Fertilization in angiosperms powerpoint presentation

fertilisation

1. Fertilization Dr . R. S. Bhosale

2. Fertilization: The process of fusion of male gametes and female gametes in embryo sac during sexual reproduction to produce zygote is called fertilization. Fertilization takes place after process of pollination. Pollination: Transfer of pollen grains from anther to stigma of a flower is called pollination. Pollination may take place by some vector or may occur naturally. Pollination may occur within same flower, different flowers of same plant or different plants of same species.

3. Types of Pollinations: On basis of mode of transfer of pollen grains pollinations are of two types. 1. Self Pollination (Homogamy, Autogamy) When pollen grains are transferred from anther to stigma of same flower, the process is called self pollination. Self pollination does not promote genetic diversity because both gametes are from identical flower. E.g. Argemone mexicana , Mirabilis jalapa . 2. Cross Pollination ( Allogamy ) When pollen grains are transferred from an anther of one flower to stigma of another flower, the process is called cross pollination. Cross pollination promotes genetic diversity because both gametes are from different flowers and are genetically different. E.g. Michelia champaka . Self pollinated flowers that produce viable seeds are known as self compatible . Flowers that are self pollinate but does not perform properly on carpel are called self incompatible . Self pollinated plants are called inbreed while those only cross pollinate are called outcross .

4. Mechanism of Pollination (Refer text book for detail notes on mechanism of pollination) Pollination is carried out by several agents or vectors like wind, water, insect, birds and animals. Depending on agents of pollination it is classified into different types. 1. Anemophily or Anemogamy : Pollination with the help of wind. Such plants are called anemophilious plants. It takes place in large and highly populated plants. 2. Hydrophily or Hydrogamy : Pollination through water. Hydrophily is of two types depending on position or site of pollination through water. Hypohydrogamy - Pollination takes place inside water. E.g. Ceratophyllum , Najas sp. Epihydrogamy – Pollination on the surface of water. E.g. Vallisneria spiralis .

5. 3. Zoophilia or Zoogamy or Zoidiophily : Pollination takes place with the help of animals. On basis of pollinating animals it is subdivided as follows. Entomophily or Entomogamy - Pollination by insects like bee, butterfly, beetles etc. Malacophily or Malacogamy - Pollination by animals like slug, snails and squirrel etc,. It takes place in flowers that open at ground level. Ornithophily or Ornithogamy - Pollination with the help of birds. E.g. Hibiscus, Erythrina . Chiropterophily or Chiropterogamy - Pollination carried out by bats. E.g. Bauhinia megalandra .

6. Germination of Pollen grain. When pollen grain lands on stigma of flower of same species it rehydrates and starts germination. When pollen grain of different species lands of stigma it does not rehydrate because stigma produces poisonous substances or dries up. After proper combination pollen grain starts germination and produces pollen tube. Pollen tube is produced by inner wall of pollen grain called intine and emerges out through germ pore. One pollen grain produces only one pollen tube ( monosiphonous ). In some plant more than one pollen tube are produced ( polysiphonous ) for e.g. Malvaceae , Cucurbitaceae . Some times pollen tube may show branching.

7. Growth of Pollen Tube. pollen grain germinates and grows through stigma, passes through style and reaches ovary to reach ovule the length of pollen tube depends on lenth of style and fertility of stigma. Inner wall of style is lined with special tissue called transmission tissue. Based on this tissue style are of three types Open style: has wider stylar canal. Inner epidermal tissue helps in nutrition and conduction of pollen grain. E.g. Papavaraceae . Half closed style: Stylar canal is surrounded by rudimentary transmitting tissue made up of glandular cells. E.g. Cactaceae. Closed style: In this type stylar canal is made up of solid cells filled up with protoplast. E.g. Datura , Gossypium .

8. Factors affecting Rate of Growth of Pollen Tube. Factors that affect rate of growth of pollen tube are Environmental factors : Temperature plays important role in rate ofpollen germination. Different plants show maximum germination at different temperature. Generally pollens moderately germinate during morning and evening. High temperature and cool climate may reduce pollen germination in many plants. Compatibility : The compatibility between male gametophyte and stelar tissue plays important role in rate of pollen germination. In compatible species germination is rapid while in incompatible species the rate is very poor.

9. Entry of pollen Tube and Discharge of gametes. Pollen tube enters ovule by three possible ways a. Porogamy : Pollen tube enters ovule directly through micropyle. It occurs mostly in angiosperms. b. Chalazogamy : Pollen tube enters ovule through chalaza. c. Mesogamy : Pollen tube enters directly through finiculus or integuments directly. It occurs in Cucurbita.

10. Pollen tube enters the ovule. Tip of pollen tube bursts in the cavity of ovule. Two male gametes are discharged in the ovule. One gamete fuses with the egg nucleus to develop zygote.

11. Mechanism of Fertilization The process of fertilization or fusion of male gametes is a very fast process. Therefore it is very difficult to study actual process of fertilization. This process can be explained by proposed mechanism by Girassimova as follows. When male gamete enters ovule it appears like a thread rolled on ball. male gamete unrolls and lies on the surface of the female gamete or egg. Male and female gamete unites completely . Male gamete is in the form of chromatin with nucleolus. Finally nucleolus of male and female gametes fuses. Second male gamete fuses with secondary nucleus to form primary endosperm cell. This process is known as double fertilization or triple fusion.

12. Double Fertilization The process of double fertilization was proposed by S. G. Nawaschin (1898). According to him both nuclei of male gametes are involved in process of fertilization. One nucleus fuses with egg to for zygote. This process is called syngamy . Other nucleus fuses with secondary nucleus which has two nuclei to form endosperm. In this process fusion occurs twice therefore it is termed as double fertilization. Double fertilization is only found in angiosperms.

13. Triple Fusion. When pollen tube enters the ovule it releases two male gametes or nuclei. One nuclei fuses with the egg and forms zygote. Other nucleus or male gamete is still present in ovule. This nucleus fuses with secondary nucleus which is made up of two nuclei. Therefore second fusion of secondary nucleus and male gamete involves fusion of three nuclei, i.e. one from male gamete and two from secondary nucleus. Secondary nucleus is diploid (2n) and male gamete is haploid (n). This fusion results in triploid endosperm (2n+n = 3n) therefore it is called triple fusion. Endosperm is nutritive in function and provides nutrition to zygote up to development of embryo.

14. Significance of Double fertilization or Triple Fusion It plays an important role in process of seed formation. Nutritive tissue or endosperm is formed due to double fertilization. Endosperm provides nutrition to growing embryo. Endosperm is triploid therefore it is metabolically active and contains more reserve food. It helps to prevent unnecessary wastage of energy by plants. Endosperm provides nourishment to resting embryo during seed dormancy. It also provides nourishment to germinating embryo.

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