order of dicotyledonous flowering plants, a division of the subclass Rosidae. It includes the citrus and other species important for their fruits. Most of its members are trees (some of which are prized as sources of timber), shrubs, and woody vines.

Distribution and abundance

The Sapindaceae (soapberry family), with about 150 genera and 2,000 species, occurs mainly in the tropical areas of the world and is especially abundant in the American tropics. Species range from trees and shrubs to lianas or herbaceous vines. The family is found throughout the wetter tropics and subtropics, extending north to Japan and south to New Zealand. The largest genera are Serjania (about 230 species), which occurs from the southern United States to tropical South America and has a main centre of diversity in southeastern Brazil, and Paullinia (about 190 species) in the American tropics and subtropics. Both are lianas or vines. Allophylus, another tropical and subtropical vine genus, may have as many as 190 species or as few as 1, depending on the authority.

The Rutaceae (the citrus, or rue, family) consists of shrubs, trees, and a few herbs. It has about 160 genera and 1,700 species, which are widespread in distribution but occur primarily in tropical and warm-temperate areas. The largest numbers of genera and species are found in southern Africa and Australia. Many of these grow in semiarid woodlands. The largest genus, Zanthoxylum (Fagara), has about 280 species and occurs in temperate North America and East Asia and throughout the tropics. Agathosma (135 species) is endemic to South Africa. Other genera with about 70 species each are Pelea, which occurs in the Pacific Islands (mainly the Hawaiian Islands); Haplophyllum, which occurs from the Mediterranean region to eastern Siberia; Melicope, occurring from Indo-Malaysia through Australia and New Zealand to the Pacific Islands; and Boronia, one of the largest endemic Australian genera.

The Anacardiaceae (the sumac family) consists of perhaps 70 genera and 650 species of trees, shrubs, or woody vines. They occur mostly in the tropics and subtropics, but a few genera extend into both the North and South Temperate zones. The largest genus by far is Rhus, with about 250 species in the subtropics and warm-temperate areas of the world. There are no other genera of comparable size, but Semecarpus (occurring from Indo-Malaysia to Micronesia) has about 60 species, Mangifera (occurring in Southeast Asia and Indo-Malaysia to the Solomon Islands) has 40, and Schinus (occurring from Mexico to Argentina) has 30.

The Burseraceae (the frankincense family) has 16 genera and 600 species of trees and shrubs. The family occurs throughout the tropics and is especially common in tropical America and northeastern Africa, Madagascar, and the Arabian Peninsula. Many species dominate the forests or woodlands in which they grow. The largest genus, Commiphora (150–200 species), is found mostly in the drier areas of northeastern Africa, Madagascar, and from Arabia to India. Three genera with about 100 species each are: Bursera, occurring in tropical America with its centre of diversity in Mexico; Protium, occurring mostly in wet lowland tropical America but with a few species in Madagascar and Malaysia; and Canarium, occurring in the forests of the Old World tropics.

The Meliaceae (the mahogany family) has 51 genera and about 575 species of trees and shrubs and a very few perennial herbs. Many are large, widespread, and common or dominant trees in tropical and subtropical, primary and secondary forests, with only a few species in temperate areas. About three-quarters of the species occur in the 6 largest genera: Aglaia (100 species) in Indo-Malaysia (a region comprising India, South China, and Southeast Asia) and tropical Australia; Trichilia (about 90 species), which occur commonly as understory trees in lowland forests from Mexico to the West Indies, tropical South America, and tropical Africa; Dysoxylum (about 75 species) from Indo-Malaysia to the islands of the Pacific; Turraea (60 species) in tropical and southern Africa to Australia; Chisocheton (51 species) in Indo-Malaysia; and Guarea (40 species) in tropical America and tropical Africa.

