Plants create more of their own kind by either sexual reproduction or asexual reproduction. In sexual reproduction, a male sperm cell joins with a female egg cell to produce a new plant. Both the egg and the sperm cells contain genes (hereditary material). Genes determine many of the characteristics of a plant. A plant that is produced by sexual reproduction inherits genes from both parent plants. It is a unique individual and has traits that may be different from either parent. Asexual reproduction can occur in many ways. It often involves the division of one plant into one or more parts that become new plants. These plants inherit genes from only one parent and have exactly the same characteristics as the parent plant. This type of asexual reproduction is called vegetative propagation. Many plants reproduce both sexually and by vegetative propagation.
Sexual reproduction. Sexual reproduction in plants occurs as a complex cycle called alternation of generations. It involves two distinct generations or phases. During one phase of the life cycle, the plant is called a gametophyte, or gamete-bearing plant. In most species of plants, the gametophyte is barely visible and is rarely noticed by people. It produces gametes-that is, the sperm and egg cells. It may produce sperm cells or egg cells, or both, depending on the species of plant.
When the sperm and egg cells unite, the fertilized egg develops into the second phase of the plant's life cycle.In this phase, the plant is called a sporophyte or spore-bearing plant. When people see a plant it is most often the sporophyte phase. Sporophytes produce tiny structures called spores through a process of cell division called meiosis. The spores form in closed capsulelike structures called sporangia. Gametophytes develop from the spores, and the life cycle begins again.
In seed plants, which include flowering and cone-bearing plants, alternation of generations involves a series of complicated steps. Among these plants, only the sporophyte generation can be seen with the unaided eye. Spores are produced in the male and female reproductive organs of a plant. The spores grow into gametophytes, which remain inside the plant's reproductive organs.
In flowering plants, the reproductive parts are in the flowers. A plant's stamens are its male reproductive organs. Each stamen has an enlarged tip called an anther. The pistil is the plant's female reproductive organ. The ovary, which forms the round base of the pistil, contains the ovules. The anthers consist of structures called microsporangia, and the ovules contain structures called megasporangia. Cell divisions in the microsporangia and the megasporangia result in the production of spores.
In most species of flowering plants, one spore in each ovule grows into a microscopic female gametophyte. The female gametophyte produces one egg cell. In the anther, the spores, called pollen grains, contain microscopic male gametophytes. Each pollen grain produces two sperm cells. For fertilization to take place, a pollen grain must be transferred from the anther to the pistil. This transfer is called pollination. If pollen from a flower reaches a pistil of the same flower, or a pistil of another flower on the same plant, the fertilization process is called self-pollination. When pollen from a flower reaches a pistil of another plant, the fertilization process is called cross-pollination. In cross-pollinated plants, the pollen grains are carried from flower to flower by such animals as birds and insects, or by the wind. Many cross-pollinated plants have large flowers, a sweet scent, and sweet nectar. These features attract hummingbirds and such insects as ants, bees, beetles, butterflies, and moths. As these animals move from flower to flower in search of food, they carry pollen on their bodies. Most grasses and many trees and shrubs have small, inconspicuous flowers. The wind carries their pollen. It may carry pollen as far as 100 miles (160 kilometers). Some airborne pollen causes hay fever and other allergies.
If a pollen grain reaches the pistil of a plant of the same species, a pollen tube grows down through the stigma and the style to an ovule in the ovary. In the ovule, one of the two sperm cells from the pollen grain unites with the egg cell. A sporophyte embryo then begins to form. The second sperm cell unites with two structures called polar nuclei and starts to form the nutrient tissue that makes up the endosperm. Next, a seed coat forms around the embryo and the endosperm.
SEED. In conifers, the reproductive parts are in the cones. A conifer has two kinds of cones. The pollen, or male, cone is the smaller and softer of the two. It also is simpler in structure. Seed, or female, cones are larger and harder than the male cones. A pollen cone has many tiny sporangia that produce pollen grains. Each of the scales that make up a seed cone has two ovules on its surface. Every ovule produces a spore that grows into a female gametophyte. This tiny plant produces egg cells. The wind carries pollen grains from the pollen cone to the seed cone. A pollen grain sticks to an adhesive substance near an ovule. It usually enters the pollen chamber of the ovule through an opening called the micropyle. The pollen grain then begins to form a pollen tube. Two sperm cells develop in the tube. After the pollen tube reaches the egg cell, one of the sperm cells fertilizes the egg. The second sperm cell disintegrates. The fertilized egg develops into a sporophyte embryo, and the ovule containing the embryo becomes a seed. The seed falls to the ground and, if conditions are favorable, a new sporophyte begins to grow.
