development of male and female gametophyte

The gametophyte is the sexual phase in the life cycle of plants and algae. In each microspore, a single gametophyte is produced, consisting of four haploid cells produced by meiotic division of a diploid microspore mother cell. They exist in their earliest life stages as spores, released with millions of others from the parent kelp, the sporophyte. [19] Just like in gymnosperms, the tube cell in angiosperms obtains nutrients from the sporophytic tissue, and may branch out into the pistil tissue or grow directly towards the ovule. Seeds are a great evolutionary development, since, unlike spores, they are multicellular and contain nutrition for the new, developing plant, all within a protective coat. However, in some groups, notably the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. The cell number of each mature pollen grain varies between the gymnosperm orders. (a) This male cone, shown in cross section, has approximately 20 microsporophylls, each of which produces hundreds of male gametophytes (pollen grains). Plant sexual reproduction usually depends on pollinating agents, while asexual reproduction is independent of these agents. Other terms that apply are vegetative propagation, clonal growth, or vegetative multiplication. [16] One cell is the tube cell, and the remaining cell/cells are the sperm cells. The immature anther (top) contains four microsporangia, or pollen sacs. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. In gymnosperms the megagametophyte consists of several thousand cells and produces one to several archegonia, each with a single egg cell. Vegetative reproduction is also utilized by horticulturists to ensure production of large quantities of valuable plants. In Isoetes and Selaginella, which are heterosporous, microspores and megaspores are dispersed from sporangia either passively or by active ejection. In extant land plants, either the sporophyte or the gametophyte may be reduced (heteromorphic). Seed plant gametophytes are not independent organisms and depend upon the dominant sporophyte tissue for nutrients and water. In bryophytes the process requires the production of male gametes (sperm), female gametes (eggs) and some means of getting the sperm to the eggs. In seed plants, the microgametophyte (pollen) travels to the vicinity of the egg cell (carried by a physical or animal vector), and produces two sperm by mitosis. During its transit inside the pollen tube, the generative cell divides to form two male gametes. The term gametophyte may refer to the gametophyte phase of the plant life cycle or to the particular plant body or organ that produces gametes. They form a multicellular embryo during development from the zygote. In the homosporous families Lycopodiaceae and Huperziaceae, spores germinate into bisexual free-living, subterranean and mycotrophic gametophytes that derive nutrients from symbiosis with fungi. As with angiosperms, the life cycle of gymnosperms is also characterized by alternation of generations. At a minimum, two of these cells are egg cells and the rest are halploid somatic cells, but more egg cells may be present and their ploidy, though typically haploid, may vary. (f) Within this single ovule are the megaspore mother cell (MMC), micropyle, and a pollen grain. The first phase involves the megasporogenesis, where a single diploid mother cell undergoes meiosis to form haploid megaspore tetrad out of … Unlike angiosperms, ovaries are absent in gymnosperms, double fertilization does not take place, male and female gametophytes are present on cones rather than flowers, and wind (not animals) drives pollination. In conifers such as pines, the green leafy part of the plant is the sporophyte; the cones contain the male and female gametophytes. In Selaginella, usually only four large megaspores are produced in a megasporangium. First, in the process of megasporogenesis, a single cell in the diploid megasporangium undergoes meiosis to produce four megaspores, only one of which survives. [8]  At maturity, each microspore-derived gametophyte become a pollen grain. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage. The female gametophyte in gymnosperms differs from the male gametophyte as it spends its whole life cycle in one organ, the ovule located inside the megastrobilus or female cone. The mature pollen grain is composed of two cells: the pollen tube cell and the generative cell, which is inside the tube cell. Flowers that contain both an androecium and a gynoecium are called perfect, androgynous, or hermaphrodites. In plants with heteromorphic gametophytes, there are two distinct kinds of gametophytes. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. (d) This cross section of a female cone shows portions of about 15 megasporophylls. In hornworts and thallose liverworts the gametophyte is the flattish sheet. The life cycle consists of alternating haploid gametophyte and diploid sporophyte generation. [10] This occurs because in some gymnosperm orders, the germ cell is nonmobile and a direct pathway is needed, however, in Cycadophyta and Ginkgophyta, the germ cell is mobile due to flagella being present and a direct tube cell path from the pollination site to the egg is not needed. [10] In some gymnosperms, the tube cell will create a direct channel from the site of pollination to the egg cell, in other gymnosperms, the tube cell will rupture in the middle of the megastrobilus sporophyte tissue. In gymnosperms, a leafy green sporophyte generates cones containing male and female gametophytes; female cones are bigger than male cones and are located higher up in the tree. [5] If any of the four parts is missing, the flower is known as incomplete. The basic function of a flower is to produce seeds through sexual reproduction. The integument will develop into the seed coat after fertilization, protecting the entire seed. If both male and female flowers are borne on the same plant (e.g., corn or peas), the species is called monoecious (meaning “one home”). With their bright colors, fragrances, and interesting shapes and sizes, flowers attract insects, birds, and animals to serve their pollination needs. Outline the components of a flower and their function. Eggs develop in archegonia and sperm in antheridia.