Stomata - Water Loss
The cross-section of a leaf shown in Figure shows the position of a typical stoma (plural stomata) which however, differs from species to species, with respect to the size of the pore, structure and size of the guard cells and depth and size of the stomatal cavity. As indicated in the diagram b, water evaporates from wet mesophyll cell walls that border intercellular spaces, the vapours then diffuse out through sub-stomatal cavity and stomatal pores to the air outside the leaf.
The water potential gradient develops in the sub-stomatal cavity, stomatal pore, boundary layer and the atmosphere. During transpiration the sub-stomatal cavity has relatively much higher water potential as compared to the atmosphere, therefore, the water vapours move out. This is turn lowers the water potential of the sub-stomatal cavity. Consequently, the cells surrounding the sub-stomatal cavity evaporate water through their cell walls. Depending upon the water potential of the environment, the water potential of the sub-stomatal cavity and the surrounding cells is lowered. This gradient eventually acts as a 'pull' on the water column which maintains continuity through the vascular bundles of the leaf. The intercellular spaces also play an important role in this respect because they are in continuity with the sub-stomatal cavity and cause a gradient quickly.