The Zygophyllaceae (the bean caper, or creosote bush, family) contains 27 genera and 270 species of mostly shrubs, but a few are trees or annual or perennial herbs. They are widespread in the warmer, drier areas of the world, where some species are extremely abundant. Creosote bush (Larrea tridentata) is the most common species in the warm deserts of North America. The largest genus, Zygophyllum, contains about 100 species and is found from northern and southern Africa to Central Asia, India, and Australia. Fagonia (which has 40 species) is widespread, occurring in the warm deserts of North and South America, the Canary Islands, the Mediterranean region, southwestern Africa, and southwestern Asia to northwestern India. Balanites (containing 25 species) is found from tropical Africa to Myanmar, and is separated into its own family, Balanitaceae, by some authorities.

The Aceraceae (the maple family), with 2 genera and about 200 species, is not a large family, but its trees dominate many areas of North Temperate forest. Acer (maple), with about 200 species, is found across the North Temperate region from western North America to Japan, and trees of the genus form an important component of the deciduous forests of North America, Europe, and Asia. The genus dips southward to Guatemala and through Southeast Asia to Malaysia and the Philippines, where species may be abundant in tropical mountain forests. The major centre of distribution for the genus is China, where half of the species occurs; about 90 percent of the species are Asian. There are eight species of Acer with compound leaves that are sometimes placed in a separate genus, Negundo. Dipteronia is a genus of central and southern China with only one or two species.

The Simaroubaceae (the quassia family) is a family of trees and shrubs with about 20 genera and 150 species that are mostly tropical in distribution. Picramnia, shrubs and small trees found from Mexico and the West Indies to tropical South America, has about 55 species. The only other genus of any size is Quassia, with 40 species in the rain forests of tropical America and Africa.

The Staphyleaceae (the bladdernut family) contains 5 genera and about 60 species of mainly North Temperate and tropical trees and shrubs that occur irregularly from western North and South America to Europe, eastern Asia, and Japan. The largest genus, Turpinia, contains about 40 species in Central and South America and in Asia from Sri Lanka to Japan. Staphylea has 9 or 10 species in the North Temperate region.

The Hippocastanaceae (the horse chestnut family) has only 2 genera of trees and shrubs. Aesculus, with about 13 deciduous species, has an interrupted distribution in temperate forests from western and eastern North America (7 species) to the Balkan Peninsula in Europe (1 species) and in Asia from India to China and Japan (5 species). The 2 evergreen species of Billia occur as isolated trees in tropical forests from southern Mexico to northern South America.

The Melianthaceae (the honeybush family) has 2 genera of shrubs or small trees. Melianthus has 7 species in southern Africa. Bersama has a great deal of morphological variability. Some botanists recognize 2 species; others recognize 30. These are widespread in southern and west-tropical Africa.

The Julianiaceae (the huachalata family) contains 2 genera and 5 species of tropical shrubs or small trees. Amphipterygium (with 4 species) occurs in southern Mexico and Guatemala, while Orthopterygium (with 1 species) grows in Peru. They are characteristic trees of tropical deciduous forests.

The Cneoraceae (the spurge olive family) has only 2 genera and 3 species of shrubs: Cneorum, with 2 species, one in the western and central Mediterranean and one in Cuba, and Neochamaelea, with one species in the Canary Islands.

The Bretschneideraceae (the bretschneidera family) is one of the two families of the Sapindales with only one genus and one species, Bretschneidera sinensis, a deciduous tree found in low mountain forests of southwestern China. The Akaniaceae (the akania family) is the other family in the order with only a single genus and species. Akania lucens is a small tree common in subtropical coastal rain forests in eastern Australia.

Economic and ecological importance

Many members of the order are important economically, particularly for their timber or fruits. A few tropical species of the family Sapindaceae produce useful wood for construction, furniture, or fuel, but many are better-known for their fruits. Akee (Blighia sapida) from West Africa, wild prune (Pappea capensis) from tropical and southern Africa, and Pometia pinnata from New Guinea are the larger trees of the family that provide timber. The akee and Pometia also have edible fruits. The akee, which looks and tastes like scrambled eggs when cooked, is the national fruit of Jamaica, where it is widely grown and eaten; it is, however, poisonous if not cooked at the correct stage of ripeness—i.e., after the fruit has opened naturally and is still fresh. Only the fleshy aril around the seed is eaten.

