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The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity) through the xylem of plants. It was proposed in 1894 by John Joly and Henry Horatio Dixon .
The forces of cohesion and adhesion cause the water molecules to form a column in the xylem. Water moves from the xylem into the mesophyll cells, evaporates from their surfaces and leaves the plant by diffusion through the stomata
Although several mechanisms have been proposed to explain how sap moves through the xylem, the cohesion-tension mechanism [1] has the most support. Although cohesion-tension has received criticism due to the apparent existence of large negative pressures in some living plants, experimental and observational data favor this mechanism. [2] [3]
In plants, the transpiration stream is the uninterrupted stream of water and solutes which is taken up by the roots and transported via the xylem to the leaves where it evaporates into the air/apoplast-interface of the substomatal cavity. It is driven by capillary action and in some plants by root pressure.
According to cohesion-tension theory, water transport in xylem relies upon the cohesion of water molecules to each other and adhesion to the vessel's wall via hydrogen bonding combined with the high water pressure of the plant's substrate and low pressure of the extreme tissues (usually leaves).
Henry Horatio Dixon FRS [1] (19 May 1869, Dublin – 20 December 1953, Dublin) was a plant biologist and professor at Trinity College Dublin.Along with John Joly, he put forward the cohesion-tension theory of water and mineral movement in plants.
Hydraulic signals in plants are detected as changes in the organism's water potential that are caused by environmental stress like drought or wounding. [1] The cohesion and tension properties of water allow for these water potential changes to be transmitted throughout the plant. Plants respond to external stimuli through thigmomorphogenesis.
Cohesion, along with adhesion (attraction between unlike molecules), helps explain phenomena such as meniscus, surface tension and capillary action. Mercury in a glass flask is a good example of the effects of the ratio between cohesive and adhesive forces.