Mechanics of Auxin-induced Groivth 



309 



X 



»- 

 o 



(T. 



0.5 - 



0.6 



EXTERNAL CONCN., M 



Fio-. 2. Initial growth rate of Avena coleoptile sections as a function of external os- 

 motic concentration. Growth in mm/section/4 hrs. Osmotic solutes made up of 

 sucrose, Q.09M plus varied concentrations of mannitol. After Ordin, Applewhite, 

 and Bonner (20). 



The DPD of Avena coleoptile sections growing in solutions over 

 the concentration range of Figure 2 has been shown to be null, 

 within the precision of measurement (Ordin et ah, 20) . The grow- 

 ing sections are therefore in osmotic equilibrium with the solution in 

 all cases. In addition O.P.Tissue is identical, or nearly so, for sections 

 in solutions of different O.P.Externai- We may conclude therefore that 

 it is the tension to which the cell walls of the tissue are subject which 

 is altered by changing O.P.Externai 



W.P. = O-P-Tissue — O.P.Externai 



and that rate of section growth as a function of O.P.Externai is in fact 

 a measure of the rate of cell wall deformation as a function of cell 

 wall tension. Our first conclusion is therefore that the rate of cell 

 extension depends on the tension to which the wall is subjected by 

 the osmotic pressure of the cell contents. 



One further conclusion can be drawn from the information in 

 Figure 2. This concerns the difference in growth rate between lAA 

 treated and control sections. Since tissue osmotic concentration is not 



