Pollination is the transfer of pollen to the receptive female reproductive structures of a seed-bearing plant. In angiosperms, or flowering plants, pollen moves from an anther to a stigma; in gymnosperms, it reaches structures associated with exposed ovules, commonly within female cones. Transport may involve animals, wind, or water. Pollination precedes fertilization but is distinct from it: pollen delivery alone does not guarantee the fusion of reproductive cells or the production of seeds. (openstax.org)
Biological process
A pollen grain is a reduced male gametophyte, part of the alternation of generations characteristic of plants. It carries or produces the sperm cells needed for sexual reproduction. In flowering plants, the anther produces pollen, while the stigma provides the receiving surface. The stigma, style, and ovary form parts of the female reproductive structure; the ovary contains ovules. (openstax.org)
When compatible pollen reaches a receptive stigma, it can germinate and form a pollen tube. The tube grows through the style toward an ovule and delivers sperm cells. In the characteristic double fertilization of flowering plants, one sperm fuses with the egg to form a zygote; another fuses with the central cell to initiate endosperm, which nourishes the developing embryo. The fertilized ovule develops into a seed, and the ovary commonly develops into a fruit. (openstax.org)
These stages need not occur immediately after pollen arrives. In some gymnosperms, more than a year may separate pollination from fertilization. This interval illustrates why the two processes are treated separately in reproductive biology. (openstax.org)
Self-pollination and cross-pollination
Self-pollination transfers pollen within one flower or between flowers on the same individual plant. Cross-pollination transfers it between different individuals, ordinarily of the same species. The distinction concerns the pollen’s origin, not its carrier: an insect can move pollen between flowers on one plant, while wind can transport it between plants. (fs.usda.gov)
Flower structure and reproductive timing influence which pathway occurs. Differences in the positions of anthers and stigmas can favor pollen exchange between flowers. Other plants bear male and female flowers on separate individuals, making transfer between plants necessary for sexual reproduction. (openstax.org)
Some plants also possess self-incompatibility, genetically controlled recognition mechanisms that prevent self-fertilization even when self-pollen reaches the stigma. Consequently, pollen deposition and reproductive compatibility are separate requirements. In orchards, many apple cultivars require a compatible second cultivar with an overlapping flowering period. A pollinizer supplies the pollen; a pollinator transports it. (open.lib.umn.edu)
Agents and mechanisms
Animal-mediated pollination involves insects—including bees, flies, beetles, butterflies, moths, and wasps—as well as birds, bats, and some other vertebrates. Pollen adheres to an animal during a flower visit and may subsequently contact a receptive stigma. Flowers often provide nectar or pollen as food, so pollen transport can occur as a consequence of feeding rather than deliberate reproductive assistance. (extension.umd.edu)
Floral color, scent, shape, and accessibility influence animal visits. Not all flowers provide rewards: some orchids attract visitors through deceptive scents or appearances, including signals resembling those of potential mates. Such systems can achieve pollen transfer without supplying food. (openstax.org)
In buzz pollination, certain bees vibrate flowers using their flight muscles, dislodging pollen from the anthers. Bumblebees perform this behavior on crops such as tomatoes and blueberries. Pollinator effectiveness therefore depends partly on whether an animal’s behavior matches the flower’s pollen-release mechanism. (extension.umn.edu)
Abiotic pollination uses nonliving carriers. Wind moves pollen in grasses and some trees; maize is a familiar wind-pollinated crop. Water also transports pollen in certain plants. These mechanisms do not depend on flower-visiting animals, although animals may still visit the plants. (extension.umn.edu)
Ecological and agricultural importance
Animal pollination supports biodiversity by enabling reproduction in many wild flowering plants. The 2016 assessment by IPBES estimated that approximately 87.5 percent of wild flowering plant species depend at least partly on animal pollination. Their reproduction helps sustain vegetation that supplies food and shelter to other organisms, linking pollination to food webs and ecosystem functioning. (files.ipbes.net)
In agriculture, pollination is an important ecosystem service. The same assessment reported that more than three-quarters of leading food crop types benefit from animal pollination, while crops with some dependence account for about 35 percent of global crop production volume. These figures describe different quantities; neither means that 75 percent of all food production would disappear without pollinators. (files.ipbes.net)
Dependence also varies with the harvested product. Apples, pumpkins, and many berries require effective pollination for satisfactory fruit production. Inadequate pollination can produce small or misshapen fruits. Crops harvested for roots, leaves, or tubers may not require pollination to produce those edible parts, although pollination can remain important for seed production and breeding. (extension.umn.edu)
Threats and management
Pollination systems are affected by habitat fragmentation, land-use change, pesticides, parasites, diseases, invasive species, and climate change. These pressures can interact, affecting both pollinator populations and their relationships with flowering plants. Their effects vary among species and locations rather than constituting a uniform global response. (cbd.int)
Management addresses floral resources, nesting habitat, pesticide exposure, and the requirements of particular crops and pollinators. Agricultural systems may combine wild pollinators with managed bees. Different bee species vary in the flowers they pollinate effectively, their nesting requirements, and the weather conditions under which they forage, so pollination management considers more than the number of visiting insects. (extension.umn.edu)