It should be clear by now that the addition of new cells by division in meristematic zones represents an important component of growth. But the dividing of a mother cell in two does not necessarily result in an increase in size. For such an increase to occur, division must be coupled with cell expansion. Without expansion, a dividing cell is just partitioned into smaller and smaller elements.

The force that powers cell expansion is turgor pressure, a positive hydrostatic pressure that builds up within the cell due to the uptake of water. A typical leaf cell in a well-watered plant has a turgor pressure between 0.1 and 3.0 MPa (MPa = megapascals). For reference, a typical car tire is inflated to 30 psi (pounds per square inch), which is equivalent to 0.2 MPa. While such a tremendous pressure would cause most cells to burst, plant cells maintain such high pressures through the resistance of the cell wall.

Cell expansion, then, represents a carefully-controlled balance between wall resistance and wall yielding. For each cell, there is a minimum of pressure required to allow expansion, and above that minimum, expansion proceeds depending on the wall extensibility:

where the rate of change in volume, , is equal to the product of effective pressure (turgor pressure (P) minus wall yield threshold (Y)) and wall extensibility (m). This is a version of the Lockhart equation, first published in 1965¹.

Of these factors, the cell has direct temporal control over wall extensibility. One means of regulation over extensibility is by acidification of the cell wall through proton pumping. Lowering the pH of the wall is associated with increased extension. Another means of control is through the production, secretion, and stimulation of wall-bound proteins that loosen bonds between the structural components of the wall. Foremost among these is a family of proteins called expansins, that appear to temporarily loosen the non-covalent linkages between cellulose and other wall polysaccharides. By loosening these linkages, expansins allow the load-bearing members of the wall to slide past each other and permit cell expansion in a very controlled manner.

The wall loosening carried out by the expansins takes place in specific wall locations under specific conditions, allowing the cell to control the timing of expansion and ultimate shape of the cell. Without such control, cells would be free to expand in any direction and at any time, which would have clear implications for tissue and organ form in the mature plant. In addition, the direction of cell expansion is heavily influenced by the pattern of deposition of cellulose microfibrils, which can also serve to constrain the kinds of final shapes a particular cell can achieve.

There is a significant brake in this system of turgor-driven cell expansion, and that is a constraint imposed by biophysics. As turgor pressure drives cell expansion, the volume of the cell increases. This increase in volume reduces the turgor pressure pushing on the wall and providing the force for expansion. Because even a slight increase in volume reduces turgor pressure, water must be taken up continuously to maintain the turgor pressure required to sustain growth.