The organs of a plant that are involved in its sexual reproduction are its flowers. Figure 3 shows the structure of a typical flower

Sexual reproduction depends on a male gamete fusing with a female gamete. In flowers, the male gametes are inside the pollen grains in the anthers, while the female gametes are in the ovules in the ovary.

The journey of a male gamete to a female gamete in a flower takes place in two stages. First, the male gametes are carried inside the pollen grains, to a stigma. Second, the pollen grain grows a tube from the stigma to the ovule, through which the male gamete can safely travel to the female gamete.

The first stage of this journey, in which pollen grains are transferred from anther to stigma, is called pollination. We can define pollination as the transfer of pollen grains from the male part of a flower (anther or stamen) to the female part of a flower (stigma).

Insects come to the flower to collect nectar or pollen for food. The coloured petals, and perhaps a scent, advertise the presece of this food and therefore attract insects to the flower.

As the insects push down towards the nectaries, they brush past the anthers. Pollen grains stick to their bodies. When the insects feed at a second flower, some of the pollen grains may be brushed onto the stigma.

Some flowers fo not use insects for pollination, however. They rely on the wind. These flowers do not need to produce nectar, nor do they need brightly coloured petals. Wind-pollinated flowers are usually dull green or brown. Their anthers and stigmas hang outside the flower, to catch the wind. They often produce larger amounts of pollen than insect-pollinated flowers, to allow for the wastage that occurs. The wind, unlike an insect, will not carry the pollen directly to another flower. Pollen grains from wind-pollinated flowers tend to be very smooth and light, whereas insect-pollinated flowers often produce pollen with spikes that can stick onto an insect's body.

Pollen can be carried from an anther to a stigma on the same plant. This is called self-pollination. It cane ven be carried to the stigma on the same flower.

Alternatively, pollen can be carried from an anther to a stigma on a different plant of the same species. This is called cross-pollination. 

Both self-pollination and cross-pollination are part of sexual reproducton. Both of them involve gametes (insde the pollen grain and inside the ovules) and fertilisation. Even self-pollination is apart of sexual reproduction, even though the male and female gametes come from just one individual.

Once it has arrived on a stigma, a pollen grain will begin to grow a tube. It is stimulated to do this by chemicals, usually including sugars, secreted by the stigma, Different species of flowers secrete slightly different combinations of chemicals, so pollen grains will normally only grow tubes if they land on a stigma of their own species.

The pollen tube grows right down through the style, towards and ovule inside the ovary. Enzymes are secreted from the tip of the pollen tube and these enzymes digest the tissues of the style. Inside the tube, the male gamete travels to the ovule. The male gamete is not a complete cell like a sperm cell; it is simply a haploid nucleus

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