4 POLLINATION and FERTILIZATION As You Have Read In
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BIOLOGY ASSIGNMENT CLASS 9th Chap -4 POLLINATION AND FERTILIZATION As you have read in the previous chapter that male and female gamete are borne on different parts of the flower For reproduction it is required that pollen grain must be transferred from anther to stigma of the carpel to bring abput Fertilization. These pollens are transferred by various agents by a process called as Pollination. POLLINATION The transfer and deposition of pollen grain from anther to stigma of the carpel in a flower is called Pollination. It is of two types – 1. Self Pollination 2. Cross Pollination 1.SELF POLLINATION or AUTOGAMY : When the pollen grain from anther of one flower are transferred to the stigma of the same flower or flower borne on same plant ,it is called as Self Pollination or autogamy. Self pollination mostly occur in Bisexual flower or monoecious plant. Advantages of Self Pollination : i. Chances of pollination increases if flower show homogamy. ii. Parental traits are preserved from generation to generation . iii. Very large number of pollen need not to be produced . Small number of pollen will do the same. iv. Flower need not to be large and showy. v. Scent and nectar need not to be present in flower to attract insects for pollination. Disadvantages of self Pollination : i. Repeated self pollination effects the vigour and vitality of the plant. ii. The weaker or defective character are inherited as such that weaken the species. iii. Genetic improvement can not take place. 2. CROSS POLLINATION When the pollen grains from the anthers of one flower are transferred on the stigma of flower borne on different plants belonging to same species , it is called Cross pollinaton or Allogamy or Xenogamy. Cross Pollination usually occur in Diecious plant having Unisexual flower. Advantages of Cross Pollination : i. It results in healthier offsprings. ii. New and improved varieties of plants can be produced by this method. iii. The seeds from cross pollination help in producing disease resistant varieties of plants. iv. The seeds produced by cross pollination are numerous, much better, healthy and vigorous. v. The yield by such seeds are very high . Disadvantages of Cross Pollination : i. It is not a sure method . Plants depends upon agents for pollination. ii. A large number of piollens need to be produced to ensure cross pollination. iii. It is less economical .Plants have to develop various devices like scent , nectar .vibrant coloured petals to attract various pollinating agents. iv. Many good character of plants are lost during recombination. Agents of Cross Pollination : For transfer of pollen grain from anther of one flower to stigma of another flower various biotic and abiotic agents are required. The most common agents are the Wind and Insects. Some agents of Pollination are : Agents of Pollination Term used 1. Wind Anemophily 2. Water Hydrophily 3. Animal Zoophily a. Insects Entomophily b. Birds Ornithophily c. Bats Chiropterophily d. Snail Malacophily e. Man Artificial pollination Some important adaptation for flower by various agents : ADAPTATION OF FLOWERS POLLINATED BY WINDS ( ANEMOPHILY ) : Flowers pollinated by winds have following adaptations : i. Flowers are neither large , nor showy or bright coloured .Corolla is usually small and conspicuous. ii. Stamens have long filament so they easily sway with the winds. iii. Pollens are produced in very large quantity. iv.The pollen grain produced are very light in weight. v. The stigmas are usually large and feathery and style is long enough to expose the flower. vi. Often ,the flowers appear and mature before new leaves arise. Example – pollination in maize , bamboo , Pine etc. ADAPTATION OF PLANTS POLLINATED BY WATER ( HYDROPHILY ) : Flowers pollinated by water generally the aquatic plants have following adaptations : i. Plants are dioecious. ii. Flower are small and inconspicuous .They are not scented . iii. Pollen grains are produced in large quantities. iv. The pollen grain have same specific gravity as that of water so that they can float in water. v.In some cases the male flower detach and floaton water till it meets the female flower . Example : Pollination in Hydrilla , Vallisnaria. ADAPTATION OF PLANTS POLLINATED BY ANIMALS : 1. ENTOMOPHILY : Insect pollinated flower have following characterstics : i. Day blooming flowers have brightly coloured large showy petals eg. Rose ,sunflower etc. ii. Night blooming flowers are often white and strongly scented . eg. Jasmine , coffee etc. iii. Flower have nectarines which have a juicy sugary necter that attracts insects . iv. The pollen grain have rough , sticky surface. v. The surface of stigma is also sticky. 2. ORNITHOPHILY : Very few plants are pollinated by birds. These plants have following characterstics : i. The flower are usually present in groups. ii. The flowers are brightly coloured and produce abundant nectar. iii. Pollen grains are produced in large quantity. iv. Floral parts are leathery. v.Corolla is funnel shaped. 