52 PLANT GROWTH SUBSTANCES 



the surroundings. Every decrease in TiT below Tmay thus be of relatively 

 great importance even if its absolute amount is small. 



In fact, when the water deficit of a cell is large and the wall pressure 

 is low a decrease of the latter permits a relatively much greater absorp- 

 tion of water and an increase in volume of the cell than when the cell is 

 more nearly water saturated. That follows from the shape of the W^-curve 

 in Figure 3. To obtain the same increase in volume only by a water ab- 

 sorption without active changes of the wall, enormous forces of water 

 absorption would be required, irrespective of whether they were of 

 osmotic or so-called nonosmotic origin. 



There remain to be considered inevitable changes of the wall pres- 

 sure as a starting point of the elongation. It is hardly necessary to 

 emphasize the fact that if there is an active change in the wall structure, 

 it is futile to speak of plastic extension as the cause of the elongation 

 because this presupposes only a passive stretching under the influence of 

 an external load. 



Elongation and Changes in the Cell Wall 



The next step is to find out what direct evidence there is of active 

 changes within the cell wall. One fact seems to be firmly established — 

 that the walls of growing cells always have a tubular structure (8). This 

 means that the micellae are generally orientated in a transverse direction, 

 neither the stretching during the first phase of the elongation nor the 

 intussusception seems to alter this structure of the wall. 



As already mentioned it is probable that the growth during the first 

 phase involves an increase in surface of the cell wall partly at the expense 

 of its thickness. This means that wall material is translocated within 

 the wall itself, and the increase in surface depends upon a kind of growth 

 by intussusception, even though the material deposited in the wall is 

 not delivered from without. If this were a mere plastic remodeling of 

 the wall substances this fact would become apparent from changes in 

 elastic properties of the wall which are most conspicuous signs of some- 

 thing happening in the wall at the start of the elongation. 



To take some figures from one series of measurements on roots, the 

 cell length increases from 20 to about 400/i. The elastic stretching under 

 a constant turgor pressure increases, as already mentioned, from about 

 3 to a maximum of about 40/i per cell when the cell reaches a length of 

 about i20/i. From then on the elastic stretching remains constant. 



