Plants need water to grow because plants require internal water pressure to strengthen the stems and leaves, and plants need water to create energy. Water is also necessary for photosynthesis, which occurs in the plant's leaves with water from the stem.
All living things need water to grow. Plants are made up of about 90 percent water and therefore require more water than animals. Water pressure inside a plant is called turgor. Without turgor, the plant wilts and dies. The internal water pressure is created when the plant absorbs water from the ground, up through the stem, to the leaves. The water is needed to create a skeleton for the plant so the plant can remain upright.
Once the water reaches the leaves, photosynthesis occurs during the daylight hours. During photosynthesis, the plant creates food. Without water, the plant could not create the sugar it needs for energy. Water travels up the stem to the leaves, and there, the water is exchanged for carbon dioxide during transpiration. This natural process only occurs when sunlight is present. Transpiration also serves to cool down the plant, much like sweating cools a human being. The water exits the leaves through small holes called stomata.