Transport of Water

After entering the plant at the root hair, there are two routes that water can take to reach the center of the root. In the apoplastic route, water moves through plant cell walls without ever entering the individual cells. In the symplastic route, water moves through the plant cells by the way of the plasmodesmata.

There are three mechanisms that describe how water gets from the roots to the leaves.

• Capillary action involves the rise of liquids in narrow tubes. The liquids are attracted to the walls of the tube. This attraction, called adhesion, pulls liquids up the tube. Capillary action, however, is not strong enough to account for much of the movement of water to the leaves.
  
  
• Root pressure is another mechanism for moving water up the xylem. In this theory, water entering the root exerts a pressure on the water already in the xylem above it pushing it further upwards. Root pressure, however, also does not account for moving water large distances.
  
  
• Most water movement through the xylem is accomplished through the process described in the cohesion-tension theory. Water, due to its polarity, forms a chain from the leaves to the roots. As water evaporates from the leaves in transpiration, the chain is drawn upwards.