Theory:

The transfer of pollen grains from the anthers of a flower to the stigma of a flower on another plant of the same species is known ascross pollination.
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Cross-pollination
 
In unisexual flowers, cross-pollination is unavoidable. Nature favours cross-pollination in bisexual flowers as well.
 
Advantages of cross-pollination:
  • Cross-pollination results in the formation of new varieties because the seeds produced by cross-pollination develop and germinate into better plants.
  • Comparatively, more viable seeds are produced.
  • Offspring can be healthier and disease resistant
Disadvantages of cross-pollination:
  • There are high chances for pollination failure due to the distance barrier.
  • Pollen grains are wasted in high amounts.
  • Cross-pollination may result in the generation of unwanted characters.
  • It depends on external factors for pollination.
  • Flowers should have bright-coloured petals, long stamen and pistils, fragrance, or  they should produce nectar to attract the pollinating agents.
Agents of cross-pollination:
Cross-pollination occurs only when a pollen grain is transferred from one flower to another by means of agents. These agents can be categorized into two categories.
  1. Biotic agents (zoophily): These agents are living organisms.
  2. Abiotic agents: Non-living factors act as agents.
Biotic agents
Pollination (Zoophily)
InsectsEntomophily
BirdsOrnithophily
BatsChiropterophily
SnailMalacophily
 
Abiotic agents
Pollination
WindAnemophily
WaterHydrophily
The type of pollination in which transfer of pollen grains occurs by living organisms such as insects, birds, bats and snails is known as biotic pollination.