In seed plants, which include flowering and cone-bearing plants, alternation of generations involves a series of complicated steps. Among these plants, only the sporophyte generation can be seen with the unaided eye. Spores are produced in the male and female reproductive organs of a plant. The spores grow into gametophytes, which remain inside the plant's reproductive organs.
In flowering plants, the reproductive parts are in the flowers. A plant's stamens are its male reproductive organs. Each stamen has an enlarged tip called an anther. The pistil is the plant's female reproductive organ. The ovary, which forms the round base of the pistil, contains the ovules. The anthers consist of structures called microsporangia, and the ovules contain structures called megasporangia. Cell divisions in the microsporangia and the megasporangia result in the production of spores.
In most species of flowering plants, one spore in each ovule grows into a microscopic female gametophyte. The female gametophyte produces one egg cell. In the anther, the spores, called pollen grains, contain microscopic male gametophytes. Each pollen grain produces two sperm cells. For fertilization to take place, a pollen grain must be transferred from the anther to the pistil. This transfer is called pollination. If pollen from a flower reaches a pistil of the same flower, or a pistil of another flower on the same plant, the fertilization process is called self-pollination. When pollen from a flower reaches a pistil of another plant, the fertilization process is called cross-pollination. In cross-pollinated plants, the pollen grains are carried from flower to flower by such animals as birds and insects, or by the wind. Many cross-pollinated plants have large flowers, a sweet scent, and sweet nectar. These features attract hummingbirds and such insects as ants, bees, beetles, butterflies, and moths. As these animals move from flower to flower in search of food, they carry pollen on their bodies. Most grasses and many trees and shrubs have small, inconspicuous flowers. The wind carries their pollen. It may carry pollen as far as 100 miles (160 kilometers). Some airborne pollen causes hay fever and other allergies.
If a pollen grain reaches the pistil of a plant of the same species, a pollen tube grows down through the stigma and the style to an ovule in the ovary. In the ovule, one of the two sperm cells from the pollen grain unites with the egg cell. A sporophyte embryo then begins to form. The second sperm cell unites with two structures called polar nuclei and starts to form the nutrient tissue that makes up the endosperm. Next, a seed coat forms around the embryo and the endosperm.
SEED. In conifers, the reproductive parts are in the cones. A conifer has two kinds of cones. The pollen, or male, cone is the smaller and softer of the two. It also is simpler in structure. Seed, or female, cones are larger and harder than the male cones. A pollen cone has many tiny sporangia that produce pollen grains. Each of the scales that make up a seed cone has two ovules on its surface. Every ovule produces a spore that grows into a female gametophyte. This tiny plant produces egg cells. The wind carries pollen grains from the pollen cone to the seed cone. A pollen grain sticks to an adhesive substance near an ovule. It usually enters the pollen chamber of the ovule through an opening called the micropyle. The pollen grain then begins to form a pollen tube. Two sperm cells develop in the tube. After the pollen tube reaches the egg cell, one of the sperm cells fertilizes the egg. The second sperm cell disintegrates. The fertilized egg develops into a sporophyte embryo, and the ovule containing the embryo becomes a seed. The seed falls to the ground and, if conditions are favorable, a new sporophyte begins to grow.
In ferns and mosses, the sporophyte and gametophyte generations consist of two greatly different plants. Among ferns, the sporophytes have leaves and are much larger than the gametophytes. Clusters of sporangia called sori form on the edges or underside of each leaf. Spores develop in the sporangia. After the spores ripen, they fall to the ground and grow into barely visible, heart-shaped gametophytes. A fern gametophyte produces both male and female sex cells. If enough moisture is present, a sperm cell swims to an egg cell and unites with it. The fertilized egg then grows into an adult sporophyte. Among mosses, a sporophyte consists of a long, erect stalk with a podlike spore-producing container at the end. The sporophyte extends from the top of a soft, leafy, green gametophyte. It depends on the gametophyte for food and water. The gametophyte is the part of the plant community recognized as moss.
Vegetative propagation. Plants can spread without sexual reproduction. Through vegetative propagation, a part of a plant may grow into a complete new plant. Vegetative propagation can take place because the pieces of the plant form the missing parts by a process called regeneration. Any part of a plant-a root, stem, leaf, or flower-may be propagated into a new plant. A plant may even grow from a single cell of another plant. Propagation occurs most often in plants with stems that run horizontally just above or below the ground. The strawberry plant, for example, sends out long, thin stems called runners that grow along the surface of the soil. The runners, at points where they touch the ground, send out roots that produce plantlets (new leaves and stems). These plantlets are actually part of the parent plant. New plants form only when the plantlets are separated from the parent plant. Ferns, irises, many kinds of grasses, blueberries and some other shrubs, and some species of trees propagate from underground stems.
