BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. TRANSPORT IN PLANTS

13.2. Water Movement in Flowering Plants

Water within a plant is in direct contact with soil water and water vapor in the surrounding air. As previously discussed, it moves along a water potential gradient from higher to lower potential. Within the plant, this potential decreases continuously from the soil to the air (Fig. 13.7). The water potential of moderately dry air is considerably lower than that of plant Tissues; consequently, water tends to move out of the tissues and into the atmosphere.

The vast majority of water enters the plant through ROOT hairs. From there, it travels across the root cortex into the xylem, ascends through the xylem to the leaves, evaporates from the surfaces of mesophyll Cells, and finally diffuses out into the atmosphere through the Stomata. Water loss from the plant surface is known as Transpiration, and its upward movement from the roots to the stomata is referred to as the transpiration stream. Estimates suggest that plants typically lose more than 99% of the water they absorb.

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Fig. 13.7. Water movement through a plant. The WATER POTENTIAL OF the soil solution is high, whereas that of atmospheric vapor is low. Water flows through the plant along a water potential gradient, which is sustained by solar energy and the evaporation of water from the plant surface (transpiration).