Sexual Reproduction in Flowering Plants
Flowers are the site of sexual reproduction in flowering plants. The flowers are modified shoots, which have male and female reproductive organs along with accessory organs. A typical flower has four whorls, outer two whorls consist of, accessory organs and inner two whorls consist of essential organs.
The accessory organs consist of sepals and petals also known as calyx and corolla respectively. The essential organs are stamens or micro-sporophylls which form the androecium that is the male reproductive organ of the flower and the carpels or mega-sporophylls which form the gynoecium, i.e. female reproductive organ of the flower.
In a flower, androecium consists of stamens and gynoecium consists of pistils.
A typical anther is bilobed, dithecous and tetrasporangiate. Inside the microsporangia, pollen grains develop.
In angiosperm, the male gametophytic generation is represented by pollen grain. The pollen grain consists of two layers: the outer exine and inner intine.
Pollen grains contain either two cells that is a vegetative cell and generative cell or three cells that is vegetative cell and two male gametes.
The pistil consists of three parts – the stigma, style and the ovary. The ovules are present in the ovary. The ovule is made up of a stalk called funicle, protective integument, and an opening called micropyle.
The Pollination is the process of transfer of pollen grains from the anther to the stigma. Pollination is facilitated by pollinating agents that transfer pollens from the male part, anther to the female part, stigma to accomplish fertilisation.The pollinating agents can be: abiotic (wind and water) or biotic (animals). On the basis of pollinating agents, pollination is classified into Anemophily, Hydrophily and Zoophily. Anemophily or wind pollination is a form of pollination whereby pollen is transferred by wind.Hydrophily or water pollination is a form of pollination whereby pollen is transferred by water.Zoophily or animal pollination is a form of pollination whereby pollen is transferred by animals.Insect pollinated plants are called entomophilous plants.Bird pollinated plants are also called ornithophilous plantsBat pollinated plants are called chiropterophilous plants.
After compatible pollination, pollen grain germinateson the stigma. The pollen tube grows through the style, enters the ovules and releases two male gametes.
In angiosperms, double fertilization and triple fusion occurs.
The products of double fertilization and triple fusion are the diploid zygote and the triploidprimary endosperm nucleus. After fertilization the zygotedevelops into the embryo and the primary endosperm cell forms the endosperm tissue. The mature dicotyledonous embryo consists of two cotyledons while embryos of monocotyledonous plant has single cotyledon. After fertilisation, ovary develops into fruit and ovulesdevelop into seeds.
The process of formation of seeds without fertilization is called apomixes. It is found in some angiosperms, particularly in grasses.
Parthenogenesis refers to the development of embryo from the unfertilized egg cell.
Some angiosperms produce more than one embryo in their seed. This phenomenon is called polyembryony.