Cohesiontension

Cohesion-tension theory is the predominant explanation for how water moves upward through the xylem of vascular plants. In its modern form, it was proposed by Henry Dixon and John Joly in the late 19th century to account for the ascent of water to great heights without a mechanical pump. The theory states that transpiration from leaves generates a negative pressure, or tension, within the xylem conduits.

Water molecules are cohesive, forming a continuous column through hydrogen bonding. As transpiration draws water out of the leaf mesophyll, the resulting tension is transmitted down the water column to the roots. The xylem’s walls are typically hydrophilic, allowing adhesion that helps counteract gravity and maintain a continuous column. The net effect is a negative water potential that pulls water from the roots toward the shoots, aided by capillary action and the connectivity of vessels and tracheids.

The driving force is leaf transpiration, which lowers the water potential in the leaf and creates a suction that propagates through the plant. This mechanism explains water transport across long distances and under varying soil moisture conditions. Evidence includes measurements of negative pressures in living xylem tissue and the correlation between transpiration rate and water transport.

A known limitation is cavitation, the formation of air embolisms when tension becomes excessive, which can disrupt the water column. Plants mitigate this risk through xylem anatomy—such as narrower vessels and regulated pit membranes—and by stomatal control to balance water loss with the need for cooling and nutrient transport. Cohesion-tension remains a foundational concept in plant physiology, distinguishing xylem transport from other processes like phloem-based nutrient movement.