Many plants that grow as weeds are able to spread rapidly by vegetative propagation. These plants are sometimes difficult to kill because they often can regrow their lost parts by regeneration. For example, a dandelion will regrow new stems and leaves even if only part of its roots are left in the soil. Farmers use vegetative propagation to raise many valuable food crops, such as apples, bananas, oranges, and white potatoes. For example, they cut potatoes into many parts, making sure that each part has at least one eye (bud). Each piece of potato will grow into a new potato plant. Propagation by this method produces new potato plants more quickly than do the seeds of a potato plant. Vegetative propagation is also widely used in gardening. Many plants, including gladioli, irises, lilies, and tulips, are propagated from bulbs or corms. These plants take longer to reach the flowering stage when grown from seeds.
People propagate many plants by three chief methods. These methods are: (1) cuttage, (2) grafting, and (3) layering.
Cuttage involves the use of cuttings (parts of plants) taken from growing plants. Most cuttings are stems. When placed in water or moist soil, the majority of cuttings develop roots. The cutting then grows into a complete plant. Many species of garden plants and shrubs are propagated by stem cuttings.
Grafting also involves cuttings. But instead of putting the cutting into water or soil, it is grafted (attached) to another plant, called the stock. The stock provides the root system and lower part of the new plant. The cutting forms the upper part. Farmers use grafting to grow large numbers of some kinds of fruit, including Delicious and Winesap apples. They take cuttings from trees that have grown the type of apples they want and graft them onto apple trees with strong root systems. For a discussion of various methods of grafting.
Vegetative propagation. Plants can spread without sexual reproduction. Through vegetative propagation, a part of a plant may grow into a complete new plant. Vegetative propagation can take place because the pieces of the plant form the missing parts by a process called regeneration. Any part of a plant-a root, stem, leaf, or flower-may be propagated into a new plant. A plant may even grow from a single cell of another plant. Propagation occurs most often in plants with stems that run horizontally just above or below the ground. The strawberry plant, for example, sends out long, thin stems called runners that grow along the surface of the soil. The runners, at points where they touch the ground, send out roots that produce plantlets (new leaves and stems). These plantlets are actually part of the parent plant. New plants form only when the plantlets are separated from the parent plant. Ferns, irises, many kinds of grasses, blueberries and some other shrubs, and some species of trees propagate from underground stems.
Many plants that grow as weeds are able to spread rapidly by vegetative propagation. These plants are sometimes difficult to kill because they often can regrow their lost parts by regeneration. For example, a dandelion will regrow new stems and leaves even if only part of its roots are left in the soil. Farmers use vegetative propagation to raise many valuable food crops, such as apples, bananas, oranges, and white potatoes. For example, they cut potatoes into many parts, making sure that each part has at least one eye (bud). Each piece of potato will grow into a new potato plant. Propagation by this method produces new potato plants more quickly than do the seeds of a potato plant. Vegetative propagation is also widely used in gardening. Many plants, including gladioli, irises, lilies, and tulips, are propagated from bulbs or corms. These plants take longer to reach the flowering stage when grown from seeds.
People propagate many plants by three chief methods. These methods are: (1) cuttage, (2) grafting, and (3) layering.
Cuttage involves the use of cuttings (parts of plants) taken from growing plants. Most cuttings are stems. When placed in water or moist soil, the majority of cuttings develop roots. The cutting then grows into a complete plant. Many species of garden plants and shrubs are propagated by stem cuttings.
Grafting also involves cuttings. But instead of putting the cutting into water or soil, it is grafted (attached) to another plant, called the stock. The stock provides the root system and lower part of the new plant. The cutting forms the upper part. Farmers use grafting to grow large numbers of some kinds of fruit, including Delicious and Winesap apples. They take cuttings from trees that have grown the type of apples they want and graft them onto apple trees with strong root systems. For a discussion of various methods of grafting.
Layering is a method of growing roots for a new plant. In mound layering, soil is piled up around the base of a plant. The presence of the soil causes roots to sprout from the plant's branches. A branch is then cut off and planted. In air layering, a cut about 3 inches (8 centimeters) long is made about halfway through a branch. A type of moss called sphagnum moss is placed in the cut to keep it moist, and this portion of the branch is wrapped in a waterproof covering. New roots form in the area of the cut. After they have sprouted, the branch is cut off and planted.
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