Chapter 24, Problem 17STP

Summary Introduction

To write:

Egg development and fertilization in flowering plants.

Introduction:

Anthophytes are widespread and diverse group of plants and are adapted to live in terrestrial as well as aquatic environments. They are flower bearing plants. They have distinct life cycles and exhibit alternation of generations. The sporophyte generation is dominant and supports the gametophyte generation.

Explanation of Solution

In most flowering plants, the megaspore's nucleus undergoes mitosis three times, producing eight haploid nuclei. These eight nuclei make up the female gametophyte. Three of the nuclei moves to one end and develop into the antipodal cells which eventually degenerates. The nucleus nearest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells. The two remaining nuclei known as polar nuclei are enclosed in a single cell known as central cell. It is located in the centre of the gametophyte. The synergids help to guide the pollen tube during fertilization and eventually disintegrates. The central cell containing the polar nuclei fuses with a sperm to form the triploid nucleus that divide many times and eventually forms the endosperm.

There are two haploid nuclei, a tube nucleus and generative nucleus inside each pollen grain. The tube nucleus is involved in the growth of the nucleus down the pistil to the ovary. The generative nucleus produces two sperm nuclei by mitosis. The sperm nuclei move to the ovary through the micropyle. One sperm fuses with the egg forming the zygote, while the other sperm and the polar nuclei fuse to form a triploid cell.

Conclusion

The female gametophyte has eight nuclei out of which one forms the egg cell and two nuclei forms the polar nuclei. One of the two sperms fuses with the egg cell to form the zygote and the other sperm and the polar nuclei fuse to form the triploid cell that divide many times and eventually forms the endosperm.