--- title: "Cohesion-Tension and Water Movement in the Xylem" description: "Evaporation from leaf cells pulls a continuous column of water up the xylem, held together by hydrogen bonds. The column is under tension, which is why a trunk narrows slightly on a hot day." canonical: https://lightmysky.com/learn/science/cohesion-tension-and-water-movement-in-the-xylem-mt_9_TORolYIl source: https://lightmysky.com/learn/science/cohesion-tension-and-water-movement-in-the-xylem-mt_9_TORolYIl.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Cohesion-Tension and Water Movement in the Xylem Evaporation from leaf cells pulls a continuous column of water up the xylem, held together by hydrogen bonds. The column is under tension, which is why a trunk narrows slightly on a hot day. Subject: Science · Area: Organisms & Life Processes · Ages 17 to 18 Page: https://lightmysky.com/learn/science/cohesion-tension-and-water-movement-in-the-xylem-mt_9_TORolYIl ## Ready when they can - Explains where the pulling force comes from and where in the plant it acts. - Links cohesion in the column back to hydrogen bonding between water molecules. - Uses the theory to predict what happens to the flow when the stomata close. ## Lesson: How leaves pull water skywards Tall trees lift water tens of metres with no pump at all, and the engine sits in the leaves. Water evaporates from the wet walls of the mesophyll cells into air spaces and out through the stomata. Each departing molecule tugs on the one behind, so the whole xylem column feels an upward pull. That pull is called tension, and it acts from the top of the plant. The column holds together because water grips itself. Hydrogen bonds chain each water molecule to the next, so the column hangs like a rope hauled from above. This grip has a name: cohesion. Sticking to the narrow xylem walls, called adhesion, steadies the rope further. Pull from the leaves plus grip within the column is the whole cohesion tension story. **Example.** The theory proves itself when you change the weather. On a hot, dry, windy day, evaporation races and uptake at the roots speeds up in lockstep. If the stomata close on a dry afternoon, evaporation halts, the pull vanishes, and the transpiration stream slows to a trickle. Pushing from the roots is far too weak for a tall tree, while pulling from above has no such limit while the column holds. **Tip.** Answer in one chain: stomata, transpiration rate, tension, xylem flow. Every factor that changes transpiration changes uptake at the roots the same way. Leaves are built for this job, with stomata that open and shut and a vast wet mesophyll surface feeding the pull. **Recap.** Leaves pull through evaporation, hydrogen bonds hold the column, and stomata set the pace. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Water and Hydrogen Bonding in Living Systems](https://lightmysky.com/learn/science/water-and-hydrogen-bonding-in-living-systems-mt_79dzlo6fD5) - [Transpiration and Transport in Plants](https://lightmysky.com/learn/science/transpiration-and-transport-in-plants-mt_AqN0vL8QRu) - [Water Potential and Osmosis Quantified](https://lightmysky.com/learn/science/water-potential-and-osmosis-quantified-mt_dFBz-cE6W0) ## Opens up - [Translocation and the Mass Flow Hypothesis](https://lightmysky.com/learn/science/translocation-and-the-mass-flow-hypothesis-mt_WZ8id0kou4)