transport system[8]
6:30 AM
Xylem vessels are like long drainpipes reaching all the way from the roots to the tip of every leaf. They are made up of hollow dead cells joined end to end.
Water and dissolved mineral salts are drawn up the xylem vessels when water from the leaves evaporates and diffuses out through tiny holes called stmata into the air. This process whereby plants lose water due to evaporation is called transpiration. As transpiration continues, more water is pulled up the xylem as though someone is "sucking" water up the plant. The movement of water with dissolved mineral salts through the plants is called the transpirational pull. Other phenomena that cause xylem sap to flow are the following- cohesion and adhesion & root pressure.
Transpirational pull- As water evaporates from the leaves of the plant,there is a suction created by this action resulting in a pull drom the roots..
Cohesion and Adhesion- The water molecules are cohesed(attracted) to each other forming a continuous column of water. They also are adhesed(attracted) to the xylem vessel(a thin tube) in the plant.
Root pressure- The water molecules surounding the root are very much eager to enter the root(due to less water potential in the root).Thus the root exerts a pressure towards the water in the xylem vessel.
the transpirational pull
image courtesy of:
www.progressivegardens.com/.../transpiration.jpg
