"Vital force theory" is a proposed mechanism for the ascent of sap through the xylem tissue of plants.
According to the vital force theory, the conduction of water up the xylem vessel is a result of vital action of the living cells in the xylem tissue.
Jagadish Chandra Bose suggested a mechanism for the ascent of sap in 1927. He found electrical ‘pulsations’ or oscillations in electric potentials, and came to believe these were coupled with rhythmic movements in the telegraph plant Codariocalyx motorius (then Desmodium). On the basis of this Bose theorized that regular wave-like ‘pulsations’ in cell electric potential and turgor pressure were an endogenous form of cell signaling. According to him the living cells in the inner lining of the xylem tissue pump water by contractive and expulsive movements similar to the animal heart circulating blood.
This mechanism has not been well supported, and in spite of some ongoing debate, the evidence overwhelmingly supports the cohesion-tension theory for the ascent of sap.