Stamen consists of an anther containing four pollen sacs or microsporangia, supported by a slender filament. Each sporangium contains mass of large cells showing prominent nucleus and abundant cytoplasm. These are the sporogenous cells or the microspore mother cells. Each microsporangium when mature, has a wall made up of distinct layers of cells.
Microspore mother cells undergo meiosis and each of them forms four haploid microspores (each of which represents first cell of male gametophyte or the pollen grain) arranged in a tetrad.
Development of male gametophyte (pollen grains) from a microspore
The wall of the microspore consists of two principal layers.
1. Outer exine, with some thin spaces (germ pores). Exine is made up of extremely durable substance called sporopollenin. The pollen tube grows out of the pollen grain through the germ pores.
2. Inner, thin cellulosic wall, the intine.
The microspore nucleus moves towards periphery and the cell divides into a large vegetative cell and a small generative cell.
At this stage pollen grains are released by the rupture of the stomium along the line of dehiscence of the anther. The Pollen grain itself is not, the male gamete. It is a structure which produces male gametes, therefore pollen grain is regarded as the male gametophyte in the flowering plants.
The pistil, megasporangium and embryo sac
The main part of the ovule is enclosed by two integument (covering) leaving an aperture (micropyle). The ovule is attached to ovary wall by a stalk (funiculus). The region of the ovule opposite the micropyle is called Chalaza.
The gynoecium or pistil represents the female reproductive part in the flower. Each pistil consists of a stigma, style and ovary. The ovary contains one or more ovules (integumented megasporangia) which after fertilization, give rise to the future seeds.
An ovule develops as a projection on the placenta in the ovary. It consists of a parenchymatous tissue called the nucellus which is covered by one or two coverings called integuments. The integuments surround the nucellus all around but leave a narrow passage, the micropyle, through which a pollen tube may enter at a later stage. As the ovule grows it is raised on a stalk like structure called funiculus which is attached to the placenta borne on the inner wall of ovary.
Development of female gametophyte
Within the nucellus, a single hypodermal cell (below the epidermis) enlarges and becomes the megaspore mother cell, which undergoes meiotic division and gives rise to four haploid megaspore cells, usually three of them degenerate and the remaining one becomes the functional megaspore.
The functional megaspore enlarges and its haploid nucleus undergoes three successive mitotic divisions. As a result 8 haploid nuclei are formed. This enlarged oval shaped structure with eight haploid nuclei is referred as the young embryo sac. These nuclei then migrate and get arranged into three groups. Three nuclei reach the micropylar end of the embryo sac and other three move in the opposite direction (i.e. the chalazal end) and the remaining two remain in the centre. The cell membranes and cell walls develop around all the nuclei excepting the two at the centre of the embryo sac which now is called the central cell.
Thus, in a mature ovule the embryo sac contains eight haploid nuclei but only seven cells. Three cells at the micropylar end, form the egg apparatus and the three cells at the chalazal end, are the antipodal cells. The remaining two nuclei called the polar nuclei may fuse to form the diploid secondary nucleus. In the egg apparatus one is the egg cell (female gamete) and remaining two cells are the synergids.
A fully developed embryo sac with the nucellus, integuments and funiculus, together constitute the mature ovule. In this condition the ovule awaits fertilization which must be preceded by pollination.
Function of cells and nuclei of embryo sac
Secondary Nucleus: During fertilization, the secondary nucleus fuses with one sperm to form a triple fusion nucleus (2n+n = 3n). This is called primary endosperm nucleus. It gives rise to the food storing endosperm of the seed in many plants.
Egg Cell: Fuses with the second male gamete (sperm) to give rise to the zygote, which develops into the embryo. This is called double fertilization.
Synergid Cells: Considered to help in fertilization by directing the pollen tube to the egg cell.
Antipodal Cells: Degenerate just before fertilization and contribute nutrition for the young embryo.