All living things need water to stay alive, but plants use much more water than animals do. Plants are 90 percent water, with animals as little as 75 percent water by weight. Plants use water directly when they capture the light energy of the sun and transform it into useful food molecules. And plants transpire. Transpiration is when plants exchange water for carbon dioxide.
All leaves have microscopic openings, called stomata.
When open, water evaporates out. Carbon dioxide, which plants need to make food, can also diffuse in through the open holes. When the pores close, little carbon dioxide or water enters or leaves the plant. The plant balances the opening and closing of the stomata to acquire enough carbon dioxide and not lose too much water, so it can stay alive. How much water plants use depends on its type, how much light it gets, and how old it is.