key concept 34.2 Water and Minerals Are Transported in the Xylem

Once water has arrived in the xylem, it is all “uphill” from there. Before considering the ascent of water and minerals to the leaves, reacquaint yourself with the cells that make up the xylem: the tracheids and vessel elements (see Figure 33.8A and B). Recall that these xylem cells are dead and lack all cell contents. When fused end-to-end, they form long tubular “straws” of lignified cell walls called xylem vessels. These vessels provide both structural support and the rigidity needed to maintain a gradient of pressure.

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  • Xylem vessels must move large amounts of water upward for long distances.

  • Experiments ruled out two possible mechanisms for movement of water upward through the xylem.

  • The currently accepted mechanism for water transport through the xylem is the transpiration–cohesion–tension mechanism.

  • There is evidence to support all parts of the transpiration–cohesion–tension mechanism.

Consider the magnitude of what the xylem accomplishes. A single maple tree 15 meters tall was estimated to have some 177,000 leaves, with a total leaf surface area of 675 square meters—about one and a half times the area of a basketball court. During a summer day, that tree loses 220 liters of *water per hour to the atmosphere by evaporation from the leaves. So to prevent wilting, the xylem needs to transport 220 liters of water up to 15 meters from the roots to the leaves every hour. (By comparison, a 50-gallon drum holds 189 liters.)

*connect the concepts As you learn about water movement in the xylem, keep in mind the properties of water described in Key Concept 2.4.