[4]. A typical flower has four main parts, or whorls: the calyx ( sepals ), corolla (petals), androecium (male reproductive structure), and gynoecium (female reproductive structure). These male and female sex cells, also known as eggs and sperm, unite during fertilization to form a diploid zygote. The flower is the reproductive organ of plants classified as angiosperms. Pollen from male cones blows up into upper branches, where it fertilizes female cones. These antheridia are structures in male gametophytes that produce and release sperm. The seed plant gametophyte life cycle is even more reduced than in basal taxa (ferns and lycophytes). Cycadophyta have 3 celled pollen grains while Ginkgophyta have 4 celled pollen grains. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Seed plant microgametophytes consists of several (typically two to five) cells when the pollen grains exit the sporangium. A double-layered integument protects the megasporangium and, later, the embryo sac. Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine. Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell. The gametophyte itself is surrounded by layers of sporangia and integument; all of these elements comprise an ovule, which is found on the surface of a female cone. In angiosperms, the megagametophyte is reduced to only a few nuclei and cells, and is sometimes called the embryo sac. The bryophyte gametophyte is longer lived, nutritionally independent, and the sporophytes are attached to the gametophytes and dependent on them. The whole structure is protected from desiccation and can reach the female organs without dependence on water. [21], The female angiosperm gametophyte develops in the ovule (located inside the female or hermaphrodite flower). Some believe it is neither.[19]. [6] Microspores produce microgametophytes which produce sperm. Vegetative reproduction is a type of asexual reproduction that results in new plant individuals without seed or spore production. Further mitosis of the microspore produces two nuclei: the generative nucleus and the tube nucleus. When this happens, a male gametophyte (sperm cell) enters the female flower, fertilizes the egg and, long story short, a seed starts developing. CC licensed content, Specific attribution, http://en.wikipedia.org/wiki/vegetative%20reproduction, http://en.wikibooks.org/wiki/Botany/Plant_reproduction, http://cnx.org/content/m44720/latest/?collection=col11448/latest, http://cnx.org/content/m44720/latest/Figure_32_00_01abc.jpg, http://en.wiktionary.org/wiki/megasporophyll, http://cnx.org/content/m44722/latest/?collection=col11448/latest, http://en.wiktionary.org/wiki/microsporophyll, http://cnx.org/content/m44722/latest/Figure_32_01_08.png, http://cnx.org/content/m44722/latest/Figure_32_01_09.jpg, http://www.boundless.com//biology/definition/perianth, http://cnx.org/content/m44722/latest/Figure_32_01_02.png, http://cnx.org/content/m44722/latest/Figure_32_01_04ab.jpg, http://cnx.org/content/m44722/latest/Figure_32_01_03.jpg, http://cnx.org/content/m44722/latest/Figure_32_01_05.jpg, http://cnx.org/content/m44722/latest/Figure_32_01_06f.jpg, http://cnx.org/content/m44722/latest/Figure_32_01_07.png. The androecium has stamens with anthers that contain the microsporangia. Because the two gametophytes differ in form and function, they are termed heteromorphic, from hetero- "different" and morph "form". Once fertilization is complete, the resulting diploid zygote develops into the embryo; the fertilized ovule forms the other tissues of the seed. Due to this complex relationship and the small size of the gametophyte tissue, in some situations single celled, differentiating with the human eye or even a microscope between seed plant gametophyte tissue and sporophyte tissue can be a challenge. [1], In land plants, anisogamy is universal. Its precursor is a diploid megaspore that undergoes meiosis which produces four haploid daughter cells. There are two types of incomplete flowers: staminate flowers contain only an androecium; and carpellate flowers have only a gynoecium. Pollen grains are male gametophytes carried by wind, water, or a pollinator. Similar to the male gametophyte, the female gametophyte normally is fully dependent on the surrounding sporophytic tissue for nutrients and the two organisms cannot be separated. The male gametophyte lands on the female cone, forming a pollen tube through which the generative cell travels to meet the female gametophyte. Some botanists consider this endospore as gametophyte tissue with typically 2/3 being female and 1/3 being male, but as the central cell before double fertilization can range from 1n to 8n in special cases, the fertilized central cells range from 2n (50% male/female) to 9n (1/9 male, 8/9th female). October 17, 2013. The tapetum supports the development and maturation of the pollen grains. The male gametophyte develops and reaches maturity in an immature anther. October 17, 2013. The sperm, guided by the synergid cells, migrates to the ovary to complete fertilization; the diploid zygote develops into the embryo, while the fertilized ovule forms the other tissues of the seed. The gametophyte even has special structures to produce the eggs and sperm. