Transport213-page11 - twigs than in the trunk of trees •...

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Transport in Plants - 11 In case you were curious about if this idea of sucking water up the tree “holds water”, there are ways of measuring all of this. Pressure bombs measure the hydrostatic pressure within the xylem. When a twig is cut, water recedes into the interior. The twig is put into a pressure chamber and gas pressure is applied until water appears at the cut end of the twig. That pressure can be measured. It is a positive pressure that equates to the tension within the xylem. Thermocouples have been used to measure the velocity of movement of small amounts of “heated” xylem. Xylem movement is always detected earlier in
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Unformatted text preview: twigs than in the trunk of trees. • The negative transpiration pressure is reflected by a narrowing of the diameter of the tree as xylem cells narrow in diameter from the tension generated. In addition, the rate of xylem movement varies. Virtually no movement occurs at night, when there is no transpiration. In the daytime, the rate of xylem movement depends on environmental conditions such as temperature, wind velocity, and light intensity, as well as solutes in the sap. Studies have shown that the rate of xylem movement increases with K + concentration in the sap, and slows when K + diminishes....
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