Other popular tropical fruits from the Sapindaceae come from litchi (Litchi chinensis), longan (Dimocarpus longan), rambutan (Nephelium lappaceum), and Senegal cherry (Aphania senegalensis) from the Old World, and mamoncillo (Melicoccus bijugatus) from the New World. Many of these are widely cultivated. Useful oils are expressed from the seeds of wild prune and Ceylon oak (Schleichera oleosa); the latter is the source of macassar oil, and this species harbours the lac insect (Laccifer lacca), which produces lac, a resinous excretion that is a source of shellac. The seeds of the Brazilian and Paraguayan vine guarana (Paullinia cupana) are ground to make a beverage, guarana, a popular drink in the region that contains three times more caffeine than does coffee. The bark of yoco (P. yoco) is used for the same purpose.

A few species of Sapindaceae also are grown as ornamentals. Golden rain tree (Koelreuteria paniculata), a small tree from China, Korea, and Japan, is commonly cultivated in temperate regions for its large pyramidal clusters of yellow flowers and conspicuous bladderlike fruits. Balloon vine (Cardiospermum halicacabum), an annual from the tropics and subtropics, is grown for its small balloonlike fruits in many areas, where it sometimes escapes and becomes naturalized. Hopbush (Dodonaea viscosa), a widespread tropical shrub, is cultivated in warmer areas for its colourful foliage. The akee is grown not only for its fruits but also as a shade tree.

Many tropical species of the family Meliaceae are important timber trees. The best known, West Indian mahogany (Swietenia mahagoni), a native of the West Indies, is one of the world's most valuable woods. Honduras mahogany (Swietenia macrophylla), from Mexico to Bolivia, is also well known. Other valuable timbers of the Meliaceae from the American tropics are derived from species of Cedrela; West Indian cedar (C. odorata), for example, is the source of wood used for making cigar boxes.

The eight species of Khaya in tropical Africa and Madagascar, including African mahogany (K. senegalensis), produce a wood like that of mahogany, as do the nine species of omu, sapele, and utile mahoganies (Entandrophragma). Important Asian timber trees are toon (Toona ciliata) in India, chittagong wood (Chukrasia tabularis) in China to Indo-Malaysia, and Indian redwood (Soymida febrifuga) in Indo-Malaysia. Australian mahogany (Dysoxylum fraserianum) is a timber tree of the forests of northern and eastern Australia. The wood of these genera, all of Meliaceae, is typically reddish in colour, lustrous, easy to work, and free of warping under changes in relative humidity. Such characteristics make the wood highly suited for cabinetmaking.

Some species of Meliaceae have medicinal properties. The bark of the Indo-Malaysian neem, nim, or margosa (Azadirachta indica) is astringent and may also be used to make an insecticide. The bark of cocillana (Guarea rusbyi) from South America is an emetic. The seeds of other species yield useful oils. Several species of Carapa produce oils used for soap: in South America (crabwood, C. guianensis), in Indo-Malaysia (C. procera), and in East Africa (cape mahogany, Trichilia emetica). The seeds of the Malayan Chisocheton macrophyllus are a source of an oil used for burning in lamps.

A few Meliaceae have edible fruits. Popular in Southeast Asia are langsat (Lansium domesticum) and santol (Sandoricum koetjape), while the flowers of Aglaia odorata are used to flavour tea in East Asia. Chinaberry (Melia azedarach), a Himalayan tree, is widely grown as an ornamental and is particularly common in the southeastern United States. It has clusters of fragrant, lilac-coloured flowers and attractive but poisonous yellow berries.

