1. Introduced by Dixon and Jolly in 1894, this theory currently stands as the widely accepted explanation for the upward movement of sap in plants.
2. Grounded in two fundamental principles—cohesion and adhesion, along with transpiration pull—this theory delineates the mechanism behind sap ascent.
3. Cohesion and adhesion dynamics involve the cohesive force among water molecules and the adhesive force between water molecules and the lignified wall of the xylem vessel, resulting in a continuous water column from roots to the topmost leaf.
4. Cohesion and adhesion contribute to the formation of an uninterrupted water column in the xylem, extending from the roots to the highest leaf.
5. The transpiration pull, generated in leaf vessels, is conveyed downward to the roots, facilitating the upward movement of sap.
6. Excessive water vapor loss through leaf stomata raises the D.P.D. of mesophyll cells, prompting water withdrawal from the leaf’s xylem.
7. Continuous transpiration establishes a suction pressure gradient (D.P.D.) from guard cells to the leaf’s xylem, creating tension (negative pull or transpiration pull) within the xylem.
8. This tension in the xylem actively draws the water column upward against gravity, resulting in the passive ascent of sap.