Plants obtain water through their roots. Water present in the soil (or air, in the case of air plants) enters the plant through the epidermis of the root. The epidermis is a very thin single layer of cells. The water passes through the membranes of plant cells and also fills the spaces in between the cells. Because the cells absorb the water directly, the soil must be in contact with the roots in order for the roots to absorb the water. In the case of air plants, the air must have enough humidity to provide the roots with water molecules. Roots also have tiny hairs that seek out and find minerals and other nutrients in soil. These nutrients travel along the hairs to the center of the root, or the stele, where they combine with the water molecules.
Next, the nutrient-rich water enters the stem of the plant, and moves from there to lateral branches and leaves. Note that a plant does not need roots in order to "drink" water, as evidenced by the fact that cut flowers can live for a time when placed in a vase full of water. The process of water traveling up a stem is called "transpiration pull." The strong attraction between water molecules aids the process, as does the constant evaporation of water from a plant's leaves. This continual path of water moving through the plant keeps the plant upright, which is one of the reasons why flowers wilt when they don't get enough water.
As the water moves through a plant, most of it is lost through the leaves by evaporation. Only a small percentage of the water taken up by the plant is actually used by the plant to transport minerals and in the process of photosynthesis. Evaporation is very important to a plant, however. It keeps a plant cool and allows for carbon dioxide to enter the plant as the water is leaving it. The evaporated water falls to the earth as precipitation, some of which ends up in the soil to nourish the plant again.
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