Question

Explain the transpiration pull theory for an ascent of sap

Answer 1

Transpiration pull theory: Water is lost from the mesophyll cells to the intercellular spaces and eventually outside as a result of transpiration. Due to loss of water from mesophyll cells, DPD increases, and such cells, absorb water from the adjoining cells. Ultimately water is absorbed from xylem elements of vascular bundles. Since there is a continuous column of water in the xylem elements, a tension or pull is transmitted from the petiole to stem and finally to roots the resulting in an upward movement of water.