Mechanics of Auxin-induced Growth 



317 



Table 1 . Comparison of parameters which characterize interaction of calcium 

 ions and Arena coleoptile sections with respect to cell wall stiffening and exchangeable 

 binding. 



(nonesterified) pectic carboxyl groups of the cell wall. It may be con- 

 cluded therefore that, in the presence of calcium ions at least, it is 

 the cell wall pectic material which limits cell wall deformability. 

 Whether this is also true in the absence of added calcium ions cannot 

 be deduced. It is of interest, however, that the cell walls of sections 

 taken from Avena seedlings germinated in distilled water contain 

 measurable calcium and, in fact, a quantity sufficient to bind with 

 approximately one-fifth of the free pectic carboxyl groups of the 

 cell wall. 



The pectic material of the Avena coleoptile makes up but 5 per 

 cent of the total weight of cell wall substance (Jansen et a\., 16). Cellu- 

 lose constitutes another 25 per cent (Bishop et aL, 3), while polysac- 

 charides which yield on hydrolysis xylose, arabinose, glucose, and 

 galactose make up the bulk of the remainder. It appears remarkable 

 that a minor constituent such as pectic material should play a major 

 role in determining cell wall mechanical properties. It is indeed clear 

 that we have inadequate knowledge of the structural arrangements of 

 cell wall components other than cellulose, and such knowledge will 

 be needed before detailed discussions of cell wall properties can be 

 fruitfully undertaken. 



INFLUENCE OF lAA ON PECTIC METABOLISM 



Since lAA increases rate of cell extension by causing increased 

 cell wall deformability, it is evident that lAA in some way alters cell 



Table 2. Demonstration that the exchangeably bound calcium oi Avena coleoptile 

 sections is held principally by free (nonesterified) pectic carboxyl groups of the cell 

 wall. After Jansen rf a/. (16). 



Cation exchange capacity of living coleoptile 

 sections 



Cation exchange capacity of cell walls 



Cation exchange capacity of cell walls calculated 

 on basis of free pectic carboxyl groups 



Cation exchange capacity of living sections pre- 

 dicted on basis of 2 



Cation exchange capacity of living sections 



predicted on basis of 3 



3.1 X 10-3 Mequiv/g fresh wt. 

 0.18 Mequiv/g dry wt. 



0.165 Mequiv/g dry wt. 



3.6 X 10"3 Mequiv/g fresh wt. 



3.3 X 10-3 Mequiv/g fresh wt. 



