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by Sharon Kahkonen Copyright by Harcourt, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Requests for permission to make copies of any part of the work should be addressed to School Permissions and Copyrights, Harcourt, Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887-6777. Fax: 407-345-2418. HARCOURT and the Harcourt Logo are trademarks of Harcourt, Inc., registered in the United States of America and/or other jurisdictions. Printed in Mexico ISBN-13: 978-0-15-362468-1 ISBN-10: 0-15-362468-X 2 3 4 5 6 7 8 9 10 126 10 09 08 Visit The Learning Site! www.harcourtschool.com
Plop. The cool autumn day is interrupted by a ripe acorn falling to the forest floor. A squirrel snatches the acorn and carries it to an open spot, where it is buried snugly beneath the leaves. Meant to be a winter snack, the acorn remains beneath a carpet of leaves and snow. The acorn contains a tiny, living, embryo plant and its food supply. At the pointed end of the acorn, there is a tiny root, stem, and a tuft of white leaflets. Just below the tiny embryo are two big cotyledons the food supply for the embryo when it starts to grow.
In the spring, the snow melts and the days get warmer and longer. The thick acorn shell becomes wet and soft. It swells and bursts open. Then water easily seeps into the acorn and reaches the embryo. After absorbing the moisture, the embryo begins to swell. A tiny root pokes out and begins to grow downward. The tiny shoot thrusts upward. The cotyledons inside the acorn shrink as the growing root uses this food supply. The root grows downward, probing for water deep in the soil. It becomes an anchor for the young oak. Then the first tiny leaves unfurl. As soon as sunlight strikes the leaves, they begin to make chlorophyll and turn from white to bright green. The chlorophyll will allow the plant to make its own food.
This is not the only acorn to have germinated in this opening in the forest. An old oak tree once stood here, but it has recently fallen, leaving a break in the forest canopy. Now there is a race among oak seedlings to claim this spot in the sun. To survive, the young oak must rapidly put out new leaves so that it can make enough food to grow. By growing faster and bigger than the other seedlings, it wins the competition for sunlight and grows into a sapling. There are other hurdles that the sapling must overcome during its first years of life. It must compete with other plants for water and nutrients. The sapling is still young and tender, so it might be eaten by a caterpillar or a deer. But an extra dose of a bitter-tasting chemical called tannin discourages browsers from taking more than one bite. There are periods of no rain in summer and intense cold in winter. Most oak saplings do not survive these hardships, but this one grows into a strong, young tree. The sapling s main root grows four or five feet deep. When there is no water at the surface, the root can draw water from the soil below. The water is in small holes between the soil particles. The root also stores food over the long winter. In the spring, this ready food supply can be used for speedy growth. Feeder roots grow many branches just beneath the soil surface. Here they have a supply of nutrients from the layer of dead leaves that covers the forest floor. Dark surface soil also is rich in nutrients. The roots can get water from rains and melting snow.
The roots grow rapidly at their tips. A tiny root cap protects each delicate root tip as it pushes through the soil. Farther from the root tip are the mature root cells that make up root tissues, each with its own role to play. A thin outer layer keeps the roots from drying out. At the center of the root are the xylem and phloem. These make up the tubes through which water and minerals flow. Food also flows to every part of the young tree. Encircling the xylem and phloem is a layer of cells that produce new roots. Around this is another layer of cells that regulate the flow of water into and out of the tubes. Food is stored in the cortex. A furry coat of root hairs projects from the mature root tissue. The root hairs absorb water and nutrients from the soil.
Once inside the root, water and minerals move into the xylem tissue. The xylem tissue is part of a complex system of tubes that extends from near the tips of the roots all the way to the surface of leaves. Water absorbed by the roots must reach all the way to the highest leaves. Water molecules attract each other strongly. Water molecules also stick to the smooth walls of the xylem. These two forces hold the water together so that it forms one continuous column inside the xylem, from the roots all the way to the leaves. On the undersides of the leaves are small pores. Water evaporates from the pores. More than 95 percent of the water taken in by the roots passes directly out of the tree through these pores. This evaporation, called transpiration, is what pulls water and dissolved nutrients up through the tree s xylem, from the roots all the way to the leaves.
The oak s trunk, limbs, and branches are the framework that holds the leaves up to the sunlight. They also contain the tubes for carrying water, minerals, and food from one place to another inside the tree. Water and minerals travel through the xylem tissue from the roots to the leaves. Food from the leaves moves through the phloem to places where the tree is growing, or to the roots, where it is stored. At the center of the tree s trunk, limbs, and branches is the heartwood. The heartwood is made up of dead xylem tissue that no longer transports water. The dead cells are reinforced with a substance called lignin, which makes them strong and stiff.
