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Unformatted text preview: Roots actively move nutrient ions from the soil all of the time, and when transpiration is low, the increasing concentration of ions within the cortex cells creates a water potential gradient for moving water into the roots. This results in a positive pressure. Simple diffusion pressure in roots moves H 2 O upward often forcing the H 2 O to be exuded from vein tips in leaves, a phenomenon called guttation. The special leaf tip cells are called hydathodes. Guttation is limited. Positive root pressure is soon matched by the atmospheric pressure, and in daylight, transpiration rates rapidly exceed any positive pressure generated. Root Pressure Guttation...
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- Fall '09