Like the Sapindaceae, members of the Rutaceae are best known for their fruits. Citrus is the most important genus in the family; its fruits are an important source of vitamin C. A genus of small evergreen trees from South China to Indo-Malaysia, Citrus species are now cultivated throughout the warm temperate and tropical areas of the world, especially lime (Citrus aurantifolia), Seville orange (C. aurantium), lemon (C. limon), citron (C. medica), mandarin orange (C. nobilis), grapefruit (C. paradisi), tangerine (C. reticulata), and sweet orange (C. sinensis). A few other genera also yield edible fruits. The Asian bael (Aegle marmelos), kumquat (Fortunella japonica), and elephant apple (Limonia acidissima) have fruits that are widely eaten, while those of Japan pepper (Zanthoxylum piperitum) are used as a condiment.

The fruits of Aegle are used medicinally, as are those of the Chinese hardy orange (Poncirus trifoliata). Many species of Rutaceae have bitter bark that is medicinal, including the tropical American angostura (Angostura febrifuga) and casparia (Galipea officinalis). The bark of several North American species of Zanthoxylum is chewed for toothache.

Essential oils found in the tissues of Rutaceae are the source of the characteristic flavours of fruits, bark, gums, and leaves. The leaves of rue (Ruta graveolens) are a flavouring herb in Europe, and oil of rue is distilled from them. Leaves of orange jasmine (Murraya koenigii) are used in curries. Citrus oils are commonly expressed from the rinds of different citrus fruits or from their flowers. Perhaps the best known is oil of neroli from the Seville orange, widely used in perfumery in France.

A few species of Rutaceae are grown as ornamentals in temperate regions. Poncirus is a spiny hedge shrub, while Japanese skimmia (Skimmia japonica) and Chinese skimmia (S. reevesiana) have white flowers and red berries, the North American hoptree (Ptelea trifoliata) has attractive winged fruits, and the northeast Asian Amur cork tree (Phellodendron amurense) is a valuable shade tree. Burning bush (Dictamnus albus) is one of the few members of the family that is not woody. This poisonous perennial herb, native from southern Europe to northern China, is grown not only for its attractive whitish flowers but also for its ethereal oil; the glands of this plant can be squeezed and the oil expressed into the air and ignited by a match—hence the common name.

Only a few species of Rutaceae are important timber trees. The Indo-Malaysian satinwood (Chloroxylon swietenia), eastern Australian yellow wood (Flindersia oxleyana), and Caribbean West Indian silkwood (Zanthoxylum flavum) are used for veneering and cabinetmaking.

The Anacardiaceae, like the Rutaceae and Sapindaceae, are known for their fruits. Cashew (Anacardium occidentale), a tropical South American tree, was one of the first fruit trees to be distributed throughout the tropics by early Spanish and Portuguese adventurers. The seed produces the cashew nut, while the reddish, swollen axis under the fruit proper is the cashew apple. The former is roasted and eaten, while the latter is crushed, strained, and added to water and sugar to make a refreshing drink consumed throughout tropical America. Mango (Mangifera indica), native to Indo-Malaysia, now also is cultivated throughout the tropics. The fruit of this large tree has a thick, aromatic, tasty flesh; it has been called the queen of the tropical fruits. Several species of the tropical genus Spondias (hog plum, Jamaica plum, mombin, otaheite apple, and Spanish plum) are cultivated for their fleshy fruits, eaten fresh or cooked. The pistachio nut (Pistacia vera) is a temperate tree, native from the Mediterranean to Afghanistan, but now also cultivated in North America for its edible seeds.

The Peruvian pepper tree (Schinus molle) has fruits that are not eaten directly, but they are the source of a fermented alcoholic drink. This is a commonly grown ornamental in subtropical America, where it has frequently become part of the natural habitat. It is grown for its feathery, evergreen, compound leaves and red berries (of the female tree). The ground seeds are used as a condiment or as an adulterant in pepper. It grows well on poor soils. The dhobi nut (Semecarpus anacardium) has young fruits with a black resin insoluble in water that is used as a marking ink in Southeast Asia.