Thus, the heartwood is a strong central column that supports the tree. Outside the heartwood is the sapwood, so named because its xylem cells still transport water and minerals. Each year, a new layer of xylem grows that is larger in circumference than the previous year s xylem. This enables the xylem to transport more sap each year, which provides water and minerals to an increasing number of leaves. Just under the bark is the living phloem tissue, which transports food from the leaves to other parts of the oak. Growth also occurs at the tips of the branches, where new cells are made. This is also where buds develop. The buds give rise to leaves and flowers. Each autumn, before the leaves fall, buds for the following spring have already been made.
The purpose of leaves is to make food for the oak tree. Leaves use a process called photosynthesis. Sunlight falls on the oak s leaves and is captured by chlorophyll. Chlorophyll is a substance that gives the leaves their green color. Energy from sunlight is used to combine water and carbon dioxide to make food in the form of sugar. Water for photosynthesis is taken up from the soil by the roots. The carbon dioxide enters the leaves through pores. A product of photosynthesis is oxygen, which passes out of the leaves.
The oak, like all other plants, respires. Respiration is almost the reverse of photosynthesis. Oxygen is taken in and carbon dioxide is released. The carbon dioxide and oxygen pass through the pores in the oak tree s leaves. The oxygen is used to break down sugars to produce energy the oak needs to live and grow. Because respiration does not require sunlight, the oak tree respires constantly. During the day, when photosynthesis is occurring, the tree produces oxygen and uses more carbon dioxide than it releases. At night, the tree stops producing oxygen. Only some carbon dioxide is released.
An oak leaf is designed perfectly for its food-making function. It has a protective outer surface covered with wax, which keeps it from drying out. Just under this protective layer is a layer of cells filled with green chlorophyll. This layer is where photosynthesis takes place. The next layer has loosely packed cells and the xylem and phloem tissue that make up the leaf s veins. The phloem tissue in the veins whisks away the sugars that are made during photosynthesis. On the underside of the leaf are hundreds of pores, through which water and gases can pass in and out of the leaf.
In 15 years the oak is a mature tree, more than 30 feet tall. Each spring, before tree leaves sprout, it is decorated with countless blossoms. The male flowers are called catkins because they look like little cat tails. The tiny female flowers are hardly noticeable. They are found in the angle between a new leaf and twig. On windy days, clouds of pollen blow off the catkins. Stigmas covered with a sticky glue protrude from the female flower. Pollination occurs when oak pollen lands on the stigmas. After pollination has occurred, acorns begin to form. The acorns get bigger as more and more food is stored in them.
Through the years, the oak grows large. Its canopy of leaves reaches out over the forest, casting the forest s floor in dim shade. Its many leaves rustle in the breeze each summer and fall to the ground each autumn. The fallen leaves provide a carpet of litter that is home to insects, worms, and many other organisms. In time, these leaves decay and make their nutrients again available to the mighty oak. During winter, the oak stands dormant. Its leaves have fallen, and its branches are exposed. The oak does not make food during winter. There are no green leaves within which photosynthesis can occur. Instead, the tree remains in a state of dormancy until the days grow warm and long again in the spring.
Six hundred years come and go. The old oak is five feet in diameter and 150 feet tall. Each spring the mighty oak bears its flowers, and each fall it drops its acorns. But over the years, old age, violent weather, and disease take their toll. There are jagged holes in the bark where limbs have fallen away. A squirrel family makes its nest in one of the holes. Shelf fungus grows from the dying trunk. Bark beetles and grubs feed on the bark. Woodpeckers search for insects inside the tree.
Then one spring day the weakened tree can no longer stand. It crashes to the forest floor. Millions of bacteria, fungi, earthworms, and other inhabitants of the forest floor begin to digest the wood from the dead oak. They are nature s recyclers, making the nutrients from the old oak available for brand new oak trees. Sunlight streams through the opening in the forest canopy where the old oak used to stand. The sun strikes the leaves of a tiny sapling. It has just sprouted up from an acorn that a squirrel has buried and forgotten.
Compare the life cycle of an oak tree to the life cycle of a human. How is photosynthesis almost the opposite of respiration? Describe how the structures of the leaves, the trunk, and the roots of an oak are suited for their functions. Trees are important to your community. During summer, trees keep streets and homes cooler. Trees beautify yards, parks, and roadsides. Do research to find out more about the importance of trees. Then write a letter to the editor of your local newspaper about the value of trees in your community. Color some water with red food dye. Make a fresh cut across the stem of a stalk of celery. Leave the celery stalk in the colored water for a day. What happens to the leaves? Cut across the celery stem and look at it through a hand lens. Find the tiny red xylem tissue that carried the colored water. Try to separate a single tube from the celery stem. With a family member, take a hike through a forest or a park near your home. Use a field guide to identify as many types of trees as you can. Study the trees leaves, and make drawings of the different kind of leaves you see. In each drawing, include the pattern of veins in the leaf. Explain to your family member why leaves have veins.
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