The stomata of leaves are surrounded by guard cells. The guard cells help the leaves to regulate the rate of transpiration of water from the leaves by opening and closing the stomata. When water enter the guard cells, they swell and bulge and this makes the stomata to open. So, with high water pressure, the guard cells will stimulate the stomata to open. The reverse will be the case if the water pressure is low.
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(D) Nonpoint source pollution is difficult to regulate, because its source is difficult to track.
Zippy Xylem. The xylem of a plant is the system of tubes and transport cells that circulates water and dissolved minerals.