Plants grow by producing more cells, and cells grow by taking in water. Plants also need water to produce their own food, using the sun's energy to split water molecules into oxygen and hydrogen.
Plants discard the oxygen from the split water molecules as waste and use the hydrogen to manufacture sugar from carbon dioxide present in the air. Without enough water, food production, photosynthesis, stops, making water the most important factor in plant growth, states the Michigan State University Extension website.
Water moves freely in and out of a plant's roots before it travels through the stems to the leaves. Cutting off or limiting a plant's water supply robs it of moisture and the nutrients it takes from the soil. Water also carries the sugars manufactured during photosynthesis throughout the plant, and provides strength so it can support its own weight.
Water also helps plants by keeping them at the right temperature during evaporation. As it exits the plant's leaves into the air, more water is drawn up from the roots in a constant cycle of replacing what is lost.