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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.(1)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.

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 Iife-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 (2)wilt and eventually die.

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 cell, water molecules (3)adhere to each other and are pulled upward through the trunk, into the branches and toward the cells and air spaces of the leaves.

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 (4)gummy waste products and can no longer transport fluids. A similar situation occurs in 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.

問 英文の内容に合うように, (1)~(6)の各文に続く ものとして最も適したものをそれぞれ選択肢1~4の中から選びなさい。 また(7)~(10)は質問に対する答えとして最も適したものをそれぞれ選択肢1~4の中から選びなさい。

• (1) The word (1)This refers to            .
  • 1. Phloem
  • 2. inner bark
  • 3. vascular cambium
  • 4. naked eye
• (2) The word (2)wilt is closest in meaning to            .
  • 1. melt
  • 2. grow
  • 3. swell
  • 4. sag
• (3) The phrase (3)adhere to is closest in meaning to            .
  • 1. depend on
  • 2. stick to
  • 3. warm up
  • 4. respond to
• (4) The word (4)gummy is closest in meaning to            .
  • 1. sticky
  • 2. liquid
  • 3. smelly
  • 4. fluffy
• (5) It can be inferred from paragraph 1 that the xylem is located            .
  • 1. on the surface of the outer bark
  • 2. inside the phloem and the vascular cambium
  • 3. next to the inner bark
  • 4. between the vascular cambium and the phloem
• (6) All of the following are functions of the xylem EXCEPT            .
  • 1. transporting food from the leaves to the trunk
  • 2. taking in chemical nutrients from the soil
  • 3. forming part of the tree's structural support
  • 4. moving water upward through the trunk
• (7) What are the primary components of the tree's circulatory system?
  • 1. Water, minerals, and organic substances
  • 2. Xylem and phloem
  • 3. Leaves, branches, and trunk
  • 4. Roots and heartwood
• (8) What can be inferred from paragraph2 about xylem sap?
  • 1. It is composed mainly of water.
  • 2. It causes waterless by transpiration.
  • 3. It gives leaves their green color.
  • 4. It is manufactured in the leaves.
• (9) Why is the process of transpiration essential the tree's circulatory system?
  • 1. It supplies the hydrogen and oxygen that trees need to live and grow.
  • 2. It produces new phloem and xylem in the trunk, branchcs, and roots.
  • 3. It causes the negative pressure that moves water through the xylem.
  • 4. It replaces the water vapor that is lost through the leaves' air spaces.
• (10) Why does the author mention arteries in the aging human body in paragraph 4?
  • 1. To show that trees and people get the same diseases.
  • 2. To imply that trees might provide a solution to human problems.
  • 3. To compare what happens in two ageing circulatory systems.
  • 4. To explain the cause of death in most trees.