Once seeds are fully developed, they need a good place to grow. If they just fell to the ground beneath their parent plant, they would struggle, competing against each other for sunlight, water, and minerals. Most seeds need to travel—by wind, water, or with the help of insects and other animals—to better places to germinate, or start to grow into new plants. Some seeds, like those from conifer and maple trees, have wings attached. Others, like those of dandelions, have parachutes made of tiny hairs. Both features allow the seeds to be carried great distances by the wind, and they sometimes land in spots that are good for germination. Water carries other seeds to good growing places; the hard, watertight shell of a coconut, for instance, allows it to travel many miles at sea before finding a beach where conditions are suitable for growth.
Seeds sometimes have to wait a long time before they find good places to grow, places where the sun, moisture, and temperature are right. Most seeds are designed for the wait, protected by a hard outer pod (except those of conifers). Some seeds wait years to germinate, and some just never do. But inside each seed pod is a baby plant, or embryo, and endosperm, a supply of starchy food that will be used for early growth if germination takes place. Then a tiny root will reach down into the soil, and a tiny green shoot will reach up, toward the light.
As people age, their bodies change in many ways that affect the ways their cells and organ systems function. These changes occur little by little, progress over time, and are different for every man and woman. We all lose height as we age, and by the time we reach 80 years old, our height may have decreased by two inches (five centimeters). Changes in posture, in the growth of our spine’s vertebrae, and joint changes all contribute to our loss of height. With aging, the hair follicles produce less melanin, the pigment that gives hair its color.
Hair becomes lighter, gray, and eventually turns white. The nails also change with aging: they grow slower, may become dull and brittle, and may become yellowed and opaque. With aging, the outer skin layer (epidermis) thins and the number of pigment-containing cells (called melanocytes) decreases, but the remaining melanocytes increase in size. Aging skin thus appears thinner, more pale, and translucent. Changes in the connective tissue reduce the skin’s strength and elasticity, resulting in a wrinkly, leathery skin.
The immune system protects the human body against germs, which are microorganisms that cause sickness and disease. There are four major types of germs— harmful bacteria (pathogens), viruses, fungi, and protozoa. This defense system begins with the skin, which stops germs from getting into your blood or tissues. If germs get into your body, for example through your nose or mouth, white blood cells called phagocytes and lymphocytes attack them. Phagocytes scout out and destroy invaders, and long-living lymphocytes remember the invaders and release chemicals called antibodies to make the body resistant, or immune, to them.
White blood cells live in the bloodstream, lymphatic system, and spleen. The lymphatic system (or lymph system, for short) is a far-reaching network that extends throughout your entire body. A clear liquid called lymph runs throughout the system, washing the body’s cells with nutrients and water and detecting and removing pathogens. Lymph is filtered through the lymph nodes, and then passes into the body’s bloodstream.
Moss is a type of plant that does not have traditional roots, stems, or leaves. Because they have no true roots, mosses use delicate growths called rhizoids to anchor them to soil, rocks, or tree bark. Moss grows along the ground where it is moist, absorbing water and nutrients from the air.
Like their cousins the ferns and liverworts (leafy mosses), mosses reproduce from spores, not seeds, and need to be moist in order to reproduce. They grow in soft cushions or small clumps, and can spread out like a blanket along the ground.
Crocodilians—scaly, carnivorous reptiles that include crocodiles, alligators, caimans, and gharials—are descendents of the archosaurs who lived on Earth with the dinosaurs 200 years ago.
Today’s modern crocodiles are semi-aquatic predators that have remained relatively unchanged since the Triassic period. Besides birds, they are the dinosaurs’ closest living relatives.
Rain forests—thick forests of trees and other plants found in the lowland areas of the Tropics around the world—exist in parts of Australia, Indochina, India, the Malay Peninsula, the East Indies, in central and western Africa, and in Central and South America. Unlike forests in many other parts of the world, which have been affected by global climate changes like the Ice Age, tropical rain forests have been growing uninterrupted in some places for millions of years. During that time an unimaginable number of different types of plants and animals have evolved to use every food source and live in every spot there. Tropical rain forests have more plant and animal species than the rest of the world combined, and scientists continue to discover new species. Because tropical rain forests are located near the equator, their climate is warm.
