Question

Which theory explain upward movement of water in tall trees

Answer 1

Answer:The ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the crown. Xylem is a complex tissue consisting of living and non-living cells. The conducting cells in xylem are typically non-living and include, in various groups of plants, vessels members and tracheids. Both of these cell types have thick, lignified secondary cell walls and are dead at maturity. Although several mechanisms have been proposed to explain the phenomenon, the cohesion-tension mechanism has the most evidence and support. Although cohesion-tension has received criticism, for example due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism.

An alternative theory based on the behavior of thin films has been developed by Henri Gouin, a French professor of fluid dynamics. The theory is intended to explain how water can reach the uppermost parts of the tallest trees, where the applicability of the cohesion-tension theory is debatable.

The theory assumes that in the uppermost parts of the tallest trees, the vessels of the xylem are coated with thin films of sap. The sap interacts physically with the walls of the vessels: as a result of van der Waals forces, the density of the film varies with distance from the wall of a vessel. This variation in density, in turn, produces a "disjoining pressure", whose value varies with distance from the wall.

Answer 2

Answer: The cohesion-tension theory states that transpiration is the primary force behind water movement in the xylem. At the leaf surface, it generates tension equal to -2 MPa. and this tension ultimately draws water up from the roots.

Explanation:

• The transfer of water and minerals from the root to the crown is referred to as the ascent of sap in the xylem tissue of plants. A complex tissue made up of both living and dead cells is called xylem.  
• The cohesion-tension mechanism has the most evidence and support, despite the fact that many different explanations for the phenomenon have been put forth.
• Henri Gouin, a French professor of fluid dynamics, has created an alternative hypothesis that is based on the behavior of thin films. The cohesion-tension theory may not apply to the highest regions of the tallest trees, thus the theory's purpose is to explain how water can get there.
• According to the notion, thin sap films are present on the xylem vessels in the uppermost regions of the tallest trees. The sap physically interacts with the vessel walls: van der Waals forces cause the density of the film to change with distance from the vessel wall.
• With respect to distance from the wall, this difference in density results in a "disjoining pressure" that has a variable value.

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