recap

37.1 recap

Flowers contain the organs for sexual reproduction in angiosperms. Plants that use pollen for reproduction have several selective advantages. After fertilization, the flower develops into seed(s) and fruit. The selective advantages of seeds and fruits include long-term viability and multiple modes of dispersal.

learning outcomes

You should be able to:

  • Compare the processes of male and female gamete formation.

  • Describe the mechanisms that guide the growth of a pollen tube.

  • Describe and compare two methods for preventing self-fertilization in angiosperms.

  • Analyze the relationship between the diversity of fruits and their ability to disperse seeds.

  • Relate fruit development to seed development.

Question 1

How would plant reproduction be affected if the pollen tube failed to grow and develop properly?

The pollen tube is required to deliver the sperm cell from the stigma of the plant to the egg nucleus in the embryo sac, which is located in the ovary at the bottom of the style. Failure of the pollen tube to grow properly and reach the egg nucleus will result in failure of reproduction, including failure of seed and fruit formation.

Question 2

How do the final products of meiosis in megagametophytes and microgametophytes differ, and how are these differences important to sexual reproduction?

In the megagametophyte, one of the original nuclei becomes an egg cell; the remainder form the protective and nutritive embryo sac. In the microgametophyte, the eight original cells from meiosis become four two-celled pollen grains, each containing a sperm cell as well as a tube cell, which forms the pollen tube. The female structure is larger and contains nutrients for the developing embryo. The pollen grains are tiny and numerous, making movement easier and pollination more likely.

Question 3

Thompson Seedless grapes are produced from triploid plants. Explain why these grapes are seedless. Describe the role played by the flower in fruit formation when no seeds are being formed.

In triploid cells undergoing meiosis, there cannot be pairing of homologous chromosomes in meiosis I, so meiosis I is abnormal and functional gametes do not form. A fruit is formed from the ovary wall of the flower. Seedless grapes are probably propagated by cuttings (vegetative reproduction).

Question 4

What is the selective advantage for the development of many diverse types of fruits?

All fruits protect the seed and aid in seed dispersal, but the variety of fruits means they can fulfill these functions in many different ways, depending on locations and environmental factors. Some seeds, such as many tree seeds, fall to the ground and remain near their parent. Others are much smaller and have structures (wings, burs) that enable their dispersal by wind or animals. In this way, they can move long distances, increasing their range and chances of survival.

793

We have now traced the sexual life cycle of angiosperms from the flower to the gametophytes, pollination, fertilization, and the dispersal of seeds. We discussed seed germination and seedling development in Chapter 36, and vegetative plant growth in Chapter 33. Next we look at the rest of the angiosperm life cycle—the transition from the vegetative to the flowering state—and how this transition is regulated.