310 



/. Bonner 



altered by lAA treatment, the wall tension developed by lAA treated 

 sections must be identical with that developed in control sections. The 

 more rapid growth of lAA treated sections must therefore be due 

 to more rapid yielding of the wall. Our second conclusion is therefore 

 that lAA treatment increases the deformability of the cell wall. 



ROLE OF OSMOTIC CONCENTRATION 



The factors which immediately control the rate of Avena coleop- 

 tile section growth are then the osmotically induced load or tension 

 on the cell wall and the resistance of the wall to deformation under 

 load. If grow^th rate of the section is to remain constant with time, 

 as is often desirable in growth studies, it is evidently essential to ar- 

 range circumstances under which both of these factors remain con- 

 stant. Thus the growth rate of sections which elongate in water (con- 

 taining lAA) steadily decreases with time as shown in Figure 3. This 

 is due to the fact that as the section takes up water and elongates, its 

 osmotically active solutes are progressively diluted, as is indicated in 

 Figure 3. The tension to which the cell wall is subjected therefore 



O 



o 



I I 



FINAL OSMOTIC CONCN. 



0.49 M 



0.32 M 



12 18 



TIME IN HOURS 



24 



Fig. 3. Progress curves for growth of Avena coleoptile sections in lAA, 2 X 10 ''.ll, 

 in presence or absence of sucrose, 0.08.\/. Initial osmotic concentration, 0.42M. 



