Essay About Water Movement And Cut Portion Of Upper Stem

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Transport in the Xylem
Cohesion-Tension Theory of water movement.
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Water in leaves develops large negative ΨP (tension)
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This tension pulls the water up through the xylem
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Cohesion is required to maintain the water column integrity
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Cohesion allows the pull from the leaves to extend all the way through the xylem to the roots and ultimately to the soil.
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This also depends on adhesion of water to the apoplast
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The pulling force arises from differences in ΨW.
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Neither capillary action nor air vacuums are sufficient to explain the large pull from the leaves.
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Instead, this results from fact that air has an extremely low ΨW, around – 95MPa.
Evidence for theory
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Evidence that transport is in the xylem: put radioactive potassium in soil or water. Ask where radioactivity goes. It is found in the xylem.
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Evidence of pulling:
Cut stem and place cut portion of upper stem in water with dye. Dye moves up the stem in the xylem.
Measure trunk diameter at the top and bottom of a tree over the course of a day. The top of the trunk shrinks before the bottom.
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Evidence that water is cohesive enough to withstand forces in the xylem: Place water in a capillary tube and spin in a centrifuge. Increase spin until the water in the column separates