Pollination is the transfer of pollen from a male part of plant to a female part of plant, later enabling fertilization and the production of seeds. This process occurs from flower to flower in angiosperms or cone to cone in gymnosperms through two main types of pollination: self-pollination (also called selfing) and cross-pollination (also known as outcrossing).
Self-pollination occurs when the pollen from the anther is deposited on the stigma of the same flower (autogamy) or another flower on the same plant (geitonogamy), leading to the creation of a genetic clone through asexual reproduction. It happens in flowers where the stamen and carpel mature at the same time and are positioned so that the pollen can land on the flower’s stigma–a process often facilitated by the wind or the movements of insects. Some flowers–called cleistogamous–are self-pollinated while remaining closed, others before they open. Overall, self-pollination increases the reliability of producing seeds, the rate at which they can be produced, and lowers the amount of energy needed. It does not require an investment from the plant to provide nectar and pollen as food for pollinators. However, this pollination mechanism limits genetic variation, can lead to inbreeding depression–a phenomenon caused by the expression of deleterious recessive mutations–as well as the reduced health of the species due to the breeding of related specimens.
It is important to note that most of the self-pollinated plants are not exempt from being pollinated by insects, especially in environment where pollinator activity is important. Some insects such as bumblebees can still pry open the closed flower petals in search of nectar while depositing pollen from another plant, therefore increasing the risk of pollen contamination. Plant varieties where the end of the stigma extends beyond the fused, cone-shape stamens of the flower (like in some tomato varieties) can also be subject to cross-pollination as it becomes more accessible to pollinators. By the same token, selfers of different varieties that are planted too close together can significantly increase the risk of crossing, especially in cases where the plants are touching (read Module IV – Mainting Genetic Purity, Diversity, and Trueness-to-Type for more information about isolation techniques).
Cross-pollination (or allogamy) is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Because the genetic make-up of the sperm contained within the pollen from the other plant is different, their combination will result in a new, genetically distinct, plant, through the process of sexual reproduction. Those plants will show variation in their physiological and structural adaptations, making the population as a whole more resistant to adverse events in the environment and thus increasing the survival of the specie. For this reason, cross-pollination is usually the pollination method preferred by most flowers.
To avoid self-pollination as much as possible, numerous flowers have evolved while developing several mechanisms and methods through their morphology and behavior in order to facilitate the transfer of pollen and promote cross-pollination. Among others, dioecy (plant bearing either staminate or pistillate flowers), the maturation of pollen and ovaries at different times, as well as heterostyly (the condition of having unequal male and female reproductive organs) are such methods. The pollination of those flowers therefore mainly dependent on external factors and vectors. Most flowers can then be divided between two broad groups of pollination methods: biotic pollination, using vectors that attract and use insects (entomophily), birds (ornithophily), bats (chiropterophily) or other animals to transfer pollen from one flower to the next; and abiotic pollination, using non-living vectors such as the wind (anemophily) or, much less commonly, water (hydrophily).
A list of common pollinators can be found in Appendix I – Glossary.