What is meant by cross-pollination? Why do plants need to cross breed?

Explanation

Cross-pollination involves the transfer of pollen grains from the anther of one plant to the stigma of another plant. With the help of certain biotic or abiotic agencies. It is also known as xenogamy.
Continued self-pollination leads to chances of inbreeding depression. Thus, flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination, which is the origin of new variations in progeny and helps in their survival.
Adaptations in plants to facilitate cross-pollination are as follows:
(i) Dichogamy In some plant species, receptivity of stigmas and pollen release is not synchronised, i.e. the pollen is often released before the stigma becomes receptive (protandry) or stigma becomes receptive before the release of pollen (protogyny). This condition is called dichogamy.
(ii) Heterostyly In some plant species, the anther and stigma are placed at different positions, so that the pollen cannot come in contact with the stigma of same flower. This condition is called heterostyly.
(iii) Self-incompatibility or Self-sterility is the third device to prevent inbreeding. It is a genetic phenomenon of preventing the pollen from same flower fertilising ovules by inhibiting pollen germination or pollen tube growth in the pistil.
(iv) Dicliny or Unisexuality effectively prevents self-pollination. It is the presence of unisexual flowers in plants that prevent autogamy but not geitonogamy, e.g. castor, maize etc.
(v) Herkogamy is seen in orchids where male or female sex organs themselves prove as a barrier to prevent self-pollination by some structural abnormalities.