Water is essential to plants in many ways. It first provides the major substance for living, to keep cells from shriveling up and dying. The second major function is to keep the plants rigidity. As plant cells become turgid, full of water, the cells expand, filling the extent of their cell walls, which are kept taught with turgor pressure. If the cells lose water, two problems occur. First, the cells dehydrate, causing the organism to die. Second, turgor pressure is lost as cells become flaccid, limp and unfilled, causing a loss of support for the plants structure which makes it appear wilted.
             As aquatic plants evolved into large complex land plants, an adaptation occurred in the center of plants to allow full growth without the problem of water loss. A system of vascular bundles extending from the tips of the furthest leaves to the deepest roots of each plant developed, carrying water in xylem sap and sugar in phloem. While phloem can transport sugar in any direction within the plant, xylem can only move water up, from root to leaf. Once in the leaf, the water evaporates through stomata-tiny gaps in the lower epidermis of each leaf, which are regulated by guard cells-a process called transpiration
             The movement of water into and out of the xylem involves water pressure factors in different sections of the plant. As water slips into the roots through osmosis, a positive water pressure gently pushes the water into the plants roots and supplies a jumpstart for the water's journey up the vascular bundle. However, it is not this pressure that supplies a great force towards the upward movement of water; it is the evaporation of water from the stomata that pulls water upward and out. When the stomata are open to take in carbon dioxide for carbohydrate production, water begins to evaporate and seep out of the tiny holes in each leaf. With a constant pull of water outward, other water molecules are pulled up to repla...