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24.1 Reproduction in Flowering Plants

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1 24.1 Reproduction in Flowering Plants
Lesson Overview 24.1 Reproduction in Flowering Plants

2 The Structure of Flowers
Flowers are reproductive organs that are composed of four different kinds of specialized leaves: sepals, petals, stamens, and carpels. This diagram shows the parts of a typical angiosperm flower.

3 Sepals and Petals The outermost circle of floral parts contains the sepals. Sepals enclose the bud before it opens, and they protect the flower while it is developing.

4 Sepals and Petals Petals, which are often brightly colored, are found just inside the sepals. The colors, number, and shapes of such petals attract insects and other pollinators to the flower.

5 Stamens The stamens are the male parts of the flower—each stamen consists of a stalk called a filament with an anther at its tip. Anthers are the structures in which pollen grains—the male gametophytes—are produced.

6 Carpels The innermost floral parts are the carpels, which produce and shelter the female gametophytes and, later, seeds. Each carpel has a broad base forming an ovary, which contains one or more ovules where female gametophytes are produced.

7 Carpels The diameter of the carpel narrows into a stalk called the style. At the top of the style is a sticky or feathery portion known as the stigma, which is specialized to capture pollen.

8 Carpels Botanists sometimes call a single carpel or several fused carpels a pistil.

9 The Angiosperm Life Cycle
Angiosperms have a life cycle that shows an alternation of generations between a diploid sporophyte phase and a haploid gametophyte stage. Male and female gametophytes live within the tissues of the sporophyte.

10 Development of Male Gametophytes
The male gametophytes—the pollen grains—develop inside anthers. First, meiosis produces four haploid spore cells.

11 Development of Male Gametophytes
Each spore undergoes one mitotic division to produce the two haploid nuclei of a single pollen grain. The two nuclei are surrounded by a thick wall that protects the male gametophyte.

12 Development of Female Gametophytes
Female gametophytes develop inside each carpel of a flower. The ovules—the future seeds—are enveloped in a protective ovary—the future fruit.

13 Development of Female Gametophytes
A single diploid cell goes through meiosis to produce four haploid cells, three of which disintegrate.

14 Development of Female Gametophytes
The remaining cell undergoes mitosis, producing eight nuclei. These eight nuclei and the surrounding membrane are called the embryo sac. The embryo sac, contained within the ovule, makes up the female gametophyte of a flowering plant.

15 Development of Female Gametophytes
Cell walls form around six of the eight nuclei. One of the eight nuclei, near the base of the gametophyte, is the nucleus of the egg—the female gamete. If fertilization takes place, this egg cell will fuse with the male gamete to become the zygote that grows into a new sporophyte plant.

16 Pollination Pollination is the transfer of pollen to the female portions of the flower. Some angiosperms are wind pollinated, but most are pollinated by animals. Because wind pollination is less efficient than animal pollination, wind-pollinated plants, such as oak trees, rely on favorable weather and sheer numbers of pollen grains to get pollen from one plant to another.

17 Pollination Animal-pollinated plants have a variety of adaptations, such as bright colors and sweet nectar, to attract and reward animals. Animals have evolved body shapes that enable them to reach nectar deep within certain flowers.

18 Pollination Insect pollination is beneficial to insects and other animals because it provides a dependable source of food—pollen and nectar. Plants benefit because the insects take the pollen directly from flower to flower. Insect pollination is more efficient than wind pollination, giving insect-pollinated plants a greater chance of reproductive success.

19 Fertilization If a pollen grain lands on the stigma of a flower of the same species, it begins to grow a pollen tube. Of the pollen grain’s two cells, one cell—the “generative” cell—divides and forms two sperm cells. The other cell becomes the pollen tube.

20 Fertilization The pollen tube contains a tube nucleus and the two sperm cells. The pollen tube grows into the style, where it eventually reaches the ovary and enters an ovule.

21 Fertilization Inside the embryo sac, two distinct fertilizations take place—a process called double fertilization. First, one of the sperm nuclei fuses with the egg nucleus to produce a diploid zygote, which will grow into the new plant embryo.

22 Fertilization Second, the other sperm nucleus fuses with two polar nuclei in the embryo sac to form a triploid (3N) cell. This cell will grow into a food-rich tissue known as endosperm, which nourishes the seedling as it grows.

23 Fertilization By using endosperm to store food, the flowering plant spends very little in the way of food resources on producing seeds from ovules until double fertilization has actually taken place. The resources saved can be used to make many more seeds. The endosperm and embryo of a corn seed are shown.

24 Vegetative Reproduction
Many flowering plants can reproduce asexually. This process, known as vegetative reproduction, enables a single plant to produce offspring genetically identical to itself by mitosis. It does not require gametes, flowers, or fertilization. This process takes place naturally in many plants.

25 Types of Vegetative Reproduction
New plants may grow from roots, leaves, stems, or plantlets. A potato is an underground stem that can grow whole new plants from buds, called “eyes.” Because vegetative reproduction does not involve pollination or seed formation, a single plant can reproduce quickly. Strawberry plants send out long, trailing stems called stolons. Nodes that rest on the ground produce roots and upright stems and leaves. Many cactus species can reproduce by dropping sections of their stems. The small individuals growing at the base of the larger adults are clones

26 Types of Vegetative Reproduction
Asexual reproduction allows a single plant to produce genetically identical offspring, enabling well-adapted individuals to rapidly fill a favorable environment. One drawback of asexual reproduction is that it does not produce new combinations of genetic traits, which may be valuable if conditions in the physical environment change.

27 Plant Propagation To propagate plants with desirable characteristics, horticulturists use cuttings or grafting (shown) to make many identical copies of a plant or to produce offspring from seedless plants.

28 Plant Propagation One of the simplest ways to reproduce plants vegetatively is by cuttings. A grower cuts from the plant a length of stem that includes a number of buds containing meristem tissue. That stem is then partially buried in soil or in a special mixture of nutrients that encourages root formation. Grafting is a method of propagation used to reproduce seedless plants and varieties of woody plants that cannot be propagated from cuttings. To graft, a piece of stem or a lateral bud is cut from the parent plant and attached to another plant, as shown.

29 Plant Propagation Grafting is a method of propagation used to reproduce seedless plants and varieties of woody plants that cannot be propagated from cuttings. To graft, a piece of stem or a lateral bud is cut from the parent plant and attached to another plant, as shown.

30 Plant Propagation Grafting works only when the two plants are closely related, such as when a bud from a lemon tree is grafted onto an orange tree. Grafting usually works best when plants are dormant, which allows the wounds created by the cut to heal before new growth starts.

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