Lesson Overview

1. 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.

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.

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.

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.