Results may vary depending on season, location, temperature and humidity. Return to the top of this page. Since their colonization of land, plants have been faced with a constant problem. The stoma is a pore formed by a pair of guard cells. Research has indicated that plants contain a range of 10,000 - 100,000 stomata per cm 2 .
Stomatal opening is regulated by turgor in the guard cells.
The inner walls of the stomata are fused at both ends, and, as the cells fill with water, the fused walls remain the same length, while the outer walls stretch. As demonstrated by the taped balloons, this stretching is limited by the orientation and strength of lignin fibers surrounding the cells. Water enters and leaves guard cells in response to changes in solute concentrations. Potassium is actively pumped into the guard cells from surrounding cells. Return to the top of this page.