Other species of Anacardiaceae are also grown as ornamentals. The smoke tree (Cotinus coggygria), from southern Europe to central China, is a shrub with purplish foliage and large, diffuse inflorescences that give the “smoky” appearance. It is commonly planted in temperate regions. Several species of sumac (Rhus), particularly those from North America, are cultivated as shrubs, especially for their colourful reddish autumn foliage.

Few Anacardiaceae species are exploited for their wood for building or for cabinetmaking; however, many of them have valuable tannins that are extracted for tanning leather. Quebracho (Schinopsis quebracho-colorado) is a major source of tannins in South America, as is sumac (Rhus coriaria) in southern Europe. Species of Cotinus, Pistacia, Rhus, and Schinopsis are important sources of tannins for the leather industry.

Exudates from the stems of various species of Anacardiaceae yield lacquers, resins, or gums. The art of lacquering began in China centuries ago, reaching its climax of development during the Ming dynasty, AD 1368–1644. The lacquers used were obtained from the varnish tree (R. vernicifera). The milky exudate from this tree darkens and thickens rapidly on exposure to air. Lacquer, when applied as a varnish, provides remarkable protection, as it is unchanged by acids, alkalies, alcohol, or heat up to 70° C (158° F). The Burmese lacquer tree (Gluta usitata) produces a more slowly drying lacquer. The mastic tree (Pistacia lentiscus), from the Mediterranean region, produces mastic, a varnish used for coating metals and oil and watercolour pictures. It is one of the most expensive resins to produce. The tropical American Anacardium humile, A. occidentale, and A. nanum produce cashew gum, a substitute for gum arabic. Not only are they good adhesives, but they also contain a small amount of cashew oil, which can be used as an insect repellent or as a lubricant in the electrical insulation of airplanes.

Like some of the Anacardiaceae, most of the Burseraceae are known for their aromatic resins or gums. The most famous of these are Boswellia carteri and related species, the sources of frankincense, while Commiphora abyssinica and related species yield myrrh. C. opobalsamum furnishes balm of Gilead. All grow naturally or are cultivated in arid areas from Ethiopia to India with other species that produce resins, also used in incense and perfumes. Resin collecting is an important part of the economy in Ethiopia and Somalia. In tropical America, copal (Protium copal) and other protiums are tapped for their resins, which have been used in Central America as incense for religious purposes since pre-Columbian times. Likewise, indio desnudo (Bursera simaruba) and other burseras are exploited for turpentine or elemi (an oily resin) in tropical America. Some contain such large amounts of resin and burn so fiercely that they are known as torchwoods. The Indian black dammar tree (Canarium strictum) and Java almond (C. commune) of Indo-Malaysia, a source of Manila elemi, also produce commercially valuable resins. The seed of the latter, which is cultivated in Australia, is edible, as are those of several other East Asian species, which also may be processed to produce cooking oil. The fruits of C. album are eaten like olives.

A few Burseraceae are important timber trees. Probably the most important of these is gaboon mahogany (Aucoumea klaineana), from West Africa, used for veneers and plywood.

Within the family Zygophyllaceae are several valuable New World timber trees. The most famous of these is lignum vitae (Guaiacum officinale), a Caribbean tree with very hard, dense, and durable wood. It is used in making the bushings for ships' screws and for mallets. The wood contains gum guaiac, a resin that has been used medicinally since the 15th century as a specific, but ineffective, cure for syphilis. The guaiac test is still used, however, to detect blood in feces. The wood of holywood lignum vitae (Guaiacum sanctum), from the northern Caribbean and Central America, is used for making small objects that require weight, hardness, and strength. Maracaibo lignum vitae (Bulnesia arborea) is utilized for the same purpose in Colombia and Venezuela. The wood of Paraguay lignum vitae (B. sarmientoi) is the source of guaiac wood oil, which has a roselike scent and is used in soaps and perfumes.

