There are several processes that enable the water to move up a plant. These processes include root pressure, transpiration pull, cohesion, adhesion and capillarity;-
1. Root pressure; As long as the soil is damp, there will be water taken in by the root hairs. As more water is takenin, the water that is already in the xylem vessel will be pushed up the plant. This is called rootpush or root pressure and it helps to push water up to the leaves. Root pressure is capable, under ideal atmospheric conditions, of pushing water one or two feet above the ground. Since root pressure is not strong enough to move water up very high.
2. Transpiration pull; Transpiration is the loss of water through the leaves and other parts of the plant. Most transpiration occurs through openings, called stomata, on the underside of the leaves. As transpiration occurs, water is lost. This water is replaced by water in the xylem vessels. This causes an upward pull (transpiration pull or transpiration stream) on the water in the vessels.Thus, water is pulled up through the plant, and more enters by the roots to replace it.
3. Cohesion; Cohesion is the force of attraction between similar molecules. Transpiration pull is possiblebecause water molecules cling to each other by cohesion. When water molecules cling to eachother as they move up the stem and into the leaves, they pull up more water molecules up theplant. This process, however, is facilitated by transpiration pull since the water molecules lostthrough transpiration is being replaced by more water molecules absorbed by the roots.
4. Adhesion; Adhesion is the force of attraction between different molecules. As water molecules are stucktogether by cohesion, the entire column of water in the xylem adheres to the sides of the xylem.It is said that the water in under tension as the column moves up the xylem. At the same time, thexylem tube narrows because of the tension.
Cohesion and adhesion forces maintain a continuous column of water in the xylem vessels fromthe roots to the leaves of plants.
5. Capillarity; Capillarity is the tendency of water to rise through narrow tubes. The lumen of xylem tracheidsand vessels is very narrow and this enables water to rise through it by capillarity. Capillarity isassisted by adhesion and cohesion forces.