Plants and Sources of Energy

            The energy for the beginning growth and push of the plant comes from food stored inside the seed. But the plant could not keep on growing unless it was able to manufacture more food. This is used by the plant to supply chemical energy for further growth.
            In order to manufacture food, the plant must have a source of energy outside itself. This energy is supplied by the sun. If the right conditions prevail, the plant inside a sunflower seed may supply itself with enough chemical energy to grow 10 or more feet tall.
            The blossom slowly turns to face the sun. As the sun sets, the flower will be facing west. At sunrise it will swing back to the east to face the rising sun.
            Green plants make use of the sun’s energy to manufacture food in their leaves. But light energy is not all that plants need. In fact, in most cases no light energy reaches seeds when the tiny plants inside them first begin to grow. It is dark down in the soil where seeds are planted.

Plant Growth and Water

            Water is needed by plants to manufacture their food. Water does not supply the energy for plant growth directly. But without water plants would not be able to manufacture their energy supply.
            More than 70% of the weight of certain plants is made up of water. Seeds also contain water, although the water present in seeds may fall as low as 10% or even less. The young plant in a seed will not start to grow until additional water is provided for it.
            Keeping the seeds damp will cause them to split open with tiny roots. Plants take in water through their roots. In most cases the water that plants use comes to them from the soil. Their roots extend down into the soil and draw water from it. Water is not taken in by the whole roots, however. There are certain little extensions on roots through which water enters. These are called root hairs. They are thin tubes, closed at the free end, and are capable of absorbing water through their surface. The length of root hairs varies. Some are quite small. Others may be as long as ¼ inch.
            Root hairs do not live very long, even on a plant which is not moved or disturbed. Each root hair works well for only a few days or weeks. Then it is no longer of any use to the plant. It shrivels and dies. But new hairs are always being formed by the plant as it grows. In this way there are enough root hairs to keep a good supply of water moving into the plant.
            Some plants have roots that grow deep down into the soil. The roots of other plants are near the surface. There are examples of both kinds of roots among the many plants that grow in deserts. Desert soil usually contains very little water. Some desert plants have long roots, which reach down to water that may be deep underground. The roots sometimes grow to be 40 feet long. Many cacti have roots that spread out just below the surface of the soil. These plants can get water quickly when showers come.
            In addition to water there are mineral elements which enter plants through their root systems. They are found in good topsoil and are necessary to plants for proper growth. Without them a plant may develop so poorly that it will die.
            There are seven mineral elements that most scientists consider very important. These are potassium, calcium, nitrogen, iron, sulfur, phosphorus, and magnesium. There are other minerals that may be as important in the growth of healthy plants, but plants need only very small amounts of each.
            Each mineral is important to a plant in a particular way. For example, there is magnesium in chlorophyll. Chlorophyll is a chemical in the leaves of plants which they use in manufacturing food. Without magnesium a plant would have no chlorophyll and would not be able to make food. So a decrease in magnesium would mean a decrease in the amount of energy a plant has for growth.
            Some of the minerals in soil dissolve in water. They are taken into plants with the water which the roots absorb. But plants take some minerals directly from soil. Their roots are able to dissolve minerals and take them in through the root surfaces.
            The mineral elements found in soil are highly necessary to plants. But plants can be grown without soil if water and the necessary elements are supplied. This kind of agriculture is called hydroponics.
            Water passes through the root hairs and moves up through the plant. You might say that there is a stream of water moving through the plant from roots to leaves. In the leaves of the plant some of the water is used in the manufacture of food. Much of the rest of the water is given off by the leaves.
            In late summer or early autumn many trees lose all their leaves. These trees are called deciduous. Cottonwoods, maples, elms, and birches are deciduous trees. Deciduous trees give off much water through their leaves. When there is little water that trees can use in the ground, the trees may lose too much water through their leaves.
            Autumn is the season of the year when the days begin to grow shorter than the nights. There is less light and heat energy. When less of the sun’s energy reaches a region of the earth, the ground grows cold, and there is less water in the soil for trees to use. As times of lessened heat and light energy approach, the leaves of deciduous trees are dropped. A thin separation layer grows at the base of each leafstalk. It develops between the stalk of the leaf and the branch on which the leaf is growing. As soon as the separation layer is completely formed, it cuts off the supply of water, and the leaf drops. There is no opening where each leaf has been. A layer of tissue grows over the spot where the leaf was attached to the branch. This spot is called a leaf scar. Certain kinds of oak trees and other plants keep some of their dead leaves most of the winter. On those plants the separation layers may not have completely formed. Sometimes the veins which go from the twigs through the stems of the leaves may not have broken. The leaves usually fall off by spring.
            Some trees stay green as long as they live. They never lose all their leaves at once. The leaves of some evergreen trees live for three, four, or more years. Some new leaves grow each year, and each year some of the older leaves drop off. It is possible for evergreen plants to lose some of their leaves at any time during the year because the separation layers may form at any time. The leaves of some evergreen plants fall at regular times during the year. The white pine sheds many of its two-year-old or three-year-old leaves in late autumn. But there are always some leaves left on the plant. Many evergreen plants have needle-like leaves. These plants lose less water than do most plants with broad leaves. A live oak is an evergreen which has quite different leaves. Its leaves may be several inches long and up to 3 inches wide. They are not at all like pine needles. These leaves stay on the tree a year or longer. Then they turn brown and fall off. New leaves have already come out. A live oak has green leaves on it all year. Live oaks do not grow where the winters are severe.
            The surface from which water can go into the air is less in desert plants than in the large leaves of trees such as maples and oaks. The outside covering of the leaves of desert plants is harder and thicker than the covering of maple and oak leaves. These things help to make it possible for plants to live where there is very little water.
            A plant called an ocotillo grows in some parts of the southwestern desert of North America. This plant has produced new leaves as many as eight times a year. When no water is available, the leaves turn brown and drop off.
            Many desert plants have small, stiff leaves. The leaves of the creosote bush are hard and look as though covered with shellac. The leaves of some plants look waxy; others are covered with fine, short hairs. On some plants spines take the place of leaves. Water does not evaporate quickly from leaves like these.

