Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 28

In asexual reproduction, a parent plant gives rise to clones, which can develop from roots, stems, or leaves. Asexual reproduction is advantageous in a stable environment where plants are well adapted to their surroundings.
Sexually reproducing plants undergo alternation of generations. The diploid sporophyte generation produces spores through meiosis. The spores divide mitotically to produce the haploid gametophyte, which in turn produces haploid gametes. In different species, either the sporophyte or gametophyte generation dominates. Female gametophytes are megagametophytes, and male structures are microgametophytes. They arise from megaspores and microspores, respectively, which form in megasporangia and microsporangia.
In pines, large female cones bear two ovules on each scale. Each ovule yields three haploid cells that degenerate and one haploid megaspore that develops into a female gametophyte. Microsporangia grow on the small scales of male cones. Through meiosis, the cones produce microspores, which become pollen grains. Pollen lands on sticky female cones, and a pollen tube grows towards the egg. The pollen divides twice, and two of the resulting cells are sperm. One sperm fertilizes the egg. The megagametophyte nourishes the embryo, which becomes dormant within a tough seed coat. Location of female cones above male cones encourages outcrossing.
Flowers are reproductive structures. The calyx, made of sepals, and the corolla, made of petals, are accessory parts. Inside the corolla, the androecium consists of stamens and their pollen-containing anthers. At the center of the flower the gynoecium encloses the ovary. The stigma extends from the ovary and captures pollen.
Male and female structures produce gametes. In the anther, four pollen sacs contain microspore mother cells. Each divides meiotically to yield four haploid microspores, which each divide mitotically to yield a haploid generative nucleus and a haploid tube nucleus; these two cells and their covering are a pollen grain. Sperm cells arise from the generative nucleus. 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 eight cells. One is the egg.
Animals or wind transfers 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. 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, which nourishes the embryo. Cotyledons develop and transfer nutrients in monocots and absorb the endosperm in dicots. Apical meristems promote the growth of shoot and root in the embryo.
After fertilization, nonessential floral parts fall off, and hormones may influence the ovary to develop into a fruit. A drupe is a fleshy fruit with a hard pit; a berry is fleshy with many seeds; blackberries are aggregate fruits; and a pome is a fleshy fruit that develops from tissues surrounding the ovary. A seed germinates in response to environmental cues. Germination requires oxygen, energy, and water. When the embryo bursts from the seed coat, the plant's primary growth ensues.