Sexual Reproduction in Flowering Plants
Embedded Files
SUMMARY
Flowers are the seat of sexual reproduction in angiosperms. In the flower, androecium consisting of stamens represents the male reproductiveorgans and gynoecium consisting of pistils represents the female reproductive organs.A typical anther is bilobed, dithecous and tetrasporangiate. Pollen grains develop inside the microsporangia. Four wall layers, the epidermis, endothecium, middle layers and the tapetum surround the microsporangium. Cells of the sporogenous tissue lying in the centre of the microsporangium, undergo meiosis (microsporogenesis) to form tetrads of microspores. Individual microspores mature into pollen grains. Pollen grains represents the male gametophytic generation. The pollen grains have a two-layered wall, the outer exine and inner intine. The exine is made up of sporopollenin and has germ pores. Pollen grains may have two cells (a vegetative cell and generative cell) or three cells (a vegetative cell and two male gametes) at the time of shedding. The pistil has three parts – the stigma, style and the ovary. Ovules are present in the ovary. The ovules have a stalk called funicle, protective integument(s), and an opening called micropyle. The central tissue is the nucellus in which the archesporium differentiates. A cell of the archesporium, the megaspore mother cell divides meiotically and one of the megaspores forms the embryo sac (the female gametophyte). The mature embryo sac is 7-celled and 8-nucleate. At the micropylar end is the egg apparatus consisting of two synergids and an egg cell. At the chalazal end are three antipodals. At the centre is a large central cell with two polar nuclei.Pollination is the mechanism to transfer pollen grains from the anther to the stigma. Pollinating agents are either abiotic (wind and water) or biotic (animals). Pollen-pistil interaction involves all events from the landing of pollen grains on the stigma until the pollen tube enters the embryo sac (when the pollen is compatible) or pollen inhibition (when the pollen is incompatible). Following compatible pollination, pollen grain germinates on the stigma and the resulting pollen tube grow through the style, enter the ovules and finally discharges two male gametes in one of the synergids. Angiosperms exhibit double fertilisation because two fusion events occur in each embryo sac, namely syngamy and triple fusion. The products of these fusions are the diploid zygote and the triploid primary endosperm nucleus (in the primary endosperm cell). Zygote develops into the embryo and the primary endosperm cell forms the endosperm tissue. Formation of endosperm always precedes development of the embryo.The developing embryo passes through different stages such as the proembryo, globular and heart-shaped stages before maturation. Mature dicotyledonous embryo has two cotyledons and an embryonal axis with epicotyl and hypocotyl. Embryos of monocotyledons have a single cotyledon. After fertilisation, ovary develops into fruit and ovules develop into seeds. A phenomenon called apomixis is found in some angiosperms, particularly in grasses. It results in the formation of seeds without fertilisation. Apomicts have several advantages in horticulture and agriculture. Some angiosperms produce more than one embryo in their seed. This phenomenon is called polyembryony.
Flowers are the seat of sexual reproduction in angiosperms. In the flower, androecium consisting of stamens represents the male reproductiveorgans and gynoecium consisting of pistils represents the female reproductive organs.A typical anther is bilobed, dithecous and tetrasporangiate. Pollen grains develop inside the microsporangia. Four wall layers, the epidermis, endothecium, middle layers and the tapetum surround the microsporangium. Cells of the sporogenous tissue lying in the centre of the microsporangium, undergo meiosis (microsporogenesis) to form tetrads of microspores. Individual microspores mature into pollen grains. Pollen grains represents the male gametophytic generation. The pollen grains have a two-layered wall, the outer exine and inner intine. The exine is made up of sporopollenin and has germ pores. Pollen grains may have two cells (a vegetative cell and generative cell) or three cells (a vegetative cell and two male gametes) at the time of shedding. The pistil has three parts – the stigma, style and the ovary. Ovules are present in the ovary. The ovules have a stalk called funicle, protective integument(s), and an opening called micropyle. The central tissue is the nucellus in which the archesporium differentiates. A cell of the archesporium, the megaspore mother cell divides meiotically and one of the megaspores forms the embryo sac (the female gametophyte). The mature embryo sac is 7-celled and 8-nucleate. At the micropylar end is the egg apparatus consisting of two synergids and an egg cell. At the chalazal end are three antipodals. At the centre is a large central cell with two polar nuclei.Pollination is the mechanism to transfer pollen grains from the anther to the stigma. Pollinating agents are either abiotic (wind and water) or biotic (animals). Pollen-pistil interaction involves all events from the landing of pollen grains on the stigma until the pollen tube enters the embryo sac (when the pollen is compatible) or pollen inhibition (when the pollen is incompatible). Following compatible pollination, pollen grain germinates on the stigma and the resulting pollen tube grow through the style, enter the ovules and finally discharges two male gametes in one of the synergids. Angiosperms exhibit double fertilisation because two fusion events occur in each embryo sac, namely syngamy and triple fusion. The products of these fusions are the diploid zygote and the triploid primary endosperm nucleus (in the primary endosperm cell). Zygote develops into the embryo and the primary endosperm cell forms the endosperm tissue. Formation of endosperm always precedes development of the embryo.The developing embryo passes through different stages such as the proembryo, globular and heart-shaped stages before maturation. Mature dicotyledonous embryo has two cotyledons and an embryonal axis with epicotyl and hypocotyl. Embryos of monocotyledons have a single cotyledon. After fertilisation, ovary develops into fruit and ovules develop into seeds. A phenomenon called apomixis is found in some angiosperms, particularly in grasses. It results in the formation of seeds without fertilisation. Apomicts have several advantages in horticulture and agriculture. Some angiosperms produce more than one embryo in their seed. This phenomenon is called polyembryony.
