The microspore consists of a centrally situated nucleus, dense cytoplasm and two distinct walls, the outer exine and inner intine usually at 2 or 3 points the exine in absent, these points are called germpores. Exine is variously sculptured. Before discharge from the microsporangium, the nucleus divides forming a small cell called Generative cell which is adjacent to spore wall.
The remaining part of microspore is filled with a cell called vegetative cell (= Tube cell = Siphonogenic cell). It is in this two celled stage that pollen grains are usually released from the anther (In some plants, the generative cell may divide into two male gametes before liberation of pollen grains from anther), and reaches the stigma of the carpel.
A typical Gynoecium (= Pistil): The gynoecium (pistil) is the topmost central whorl of the flower and is made up of one or more carpel (megasporophylls). Each gynoecium consists of stigma at the top, an elongated style and the bulbous ovary. Each ovary contains one or more ovules (megasporangia). Ovary is the most important part of gynoecium.
The mature ovule is differentiated into the following parts, a centrally located large cell with eight nuclei called embryo sac (mature female gametophyte). Surrounding the embryosac is multicellular nucellus (2n). If there are 2 integument the voule is called bitegmic, if there is only one integument, the ovule is called unitegmic. Sunflower, Tagetus have only one integument. Some ovules do not have any integument, e.g.
Lovanthus, Santalum. The integuments are incomplete at the apex of ovule forming a pore called micropyle. The base of ovule from which integuments arise is called Chalaza. The ovule is attached to the placenta by a slender stalk known as funicle. The point of attachment of the body of the ovule to its stalk (funicle) is known as hilum. Sometimes the funicle continues beyond the hilum, along the body of the ovule forming a sort of ridge which is called raphe.
(1) Orthotropous (Straight):
The micropyle, chalaza and funicle lie in one straight line. e.g. Family Polygonaceae, Piperaceae.
(2) Anatropous (Inverted):
The body of the ovule is completely inverted so that micropyle and hilum come to lie very close to each other, e.g. All plants of gamopetalae.
When nucellus and integuments lie more or less at right angles to the funiculus.
When the ovule is curved, the micropyle is directed toward chalaza. Chalaza is situated at right angle to funicle. e.g. Members of Liguminosae family.
The ovule curvature is more pronounced and embryosac become horse-shaped, e.g. Families, Alismaceae, Butamaceae.
The funicle is exceptionally long and forms a complete circle around the ovules which is free from it except for a small area at the end of funicle. e.g. Opuntia and other members of families Cactaceae and Plumbaginaceae.
Either hypodermal cell at the apex of nucellus functions directly as Megaspore mother cell or hypodermal cell first divides unequally into outer parietal and an inner megaspore mother cell. The parietal cell in this case may from a variable number of wall cells.
Megasporogenesis (Megaspore Formation):
The first meiotic division of megaspore mother cell is always transverse forming a dyad of cells. Second meiotic division is also transverse resulting in a linear tetrad of megaspores. Linear tetrad of megaspores is most common, but T-shaped, 1-shaped or isobilateral tetrads may also occur.
Generally the lowermost megaspore (farthest from micropyle) is functional i.e. develops into- embryosac or female gametophyte. The first division of functional megaspore gives rise to two nuclei, the primary micropylar and primary chalazal nuclei.
The second division results in one pair of nuclei at each end, and the third division produces four nuclei at one end and four on the other end (i.e., at opposite poles of the now elongated developing embryosac). Three of the nuclei at the micropylar end become originated to form egg apparatus and one nucleus does not participate in the formation of egg apparatus and is called upper polar nucleus.
At the opposite (challazal) end of the developing-embryosac three nuclei differentiate as antipodal cells (or nuclei) and the fourth remaining nucleus as lower polar nucleus. The two polar nuclei from opposite ends (micropylar and chalazal) migrate into the centre of the embryosac giving rise to secondary nuclei (= definitive nucleus). This type of development is normal type of polygonum type. When the embryosac develops from one megaspore, it is called monocarpic embryosac. Many other variations are however found.
Structure of the Mature Embryo sac (Female Gametophyte) (Megagametogenesis)
A typical mature embryosac consists of a large cell with two polar nuclei (which on fusion give rise to secondary nucleus in the centre). At the micropylar end there is an egg apparatus (1 egg cell with 2 synergids) and at its chalazal end are found 3 antipodals. The cells of egg apparatus and antipodal cells are uninucleate and haploid but the central cell is first binucleate which fuses later forming a diploid nucleus called secondary nucleus = definitive nucleus.