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. Pollen grain structure: Pollen develops from the microspore mother cells. October 23, 2013. As in animals, female and male gametes are called, respectively, eggs and sperm. In the female cone, megaspore mother cells are present in the megasporangium. Finally, wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. In some bryophyte groups such as many liverworts of the order Marchantiales, the gametes are produced on specialized structures called gametophores (or gametangiophores). In this respect, it is a form of cloning that has been carried out by humanity for thousands of years and by plants for hundreds of millions of years. [9][8] One of these cells is typically a germ cell and other cells may consist of a single tube cell which grows to form the pollen tube, sterile cells, and/or prothallial cells which are both vegetative cells without an essential reproductive function. The second whorl is comprised of petals, usually brightly colored, collectively called the corolla. Carpellate flower are clustered in the immature ears. Flowers are often the showiest or most strongly-scented part of plants. OpenStax College, Biology. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte. The male spore will develop into a mature gametophyte that contains structures to make male gametes. Plants and sexual reproduction: Plants that reproduce sexually often achieve fertilization with the help of pollinators such as (a) bees, (b) birds, and (c) butterflies. A pollinated female plant will spend most of its resources growing lots of seeds and will stop developing the buds, thus reducing your yield significantly. [2], In bryophytes (mosses, liverworts, and hornworts), the gametophyte is the most visible stage of the life cycle. The female cone also has a central axis on which bracts known as megasporophylls are present. Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative cell migrates towards the female gametophyte through the micropyle. During megasporogenesis, four megaspores are produced with one surviving; during megagametogenesism, the surviving megaspore undergoes mitosis to form an embryo sac (female gametophyte). All plants have the means and corresponding structures for reproducing sexually. However, select female gametophytes do contain chlorophyll and can produce some of their own energy, though, not enough to support itself without being supplemented by the sporophyte. If all four whorls are present, the flower is described as complete. The innermost group of structures in the flower is the gynoecium, or the female reproductive component(s). In heterosporous vascular plants (plants that produce both microspores and megaspores), the gametophytes develop endosporically (within the spore wall). (b) In these scanning electron micrographs, pollen sacs are ready to burst, releasing their grains. A flower may have one or multiple carpels. Three of these independent gametophyte cells degenerate, the one that remains is the gametophyte mother cell which normally is composed of one nucleus. A male cone has a central axis on which bracts, a type of modified leaf, are attached. This creates a 2 or 3 celled male gametophyte which becomes known as the pollen grain once dehiscing occurs. The female cones are larger than the male cones and are positioned towards the top of the tree; the small, male cones are located in the lower region of the tree. A gametophyte (/ɡəˈmiːtoʊfaɪt/) is one of the two alternating multicellular phases in the life cycles of plants and algae. In gymnosperms, the male gametophytes are produced inside microspores within the microsporangia located inside male cones or microstrobili. Species with male and female flowers borne on separate plants (e.g., C. papaya or Cannabis)are termed dioecious, or “two homes.” The ovary, which may contain one or multiple ovules, may be placed above other flower parts (referred to as superior); or it may be placed below the other flower parts (referred to as inferior). Superior and inferior flowers: The (a) lily is a superior flower, which has the ovary above the other flower parts. Angiosperms may be monoecious or dioecious and undergo sexual reproduction. Scholars still disagree on whether the fertilized central cell is considered gametophyte tissue. [20][21] Once double fertilization is completed, the tube cell and other vegetative cells, if present, are all that remains of the male gametophyte and soon degrade. The pollen grain has two coverings: an inner layer (intine) and an outer layer (exine). The scales of the cones are closed during development of the seed. (b) Pollen grains are visible in this single microsporophyll. Describe the process of sexual reproduction in gymnosperms. In Ulva the gametes are isogamous, all of one size, shape and general morphology. Microsporangium: Shown is (a) a cross section of an anther at two developmental stages. The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains. Extant lycophytes produce two different types of gametophytes. Angiosperms that contain only male or only female gametophytes are considered to be incomplete and are either staminate (contain only male structures) or carpellate (contain only female structures) flowers. Once the microspore undergoes meiosis, 4 haploid cells are formed, each of which is a singled celled male gametophyte. Conifer life cycle: This image shows the life cycle of a conifer. The gametophytes of Isoetes appear to be similar in this respect to those of the extinct Carboniferous arborescent lycophytes Lepidodendron and Lepidostrobus.[7].

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