Q WHAT IS APOMIXIS?
Ans. Apomixis in flowering plants is defined as the asexual formation of a seed from the maternal tissues of the ovule, avoiding the processes of meiosis and fertilization, leading to embryo development.
Q What is parthenocarpy?
Ans. Parthenocarpy, development of fruit without fertilization. The fruit resembles a normally produced fruit but is seedless. Varieties of the pineapple, banana, cucumber, grape, orange, grapefruit, persimmon, and breadfruit exemplify naturally occurring parthenocarpy.
Q Difference between parthenocarpy and apomixis.
Ans. Both apomixis and parthenocarpy are asexual modes of reproduction, apomixis is the formation of seeds whereas parthenocarpy is the formation of fruits without fertilization. Apomixis produces genetically identical mother cells whereas parthenocarpy produces genetically identical offsprings.
Autogamy examples: Rice, Wheat, Barley, Oats, Chickpea, Pea, Cowpea, Lentil, Green gram, Black gram, Soybean, Common bean, Moth bean, Linseed, Sesame, Khesari, Sunhemp, Chillies, Brinjal, Tomato, Okra, Peanut, etc.
Geitonogamy Examples: Maize, corn
Xenogamy Examples: Squash, broccoli etc.
MCQs
1. The plant parts which consist of two generations-one within the other
i. pollen grains inside the anther
ii. germinated pollen grain with two male gametes
iii. seed inside the fruit
iv. embryo sac inside the ovule
a. i only, b. i, ii, iii, c. iii & iv, d. i & iv
2. In water hyacinth and water lily, pollination takes place by
a. insects or wind, b. water currents only, c. wind and water, d. insects and water
Ans.
3. Which is the most common type of embryo sac in angiosperms?
a. Tetrasporic with one mitotic stage of divisions, b. Monosporic with three sequential mitotic divisions, c. Monosporic with two sequential mitotic divisions, d. Bisporic with two sequential mitotic divisions
Ans.
4. What type of pollination takes place in Vallisneria?
a. Pollination occurs in submerged condition by water, b. Flowers emerge above surface of water, and pollination occurs by insects, c. Flowers emerge above water surface, and pollen is carried by wind, d. Male flowers are carried by water currents to female flowers at surface of water.
Ans.
5. In which one of the following, both autogamy and geitonogamy are prevented?
a. Wheat, b. Papaya, c. Castor, d. Maize
6. Pollen grains can be stored for several years in liquid nitrogen having a temperature of
a. -120 C, b. -80 C, c. -196 C, d. -160 C
Ans.
7. Which of the following has proved helpful in preserving pollen as fossils?
a. Pollen kit, b. Cellulosic intine, c, Oil content d. Sporopollenin
8. Winged pollen grains are present in
a. mustard, b. Cycas, c. mango, d. pinus
Ans.
9. Functional megaspore in an angiosperm develops into an
a. endosperm, b. embryo sac, c. embryo, d. ovule
Ans
10. Attractants and rewards are required for
a. entomophily, b. hydrophily, c. cleistogamy, d. anemophily
Ans.
Google Sites
Report abuse