Mechanics of Auxin-induced Growth 311 



decreases correspondingly. The maintenance of constant growth rate 

 requires the addition to the external medium of an absorbable solute 

 which is then taken up by the tissue and contributes to tissue osmotic 

 concentration. By appropriate choice of solute and concentration, con- 

 ditions may be arranged so that the uptake of solute balances the 

 growth-induced dilution of cell contents to maintain constant tissue 

 osmotic concentration. In the experiment of Figure 3 this has been 

 achieved by the addition to the medium of OMM sucrose, which in 

 fact constitutes the standard medium for the growth of Avena coleop- 

 tile sections. A wide range of absorbable solutes can, however, replace 

 sucrose in this function (Ordin et al, 20) . It is important that the role 

 of tissue osmotic concentration in determination of section growth 

 rate be generally understood since lack of such understanding in the 

 past has led to acrimonious dispute (Bennet-CIark and Kefford, 2; 

 Bonner and Foster, 7; Marinos, 18) . 



I 



ROLE OF CELL WALL RIGIDITY 



The effect of lAA in increasing the rate of coleoptile section 

 elongation is due, in last analysis, to the effect of lAA in decreasing 

 cell wall resistance to deformation under load as has been demon- 

 strated above. There are, however, additional reagents which may be 

 used to experimentally alter cell wall deformability. The chief of 

 these is the calcium ion. The lAA-induced growth rate of sections is 

 decreased in the presence of calcium ions (Thimann and Schneider, 

 26) as is illustrated in Figure 4. The relation of steady state growth 

 rate to calcium ion concentration is a hyperbolic one, and the Kg or 

 calcium ion concentration required to elicit half-maximal inhibition 

 is ca. 3 X 10"^ equiv. That the effect of calcium ion is upon cell wall 

 resistance to deformation is demonstrated by the fact that sections 

 in solutions of varied calcium ion concentration are all in osmotic 

 equilibrium with the solution (DPD = O) and that the initial osmotic 

 concentration of the tissue is unaffected by calcium. Wall pressure 

 therefore equals osmotic pressure in all cases. Since the force exerted 

 on the wall is then independent of calcium ion concentration, it fol- 

 lows that the reduced rate of extension of sections in calcium-contain- 

 ing solutions is due to reduced rate of deformation of the walls in 

 response to this constant force. 



The effect of the calcium ion in increasing cell wall resistance to 

 load is shared by the magnesium ion which is, however, less effective. 

 Monovalent cations such as Na+ and K+ are essentially without effect 

 on growth rate, at least in concentrations of 1 to 10 mequiv/1. Potas- 

 sium ions do, however, act as an antidote against the inhibitory effect 



