316 



]. Bonner 



O 



Q 



LU 

 OQ 



O 



H 

 C/) 



< 



8* 



40 _ 



0* 



120 



TIME IN MINUTES 



Fig. 8. Time course of inhibition of plastic deformability of Avena coleoptile 

 sections by added calcium ions (20 mequiv/1) and of reversal of the inhibition 

 by potassium ions (20 mequiv/1). Sections equilibrated in solutions for varied 

 times as indicated and plastic deformation then measured over a 4 min. period. 



walls the material which interacts with calcium limits deformability. 

 That lAA does its work upon the same material which interacts with 

 calcium is therefore implied although not rigorously demonstrated. 

 It will now be shown by independent, chemical, methods that lAA 

 and calcium ions exert their effects upon the same cell wall constitu- 

 ent, namely, the pectic material. 



The nature of the material within the coleoptile which interacts 

 with calcium ions to cause cell wall stiffening can be attacked in a 

 straightforward manner. It has been shown above that the responsible 

 ions behave as though exchangeably bound to the tissue. Living Avena 

 coleoptile sections possess a readily measurable cation exchange ca- 

 pacity. The characteristics of such binding closely resemble those of 

 the binding which causes cell wall stiffening, as is summarized in 

 Table 1. The data of Table 2 establish that the capacity of such 

 sections to bind calcium ions exchangeably is entirely due to the free 



