Pollination is the transfer of pollen from anther to stigma. Three main types based on the source of pollen and genetic consequences: Autogamy: pollen from anther to stigma of the SAME flower. Ensures seed set but produces genetically identical offspring (inbreeding). Geitonogamy: pollen transferred between different flowers but on the SAME plant. Requires pollinators but genetically equivalent to autogamy (same genome). Xenogamy: pollen from a genetically DIFFERENT plant of the same species. Only type that brings new genetic combinations. Produces genetically diverse offspring. This is the question asks about — the answer is XENOGAMY.

Xenogamy (from Greek: xenos = stranger/foreigner, gamos = marriage) is the transfer of pollen grains from the anther of one plant to the stigma of a genetically different plant of the same species. It is the only true cross-pollination. Genetic consequence: new allele combinations in offspring. Evolutionary advantage: genetic variation is the raw material for natural selection and evolution. Practical advantages: heterosis (hybrid vigour) — F1 hybrids from cross-pollination often show superior performance. Reduced expression of deleterious recessive alleles (masked by dominant alleles from other parent). Examples: most wind-pollinated plants (grasses, Pinus) practise xenogamy. Most insect-pollinated plants are adapted for xenogamy. All crops used to produce F1 hybrid seeds practise xenogamy (after emasculation of female parent).

Cleistogamy (Greek: kleistos = closed): production of flowers that never open and are self-pollinated within the closed bud. Always produces offspring genetically identical to parent. Examples: Commelina benghalensis: produces blue-petalled open flowers (chasmogamous) AND small closed underground flowers (cleistogamous). The underground cleistogamous flowers produce more seeds than aerial open flowers. Viola (violets): spring flowers are chasmogamous (open, cross-pollinated); late season flowers are cleistogamous (closed, self-pollinated). Oxalis: similar mixed strategy. Groundnut (Arachis hypogaea): flowers pollinate within closed bud → fertilised ovary buried underground by geocarpy → peanuts develop underground. Advantages of cleistogamy: seed production without pollinators (drought, cold, pollinators absent). Energy saving (no petals, nectar needed). Guaranteed reproduction. Disadvantage: no genetic variation → reduced adaptability in changing environments.

Many plants have evolved mechanisms to prevent self-pollination and promote xenogamy: Dichogamy (temporal separation): maturation of male and female parts at different times. Protandry: stamens mature before pistil (most common). Examples: Salvia (sage), sunflower (Helianthus), Ranunculus, Clerodendron. Protogyny: pistil matures before stamens. Examples: Mirabilis jalapa, rose, Ficus. Herkogamy (spatial separation): physical barriers prevent self-pollen from reaching own stigma. Glochids, hairs, or structural arrangement keeps anthers away from stigma. Example: Calotropis (anthers fused around stigma — insect must get past them). Dioecy (separate sexes on separate plants): Male plants (staminate) and female plants (pistillate) different individuals. Complete prevention of autogamy and geitonogamy. Examples: papaya (Carica papaya), date palm (Phoenix dactylifera), mulberry (Morus). Self-incompatibility: biochemical prevention (discussed in FAQs).

Insect-pollinated flowers (entomophilous) show adaptations to attract specific pollinators: Nectar guides: UV-visible patterns on petals guide bees to nectar (bees see UV). Flower colour: blue/violet/yellow for bees; red for birds (not visible to bees). Strong scent: for long-distance attraction. Landing platform: prominent lower petals for insect landing. Appropriate flower shape: tubular for long-tongued insects (butterflies, moths), open and flat for short-tongued insects (flies). Anthers and stigma positioned to touch insect body: bee enters flower → pollen deposited on bee → bee visits next flower → pollen deposited on stigma. Synchronised timing: flowers open when specific pollinators are active. Orchid specialisation: each orchid species often has one specific pollinator species (extreme specificity). Coevolution: both flower and pollinator evolve together over millions of years.

Wind-pollinated flowers (anemophilous) show opposite adaptations: Small, inconspicuous flowers (no attraction needed). No nectar (wind does not get rewarded). No scent. Large quantities of small, light, dry, smooth, non-sticky pollen. Pollen with air sacs or balloon-like extensions (e.g., Pinus pollen has two air sacs). Feathery, large stigmas to trap airborne pollen. Exposed stamens on long filaments to release pollen into wind. Often produce pollen before leaves (avoids leaf interference). Examples: all grasses (Poaceae) — wheat, rice, maize, barley, sugarcane, bamboo. Pinus, Cycas (gymnosperm). Oak (Quercus), birch (Betula), alder. Wind pollination = wasteful (most pollen lost) but effective for forest canopy trees where insects cannot efficiently move. Hay fever: allergy to wind-borne pollen — IgE-mediated type I hypersensitivity. Common allergens: Phleum (timothy grass), Ambrosia (ragweed), birch, oak.

Sexual deceit: plant mimics female of a pollinator species (visual, olfactory, tactile) to attract males without offering any reward. Ophrys orchid (bee orchid, Mediterranean region): one petal (labellum) has: same colours and patterns as female bee. Similar texture (trichomes mimic female bee hairs). Produces pheromones chemically similar to female bee sex pheromones. Male bee (Eucera/Andrena): approaches orchid → attempts to copulate (pseudocopulation) → picks up pollinia (pollen masses) on its head/body. Flies to another Ophrys → attempts copulation again → transfers pollinia to stigma. Orchid is pollinated without offering any reward to the bee. The deception: bee never succeeds in actual mating → keeps trying on orchids (only early in season before female bees emerge). Highly specific: each Ophrys species targets specific bee species. Co-evolution drives specificity.

Hydrophily: pollination by water. Two types: Ephydrophily: pollination on water surface. Vallisneria (water tape grass): female flowers have very long stalk → float on water surface. Male flowers released underwater → float to surface → carried by water currents to female flowers. After pollination, stalk of female flower spirals → draws fertilised flower underwater for seed development. Hydrilla: similar mechanism. Subhydrophily: pollination underwater (rare). Ceratophyllum (hornwort): pollen released underwater → non-sticky, specific gravity similar to water → drift to female flowers. Water-pollinated pollen: non-sticky, no exine (or reduced), elongated (ribbon-like in some). Most aquatic plants actually use wind or insect pollination (not water) — true hydrophily is rare. Example: only about 150-200 angiosperm species use hydrophily.