BENCHMARK • Q Life Cycles Life Cycles A Reading A–Z Level Q Benchmark Book Word Count: 1,154 Written by Robert Charles Illustrations by Signe Nordin Visit www.readinga-z.com www.readinga-z.com for thousands of books and materials. Life Cycles Photo Credits: Front cover, title page, pages 4 (top, bottom left), 11, 18: Corbis; back cover, pages 5, 7, 8, 9, 12, 13, 15 (bottom): PhotoDisc; pages 4 (right), 15 (top): Priya Nanjappa, USGS–Patuxent Wildlife Research Center, Laurel, MD; page 6: Cabisco/Visuals Unlimited; page 14: Jim Merli/Visuals Unlimited; page 16: Joe McDonald/Visuals Unlimited. Written by Robert Charles Life Cycles Level Q Benchmark Book Correlation Illustrations by Signe Nordin © Learning A–Z, Inc. Written by Robert Charles LEVEL Q Illustrations by Signe Nordin Fountas & Pinnell N All rights reserved. Reading Recovery 21 DRA 30 www.readinga-z.com www.readinga-z.com Introduction Whether plant or animal, all living things undergo changes as they go through life. This Table of Contents is also true for tiny living things made of only one cell. This book is about the changes living Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 things go through during their lifetime. It is about life cycles. Human Life Cycle . . . . . . . . . . . . . . . . . . . . . . . 6 Animal Life Cycles . . . . . . . . . . . . . . . . . . . . . . 12 Plant Life Cycles . . . . . . . . . . . . . . . . . . . . . . . . 17 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 3 4 A life cycle includes all the stages a living thing goes through from birth to death. All living things have a beginning, and they all must die. What happens between birth and death varies from one kind of living thing to another. Most living things have one thing in common— they begin life as a tiny single cell. The simplest forms of life never grow beyond a single cell. The life cycle of these Human Life Cycle simple life forms is really Let’s start with you. You are a human just a matter of growing a bit being, or at least I hope you are. You started larger and then dividing into out as a tiny egg inside your mother, and you two smaller cells. But most began to change when that tiny egg began to of the plants and animals split. It split and split and split and continued we know have much more splitting until it split millions and millions of complicated life cycles. We times. Before long, body parts began to form. are going to examine how You began to get legs, arms, a head, and a life cycles differ among some whole bunch of organs, and over time, you plants and animals. looked more and more like a human. 5 6 After about nine months of cell splitting In the next stage of your life cycle, you and development inside your mother, you became a toddler. That’s when you got into were born. You entered the first stage of life lots of trouble. You learned to walk and talk, as an infant. You were pretty helpless at first, and you probably got into all sorts of things. but you continued to grow and get stronger. You probably also said some things you should not have said. Then you entered childhood. This is where you are now and where you will stay for a few more years. During childhood, you learn more and more about taking care of yourself. You will become less dependent on your parents by learning how to do all sorts of things, from riding a bike to reading and writing. As a child, you can now feed yourself, go to the bathroom by yourself, and dress yourself. 7 8 The adult stage is the longest, going on for many years. In fact, the average adult stage lasts more than fifty years, and adult life spans seem to get longer and longer. Toward the end of your adult stage, you will become what many people call a “senior citizen.” This is the last stage of the human life cycle. You will stay at this stage for several years until you enter adolescence. This is the stage when children rapidly grow into adulthood This stage is also called your teen years. 9 10 The adult stage is when many people choose to have and raise children. These children will also go through a life cycle and change with each stage. When an animal produces more of its own kind, it helps ensure that that kind of animal will remain on Earth. If a kind of animal dies off faster than it can reproduce, it becomes extinct and disappears from Earth. Long ago, humans had to produce Animal Life Cycles many offspring to make sure that we would Even small animals like insects go through survive. Now, however, the number of life cycles. Most insects go through three or humans on Earth is too many for the four stages during their life cycle. The change balance of life on our planet. This is called an insect goes through during its life is called overpopulation. Many countries around the metamorphosis. All insects begin as eggs. world are working to limit the number of During the stage after the egg stage, known babies each family produces. Over time, this as the larval stage, many insects look much may help to restore the balance. different than they do as adults. As a larva, Now let’s look at some other life cycles. many insects eat huge amounts of food. 11 12 They grow larger and then enter a stage Some animals lay eggs with shells as the where they are not active. For some insects, first stage of their life cycle. Birds and reptiles this stage is spent inside a protective covering lay eggs that are covered by protective shells. called a cocoon. Other insects spend this The eggs hatch when the baby animal breaks stage inside a hard case called a chrysalis. through the protective shell. The young When insects leave this stage, known as the animal that emerges has many of the same pupa, they enter the adult stage of their life. features as the adult. Other animals, like frogs As adults, female and male insects mate. and fish, lay eggs without shells in jellylike The female then lays eggs, and the life cycles clumps. These eggs are usually laid in water. of many new insects begin. Do You Know? Do You Reptile Know? eggshells An insect known are soft as the mayfly has like leather. an adult life that is Bird only one day long. eggshells are hard and brittle. A monarch butterfly emerging from a chrysalis A snapping turtle hatching 13 14 Other animals give birth to live babies. Some of these babies are born helpless and blind. Rabbits, mice, and kittens are like this and require a lot of care from their mothers. Marsupials, or pouched animals, such as kangaroos, give birth to very tiny, helpless babies that they keep in a special pouch until the babies develop enough to survive outside the pouch. Other baby animals are born ready to walk and run. Do You Know? A newborn Some insects and giraffe is frogs lay thousands six feet of eggs in a lifetime. (2 meters) Some birds lay only tall at birth one egg a year. and is able to walk. When a frog egg hatches, a creature called a tadpole emerges. The tadpole looks more like a fish than a frog. It slowly changes during its life cycle, growing legs, developing lungs, and losing its tail. Soon it looks like a frog. Fish eggs, on the other hand, hatch into tiny fish, called fry, which more closely resemble adult fish. 15 16 Life cycle of an oak tree sapling seedling seed Do You Know? adult tree The largest living thing is the giant sequoia tree. It grows Plant Life Cycles from a tiny pinecone seed. Most plants begin their life cycle as seeds. It is hard to imagine that a giant oak tree was once a small acorn. When it first sprouts, it is Some plant life cycles last hundreds of called a seedling, but over the years it grows years. A bristlecone pine can live well over larger and becomes a young tree called a a thousand years. Plants with life cycles that sapling. After a few more years, it becomes are several years long, such as trees, grass, an adult tree capable of making seeds (acorns) and shrubs are called perennials. Other that will grow into new trees. Plants are not plants, called annuals, live through only a born, and they do not hatch. Instead, a seed single season. Many garden plants, including germinates or sprouts to form a new plant. peas, beans, corn, and tomatoes, are annuals. 17 18