Physical Force Theory

Physical force theories try to explain the ascent of sap through the dead xylem vessels alone, without invoking any living-cell activity. The capillary theory (Boehm, 1809) proposed that xylem vessels simply act as fine capillary tubes, drawing water up by ordinary capillary action under atmospheric pressure, but it was rejected once it became clear that capillary force alone can raise water only to a limited height, and also that the wider xylem vessels - which actually conduct the bulk of the water - would be expected to show weaker capillary rise than the narrower tracheids, contradicting the theory's own logic. The imbibition theory (Unger, 1876; supported by Sachs, 1878) proposed instead that water moves upward by being imbibed through the cell-wall material itself, not through the open lumen of the vessel, but a ringing experiment demonstrated that water in fact travels through the open lumen, not the cell wall, disproving this idea too. That leaves the cohesion-tension theory (Dixon and Jolly, 1894; further developed by Dixon in 1914 and 1924) as the theory now most widely accepted: water molecules possess a strong mutual cohesive force (estimated at up to about 350 atmospheres, far more than enough for even the tallest trees) that keeps an unbroken column of water intact inside the xylem, assisted by adhesive attraction between water and the vessel walls; any gas bubbles that do form and threaten to break this column (a process called cavitation or embolism) are bypassed as water simply diffuses around them into neighbouring xylem elements, so the column's overall c …