Sexual Reproduction In Flowering Plants

In nuclear type of endosperm the division of the primary endosperm nucleus and a few subsequent nuclear division are not accompanied by wall formation. So numerous nuclei are freely suspended in its sap. In coconut, the primary endosperm nucleus undergoes a number of free nuclear divisions. When the fruit is about 50 mm long before the embryo sac gets filled with a clear fluid in which float numerous nuclei of various sizes. At a later stage (about 100 mm long fruit) the suspension shows in addition to free nuclei, several cells each enclosing variable number of nuclei. Gradually these cells and free nuclei start setting at the periphery of the cavity and layers of cellular endosperm start appearing. This forms the coconut meat. In mature coconuts the liquid endosperm becomes milky and it does not contain free nuclei or cell.

Based on the destination of pollen grains, two types of pollination are recognised. When pollen grains are transferred from an anther to the stigma of the same flower the process is called self pollination or autogamy. Cross-pollination is further classified depending on whether the pollination has occurred between two flowers on the same plant (geitonogamy) or between two flowers on different plants (xenogamy).

The central cell is the largest cell of the embryo sac. It has a highly vacuolate cytoplasm which is rich in reserve food and golgi bodies. In the middle the cell contains two polar nuclei which have large nucleoli The polar nuclei fuse to form a single diploid secondary or fusion nucleus.

One hypodermal nucellar cell of the micropylar region differentiates into the sporogenous cell. It forms a diploid megaspore mother cell or megasporocyte. The megaspore mother cells undergo meiosis and forms a row of four haploid megaspores. Only the chalazal megaspore remains functional megaspore enlarges and gives rise to female gametophyte, also called embryo sac.