On a summer day, a tree can transport many hundreds of liters of water from the soil to its leaves. Impressively, this feat is accomplished without any moving parts. Even more remarkably, trees and other plants transport water without any direct expenditure of energy. The structure of vascular plants allows them to use the evaporation of water from leaves to pull water from the soil.

Fig. 29.9 shows the cross section of a sunflower stem. Like a leaf, it has a surface layer of epidermal cells. This layer encloses thin-walled, undifferentiated parenchyma cells in the interior. Notice that the stem also contains differentiated tissues that lie in a ring near the outside of the stem. These are the vascular tissues, which form a continuous pathway that extends from near the tips of the roots, through the stem, and into the network of veins within leaves. The outer tissue, called phloem, transports carbohydrates from leaves to the rest of the plant body. The inner tissue, called xylem, transports water from the roots to the leaves.

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In addition, both the xylem and the phloem transport dissolved nutrients and signaling molecules, such as hormones, throughout the plant. Importantly, the xylem is the pathway by which mineral nutrients absorbed from the soil are transported to the aboveground portions of the plant. The total concentration of solutes in the xylem, however, is very low (typically less than 0.01%). We thus focus on the xylem as a means for transporting water from soil to leaves and discuss the uptake of nutrients in section 29.5.