Water moves upward through a plant through the xylem tissue. Along with water, xylem also transports dissolved minerals, and some small organic molecules. This fluid mixture of materials flowing through the xylem is called sap. In a tree, the wood constitutes the xylem with fluid transport occurring in the outermost tree ring. In a non-woody plant, the stem contains the xylem. Three factors that influence the flow of sap through a plant include diffusion, transpiration and capillary action.
Water enters the roots through diffusion or osmosis. The root has a higher salt concentration on the plant side of the membrane than in the soil. Water flows from the low salt side to the high salt side trying to make the salt concentration equal. The root does not allow passive movement of salts across the membrane. However, it does allow water, minerals and small organic compounds to pass from the soil to the plant passively. The membrane is semi-permeable. If the salt concentration inside the root becomes too low, it will actively transport salts into the root.
Another component of water movement throughout the plant involves transpiration. Plants lose water through the leaf surface primarily from the stomata, or pores in the leaf surface used for respiration and photosynthesis. On warmer days, more water evaporates, the plant draws more water from the roots, and the roots take up more water from the soil. Wind also increases the transpiration rate by increasing the rate of evaporation. Relative humidity lowers the transpiration rate because evaporation occurs more slowly.
Xylem depends on the chemical properties of water. Equally important is the small diameter of the xylem. Water molecules have a strong attraction to each other causing the molecules to stick together. They also adhere to the surface of the xylem. As the plant transpires, it draws water up through the xylem. A water molecule evaporates, pulling another water molecule up through the xylem. The water sticks to the side of the xylem and does not fall back down. This process is called capillary action. The cohesion formed by the individual water molecules, the tension between the water molecules and the xylem creates such a strong attraction that capillary action gets water to the tips of the tallest trees.
Upward Water Movement in Plants
In order for water to move upward through the plant, diffusion, transpiration and capillary action must occur together. Water diffuses into the root creating pressure and pushing the water along the xylem. Transpiration causes water to leave the plant resulting in a force that draws water up through the xylem. Capillary action creates a cohesion among the water molecules as well as between the water molecules and the plant, overcoming the force of gravity and keeping water flowing throughout the plant.