The water absorbed by the roots goes up along the xylematic (woody) vessels up to the leaves as indicated in the drawing below.
During the daytime, when the temperature is higher, there is the maximum loss of water by transpiration. The stomata are small openings present in large numbers mainly on the underside of the leaf. Through these openings the water vapor exits the external environment. The fact that the stomata are located almost exclusively on the part of the leaf not directly exposed to the sun is not accidental: this mechanism allows to reduce the loss of water during the hot hours. Some plants in arid areas have developed very particular systems to minimize the loss of water through the leaves.
Through the stomata, all gaseous exchanges take place: the water coming from the xylem comes out in the form of vapor (perspiration), as a result of photosynthesis, it enters carbon dioxide and oxygen is eliminated.
1.cuticle; 2.epidermis; 3.mesofillo; 4lower epidermis; 5.cuticle; 6.xilema;7spaces of the lacunar tissue; 8palisade fabric Gaseous exchanges (source Iprase Trentino)
The stomata open when the cells, on guard, accumulate potassium ions (red dots) which reduce the water potential causing the water to enter the cell by osmosis. The swollen cells of water become turgid and fold back in the shape of a bean, increasing the brightness of the stomatal opening. On the contrary, a potassium ions leak from the guard cells increases the water potential: the water escapes from the stomatal cells which sag and flatten approaching until the stomatal opening is completely closed.