Sexual Reproduction

In plants, sexual and asexual reproduction unite in a single cycle called alternation of generations. During alternation of generations, a gametophyte, (a haploid gamete-producing plant), alternates with a sporophyte (a diploid spore-producing plant). In Ectocarpus, a brown aquatic alga, the two generations are equally prominent, whereas in mosses, the gametophyte generation dominates. In ferns and seed plants, the sporophyte dominate, because the sporophyte generation is better adapted to survive on land.

Mosses are small plants that lack vascular tissue and do not produce seeds, and depend on a moist environment to survive. The green leafy ground cover of mosses that we are familiar with is the haploid gametophyte. The gametophyte develops sex organs, a male antheridium and a female archegonium on the same or different plants. The antheridium produces flagellated sperm cells that swim to the egg cells in the archegonium. After fertilization, the zygote grows into a diploid sporophyte. The sporophyte consists of a foot, stalk, and capsule. It remains attached to the gametophyte. Cells in the capsule undergo meiosis and develop into haploid spores. When released, spores grow into gametophytes with rootlike, leaflike and stemlike parts.

Ferns, in the form of the familiar green leafy plants, represent the diploid sporophyte generation. Ferns have a vascular system and true roots, stems, and leaves, but they do not produce seeds. Sporangia, or spore cases, develop on the leaves of ferns, and produce haploid spores by means of meiosis. The spores germinate into haploid green gametophytes. The fern gametophyte is a tiny heart-shaped structure that bears antheridia and archegonia. Flagellated sperm swim to the eggs in a layer of ground water. Although the sporophyte is adapted to land life, this need for water limits the game-tophyte. After fertilization, the diploid zygote develops into the sporophyte.

In flowering plants, the diploid sporophytes are plants with roots, leaves, stems, flowers and seeds. Anthers within the flower contain four sporangia. Cells in the sporangia undergo meiosis and produce haploid microspores. The wall of each microspore thickens, and the haploid nucleus of the microspore divides by mitosis into a generative nucleus and a tube nucleus. These microspores are now called pollen, and each pollen grain is an immature male gametophyte. Pollination occurs when pollen escapes from the anthers and lands on the stigma of a flower, either of the same plant or a different plant. There, a pollen tube begins to grow down the style toward the ovary of the pistil, and the two nuclei move into the pollen tube. The generative nucleus divides to form two haploid sperm cells. The germinated pollen grain is now a mature male gametophyte. Finally, the pollen tube penetrates the ovary and the sperm enter. The ovary contains sporangia called ovules. Meiosis occurs within each ovule forming four haploid megaspores. Three disintegrate, and the remaining megaspore undergoes repeated mitosis to form the female gametophyte. The female gametophyte is a haploid seven-celled structure. One of the seven cells is an egg cell. Another of the seven cells contains two nuclei called polar nuclei. When the two sperm cells enter, double fertilization occurs. One sperm fertilizes the egg, forming a zygote that develops into a diploid embryo sporophyte. The two polar nuclei fuse and their product unites with the second sperm forming a triploid endosperm. The endosperm serves as stored food for the embryo sporophyte. After fertilization, the ovule matures into a seed, consisting of embryo, stored food, and seed coat. In angiosperms, the ovary usually enlarges to become a fruit. Upon germination, the seed develops into a mature diploid sporophyte plant. Internal fertilization and seeds help adapt flowering plants to life on land.