HANS BURSTROM 51 



The most elaborate theory to that effect has recently been advanced 

 by Pohl (10). By studies on plasmolysis and deplasmolysis of coleoptiles 

 he has verified in a highly convincing way the point that auxins increase 

 the permeability of the cytoplasm to water. He has concluded that the 

 cause of the elongation is an auxin-induced rise in the water permeability, 

 causing a water uptake and an increase in volume of the cell. There is a 

 difficulty in this interpretation, however. It presupposes that the increase 

 in volume of the cell is limited by the rate of water absorption. Now it 

 is generally assumed, for good reasons, that the water permeabihty of 

 the cytoplasm is very high and that it can scarcely hamper the water 

 uptake if the osmotic conditions permit an absorption of water. Of 

 course the rate may be affected but not the end result. Pohl has therefore 

 been compelled to assume that initially the tonoplasts are practically 

 impermeable to water. This is a highly hypothetical assumption which, 

 of course, ought to be verified cytologically. 



The immediate consequence of this or any water absorption is a 

 reversible elastic extension of the wall along the curve W of Figure 3, 

 and it need not be emphasized that this is different from the elongation 

 which occurs in growth. The problem is, however, whether such an 

 elastic extension may change into an irreversible, plastic deformation. 



This concept of plasticity in the physical meaning applied to living 

 cell walls has been repeatedly criticized with regard to the special 

 micellar structure of the wall and the extreme and unnatural experi- 

 mental conditions necessary for a demonstration of such a passive, 

 irreversible stretching of the cell wall. Not even Pohl was able to explain 

 the elongation by means of a water absorption alone, but he had to round 

 off his theory by assuming an additional change in the cell wall, without 

 which cell stretching is precluded. 



The reason Pohl has found it impossible to explain elongation and 

 auxin action by a change in the wall and the wall pressure only is his 

 emphatic postulation that the suction pressure of the coleoptiles amounts 

 to 6 to 7 atmospheres and the wall pressure to only i to 2 atmospheres. 

 Thus any change in the wall pressure must be of relatively httle im- 

 portance and cannot explain the rapid elongation following an auxin 

 treatment. This deduction is wrong, however. Before growth in length 

 starts the wall pressure is only some few atmospheres whereas the cell is 

 probably in equilibrium and the turgor is as low as the wall pressure. 

 There exists no turgor-over-pressure or suction deficit with respect to 



