Water in the soil enters a plant through the roots. More specifically, it enters through the epidermis of the root. The epidermis is a single layer of cells that covers the root, much like the skin of humans. The water passes through the membrane of the plant cells in the roots, filling both the plant cells and the empty spaces in between them. Although most plants get their water from the soil, roots are not necessary for plants to "drink" water, as evidenced by the classic elementary science experiment in which a piece of celery is placed in colored water, or even as evidenced by cut flowers in a vase. Without the accompanying nutrients that are in the soil, however, a plant will soon die, as water and nutrients enter the plant separately from the soil and are combined when they enter the stele, which is the central part of the root that extends into the stem.
Water moves from the root cells to the stem. It is pulled upward by a process called transpiration-pull. The loss of water through evaporation from a plant's leaves is called transpiration. This loss of water equals a loss of pressure (tension), which forces water up the stem. Another factor is the bonding of water molecules. They are attracted to each other by an incredible amount of force. In effect, they are drawn upward by the pull of other water molecules. For this reason, the transpiration-pull theory is also called the cohesion theory.
Leaves and Branches
Once in the stem, water moves laterally into a plant's leaves and branches. It travels along elongated cells called tracheids. From there, water enters the veins of leaves, traveling to every part of the foliage. Some water travels to the flower as well. Once it reaches the smallest veins in the leaves, it moves directly into the plant cells again. This is where transpiration occurs. Although some of the water is used for metabolism, most of it is lost to transpiration. In fact, less than 1 percent of the water remains in the plant. This may seem counterproductive, but it is actually very useful to the plant. Water evaporation helps keep the plant cool. It also allows carbon dioxide to enter, which is vital for the plant's food production and growth.