52 THE PLANT CELL WALL 



The processes involved in growth call for a complex array of 

 chemical transformations, as we have seen. In elongation growth, 

 there are also profound architectural changes in the cellulosic 

 frame. The walls of meristematic cells characteristically possess 

 a tubular texture which is even retained in the thin primary walls 

 of thickened cells such as bast and cotton fiber. The tubular wall 

 contains micellar strands lying at right angles to the fiber axis of 

 the cell, an arrangement which gives, rise to its optically negative 

 character. When the tubular wall is stretched mechanically, it 

 becomes optically positive. When it is stretched during growth 

 the negative character is retained. Only a small amount of elastic 

 deformation (8 per cent elongation in the Arena coleoptile) may 

 be required to reverse the optical character of the wall. Upon 

 release, the original negative orientation is approximately restored. 

 Further, mechanical deformation stretches the cell at the expense 

 of its diameter, a condition quite distinct from wall thickening in 

 elongating cells. To account for the unique features of growing 

 walls, it has been suggested that the tubular framework consists 

 of a regular network held together in part at the junctions where 

 cellulose strands meet. If in this structure the junction points are 

 loosened, the frame can be pulled apart readily and the cell will 

 elongate. A loosened, more open structure will result, but can be 

 once again rendered compact by the formation and interposition 

 of new cellulose strands (intussusception). Weakening of inter- 

 fibrillar forces of course renders the wall plastic, but no extension 

 can occur without the expenditure of energy. The driving force 

 which brings about elongation is the hydrostatic pressure exerted 

 against the wall by its contents. The maintenance of hydrostatic 

 pressure is a function of the osmotic potential energy and directly 

 or indirectly, energy derived from respiratory processes. The 

 directionality of cell enlargement can be explained if it is recalled 

 that in the tubular wall, molecular chains and therefore primary 

 valence forces are principally arranged at right angles to the long 

 axis of cell. Tn contrast, the interchain forces arranged parallel to 

 the fiber axis must consist of far weaker hydrogen bonds. Accord- 

 ingly, the wall yields preferentially in the direction of these secondary 

 forces so that the weakly coherent strands will pull apart as the 



