Water and dissolved minerals are transported through xylem tissues. Xylem vessels and tracheids of roots, stems and leaves are interconnected to form a continuous system of water and dissolved minerals conducting channels reaching all the parts of the plant.
The root cells in contact of with the soil actively take up ions from the soil. This creates a difference in the concentration of these ions between the root and the soil. Therefore, there is steady movement of water into the xylem.
This creates a column of water that is steadily pushed upward, called root pressure. However, this pressure is not enough to push water and dissolved minerals to leaves in the tall trees.
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There is continuous loss of water through stomata of the leaves in the form of water vapor. This process is called transpiration.
The water lost due to transpiration is taken up from the xylem vessels and tracheids in the leaves. This loss of water during transpiration creates a suction pressure which pulls water from the xylem cells of root.
As a result, there is enhanced absorption and upward movement of water and dissolved minerals from roots to the leaves due to transpiration.
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The effect of root pressure in transport of water is more important at night when stomata are closed.
When the stomata are open during the day, the transpiration pull is the major force in the movement of water and dissolved minerals in the xylem, especially in trees.