Water Flow

Water movement through plants requires no metabolic energy; rather, water flows passively from one place to another. Although root pressure, caused by the pumping of dissolved minerals into the roots, can push water up to a few meters, in taller plants water and dissolved minerals are pulled up through the xylem. The driving force for this movement is the transpiration of water from the leaves. The hypothesis that describes the process is known as the transpiration-cohesion hypothesis of water movement.It states that solar-driven transpiration of water dries the walls of mesophyll cells of leaves; the loss of water from the cell wall then causes water from neighboring cells to enter the leaf cell. Cells bordering tra-cheids and vessels replace their water with water from the xylem. The loss of water from xylary elements creates a negative pressure, thereby lifting the water column up the plant. The water column does not break, because water molecules cohere strongly.

The negative pressure created in the xylem by transpiration extends all the way down to the tips of roots, even in the tallest trees. The tension in the root xylem causes water to flow passively from the soil, across the root cortex, and into the xylem of the root. This water is then pulled up the xylem to leaves to replace water lost via transpiration.

Transpiration is affected by atmospheric humidity, wind, air temperature, soil, light intensity, and the concentration of carbon dioxide in the leaf. Transpiration is greatest in plants growing in moist soil on a sunny, dry, warm, and windy day. In these conditions, transpiration often exceeds the plant's ability to absorb water. As a result, many plants wilt at midday, even if the soil in which they are growing contains abundant water. Transpiration also moves solutes in plants; for example, most minerals move from roots to shoots in the transpiration stream.

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