Plants use this soil-air water potential gradient to move water to their tissues. The symbol for water potential is Y . By convention, the water potential of pure water is wet at zero. Since water always flows from higher to lower potential, the water potential of water with solutes is negative, as is the water potential of non-saturated air. The contribution of solutes to soil water potential is usually low compared to the matric forces.
Matric forces are due to the tendency of water to adhere to the walls of containers.
The narrower the pores, the higher the matric force. Y soil Plants control the flow of water by opening and closing stomates, which controls evapotranspiration. If the air is dry and windy, water, if available, has to flow more rapidly through the plant to keep up with evaporative water loss. If the water content of the soil gets too low, plant has to close its stomates to prevent losing the water in its system and allowing its cells to dry out. For most animals, evaporation from respiratory surfaces is the largest source of water loss and drinking is the major source of intake.