Plants Use Energy

            Energy from the sun provides light and heat through the year. Plants do not receive the same amount of energy from season to season. But we have plants in our gardens and fields year after year. Plants are able to make use of changing amounts of energy. The hours of sunlight and the temperature make a great deal of difference to plants. Even the water that plants need for growth depends on the energy supply. Plants frequently need different amounts of energy to develop. All plants and all parts of plants are affected by changing amounts of energy. Plants have many different ways of making use of changes in light, heat, and moisture.
            All green plants need light energy. When sunlight falls on their leaves, these plants can make their own food. Water with minerals dissolved in it is absorbed from the soil through the plant roots. The water passes through the stems to the leaves. Carbon dioxide from the air enters the leaves of the plant. The water and carbon dioxide are changed into plant material by means of green coloring matter called chlorophyll. This change could not occur if there were not light energy striking the leaves. The energy of the sun falling on the leaves of the plant makes the chemical change possible. The carbon dioxide and water are changed into a new chemical material, and the light energy is changed into chemical energy. The chemical energy now is a part of the plant. If you eat the plant for food, the chemical energy of the plant will be used by you as muscle energy. You eat food so that you will have energy to play and work. Hunger is a sign that your energy supply is low. You cannot change light energy to chemical energy. Plants can do something that you can’t do.
            There are many plants which grow with little or no sunlight. These are the non-green plants, such as toadstools, mushrooms, puff balls, and other fungi. The non-green plants too use food to supply energy for growth. But these plants are not able to manufacture their own food. They must get food ready-made. Some non-green plants attach themselves to green plants. They draw their food from inside the green plants. Others draw their food from the decayed parts of green plants and animals in the soil. Plants use light energy directly or indirectly. Green plants use sunlight to manufacture food for growth. Non-green plants use the chemical energy of green plants in order to grow.
            The amount of light energy which is received by a region has a great deal to do with the kinds of plants able to grow there. The northern regions have more hours of sunlight each day during the growing season. This makes it possible for many plants to grow well in summer in the northern part of the United States and in Canada. From 50 to 60 days after some plants begin growing, they may be ready to harvest.
            Large crops of hay, wheat, and potatoes and other vegetables grow in some parts of Alaska. The many hours of sunshine each day during the summer help these plants to grow quickly. Harvest time is over before the frosts of autumn begin. In the northern part of the Central Valley of California, oranges ripen faster than they do in southern California. In fact, the oranges are ready for market several weeks earlier than they are 400 miles farther south. This is partly due to the longer period of sunshine each day during the growing season.
            Scientists have experimented with plants to find out how the length of day affects them. They found that some plants produce flowers and fruit only when the days are long. They are called long-day plants. In order to bloom, plants such as lettuce, radishes, and red clover need more than twelve hours of sunlight each day. Other plants bloom only when the days are shorter, when there are fewer hours of sunlight. They are called short-day plants. Tobacco, soybeans, and ragweed are some plants that are short-day plants. Many plants are neither long-day nor short-day plants. They will bloom when there are many hours of sunlight each day and also when there are few. People who study plants say that most plants probably belong to this group.
            Scientists tell us that many plants can stand cold winter temperatures, but during that time they do not grow. In arctic regions some plants live through temperatures far below freezing. Some plants can also stay alive when the temperature of the air becomes as high as 122° F. However, there is a certain temperature at which each kind of plant grows best.
            Oats, melons, and apple trees need quite different temperatures in order to grow and develop seed. Winter wheat grows best where it can begin growing in the cool days of autumn. It lives through a moderately cold winter. Then it will develop, produce seeds, and ripen in early summer before the heat of midsummer arrives.
            Sometimes the temperature of the air changes very quickly. If this happens, plants may be injured. Plants may also be injured if the temperature rises very quickly.

