Hachi's posts

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Chain of water molecules can avoid these air bubbles by going through pits between adjacent tracheids or vessel elements

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Common in vessel elements (wider), occurs during drought or winter (xylem sap freezes)

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Cavitation - formation of a water vapor pocket, breaks the chain of water molecules needed for transpirational pull

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Lowers water potential in root xylem → water flows passively in from the soil

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Thick secondary walls prevent them from collapsing

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Upward pull of sap creates tension in vessel elements and tracheids → pulls the walls inward

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Adhesion of water molecules to hydrophilic cellulose in xylem walls offsets the force of gravity

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Negative pressure potential of the leaves pulls water from the xylem as a result of water moving from higher to lower potential

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Water evaporates from the film → air-water interface retreats into the cell wall and curves (due to high surface tension of water) → tension increases (more negative water pressure) → water from hydrated parts of the leaf are pulled to this area to reduce the tension → “pulling” is transferred to xylem by cohesion

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Water adheres to the cellulose microfibrils of the mesophyll cell wall → water film

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Air outside the leaf is drier (lower water potential) → water diffuses out of the leaf via the stomata

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In leaves - internal air spaces expose mesophyll to CO2, saturated with water vapor due to moist walls of cells

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Xylem sap is under negative pressure (tension)

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Cohesion-tension hypothesis (pulling xylem sap) - transpiration pulls xylem sap upwards, and the cohesion of water molecules transmits this pull along the length of the plant