SEXUAL REPRODUCTION IN FLOWERING PLANTS
Flowers are the seat of sexual reproduction in angiosperms. In the flower, androecium consisting of stamens represents the male reproductive organs 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.
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.
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.
List of Topics
|Flower-A Fascinating Organ of Angiosperms||Flower-A Fascinating Organ of Angiosperms|
|Pre-Fertilization: Structures and events||Stamen, Microsporangium and Pollen Grain.|
|a. The Pistil,|
|c. Embryo Sac|
|a. Kinds of Pollination|
|b. Agents of Pollination|
|c. Out Breeding Devices|
|d. Pollen Pistil Interaction|
|e. Artificial Hybridization|
|Double Fertilization||Double Fertilization|
|Post- Fertilization: Structures and events||Endosperm|
|Apomixis and Polyembryony||Apomixis and Polyembryony|