The transfer of pollen grains from the anther of one flower to the stigma of another flower of the same plant is called:

Pollination is the transfer of pollen grains from the anther (the male part of a flower) to the stigma (the female part of a flower). It is an essential step in sexual reproduction in flowering plants, because it brings the male gametes close to the female gametes for fertilisation.

Pollination is classified into three types, depending on the source of the pollen:

Autogamy: pollen is transferred from the anther to the stigma of the same flower. This is self-pollination in the strict sense. It requires that the anther and stigma mature at the same time and lie close together.

Geitonogamy: pollen is transferred from the anther of one flower to the stigma of another flower on the same plant. The word comes from the Greek "geiton", meaning "neighbour". Functionally, it is cross-pollination, because it involves a pollinating agent such as an insect or wind. But genetically, it is similar to self-pollination (autogamy), because the pollen comes from the same plant and is genetically identical.

Xenogamy: pollen is transferred from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species. This is true cross-pollination. It brings genetically different pollen, increasing genetic variation in the offspring.

Some plants produce two kinds of flowers. Chasmogamous flowers open and expose their anthers and stigma, while cleistogamous flowers never open, ensuring self-pollination; examples include Viola (pansy), Oxalis and Commelina.

Pollinating agents include wind (anemophily), as in grasses, maize and rice; water (hydrophily), as in Vallisneria and Hydrilla; insects (entomophily), as in many brightly coloured, scented flowers; birds (ornithophily); and bats (chiropterophily).

Plants have developed several ways to avoid self-pollination and encourage cross-pollination, such as producing male and female flowers on different plants (dioecy, as in papaya), or having anthers and stigmas that mature at different times.

Why not the other options?

• ✗Autogamy is pollination within the same flower.
• ✗Xenogamy is pollination between flowers of different plants.
• ✗Cleistogamy refers to closed flowers that self-pollinate.