308 



]. Bonner 



O 



cr 

 o 







6 12 



TIME IN HOURS 



18 



Fig. 1. Progress curves for growth of Avena coleoptile sections. Basal medium of 

 sucrose, 0.09A1 and K-malcate buffer, 0.0025Af, pH 4.5. IA.\ as indicated. Temp. 

 25° C. Initial length of sections 5.0 mm. 



coleoptile section to added lAA we have, then, not only a large re- 

 sponse but also one that is pure — uncontaminated by residual native 

 growth substance. 



THE DRIVING FORCE OF CELL ELONGATION 



We will first ask. What powers the elongation of plant tissue? 

 This is a general question and one relevant to all cell extension 

 whether auxin-controlled or not. In the case of the Avena coleoptile 

 section, the driving force of cell elongation is the osmotic pressure 

 of the vacuolar contents of the individual cells. This is clear from 

 the information in Figure 2. Sections placed in solutions isotonic 

 (0.42Af) with themselves essentially fail to elongate, either in the 

 presence or absence of lAA. Sections placed in solutions containing 

 an osmotically active solute (mannitol) in concentrations lower than 

 isotonic grow at reduced rates. The rate of section elongation is, in 

 fact, essentially a hyperbolic function of concentration of external 

 solute. 



Classical osmotic lore tells us that: 



Net DPDT„,„e = O.P.T,,sue - W.P. - O.P.Kx.ornal 



where DPD designates diffusion pressure deficit, O.P., osmotic pres- 

 sure, and W.P., wall pressure. 



