In ovule meiosis occurs in

Learning Objectives • Compare and contrast the life cycles of angiosperms (flowering plants), gymnosperms (conifers), non-seed vascular plants (ferns), and nonvascular plants (mosses) • Describe the structures and functions of the flower, seed, and fruit in the angiosperm life cycle • Explain the process, locations, and significance of angiosperm gametogenesis and fertilization, including double fertilization • Explain the process and significance of seed maturation, dormancy, and germination • Predict mechanisms of pollination based on flower characteristics and dispersal based on fruit characteristics Sexual reproduction in plants: Alternation of Generations The text below is adapted from Plants have two distinct multicellular stages in their life cycles, a phenomenon called alternation of generations (in contrast to the gametophyte and the multicellular diploid sporophyte. This is very different from most types of animal reproduction where there is only one multicellular stage: a diploid organism which produces single-celled haploid gametes. Before we revisit this life cycle, a reminder of some terms: • Gamete: a mature haploid male or female germ cell that is able to unite with another of the opposite sex in sexual reproduction to form a zygote • Spore: a minute, typically one-celled, reproductive unit capable of giving rise to a new individual without sexual fusion Gametes are always haploid, and spores are usually haploid (spores are always haploid in the plant alter...

\( \newcommand\) • • • • • • Learning Objectives • Identify structures and phases in the angiosperm life cycle; know their ploidy. • Explain how fertilization occurs within a flower. • Label a developing ovary cross section. Angiosperms have a complex life cycle. The gametophytes have been further reduced: antheridia were lost in the gymnosperms and archegonia were lost in the angiosperms. Both gametophytes are now housed within the flower, a structure composed of highly modified leaves specialized for pollination. From flowers, fruits are produced, a protective structure that (usually) develops from the ovary wall and is specialized for seed dispersal. The Microgametophyte (AKA the Pollen Grain) The microgametophyte develops and reaches maturity within the microsporangia (Figure \(\PageIndex\): Pollen develops from the microspore mother cells. The mature pollen grain is composed of two cells: the pollen tube cell and the generative cell, which is inside the tube cell. The pollen grain has two coverings: an inner layer (intine) and an outer layer (exine). The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains. Descriptive text: The mother cell undergoes meiosis to form a tetrad of cells, which separate to form the pollen grains. The pollen grains undergo mitosis without cytokinesis, resulting in four mature pollen grains with two nuclei each. One is called the generative nucleus, and the other is called the pollen tube nucleus. Two projective layers ...

In the ovule or megasporangium, during the process of megasporogenesis, a hypodermal cell functions as a archesporium cell and becomes more prominent due to large size, denser cytoplasm and large nucleus. In tenuinucellate ovules, archesporial cell directly functions as megaspore mother cell while in crassinucellate ovules, archesporial cell divides periclinally to form an outer parietal cell and inner primary sporogenous cell that functions as megaspore mother cell.

Reproductive structures and function Completion of the entire reproductive cycle, from the advent of pollination to the production of seeds with well-developed embryos, takes about 14 months. Pollination and the development of the sexual, or gametophytic, phase of the Ginkgo is dioecious, which means that pollen-producing structures and The pollen-producing strobilus is a loose, pendulous, catkinlike structure consisting of a main axis to which are attached numerous appendages, each of which usually bears two microsporangia at its tip. Meiosis occurs in cells of the microsporangia, giving rise to numerous haploid microspores. Cell divisions take place within the microspores, resulting in the formation of five-celled pollen grains (male gametophytes). Ovuliferous structures also arise in the axils of bud scales and the One of the megaspores in the ovule that results from meiosis enlarges and undergoes a The basal end of the filament-like male gametophyte becomes suspended in a cavity above the female gametophyte (called the fertilization chamber). The spermatogenous cell of a male gametophyte divides, resulting in the production of two multiflagellated sperm. The sperm and the contents of the pollen tube are released into the fertilization chamber. The sperm swim in the liquid for a brief period of time. Approximately 1,000 flagella are attached to a spiral band at the Ginkgo and the cycads are the only seed-producing plants that have motile sperm. The growth of the embryo ...

[ "article:topic", "alternation of generations", "angiosperms", "authorname:openstax", "gametophyte", "sporophyte", "gynoecium", "perianth", "androecium", "antipodals", "exine", "intine", "megagametogenesis", "megasporangium", "megasporogenesis", "megasporophyll", "micropyle", "microsporangium", "microsporophyll", "polar nuclei", "synergid", "showtoc:no", "license:ccby", "licenseversion:40", "program:openstax" ] \( \newcommand\) • • • • • • • • • • • • Skills to Develop • Describe the two stages of a plant’s lifecycle • Compare and contrast male and female gametophytes and explain how they form in angiosperms • Describe the reproductive structures of a plant • Describe the components of a complete flower • Describe the development of microsporangium and megasporangium in gymnosperms Sexual reproduction takes place with slight variations in different groups of plants. Plants have two distinct stages in their lifecycle: the gametophyte stage and the sporophyte stage. The haploid gametophyte produces the male and female gametes by mitosis in distinct multicellular structures. Fusion of the male and females gametes forms the diploid zygote, which develops into the sporophyte. After reaching maturity, the diploid sporophyte produces spores by meiosis, which in turn divide by mitosis to produce the haploid gametophyte. The new gametophyte produces gametes, and the cycle continues. This is the alternation of generations, and is typical of plant reproduction (Figure \(\PageIndex\)...