The guard cells are two small, specialised green epidermal cells surrounding a stomata. They are kidney-shaped in dicot plants and dumb-bell shaped in cereals/monocot plants. The expansion and contraction of thin-walled sides/ends of guard cells regulate the opening and closure move-ments.
Mechanism of Stomatal Movement: Stomata function as turgor operated valves because their opening and closing movement is governed by turgor changes of the guard cells. Whenever, the guard cells swell up due to increased turgor, a pore is created between them. With the loss of turgor the stomatal pores are closed. Stomata generally open during the day and close during the night with a few exceptions. The important factors which govern the sotmatal opening are light, high pH or reduced CO2 and availability of water. The opposite factors govern stomatal closure, viz., darkness, lowpR or high CO2 and dehydration.
