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Transport213-page10 - surrounding soil areas • This...

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Transport in Plants - 10 Water molecules also tend to cohere (stick to each other -the surface tension property of water). This second force tends to make water molecules take on a concave shape that resists the increasing surface area. The two forces together (adhesion and cohesion) generate a negative water pressure that literally pulls water in the leaf out of the xylem. This transpiration pull can be exerted throughout the xylem down to the roots making the root water potential negative, too. o When transpiration rate is high, the osmotic gradient across the endodermis decreases as dissolved ions are drawn upward in the xylem. The depletion of water in the root hair zone can draw water from
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Unformatted text preview: surrounding soil areas. • This combination of forces (transpiration and cohesion) is sufficient to move water upward against the forces of gravity. • In addition, the diameter of vessels and tracheids promotes cohesion. • It also means that a plant disadvantage, water lost by transpiration, can be turned around to do something beneficial for the plant. For all of this to work, one must have an unbroken column of water. Air bubbles in the column dramatically affect water movement. A rupture of the water column is called cavitation and vessels affected will no longer function. The air or water vapor blockage that forms after the cavitation is called an embolism....
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