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Sexual Reproduction in Flowering Plants

11-MAR-2016 16:15

    Sexual reproduction is the production of a new organism from two parents by making use of their gametes or sex cells. Plants also have male and female sex organs. These sex organs in plants are carried within the flower and the seeds which are inside a fruit. Such plants are called angiosperms or flowering plants as they reproduce by sexual reproduction method.

    Most of the plants contain reproductive organs of both male and female in the flowers. The same flower has both male and female reproductive organs. Such flowers make male and female gametes and ensure that fertilisation takes place so that new seeds are formed for the reproduction of plant.

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                             Parts of a Flower

    Steps for sexual reproduction in plants

    • The male organ of a flower is called stamen. It helps in making male gametes of the plant and is present in pollen grains.
    • The female organ of a flower is called carpel. It helps in making female gametes or egg cells of the plant and is present in ovules.
    • The male gametes fertilise the female gametes.
    • The fertilised egg cells grow in ovules and become seeds.
    • When germinated, these seeds become new plants.

    Various Parts of a Flower

    • Receptacle: It is a base of a flower above the flower stem. It is at receptacle that all other parts of a flower are attached.
    • Sepals: These are the green leaf-like parts which are present on the outer most part of the flower. Sepals help in protecting the flower when it is in the form of a bud. All sepals of a flower together are called calyx.
    • Petals: Petals are the colourful leaves of a flower. All the leaves of a flower together are called corolla. The petals of a flower have fragrance and they attract insects for pollination. Their function is to protect the reproductive organs present in the centre of a flower.
    • Stamen: Stamens are the male reproductive organ of a plant. They are present inside the ring of petals and are little stalk with swollen tops. Stamen is made of two parts, anther and filament. The stalk of stamen is called filament and swollen top is called an anther. The anther of a stamen produced pollen grains and stores them. These pollen grains contain male gametes of the plant. A flower has a lot of stamens in it.  

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               Stamen: Male reproductive organ of a plant. 

    • Carpel: Carpel is the female reproductive organ and is present in the centre of a plant. The shape of carpel is like a flask. Carpel is made of three parts, stigma, style and ovary. The top part of carpel is called stigma. The stigma is sticky and receives pollen from anther of stamen. The pollen grains sticks to the stigma. The centre part of carpel is called style. Style is a tube which connects stigma to ovary. The bottom part of carpel which is swollen is called ovary. It is here that the ovules are made and stored. There are many ovules in the ovary and each ovule contains one female gamete of the plant. The female gamete of the plant which is present inside the ovule is called egg or ovum. Therefore, female gametes are made in the ovary of carpel. The female organ of a plant is also called pistil. Also the carpel is surrounded by a number of stamens.

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       Carpel: Female reproductive organ of a Plant. Carpel is also known as Pistil.

    There are flowers which are called unisexual. This is because they have either stamens or carpels. Flowers of papaya and watermelon are examples of unisexual flowers. And the flowers which have both male and female sex organs are called bisexual. Flowers of hibiscus and mustard plant are called bisexual flowers.

    In order to make a new seed male gamete present in a pollen grain units with the female gamete present in the ovule. This process takes place in two steps.

    • Pollination
    • Fertilisation

    Pollination

    When pollen grains from the anther are transferred to the stigma of a carpel, it is called pollination. This is important because it is due to pollination that male gametes are able to combine with female gametes. Pollination is done by insects like bees, butterflies and birds, wind and water.

    There are two types of pollination, self-pollination and cross-pollination. When pollen grains from one flower carried to the stigma of the same flower or to another flower of the same plant, it is called self-pollination. And when pollen grains from a flower of one plant are carried to the stigma of a flower of another similar plant, it is called cross-pollination.  

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                                                    Pollination

    Insects help in pollination. This happens when an insect sits on a flower of one plant to suck nectar then the pollen grains from anther stick to its body. Now, when this insect flies and sits on the flower of another similar plant then the pollen grains are transferred and they stick to the stigma of the flower of another plant. This way insects help cause cross-pollination. Wind too helps in cross-pollination.

    Fertilisation

    After pollination, next step is fertilisation. In this step, male gametes present in pollen grains joins with the female gametes present in ovule.

    When the pollen grain falls on stigma it bursts open and a pollen tube grown which moves through the style towards the ovary and enters ovule. Male gamete moves down through the pollen tube. The tip of pollen tube bursts open in the ovule and the male gamete come out. In the ovule, male gamete combines with the nucleus of female gamete and fertilised egg is formed. This fertilised egg is called zygote.

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                                                  Fertilisation in a Flower.

    Formation of fruits and seeds

    In the ovule, the fertilised egg divides several times to form an embryo. A tough coat is developed around the ovule and it gradually develops into a seed. The ovary of flower develops to become a fruit which contains seeds inside it. Other parts of the flower like sepal, stamen, stigma and style become dry and fall off. Fruit takes place of the flower. Seed is protected by the fruit. Some fruits are soft and juicy while the other fruits are hard and dry.

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                                             A fruit contains seed of the plant inside it.

    The seed is the reproductive unit of a plant. With this seed new plant can be grown as the seed contains baby plant and food for the baby plant inside it. The part of the baby plant in seed which grown into leaves is called plumule and the part which develops into roots is called radicle. The part of the seed which stores food for the baby plant is called cotyledon. The baby plant inside a seed is in a dormant state. It is only when we provide it with suitable environment like water, air, light, etc., it germinates and a new plant grows. Wheat grains, gram, corn, peas, beans etc., are examples of seeds.

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                                                           Parts of a Seed

    Germination of seeds

    The seeds obtained from a plat are in dry and dormant state. It is only when they get water, air, soil, etc., that they begin to grow into a new plant. The beginning of the growth of a seed is called germination of seeds.

    The germination of a seed begins when it absorbs water, swells and bursts through the seed coat. It is with the help of water that enzymes function in the seed. The enzymes digest stored food and make it soluble. With the help of soluble food radicle and plumule grow.

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    Seeds germinate under suitable conditions to produce new plants. These pictures show the germination of a bean seed to form a new bean plant.

    First the radicle of the seed grows to form roots. These roots grow inside soil and absorb water and minerals from the soil. After this plumule grows upwards and shoots are formed. These shoots develop green leaves. The leaves begin to make food with the process of photosynthesis and gradually a whole new plant is developed.

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