Most people know that plants use water to grow, but in reality, plants only use between 1 and 5 percent of their intake water for growth and photosynthesis. The rest of the water evaporates through the leaves in a process known as transpiration. This process is vital for plants. The evaporation of the water keeps the plant cool and allows carbon dioxide to enter the plant. Transpiration also creates a tension, or unequal pressure, in the plant that allows water to travel up the stem, against the pull of gravity, and through the branches to the leaves and flowers of the plant.
The water molecules in the plant carry minerals to every part of the plant. For example, they carry hydrogen to the leaves, which is a crucial ingredient in the photosynthesis process. Minerals actually enter a plant through the root hairs, separately from water molecules, which enter through the epidermis of the root. The minerals are combined with the water molecules once they enter the stele, which is the middle part of the root that leads to the stem. This is why flowers placed in water will eventually die. They are not receiving a steady flow of minerals.
After photosynthesis (the process by which a plant uses light energy, water and carbon dioxide to make glucose) occurs, water carries the glucose, or sugar, back around the plant, where it is either used for growth or stored for later, as in the case of bulb or tuber plants.
Water flows in and out of plant cells through a process called osmosis. This plumps up the plant cells, making them firm and providing structure to the plant. This is why flowers and plants wilt when they do not receive enough water. Their support structure is gone.