Pollination

Pollination is defined as the transfer of pollen grains from the anther to the stigma of a flower, and it is a characteristic feature of the spermatophytes (gymnosperms and angiosperms). In gymnosperms pollination is direct, since pollen lands straight on an exposed ovule, whereas in angiosperms it is indirect, landing instead on the stigma of an enclosed pistil. Most angiosperm flowers are chasmogamous - they open and expose their mature anthers and stigma for pollination - but some are cleistogamous, completing pollination without the flower ever opening, as in Commelina benghalensis, which produces both brightly coloured, insect-pollinated chasmogamous aerial flowers and dull, self-pollinated cleistogamous flowers on its underground rhizome branches. Based on which flower's stigma the pollen actually reaches, pollination is classified as self-pollination (autogamy), the transfer of pollen onto the stigma of the same flower - possible only in bisexual flowers - or cross-pollination (allogamy), the transfer of pollen onto the stigma of another flower; cross-pollination itself has two forms, geitonogamy (pollen moves to another flower of the same plant, typically in monoecious species - functionally cross-pollination, but genetically equivalent to self-pollination since the pollen source is the same individual) and xenogamy (pollen moves to a flower of a genetically different plant of the same species). Flowers use several adaptations to actively promote self-pollination: cleistogamy itself (flowers never open, guaranteeing self-pollination, as in Commelina, Viola, Oxalis) and homogamy, where the stamens and stigma of a flower simply mature at the same time, favouring self-pollination, as in Mirabilis jalapa and Catharanthus roseus. Other flowers instead carry contrivances (outbreeding devices) that actively promote cross-pollination. Dicliny (unisexuality) forces cross-pollination outright, and comes in two forms - monoecious plants bear separate male and female flowers on the same individual (coconut, bitter gourd; in castor and maize this still allows geitonogamy even though it blocks autogamy) while dioecious plants bear male and female flowers on entirely separate individuals (Borassus, Carica papaya, date palm), blocking both autogamy and geitonogamy. Monocliny (bisexuality) instead relies on structural adaptations within a bisexual flower: dichogamy staggers the maturation times of anther and stigma, either as protandry (stamens mature before the stigma, e.g. Helianthus, Clerodendrum) or protogyny (the stigma matures before the stamens, e.g. Scrophularia nodosa, Aristolochia bracteata) - though incomplete dichogamy, where the maturation windows overlap slightly, can still allow some self-pollination; herkogamy physically arranges the stamens and stigma so self-pollination becomes impossible even though both are mature at once (the reflexed style of Gloriosa superba, the stigma projecting far above the stamens in Hibiscus); heterostyly produces two or three floral forms differing in stamen and style length so that pollination succeeds only between organs of matching length - distyly in Primula (a long-styled 'pin' form with short stamens and small pollen, paired against a short-styled 'thrum' form with long stamens and large pollen) and tristyly in Lythrum (three floral forms, where pollen from one form fertilises only the other two, never its own); and self-sterility (self-incompatibility), a genetic mechanism in which pollen from a flower is simply unable to germinate on the stigma of that same flower, as in Abutilon and Passiflora. Pollination is carried out by many agents, grouped as abiotic (anemophily by wind, hydrophily by water) or biotic (zoophily, pollination by animals - of which entomophily, pollination by insects, is the commonest form used by the majority of angiosperms). Anemophilous (wind-pollinated) flowers - grasses, sugarcane, bamboo, coconut, palm, maize - are typically borne in pendulous, catkin-like or spike inflorescences raised well above the leaves, have a reduced or absent perianth, are small, colourless, scentless and nectarless, carry numerous long, exserted, versatile stamens producing enormous quantities of light, dry pollen (in Urtica the anthers even burst violently to fling pollen into the air), and usually bear large, protruding, sometimes feathery stigmas adapted to intercept airborne pollen, since wind pollination is essentially a chance event with a great deal of wasted pollen; maize (Zea mays) illustrates this well, since its heavy pollen cannot travel far on a light breeze but is instead shaken loose from the terminal male tassel and caught by the long silk (style/stigma) of the lateral female cob below. Hydrophilous (water-pollinated) flowers are comparatively rare even among aquatic plants (most, like Eichhornia and water lily, are actually still wind- or insect-pollinated) and come in two forms - epihydrophily, where pollination happens at the water surface (Vallisneria spiralis, Elodea), illustrated by Vallisneria's dioecious mechanism in which a long coiled stalk raises the female flower to the surface where floating, detached male flowers settle into a depression around it and effect pollination before the stalk coils back underwater for fruiting; and hypohydrophily, where pollination happens entirely underwater (Zostera marina, Ceratophyllum), as in the marine seagrass Zostera, whose long needle-like pollen has the same specific gravity as seawater so it floats freely at any depth until it coils around the large, elongated stigma. Zoophilous (animal-pollinated) flowers include ornithophily by birds (Erythrina, Bombax, Syzygium, Bignonia, Sterlitzia; large, tubular/cup/urn-shaped, brightly coloured, scentless, copiously nectar-producing, tough-textured flowers pollinated by hummingbirds, sunbirds and honeyeaters), cheiropterophily by bats (Kigelia africana, Adansonia digitata; borne singly or in clusters away from foliage, nocturnal-opening, strongly scented and nectar-rich - in Adansonia the bat clasps the projecting stamen ball and stigma against its breast while feeding, transferring pollen to the next flower it visits), malacophily by slugs and snails (some Araceae; water snails pollinate Lemna) and entomophily by insects, the most important group, chiefly bees along with moths, butterflies, flies, wasps and beetles - entomophilous flowers are usually large (or aggregated into dense inflorescences if individually small, as in Asteraceae), brightly coloured (sometimes with coloured bracts, as in Poinsettia and Bougainvillea), scented and nectar-producing, and flowers pollinated specifically by flies or beetles instead often produce a foul odour. Several plants have evolved striking specialised entomophilous mechanisms: in Salvia's lever mechanism, a visiting bee pushing into the bilabiate corolla strikes the sterile end of each anther's long connective, swinging its fertile lobe down to dust pollen onto the bee's back, which is later rubbed onto the stigma of the next flower visited; in Calotropis's translator mechanism (Asclepiadaceae), the pollen of each anther lobe fuses into a pollinium, pollinia are joined by a sticky clip (corpusculum) via thread-like retinacula into a Y-shaped translator that clips onto a visiting insect's leg or proboscis and is carried to the next flower's receptive stigma; and in Aristolochia's trap mechanism, downward-pointing stiff hairs lining the tubular perianth trap visiting flies until the anthers ripen and the hairs wither, releasing the flies dusted with pollen to visit and pollinate the next flower. Self-pollination has the advantage of near-certain seed-set in bisexual flowers and lets a species survive even when individuals are rare and widely scattered or when cross-pollination fails altogether, but repeated generations of self-pollination weaken the progeny and produce very few new varieties; cross-pollination, conversely, produces healthier, better-germinating offspring with more variation and better environmental adaptabil …