Plants Continue on Earth

            All the plants which man learned to cultivate and care for existed originally as wild plants. Certain plants grow, blossom, make seeds, and die within a short time. It is possible to grow these plants year after year because each plant makes many seeds. Inside each seed there is a young plant. The young plant can stay alive inside the seed for a long time. Each seed will grow into a new plant when the right amounts of moisture, heat, and light are provided. Not all seeds are gathered and kept to be planted. Often many seeds fall to the ground. Sometimes these seeds are able to live through the cold of winter. The following spring they will start to grow.
            Some plants make seeds and store food in them. Some also store food in their underground parts. The first year, the young plants grow from seeds and develop leaves above the ground. Plants make food during the time when there is plenty of heat and light energy. This food is stored during the growing season in the underground parts of the plants. When cold weather comes, the parts of the plants that are above the ground may die. The underground parts remain alive, but in a resting stage. In this way the plants can live through the winter. The next year, when there is enough warmth, water, and light, the plants begin to grow again. Later, flowers develop and seeds form. Even if the plants die, new plants will grow from the seeds.
            Certain plants store some of the food made during the growing season in underground parts called bulbs. Onions live through the winter months as bulbs. When the growing season comes again in the spring, the onion bulbs send up stalks on which may grow flowers and then seeds.
            Plants, such as trees, that live on year after year also store food in their stems, but their stems are not underground. There is much plant food stored in the trunks of trees. Water is also stored by plants. Some evergreens hold water in their stems, or trunks, which the plants use when little water is available from the soil. Many cacti are able to store water to be used when there is no rain. Some have water stored in their stems. There are other plants whose leaves are thick and protected by a tough covering. Water can be stored in these leaves.
            Plants that go into a resting stage must have some way of starting to grow again after the winter is over. During warm weather these plants make use of some of their energy to form buds. Many deciduous trees and shrubs look as if they were dead during the winter, when their branches are brown and bare. However, the branches and roots of these plants stay alive all winter. When the first warm days of spring arrive, tiny green leaves and colored flowers appear on the branches. The buds on most trees are covered by scales which protect the leaves and flowers inside the buds from very cold weather. On some plants, the bud scales are covered with fine hairs or a waxy substance. This covering protects the buds from losing too much water through evaporation.
            Some plants that live year after year do not have woody parts as trees and shrubs do. Even plants that are not woody have buds which live through the winter. These are not the buds which you see above the ground. You do not usually see these buds for they are underground. On most of these plants the parts above the ground die when the cold days come. To live through the winter, the buds must be formed on the parts of the plants which are protected from the cold. The underground parts of these plants are not all alike. However, they all live through the winter in a resting stage. They are called rootstocks, corms, bulbs, tubers, or roots. They are protected against the cold air by the soil. Dry grass and leaves which cover the ground also help to protect them. Food is stored in these underground parts. The buds are formed on the rootstocks or bulbs or tubers or other underground parts of the plants. These buds allow the plants to grow quickly when they again receive enough heat and light energy and water.

Climates Change the Way Plants Live

            In most places many different kinds of plants are in bloom during the summer, in desert regions, however, there is often so much heat and so little rainfall that few flowers may be seen during the summer months. Some of the flowers and vegetables that grow well in northern regions will not grow at all where the weather is mild all year long.
            The words annual, biennial, and perennial describe plants that live one, two, or more years. Some plants, such as tulips and hyacinths, which are perennials in the north, must be treated as biennials in the south. New bulbs must be planted every second year in order to have these flowers. Still other plants that are annuals in the north are able to live on year after year in warm regions. This is true of tomatoes. In cool parts of the country these plants are annuals. They may become perennials in places which receive great amounts of energy from the sun throughout the seasons.
            The southernmost part of the United States receives more energy from the sun all year long than places farther north do. This means that in some places there may be no frost at all during the year. Plants growing here have a long growing season and a short rest period.
            Some kinds of perennials grow well where there is a long rest period each year. But they cannot live through a year-long growing season with hardly any rest at all. Lilacs and peonies are two perennials that do not grow well, if at all, where the weather is warm the year round.
            The amounts of heat, light, and water that plants receive vary a great deal. They vary from one part of the earth to another and from season to season. Changing amounts of energy greatly affect the way that plants grow.

Plant cells and Growth

            A plant cell is made up of many things. Most of the cell is made up of a jellylike matter. This jellylike matter is living. It grows and uses food like other living things do. The most important part of the jellylike matter is the nucleus. The nucleus controls the growth of the cell. Small green dots in the plant cell hold the chlorophyll. Chlorophyll is what makes green plants green. A plant cell has a thin, living cover. This cell cover lets things like water and oxygen pass in and out of the cell. Just outside this cover is another cover called the cell wall. It also lets things pass in and out of the cell. The cell wall is thick. It gives the cell its shape.
            Each cell grows bigger. So the plant grows bigger. Then, each cell divides into two new cells. These new cells then grow and divide. Each new cell keeps growing and dividing into new cells.
            Another thing some cells of green plants do is make food. The food the plant makes is a kind of sugar. Plant cells need chlorophyll to make this sugar. And chlorophyll must have light to work. When plant cells make food, they use water that the roots take in. they also use carbon dioxide, a gas found in air.
            Plant cells can also store the food they make. For example, carrots store food in the cells of their roots. Other plants, like cabbages, store food in the cells of their leaves. The parts of these plants that have stored food are the parts people eat.