How Does Xylem Move Water Up a Plant?
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How does xylem move water? Xylem is the plant tissue that carries water from the roots up to the leaves. The most widely cited explanation for how it does this is the cohesion-tension theory. The theory says that water is pulled upward through a continuous column rather than pushed from below, and that the pull comes from water loss at the leaves. This article explains the mechanism, the physical forces that keep the column together, the height limits the sources describe, and the evidence they report. It also notes where the sources leave the question open.
Quick answer
- The cohesion-tension theory describes water as “pulled up the plant by tension (negative pressure) from above.”
- Transpiration is the main driver: the LibreTexts chapter says that “transpiration is the main driver of water movement in the xylem.”
- Cohesion between water molecules, caused by hydrogen bonds, gives the column strength, and adhesion to the vessel walls helps hold it in place.
- A review by Sane and Singh calls cohesion-tension “the likely mechanism for sap ascent” and describes the exact mechanism as still debated.
The pull from above
The LibreTexts chapter on the cohesion-tension theory states that “In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is pulled up the plant by tension (negative pressure) from above.” The same chapter explains their reasoning: “Dixon and Joly believed that the loss of water in the leaves exerts a pull on the water in the xylem ducts and draws more water into the leaf.”
A pulling force has a limit, and the chapter states it: “even the best vacuum pump can pull water up to a height of only 10.4 m (34 ft) or so.” The chapter uses this limit to frame the question of how a column can be lifted higher than a pump could lift it.
Transpiration is the step that creates the tension. The chapter describes the gradient along the path: “Water potential decreases from the root cells (-0.2 MPa) to the stem (-0.6 MPa) to the leaf at the tip of a tree (-1.5 MPa) to the atmosphere (-100 MPa).” The drop from the roots to the air is the gradient the theory relies on.
How transpiration sets the tension
The chapter describes the exit point at the leaf: “Transpiration draws water from the leaf through the stoma.” Water evaporating from inside the leaf is also part of the pull. The chapter says that “Evaporation of water into the intercellular air spaces creates a greater tension on the water in the mesophyll cells, thereby increasing the pull on the water in the xylem vessels.”
The root end is part of the same path. The chapter says that “Negative water potential draws water from the soil into the root hairs, then into the root xylem.” Together these statements describe one continuous path from soil to leaf.
The strength of the pull depends on air conditions. The chapter states that “The water potential at the leaf surface varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH.” It also addresses night: “At night, when stomata typically shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other.”
The tension can be seen in the tree itself. The chapter reports that “the diameter of Monterey pines decreases during the day, when transpiration rates are greatest.” It gives the reason: “Because the water column is under tension, the xylem walls are pulled in due to adhesion.”
Why the water column holds together
The chapter states that “the force of cohesion between water molecules imparts great strength to the column of water.” It attributes the attraction to hydrogen bonds: “Cohesion occurs when water molecules are attracted to each other. This is due to hydrogen bonds, which form between the partially negative oxygen of one molecule and the partially positive hydrogen of another molecule.” The same passage notes that “Hydrogen bonds are a strong intermolecular force.”
Adhesion is the second part of the explanation. The LibreTexts figure caption states that “Adhesion occurs when water molecules are attracted to the walls of the vessel element, which has thick walls with lignin, a stiff substance.”
The vessels are built to withstand the pressure changes. The chapter says that “Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure.” It also states that “The mechanism of the cohesion-tension theory is based on purely physical forces because the xylem vessels and tracheids are not living at maturity.”
The chapter reports a measured limit on the tension: “water in the xylem avoids cavitation at negative pressures exceeding ~1.6 MPa.” It also gives a height example: giant redwoods “can successfully lift water 109 m (358 ft), which would require a tension of ~1.9 MPa.” The chapter adds that “such heights may be approaching the limit for xylem transport.”
What the measured evidence shows
The chapter describes an experiment in which “a German botanist who sawed down a 21-m (70-ft) oak tree and placed the base of the trunk in a barrel of picric acid solution.” The chapter reports the result: “The solution was drawn up the trunk, killing nearby tissues as it went.” It then explains the logic: “If the roots were the driving force, upward water movement would have stopped as soon as the acid killed the roots. However, the solution reached the top of the tree.”
The chapter continues the same experiment: “When the acid reached the leaves and killed them, the water movement ceased, demonstrating that the transpiration in leaves was causing the water” movement. The sequence ties upward movement to leaf activity.
The chapter also describes an effect on plants that live in salty conditions. It states that “The pulling force due to transpiration is so powerful that it enables some trees and shrubs to live in seawater.” It adds that “the remarkably high tensions in the xylem (~3 to 5 MPa) can pull water into the plant against this osmotic gradient.”
The Sane and Singh review places the theory in a broader context. Its abstract states that “Although early researchers proposed the hypothesis of cohesion-tension of water as the likely mechanism for sap ascent, the exact mechanism of transport continues to be a hotly debated topic in the field of plant physiology.”
How we chose
The two sources were selected because one is a university-hosted plant physiology chapter that walks through the theory step by step, and the other is a review that discusses the debate around it. Each factual sentence in this article is tied to a quote from one of these pages. The sources were opened on 2026-10-27 and returned HTTP 200 before use.
What we could not verify
Claims that are not stated in either fetched source are left out. The sources were not used for any claim about a specific tree species beyond the examples quoted above.
What the sources do not say
The review states that “the exact mechanism of transport continues to be a hotly debated topic in the field of plant physiology.” The LibreTexts chapter states that “such heights may be approaching the limit for xylem transport” without giving a single height for all plants.
For practical questions about a specific plant, the sources do not provide species-specific rates. Readers who want background on soil conditions can read the garden soil improvement guide, and the grapevine cuttings guide covers a related plant-growing topic.
Frequently asked questions
Is xylem movement pulled or pushed?
The LibreTexts chapter says that the theory describes water as “pulled up the plant by tension (negative pressure) from above.”
What holds the water column together?
The chapter says that “the force of cohesion between water molecules imparts great strength to the column of water,” and that cohesion is due to hydrogen bonds.
What limits how high xylem can lift water?
The chapter reports that “water in the xylem avoids cavitation at negative pressures exceeding ~1.6 MPa,” and that giant redwoods lift water 109 m, a height the chapter says may be “approaching the limit for xylem transport.”
Is xylem movement a living process?
The chapter says the mechanism is based on purely physical forces because “the xylem vessels and tracheids are not living at maturity.”
Is the cohesion-tension theory fully proven?
The Sane and Singh abstract calls the exact mechanism “a hotly debated topic in the field of plant physiology,” so the theory is presented as the likely explanation rather than a settled one in every detail.
Sources
- LibreTexts Biology, “4.5.1.3: Cohesion-Tension Theory,” bio.libretexts.org, opened 2026-10-27: https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/04%3A_Plant_Physiology_and_Regulation/4.05%3A_Transport/4.5.01%3A_Water_Transport/4.5.1.03%3A_Cohesion-Tension_Theory
- Sane, S. P. and Singh, A. K., “Water Movement in Vascular Plants: A Primer,” arXiv preprint copy, opened 2026-10-27: https://arxiv.org/pdf/1111.0404
