered that cell elongation occurs only in the 

 presence of auxins. In the absence of these substances elongation fails to take 

 place and within limits elongation is proportional to the concentration of 

 auxins, providing no other factor necessary for growth is limiting. It appears to 

 be true, however, that auxin frequently is the limiting factor in cell elongation. 

 Increase in concentration beyond the point at which some other factor becomes 

 limiting results in no further elongation (Blackman's principle, Chap. XXI). 

 Relatively high concentrations of auxins, in fact, often exert an inhibiting 

 effect on the elongation of aerial organs. The optimum range of concentra- 

 tions for cell elongation varies greatly with different tissues. 



If the extreme tip of a maize or lupine root is cut off, its rate of elongation 

 increases, although not greatly (Cholodny, 1926). Replacement of the root 

 tip in maize plants results in a retardation in elongation rate as compared with 

 decapitated roots. Furthermore, attachment of coleoptile tips of maize to 

 decapitated root tips of the same plant results in a retardation in the elonga- 

 tion rate of the root tip. These results suggest that the same concentrations 

 of auxins which accelerate elongation in coleoptiles and other aerial organs 

 retard elongation in roots. 



This supposition has been confirmed by experiments in which the roots of 

 oat seedlings were immersed in pure solutions of auxins. The growth of the 

 roots was found to be retarded in proportion to the concentration of auxin 

 used. However, when roots which contain either no auxin at all or virtually 

 none are treated with auxin solutions of very low concentration acceleration 

 of growth as compared with similar but untreated roots often results. 



The presence of auxin in roots has been demonstrated by chemical as well 

 as by biological tests, and in general it is found to be present in greatest con- 

 centration in the root tips. It is not definitely known whether or not intact 

 roots actually synthesize auxins. The evidence at present available indicates 