The name “rain forest” comes from the fact that they receive a lot of rain—between 160 and 400 inches (4 and 10 meters)—throughout the year. Plants grow very quickly under such ideal conditions. In order to get the sunlight that they need for photosynthesis (the process by which they and other green plants make their own food), rain forest trees grow very tall, up to 130 feet (40 meters) high. Their tops form a huge canopy that shades most of the ground, protecting plants on the ground from excessive sunshine as well as wind. Rain forest trees have very shallow roots, for the soil in which they grow is poor, having long been depleted of nutrients by the needs of thick plant life over millions of years. But the abundant life all around contributes organic matter (the decomposed remains of plants and animals) to the surface of the soil, which is enough to nourish these grand, ancient forests.
The word “disabled” usually refers to a person who has a physical or mental handicap that keeps him or her from doing certain tasks—or makes performing them unusually difficult. Most physical disabilities, like blindness or paralysis, are easily noticed, but many mental disabilities are harder to detect. Mental disabilities can include diseases like schizophrenia, which causes severe disturbances in people’s thoughts and emotions.
Another type of disability is a learning disability, such as dyslexia, which is a learning disorder that makes reading very difficult because the brain reverses the order of letters and words. Many disabled people prefer the term “differently abled,” a description that doesn’t divide people into categories like “normal” and “disabled” but addresses the idea that every person has different abilities.
The combine harvester saves the farmers time and labor. Before modern machinery, harvesting crops was a painstaking process. Gathering and removing mature plants from the field had to be done by hand. Farm workers used sharp-bladed, long-handled scythes and curved sickles to cut down cereal crops like wheat. Even the fastest reaper could only clear about a third of an acre a day. Because rain could ruin harvested wheat, workers called sheaf-makers quickly tied it into bundles, so that it could be safely stored if the weather turned stormy. During the long winter months farm workers used jointed wooden tools called flails to thresh or beat the dried wheat in order to separate its edible grain seeds from its stalks.
But in 1786 a machine that threshed wheat by rubbing it between rollers was invented, replacing human threshers. And around 1840 a reaping machine—whose revolving wheel pressed grain stalks against a sharp blade that cut them down— replaced human harvesters. Today, farm machines called combine harvesters do this work in much the same way. These machines are very efficient and combine all three jobs of cutting, collecting, and threshing a crop. A single combine harvester can process five acres of wheat in less than an hour!
Since flowers possess both male and female parts, some flowers can fertilize themselves— or fertilize another flower on the same plant—which is called self-pollination. Or the ovules of one flower may be fertilized by the pollen of a different flowering plant of the same species, a method called cross-pollination.
The wind, water, insects, and other animals help to carry pollen from one flower to another. Crosspollination usually produces a better plant: the offspring of cross-pollination possesses the genetic traits of two parents, which may give it new characteristics that will help it survive in an always-changing environment. Cross-pollination is so desirable, in fact, that many flowering plants have developed different ways to keep selfpollination from happening. In the flowers of a spiderwort plant, for example, the stamens are ready to release pollen grains before the pistils are ready to accept them, so the pollen has to travel to other spiderwort plants in search of a ripe pistil.
Husbands and wives divorce when they can no longer live happily together. It is usually a sad thing, because when people marry they expect to be with their partner for the rest of their lives. But over the course of a marriage things happen, people can change, and the happiness that the couple was so sure of in the beginning sometimes disappears. When couples with children get divorced it is even more unfortunate because more people are affected. Many children feel bad when their parents divorce because their family will not be the same. After a divorce, they generally do not see one of their parents as much as they did before.
Still, just because the feelings between a mother and a father change doesn’t mean that their love for their children changes in any way. It’s important to remember that divorce is something that happens between a husband and wife—it has nothing to do with the kids. Many children feel that if they adjust their behavior somehow their parents will want to stay together, but divorces are not caused by anything children do.