Water Is a Very Important Substance Obtained and Transported Throughout Plants

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Water is a very important substance obtained and transported throughout plants Transpiration stream is the movement of water through a plant. Transpiration stream is maintained by: 1. Osmosis 2. Root pressure 3. Transpiration Osmosis is the movement of water from high water concentration to low water concentration across a semi-permeable membrane. Root pressure is the force exerted by water within the xylem tissue of the roots. Transpiration is the loss of water from the aerial parts of a plant. Water and mineral uptake: Water enters the root hairs by osmosis, moving from high water concentration to low water concentration. Minerals dissolve easily in water and move into the root by diffusion - either by passive transport or by active transport (requires ATP). Water moves across the ground tissue and into xylem tissue. Water is then transported up the plant. Cohesion-tension model of water transport John Joly and Henry Dixon were two Irish scientists who first proposed the cohesion-tension model of water transport. Water moving into the xylem tissue of the root causes a pressure build up - this is called root pressure. Root pressure contributes to the upward movement of water molecules. Water molecules have hydrogen bonds between them maintaining them in the liquid form - this is cohesion of the water molecules. Water molecules also tend to stick easily to the sides of the xylem vessels - this is called adhesion of the water molecules. Transpiration of the water molecules occurs mainly from the leaves - this pulls the column of water molecules upwards through the xylem, creating a tension in the water molecules. Control of transpiration Transpiration is controlled by: Waxy cuticle - prevents direct water loss from the surface of leaves. Stomata - controls the rate of transpiration by opening and closing. Lenticels - allows a small amount of transpiration but also allows oxygen in (for respiration) and carbon dioxide out (excretion). .

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Theories of Ascent of Sap
Theories of Ascent of Sap The following points highlight the top four theories of ascent of sap. The theories are: 1. Vital Force Theory 2. Root Pressure Theory 3. Theory of Capillarity 4. Cohesion Tension Theory. 1. Vital Force Theory: A common vital force theory about the ascent of sap was put forward by J.C. Bose (1923). It is called pulsation theory. The theory believes that the innermost cortical cells of the root absorb water from the outer side and pump the same into xylem channels. However, living cells do not seem to be involved in the ascent of sap as water continues to rise upward in the plant in which roots have been cut or the living cells of the stem are killed by poison and heat. 2. Root Pressure Theory: The theory was put forward by Priestley (1916). Root pressure is a positive pressure that develops in the xylem sap of the root of some plants. It is a manifestation of active water absorption. Root pressure is observed in certain seasons which favour optimum metabolic activity and reduce transpiration. It is maximum during rainy season in the tropical countries and during spring in temperate habitats. The amount of root pressure commonly met in plants is 1-2 bars or atmospheres. Higher values (e.g., 5-10 atm) are also observed occasionally. Root pressure is retarded or becomes absent under conditions of starvation, low temperature, drought and reduced availability of oxygen. There are three view points about the mechanism of root pressure development: (a) Osmotic: Tracheary elements of xylem accumulate salts and sugars.
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Non-Destructive Estimation of Root Pressure Using Sap Flow, Stem
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