Roots. Most roots grow underground. As the roots of a young plant spread, they absorb the water and minerals that the plant needs to grow. The roots also anchor the plant in the soil. In addition, the roots of some plants store food for the rest of the plant to use. Plants with storage-type roots include beets, carrots, radishes, and sweet potatoes. There are two main kinds of root systems-fibrous and taproot. Grass is an example of a plant with a fibrous root system.
It has many slender roots of about the same size that spread out in all directions. A plant with a taproot system has one root that is larger than the rest. Carrots and radishes have taproots. Taproots grow straight down, some as deep as 15 feet (4.6 meters).
The root is one of the first parts of a plant that starts to grow. A primary root develops from a plant's seed and quickly produces branches called secondary roots. At the tip of each root is a root cap that protects the delicate tip as it pushes through the soil. Threadlike root hairs grow farther back on the root of the plant. Few of these structures are over 1/2 inch (13 millimeters) long. But there are so many of them that they greatly increase the plant's ability to absorb water and minerals from the soil. The roots of some aquatic plants float freely in the water. Other plants, such as orchids and some vines, have roots that attach themselves to tree branches. The roots of almost all land plants have a special relationship with fungi. In this relationship, known as mycorrhiza, fungi cover or penetrate the growing tips of a plant's roots. Water and nutrients enter the roots through the fungi. Fungi extend the plant's root system and improve the plant's ability to absorb water and minerals. Many botanists believe the first land plants developed millions of years ago from algae that lived in water. They think mycorrhizal relationships may have helped these plants to grow on land.
Stems of plants differ greatly among various species. They make up the largest parts of some kinds of plants. For example, the trunk, branches, and twigs of trees are all stems. Other plants, such as cabbage and lettuce, have such short stems and large leaves that they appear to have no stems at all. The stems of still other plants, including potatoes, grow partly underground. Most stems grow upright and support the leaves and reproductive organs of plants. The stems hold these parts up in the air where they can receive sunlight. Some stems grow along the ground or underground. Stems that grow aboveground are called aerial stems, and those underground are known as subterranean. Aerial stems are either woodyor herbaceous (nonwoody). Plants with woody stems include trees and shrubs. These plants are rigid because they contain large amounts of woody xylem tissue. Most herbaceous stems are soft and green because they contain only small amounts of xylem tissue.
In almost all plants, a stem grows in length from the end, called the apex. The cells that form this growth area are called the apical meristem. An apical meristem produces a column of new cells behind itself. These cells develop into the specialized tissues of the stem and leaves. A resting apical meristem and the cluster of developing leaves that surround it is called a bud. Buds may grow on various parts of the stem. A terminal bud is found at the end of a branch. A lateral bud develops at a point where a leaf joins the stem. This point is called a node. Buds may develop into new branches, leaves, or flowers. Some buds are covered with tiny overlapping leaves called bud scales. The bud scales protect the soft, growing tissue of the apical meristem. During the winter, the buds of many plants are dormant (inactive) and can be seen easily. In the spring, these buds resume their growth.
Leaves make most of the food that plants need to live and grow. They produce food by a process called photosynthesis. In photosynthesis, chlorophyll in the leaves absorbs light energy from the sun. This energy is used to combine water and minerals from the soil with carbon dioxide from the air. The food formed by this process is used for growth and repair, or it is stored in special areas in the stems or roots.
In almost all plants, a stem grows in length from the end, called the apex. The cells that form this growth area are called the apical meristem. An apical meristem produces a column of new cells behind itself. These cells develop into the specialized tissues of the stem and leaves. A resting apical meristem and the cluster of developing leaves that surround it is called a bud. Buds may grow on various parts of the stem. A terminal bud is found at the end of a branch. A lateral bud develops at a point where a leaf joins the stem. This point is called a node. Buds may develop into new branches, leaves, or flowers. Some buds are covered with tiny overlapping leaves called bud scales. The bud scales protect the soft, growing tissue of the apical meristem. During the winter, the buds of many plants are dormant (inactive) and can be seen easily. In the spring, these buds resume their growth.
Leaves make most of the food that plants need to live and grow. They produce food by a process called photosynthesis. In photosynthesis, chlorophyll in the leaves absorbs light energy from the sun. This energy is used to combine water and minerals from the soil with carbon dioxide from the air. The food formed by this process is used for growth and repair, or it is stored in special areas in the stems or roots.
Flowers contain the reproductive parts of flowering plants. Flowers develop from buds along the stem of a plant. Some kinds of plants produce only one flower, but others grow many large clusters of flowers. Still others, such as dandelions and daisies, have many tiny flowers that form a single, flowerlike head. Most flowers have four main parts: (1) the calyx, (2) the corolla, (3) the stamens, and (4) the pistils. The flower parts are attached to a place on the stem called the receptacle.
Seeds vary greatly in size and shape. Some seeds, such as those of the tobacco plant, are so small that more than 2,500 may grow in a pod less than 3/4 inch (19 millimeters) long. On the other hand, the seeds of one kind of coconut tree may weigh more than 20 pounds (9 kilograms). The size of a seed has nothing to do with the size of the plant. For example, huge redwood trees grow from seeds that measure only 1/16 inch (1.6 millimeters) long. There are two main types of seeds-naked and enclosed. Cone-bearing plants and all other nonflowering seed plants have naked, or uncovered, seeds. The seeds of these plants develop on the upper side of the scales that form their cones. All flowering plants have seeds enclosed by an ovary. The ovary develops into a fruit as the seeds mature. The ovaries of such plants as apples, berries, and grapes develop into a fleshy fruit. In other plants, including beans and peas, the ovaries form a dry fruit.
Still other plants have aggregate fruits. Each tiny section of an aggregate fruit, such as a raspberry, develops from a separate ovary and has its own seed. Seeds consist of three main parts: (1) the seed coat, (2) the embryo, and (3) the food storage tissue. The seed coat, or outer skin, protects the embryo, which contains all the parts needed to form a new plant. The embryo also contains one or more cotyledons, or embryo leaves, which absorb food from the food storage tissue. In flowering plants, the food storage tissue is called endosperm. In some plants, such as peas and beans, the embryo absorbs the endosperm, and food is stored in the cotyledons. In nonflowering seed plants, a tissue called the megagametophyte serves as a place to store food.
