Plants are living things, and all living things on Earth require water in order to survive. Plants, of course, cannot move about in search of fuel like animals can, and they cannot drink fluids in the sense the term "drink" is normally understood. But like animals, plants have evolved specific components and physiological mechanisms to ensure adequate levels of hydration under a variety of conditions.
Functions of Water in Plants
Water is one of the reactants in the chemical reaction known as photosynthesis, the other being carbon dioxide. These two compounds react under the influence of sunlight to generate glucose and oxygen. This is almost exactly the reverse of respiration in other organisms, in which oxygen is used to break down glucose and liberate energy, carbon dioxide and water.
Water is also used to transport minerals around the plant in much the same way blood moves vital substances throughout animal bodies. Water also provides plants with structural support, and also allows the plant's leaves to keep cool through the process of evaporation. In short, water serves many of the same functions in plants as it does in animals, adjusting for anatomical and other differences.
Water Transport in Plants
Water moves from the soil in which plants are anchored into the root systems of the plants through the root hair cells at the tips of the individual roots. Once a water molecule diffuses into a root, it can take one of three paths to reach the xylem, which is the conduit from the roots to the rest of the plant. The first of these paths is simply between cells in the root. The second is navigating the junctions between cells (plasmodesmata), and the third is traversing cells and repeatedly crossing different cell membranes.
Once in the xylem, analogous to veins in animals, the water moves under far less resistance in the direction of the leaves. Water ultimately leaves plants through openings in leaves called stomata (singular: stoma).
Effect of Ambient Conditions on Water Balance
Higher temperatures lead to faster transpiration (water turnover) rates. This is chiefly the result of stomata opening more robustly when the air is warmer and allowing more water to escape. Higher humidity slows water movement in plants because water cannot evaporate from the leaves into the atmosphere as easily. Wind tends to increase plant water absorption, partly by lowering humidity in the immediate vicinity. Finally, plants that grow in drier regions, such as cacti, tend to conserve water and have lower transpiration rates overall.
Reducing Water Losses
Leaves have a waxy cuticle layer on their outer surfaces, which is sometimes evident to the touch. This leads to an increase in water retention. Under certain conditions, stomata close, lowering the amount of water the plant releases into its environment.
Plants also retain water to maintain their structural integrity. More water leads to a higher level of turgidity, or firmness, which is especially important in plants that contain no woody supporting structures.