The Sexual Reproduction In Flowering Plants

Here is information on the sexual reproduction in flowering plants; includes the parts of a flower, and the steps of its reproductive cycle.

Flowers are reproductive shoots usually composed of four parts- sepals, petals, stamens, and carpels- arranged in whorls on the end of a flower stalk, or peduncle. The peduncle may terminate in a single flower or a cluster of flowers known as an inflorescence. The tip of the peduncle enlarges to from a receptacle that bears some for all of the flower parts.

All four floral parts are important in the reproductive process, but only the stamens (the "male" organs) and carpels (the "female" organs) produce gametes. A flower that has all four parts is said to be complete, whereas an incomplete flower lacks one or more of these four parts. A flower with both stamens and carpels is said to be perfect, whereas an imperfect flower has stamens or carpels, but not both.

Sepals, which make up the lowermost and outermost whorls on a floral shoot, are leaf-like in appearance and often green. Sepals cover and protect the other flower parts when the flower is a bud. As the blossom opens, the sepals fold back to reveal the more conspicuous petals. The collective term for all the sepals of a flower is the calyx.

The whorl just above the sepals consists of petals, which are broad, flat, and thin but vary in shape and are frequently brightly colored. Petals play an important role in attracting animal pollination to the flower. Sometimes petals are fused to form a tube or other floral shape. The petals of a flower are referred to collectively as the corolla.

Just inside the petals is a whorl of stamens. Each stamen is composed of a thin stalk, called a filament, and a saclike anther, where meiosis occurs to form microspores that develop into pollen grains. Each pollen grain produces two cells surrounded by a though outer wall. One cell divides to form two male gametes, or sperm cells, and the other produces a pollen tube through which the sperm cells travel to reach the ovule.

In the center of most flowers is one or more carpels, the "female" reproductive organs. Carpels bear ovules, which are structures with the potential to develop into seeds. The carples of a flower may be separate or fused into a single structure. The female part of the flower is also referred to as a pistil. A pistil may consist of a single carpel or a group of fused carpels. Each pistil has three sections: a stigma, on which the pollen grain lands; a style, a necklike structure through which the pollen tube grows; and an ovary, an enlarged structure that contains one or more ovules. Each young ovule contains a female gametophyte that forms one female gamete (an egg) and two polar nuclei. The egg and polar nuclei participate directly in fertilization, and, following fertilization, the ovule develops into a seed and the ovary into a fruit.

Flowering plants have an alternation of generations in which the sporophyte generation is larger and nutritionally independent and the gametophyte generation is microscopic in size and nutritionally dependent on the sporophyte. Flowering plants, like gymnosperms and certain other vascular plants, are heterosporous and produce two kinds of spores: microspores and megaspores. Sexual reproduction occurs in the flower.

Each young ovule within an ovary contains a megasporocyte that undergoes meiosis to produce four haploid megaspores. Three of these usually disintegrate, and one divides mitotically and develops into a female gametophyte, also called and embryo sac. The embryo sac contains seven cells with eight haploid nuclei. Six of these cells including the egg cell, contain a single nucleus each , and a central cell has two nuclei, called polar nuclei. The egg and the central cell with tow polar nuclei are directly involved in fertilization; the other five cells in the embryo sac apparently have no direct role in the fertilization process and disintegrate. It has been hypothesized, however, that as the synergids (the two cells closely associated with the egg) disintegrate, they release chemicals that may affect the direction of pollen tube growth.

Each pollen sac, or microsporangium, of the anther contains numberous microsporocytes, each of which undergoes meiosis to form four haploid microspores. Every microspore develops into an immature male gametophyte, also called a pollen grain. Pollen grains are extremely small; each consists of two cells, the tube cell and the generative cell.

The anther sacs split open and begin to shed pollen. Pollen grains are transferred to the stigma by a variety of agents, including wind, water, insects, and other animal pollinators. If compatible with the stigma, the pollen grain germinates; that is, the tube cell forms a pollen tube that grows down the style and into the ovary. Next,the generative cell divides to form two nonmotile male gametes called sperm cells. The sperm cells move down the pollen tube and are discharged into the embryo sac. Both sperm cells are involved in fertilization.

Something happens during sexual reproduction in flowering plants that does not occur anywhere else in the living world. When the two sperm cells enter the embryo sac, both participate in fertilization. One sperm cell

fuses with the egg, forming a zygote that grows by mitosis and develops into a multicellular embryo in the seed. The second sperm cell fuses with the two haploid polar nuclei of the central cell to form a triploid cell that grows by mitosis and develops into endosperm, a nutrient tissue that nourishes the embryo. This fertilization process, which involves two separate nuclear fusions, is called double fertilization and is, with two exceptions, unique to flowering plants.

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