出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/06/23 10:00:29」(JST)
Nectar is a sugar-rich liquid produced by plants. It is produced in glands called nectaries, either within the flowers in which it attracts pollinating animals, or by extrafloral nectaries which provide a nutrient source to animal mutualists, which in turn provide anti-herbivore protection. Common nectar-consuming pollinators include bees, butterflies and moths, hummingbirds and bats.
Nectar is an ecologically important item, the sugar source for honey. It is also useful in agriculture and horticulture because the adult stages of some predatory insects feed on nectar such as almost all solitary wasps. In turn, these wasps then hunt agricultural pest insects as food for their young. For example, thread-waisted wasps (genus Ammophila) are known for hunting caterpillars that are destructive to crops.[citation needed]
Nectar secretion increases as the flower is visited by pollinators. After pollination, the nectar is frequently reabsorbed into the plant.[1]
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Nectar is derived from Latin nectar, the favored drink of the gods, which in turn is the Latinized version of Greek νέκταρ, néktar, presumed to be a compound of the PIE roots *nek-, "death", and -*tar, "overcoming", i.e. has a similar etymology to ambrosia. The earliest recorded use of its current meaning, "sweet liquid in flowers," was in AD 1609.[2]
Floral nectaries are generally located at the base of the perianth, so that pollinators are made to brush the flower's reproductive structures, the anthers and pistil, while accessing the nectar.
Extrafloral nectaries (also known as extranuptial nectaries) are nectar-secreting plant glands that develop outside of flowers and are not involved in pollination.[3] They are highly diverse in form, location, size, and mechanism. They have been described in virtually all above-ground plant parts—including leaves (in which case they are known as foliar nectaries), petioles, stipules, cotyledons, fruits, and stems, among others. They range from single-celled trichomes to complex cup-like structures that may or may not be vascularized.[4]
In contrast to floral nectaries, nectar produced outside the flower generally have a defensive function. The nectar attract predatory insects who will eat both the nectar and any plant-eating insects around, thus functioning as 'bodyguards'.[5] Foraging predatory insects show a preference for plants with extrafloral nectaries, particularly some species of ants and wasps which have been observed to directly defend the plants.[3] Among passion flowers, for example, extrafloral nectaries prevent herbivores by attracting ants and deterring two species of butterflies from laying eggs.[6] In many carnivorous plants, extrafloral nectaries are also used to attract insect prey.[7]
Extrafloral nectaries were originally believed to simply be excretory in nature (hydathodes). Their defensive functions were first recognized by the Italian botanist Federico Delpino in his important monograph Funzione mirmecofila nel regno vegetale (1886). Delpino's study was inspired by a disagreement between him and Charles Darwin with whom he corresponded with regularly. Darwin believed that extrafloral nectaries were simply hydathodes, while Delpino believed they had a defensive function, especially among myrmecophilic plants.[8]
Extrafloral nectaries have been reported in over 3941 species of vascular plants belonging to 745 genera and 108 families. 99.7% of which belong to flowering plants (angiosperms), comprising 1.0 to 1.8% of all known species. They are most common among eudicots, occurring in 3642 species (of 654 genera and 89 families), particularly among rosids which comprise more than half of the known occurrences. The families showing the most number of recorded occurrences of extrafloral nectaries are Fabaceae, with 1069 species; Passifloraceae, with 438 species; and Malvaceae, with 301 species. The genera with the most number of recorded occurrences of extrafloral nectaries are Passiflora (322 species, Passifloraceae), Inga (294 species, Fabaceae), and Acacia (204 species, Fabaceae).[4] Other genera with extrafloral nectaries include Salix (Salicaceae), Prunus (Rosaceae) and Gossypium (Malvaceae).[6]
Foliar nectaries have also been observed in 39 species of ferns belonging to 7 genera and 4 families of Cyatheales and Polypodiales.[4] They are absent, however, in bryophytes, gymnosperms, early angiosperms, magnoliids, and members of Apiales among the eudicots.[4] Phylogenetic studies and the wide distribution of extrafloral nectaries among vascular plants point to multiple independent evolutionary origins of extrafloral nectaries in at least 457 independent lineages.[4]
Although its main ingredient is natural sugar (i.e., sucrose (table sugar), glucose, and fructose),[9] nectar is a brew of many chemicals. For example, the Nicotiana attenuata, a tobacco plant native to the US state of Utah, uses several volatile aromas to attract pollinating birds and moths. The strongest such aroma is benzyl acetone, but the plant also adds bitter nicotine, which is less aromatic and therefore may not be detected by the bird until after taking a drink. Researchers speculate the purpose of this addition is to drive the bird away after only a sip, motivating it to visit other plants to fill its hunger, and therefore maximizing the pollination efficiency gained by the plant for a minimum nectar output.[10] Neurotoxins such as aesculin are present in some nectars such as that of the California Buckeye.[11] All twenty of the normal amino acids found in protein have been identified in various nectars, with alanine, arginine, serine, proline, glycine, isoleucine, threonine, and valine being the most prevalent.[1]
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