Wednesday, July 1, 2015

Sexual Reproduction in Plants

Sexual Reproduction in Plants:
Sexual Reproduction in Plants

Sexual reproduction involves the production of gametes and their fusion.Gametes are produced in special structures in plants body. The major plant groups are mosses, ferns and seed plants. The seed plants include gymnosperms and angiosperms. Plants use different methods for bringing the sperm and egg cells. Therefore, these plants require water for sexual reproduction. On the other hand, gymnosperms and angiosperms have special methods for carrying their sperms to egg cells. They do not need water for reproduction.
In the life cycle of plants, two different generations alternate with each other. One generation is diploid and produces spores. It is called sporophyte generation. The other generation is haploid and produces gametes. It is called gametophyte generation. The phenomenon in which two different generation alternate with each other during life cycle is known as alternation of generations.
In most plants, a sporophyte generation is dominant. It means that it is big in size and is independent. Sporophyte produces haploid spores by meiosis. The spores develop into gametophyte. It is small in size and depends upon sporophyte. It produces gametes by mitosis. The male and female gametes fuse and form and form diploid zygote. The zygote undergoes repeated mitosis and develops into a new diploid sporophyte.

Sexual Reproduction in Flowering Plants:

In angiosperms, parent plant is diploid sporophyte generation. Flower is the reproductive structure in this generation. The flower components are arranged in the form of whorls. The outer two whorls in a flower are the non-reproductive whorls while the inner two whorls are the reproductive whorls.
Calyx is the outermost whorl. It is usually green in colour. Its individual units are called sepals. Sepals protect the inner whorls at bud stage. Corolla is the next inner whorl and is often coloured brightly. Its individual units are called petals. They serve to attract bees, birds, etc. Which are the agents of pollination.
Third whorl i.e. androecium is the male reproductive part of flower. Its units are called stamens. Each stamen has a thread-like filament at the free end of which anther is attached. Anther has pollen-sacs in which haploid microspores are produced through meiosis. Each microspore germinates into the male gametophyte nuclei. The  generative nucleus again nucleus and two sperms. All these structures are the male gametophyte generation of plant.
Fourth whorl i.e. gynaecium is the female reproductive part of flower. Its units are called carpel’s. Each carpel is made up of the basal ovary, middle style and upper stigma. Inside ovary, there are one two many ovules. Inside each ovule, one haploid macrospore is produced through meiosis. Macrospore germinates into the female gametophyte generation. During it, macrospore undergoes mitosis and produces an egg cell and some associated structures. Egg cell and associated structures are the female gametophyte generation of plant.
When pollen grains mature, they are transferred to stigma. It is called pollination. On reaching the stigma, the tube nucleus of pollen grain constructs a pollen tube. The pollen tube contains a tube nucleus and two sperms. The tube grows through style and ovary and enters ovule. Here, it bursts and releases the sperms. Both sperms enter the female gametophyte. One sperm fuses with egg and forms a diploid zygote. The other sperm fuses with diploid fusion nucleus and forms a triploid (3N) nucleus called endosperm nucleus. Since the process of fertilization involves two fusions, it is called double fertilization.
Zygote develops into embryo and endosperm nucleus develops into endosperm tissue. Ovule then becomes seed and ovary changes into fruit. When seeds mature, they are dispersed. If seeds get suitable conditions, their embryos develop into new plants.

Pollination:

Pollination is defined as the transfer of pollen grains from flower’s anther to stigma. Two types of pollination are recognised.
Self Pollination is defined as the transfer of pollen grains from the anther to the stigma of the same flower or other flower of the same plant.
Cross Pollination is the transfer of pollen grains from the flowers on one plant to the flower on other plant of the same species. Cross pollination is brought about by various agencies like wind, water, bees, birds, bats and other animals including man.
The insect pollinated and wind pollinated flowers have structural adaptation that facilitate the transfer of pollen grains between two plants.

Development and Structure of Seed:

After fertilization in the female gametophyte, zygote divides repeatedly by mitosis and develops into an embryo. At this stage, ovule changes into seed. The formation of seed completes the process of sexual reproduction in seed plants.
Angiosperm seeds consist of three distinct parts;(1) the embryo formed from zygote, (2) the endosperm tissue formed from endosperm nucleus, and (3) the seed coat which develops from the wall of ovule.
Seed coat develops from the integument, originally surrounding the ovule. It may be a paper-thin layer or thick and hard. Seed coat protects embryo from mechanical injury and from drying out. There is a scar on seed coat, called hilum. It is where the seed is attached to ovary wall. At one end of hilum, there is micropyle. This is the same opening through which the pollen tube entered ovule. Seed uses it for the absorption of water.
Embryo is actually an immature plant. It consists of a radical, a plumule and one or two cotyledons. The radical of embryo develops into new root while the plumule develops into new shoot. The embryonic stem above the point of attachment of cotyledon is called epicotyls. The embryonic stem below the point of attachment is hypocotyls. With is seed, there us as tore of nutrients for the seeding that will grow from embryo. In angiosperms, the stored food is derived from the endosperm tissue. This tissue is rich in oil or starch and protein. In many seeds, the food of the endosperm is absorbed and stored by cotyledons.
Germination of Seed:

For the germination of seeds, they must arrive at a suitable location and be there at a time favorable for germination and growth.
Seed germination is a process by which a seed embryo develops into a seeding. During germination, embryo soaks up water which causes it to swell, splitting the seed coat. Root is the first structure that emerges from the radical present in seed. It grows rapidly and absorbs water and nutrients from soil. In the next phase, plumule develops into tiny shoot which elongates and comes out of soil.

On the basis of the elongation of hypocotyls and epicotyls, there are two types of germination. In apogeal germination, the hypocotyls elongate and form a hook, pulling the cotyledons above ground. Beans, cotton and papaya are the examples of seeds that germinate this way. In hypogeal germination, the epicotyls elongate and form the hook. In this type of germination, the cotyledons stay underground. Pea, maize and coconut germinate this way.

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