Reproduction of Flowering Plants
28.1 Asexual Reproduction
1. In asexual reproduction, a parent plant gives rise to clones, which can develop from roots, stems, or leaves.
2. Asexual reproduction is advantageous in a stable environment where plants are well adapted to their surroundings.
28.2 Sexual Reproduction
3. Flowers are reproductive structures built of whorls of parts. The calyx, made of sepals, and the corolla, made of petals, are accessory parts. Inside the corolla, the male parts consist of stamens and their pollen-containing anthers. At the center of the flower are the female parts, including the ovary. The stigma extends from the ovary and captures pollen. 4. A perfect flower has male and female parts; an imperfect flower has either. In a monoecious plant, opposite-sex flowers are on the same plant. In a dioecious plant, a plant has flowers of only one sex.
5. Female gametophytes are megagametophytes, and male structures are microgametophytes. They arise from megaspores and microspores, respectively.
6. Male and female structures produce gametes. In the anther, pollen sacs contain microspore mother cells. Each divides meiotically to yield four haploid microspores, which each divide mitotically to yield a haploid generative cell and a haploid vegetative cell; these two cells and their covering are a pollen grain. The pollen grain is the immature male gametophyte. Sperm cells arise from the generative cell and a pollen tube grows from the vegetative cell.
7. In the ovary, megaspore mother cells divide meiotically to yield four haploid cells, one of which persists as a haploid megaspore that divides mitotically three times. The resulting megagametophyte, or embryo sac, contains seven cells. One is the egg.
8. Animals or wind transfer pollen from the anthers of one plant to its own or another plant’s stigma. Flower structures and odors are adapted to encourage animal or wind pollination.
9. The stigma retains pollen from the same species.
10. Once on a stigma, a pollen grain grows a pollen tube, and its two sperm move through the tube towards the ovary. In the embryo sac, one sperm fertilizes the egg to form the zygote, and the second sperm fertilizes the polar nuclei to form the endosperm. This phenomenon is termed double fertilization.
28.3 Seeds and Fruits
11. A seed is an embryo, endosperm, and seed coat. The endosperm nourishes the developing embryo.
12. As the embryo grows, cotyledons develop. In dicots, the cotyledons absorb the endosperm and become fleshy.
13. Cells in apical meristems divide to promote the growth of shoot and root in the embryo.
14. The epicotyl is the stemlike region above the cotyledons; along with its leaves it is a plumule. The hypocotyl is the stemlike region beneath the cotyledons. The radicle is the embryonic root.
15. Seeds enter a dormancy period in which the embryo postpones development.
16. After fertilization, nonessential floral parts fall off, and hormones may influence the ovary and sometimes other plant parts to develop into a fruit. Fleshy fruits have distinct exocarp, mesocarp, and endocarp layers. In dry fruits, these join to form a pericarp. Drupes, berries, and pomes are types of fruit.
17. Seed germination requires oxygen, energy, and water. When the embryo bursts from the seed coat, the plant’s primary growth ensues.