The kinetics of auxin-induced growth 



that addition of saks and also of sugars to the buffered auxin solutions in 

 which coleoptiles or mesocotyls are extending causes profound change in the 

 course of extension. Relatively small osmotic pressures of non-electrolytes 

 and still smaller external osmotic pressures of electrolytes cause marked 



Figure 1. {Initial growth rate) "^ plotted 

 against {auxin concentrations)~^ . Our data 

 are shown by a continuous line. Bonner's 

 data by a dotted line. 



[Aux/nJ 



arbitrary units 



reduction of the initial extension rate especially at high auxin levels. One 

 can by suitable choice of external concentration, thus obtain curves convex 

 or concave to the horizontal axis, or, at intermediate concentrations, nearly 



Figure 2. Per cent extension of Avena 

 coleoptiles plotted against time. The 

 osmotic pressures of sugar solutions in 

 which the auxin was dissolved are shown 

 on the individual curves. 



linear relationships are found. This is shown clearly by the data o^ Figure 2, 

 where sucrose only was added to the auxin to give solutions of the range of 

 osmotic pressures shown. 



It is, therefore, our claim that the reduced initial rates of extension and the 

 linear extension-time curves at high auxin levels as found by Bonner and 

 Foster (1955) are experimental artefacts due to the depressant action of 

 external solutes. It has been shown by Marinos (1955) that the decrease with 

 time in extension rate observed by him at high auxin levels is accompanied 

 by a corresponding decrease in respiration rate and is associated with marked 

 exosmosis of the cell contents. It seems probable that cell permeability or 

 solute pumping mechanisms are progressively interfered with or destroyed 

 when auxin concentration is high. In our laboratory we have confirmed 

 Marines' finding regarding this roughly exponential decrease in respiration 

 rate; this and the corresponding decrease in extension rate may be looked 

 upon as injury or toxicity phenomena, but they are not due to low pH of the 

 medium as they occur at pH as high as 6. 



It is difficult for the present writer to understand how the Californian 



V_ 311 



