单选题 THE CIRCULATORY SYSTEM OF TREES

1 Inside the tree's protective outer bark is the circulatory system, consisting of two cellular pipelines that transport water, mineral nutrients, and other organic substances to all living tissues of the tree. One pipeline, called the xylem--or sapwood--transports water and nutrients up from the roots to the leaves. The other, the phloem--or inner bark---carries the downward flow of foodstuffs from the leaves to the branches, trunk, and roots. Between these two pipelines is the vascular cambium, a single--cell layer too thin to be seen by the naked eye. This is the tree's major growth organ, responsible for the outward widening of the trunk, branches, twigs, and roots. During each growing season, the vascular cambium produces new phloem cells on its outer surface and new xylem cells on its inner surface.
2 Xylem cells in the roots draw water molecules into the tree, taking in hydrogen and oxygen and also carrying chemical nutrients from the soil. The xylem pipeline transports this life-sustaining mixture upward as xylem sap, all the way from the roots to the leaves. Xylem sap flows upward at rates of 15 meters per hour or faster. Xylem veins branch throughout each leaf, bringing xylem sap to thirsty cells. Leaves depend on this delivery system for their water supply because trees lose a tremendous amount of water through transpiration, evaporation of water from air spaces in the leaves. Unless the transpired water is replaced by water transported up from the roots, the leaves will wilt and eventually die.
3 How a tree manages to lift several liters of water so high into the air against the pull of gravity is an amazing feat of hydraulics. Water moves through the tree because it is driven by negative pressure--tension--in the leaves due to the physical properties of water. Transpiration, the evaporation of water from leaves, creates the tension that drives long- distance transport up through the xylem pipeline. Transpiration provides the pull, and the cohesion of water due to hydrogen bonding transmits the pull along the entire length of xylem. Within the xylem cells, water molecules adhere to each other and are pulled upward through the trunk, into the branches, and toward the cells and air spaces of the leaves.
4 Late in the growing season, xylem cells diminish in size and develop thicker skins, but they retain their capacity to carry water. Over time the innermost xylem cells become clogged with hard or gummy waste products and can no longer transport fluids. A similar situation occurs in the clogging of arteries in the aging human body. However, since the vascular cambium manufactures healthy new xylem cells each year, the death of the old cells does not mean the death of the tree. When they cease to function as living sapwood, the dead xylem cells become part of the central column of heartwood, the supportive structure of the tree.