312 



J. Bonner 



CALCIUM CONCN., MEQ./L. 



Fig. 4. Inhibition of lAA-induced growth of Avena coleoptile sections by CaCL. 

 Based on steady state rates over a 6 hr. growth period. Basal medium contained su- 

 crose, 0.09i\f. After Cooil and Bonner (12). 



of calcium ions on section growth rate. The experiment of Figure 5 

 summarizes the growth interrelations of Ca+^ and K+ ions. Sections 

 growing in the absence of either ion, when transferred to calcium- 

 containmg solution, quickly assume a new and slower steady state 

 growth rate. If such sections are transferred to water, their growth 

 continues at the rate characteristic of the calcium-containing solution 

 and only slowly increases. Transfer of the sections to K^ ion-containing 

 solution results in immediate reversal of the calcium inhibition. We 

 may summarize the information of Figure 5 by saying that growth 

 inhibition by calcium ions behaves as thougii it were mediated by ex- 

 changeably bound ions. 



MECHANICAL PROPERTIES OF COLEOPTILES UNDER 



EXTERNAL LOAD 



The elfects of lAA and of inorganic ions on the cell walls of 

 coleoptile sections can be readily and rigorously demonstrated by 

 methods which measure their deformability under artificially im- 