3. CHIROPTEROPHILY : It is a kind of cross pollination by bats. i. The flower are strongly scented. ii. It usually occur in tropical fruits. iii. They produce abundant nectar and pollens. 4. MALACOPHILY : Brought about by snail. 5. ARTIFICIAL POLLINATION : It is brought about by man artificially for producing useful varieties and to get the desired results. FERTILIZATION Successful pollination is followed by Fertilization. The fusion of male gamete with the female gametes leading to the formation of zygote or oospore is called Fertilization. STRUCTURE OF POLLEN GRAIN : The pollen grain is a microscopic unicellular structure. It is covered by two layer – Outer hard and thick exine and inner thin intine. The exine at certain places has pores called germ pore through which pollen tube emerges out during its germination.The pollen grain has two nuclei – tube nucleus and generative nucleus. STRUCTURE OF OVULE: Ovule is the small structure present in the ovary.It is attached to placenta with a small stalk called funicle. The funicle provide nourishment to the ovule. The ovule is protected by two protective layers called integument. It bears a small opening called micropyle at one end. The ovule contain structure called embryo sac. In the embryo the following structures are present – i. A female gamete called egg towards micropyle. ii. Two polar nuclei which fuse to form the secondary nucleus at the centre. iii. Two synergids iv. Three antipodal cells The embyo sac is surrounded by nutritive tissue called as the nucellus. STEPS INVOLVED IN FERTILIZATION- During Fertilization number of events takes place – i. After the pollen grains are deposited on the stigma ,they swell up by absorbing nutrients from stigma . Growth of pollen tube is stimulated. ii. From one of the germ pore in extine , the intine of pollen grain grows out into a fine tube called Pollen tube . iii. The pollen tube grows and passes from the style dissolving the tissue through its way and reaches ovary. iv. Mean while , the generative cells divide into two mal gamete or nuclei. The male gamete moves to the tip of the pollen tube and lie in the mass of cytoplasm. The tube nucleus degenerates.The pollen grain along with a polien tube and two male gamete constitute the mature Male Gametophyte v.The pollen tube carrying the two male gamete now turns towards the micropyle and enters the embryo sac.At the very time the pollen tube dissolves. vi. The two male gametes are released into the embryo sac. vii. One of the two male gamete fuses with the egg cell and forms the fertilized egg or Zygote. viii. The second male gamete fuses with the secondary nucleus ( definitive nucleus ). It forms the Endosperm nucleus. The secondary nucleus is formed by the fusion of polar nuclei. ix. The zygote develops into an embryo . Where the endosperm nucleus develops into endosperm.The endosperm provide nutrition to the developing embryo. Q. What do you mean by Double Fertilization and Triple fusion ? Ans . Fertilization characterstic of seed plant in which one SPERM NUCLEUS FUSES WITH THE EGG nucleus to form an embryo and another fuses with polar nuclei to form endosperm is called Double Fertilization The fusion of second male gamete with the two polar nuclei is often referred to as Triple fusion. Doble Fertilization : Egg cell (n ) + Male gamete ( n ) =Zygote Triple Fusion : Secondary nucleus ( 2n ) + Male gamete ( n ) = Endosperm (3n ) Q. What changes occur in an ovule after fertilization ? OR List some post fertilization changes in an ovule . Ans. After fertilization large number of changes occur in an ovule – i. The egg cell is surrounded by the cell wall and is called oospore. ii. The oospore develops into an embryo. iii. Definitive nucleus ,now called endosperm nucleus develops int Endosperm. iv. The synergids and Antipodal cells disappear. v. The two integuments develops into the seed coat – outer testa and inner tegument. vi. Complex and insoluble food is formed from simple and soluble food .This gets stored in cotyledons for future use. Vii. The ovule is converted into seed and the ovary matures into fruit. viii. The non essential whorls like clyx and corolla falls off. Q. What do you mean by Parthenocarpy ? Ans. The natural or artificial development of fruit without fertilization is called Parthenocarpy. Example : Banana, Pineapples ,Papaya ,figs ,cucumber ,grapes ,orange ,pear ,watermelon are some parthenocarpic fruits. Q.Write the biological significance of Fruits and Seeds. Ans . FRUITS : After the process of fertilization , the ovary ripens to form Fruits .The wall of ovary hardens and becomes fleshy and get covered by fruit wall called Pericarp.The fruit may contain one to many seeds . Significance of fruit : the formation of fruit has its own biological significance : i. Fruit protect the immature seeds from unfavourable climatic condition . ii. when mature the bright colors of fruit attracts the insects and animal for dispersal of seeds.