Sexual reproduction requires fusion of male and female gametes. The reproductive organ of flowering plants is the flower. Stamens (Androecium) which produce pollen are the male part. Pollen grains contains male sex cells.
There may be several stamens in each flower. Each stamen (Androecium) has two parts. The upper part is known as anther which bears pollen. It is held on the lower part called filament.
The pistil (Gynoecium) is the female part and its basal part is the ovary carrying eggs or ovules or female sex cells. The parts of the pistil are the stigma, style and ovary.
In most plants, each flower bears both male and female parts. They are termed bisexual. In some plants there are male flowers with only androecium and female flowers bearing only gynoecium. They are unisexual.
For fusion of the nuclei, pollen and ovule are brought together by several agencies like the wind, water, and insects. This transfer of pollen grain from anther to the stigma of a flower is called pollination. Self pollination is when pollen of a flower falls on its own stigma and fertilizes the ovule. In Cross pollination, pollen from one flower falls on the stigma of another flower of a different plant of the same species and then fertilizes the ovule of that flower. Agents like wind, water or insects help to transfer pollen from one flower to another.
For fertilization or fusion of nuclei of pollen and ovule, pollen is brought by any pollinating agent on the stigma of the pistil. Each pollen grain forms a pollen tube and pollen grain nucleus reaches the ovule as pollen tube pushes through the pistil. The fertilized ovules develop into seeds which are capable of germinating into seedlings and new plants.
Once seeds are formed, they get dispersed or are carried away from the parent plant and then germinate under favourable conditions.