A few species of Zygophyllaceae are edible. The fruits of several African species of Balanites and Asian species of Nitraria are sometimes eaten, and their seeds produce oils used in cooking or soap manufacture. The pickled buds of the North African bean caper (Zygophyllum fabago) are used as a substitute for capers. The seeds of harmal (Peganum harmala), a small shrub that grows naturally from North Africa to the Middle East and is a weed (not indigenous) elsewhere, are reputedly narcotic. They also produce the dye Turkey red. Some species of other genera also are weedy, but the most pernicious of these is puncture vine (Tribulus terrestris). This native of the Mediterranean region has been disseminated to all the drier warm areas of the world. It has hard fruits with sharp spines that easily attach to automobile and airplane tires and to the feet of grazing animals. The spines can injure an animal externally if touched or internally if eaten. Livestock that eat the plant may become very sensitive to light; ingestion of the fruits may cause death in these animals.

The best-known member of the Aceraceae is sugar maple (Acer saccharum). It has sugar-rich sap that is tapped in the early spring in eastern North America in order to make maple syrup and maple sugar. Sugar maple has been described as the most valuable hardwood in North America. Its figured wood (curly maple and bird's-eye maple) is valued for cabinetry and furniture; the plain wood is used for construction, flooring, and interior finish. The hard, strong, heavy, close-grained wood is often beautifully patterned. The sugar maple is a valuable ornamental and shade tree because of its thick, shapely crown and the bright yellow, orange, and red autumnal coloration. In addition, it yields valuable firewood.

Other North American maple species are less important as timber, paper pulp, sugar-producing, and ornamental trees. A few European species, and more Asian species, are sources of timber or grown as ornamentals for their foliage and leaf colours.

The Simaroubaceae is a family known for its medicinal plants, although the southern African white syringa (Kirkia acuminata) has wood that is worked into veneer, furniture, flooring, or household articles. Decoctions of the bark and wood of quassia wood (Quassia amara) are used to make an antimalarial tonic in tropical America. This species is widely cultivated for its red flowers and bitter bark. Likewise, bitters are prepared from the bark of cedron (Simaba cedron) in Central America and Jamaica quassia (Picrasma excelsa) in the West Indies. At one time, the bitter leaves and licorice-flavoured bark of the West Indian and Central American cascara amarga (Picramnia antidesma) were exported to Europe as a treatment for venereal disease. The astringent seeds of Brucea amarissima and B. sumatrana are used to treat dysentery in southeastern Asia.

The Staphyleaceae has a few members of minor economic importance. Several species of bladdernut (Staphylea) are grown as ornamental shrubs in temperate areas. Turpinia species are used locally in Asia and the West Indies for their timber. In China and Japan, the fruits of gonzin zoku (Euscaphis japonica) are used medicinally.

Next to the Aceraceae, the Hippocastanaceae probably have the most economic significance in temperate areas, in spite of the small size of the family. The horse chestnut (Aesculus hippocastanum) of the Balkan mountain region and the North American Ohio buckeye (A. glabra) and yellow buckeye (A. flava) have light, soft, tough, fine-grained wood. Once used for artificial limbs, splints, and various kinds of woodware, the wood is now most important as a source of paper pulp. The leaves and seeds are poisonous, and extracts from them are employed by certain native peoples to stun fish. Selected varieties and hybrids of Aesculus are frequently cultivated as ornamentals for their flowers or foliage or as shade trees.

The African family Melianthaceae has a few species of shrubs that are sometimes cultivated in warm-temperate areas. Honey bush (Melianthus major) and touch-me-not (M. comosus) have long racemes of reddish flowers that are attractive to honeybees. A decoction of the leaves of the honey bush is used for healing wounds, while the root, bark, and leaves of the touch-me-not are used in southern Africa for treating snake bites. Bitter bark (Bersama abyssinica), a tree, produces a hard, heavy wood suitable for house construction in West Africa.

In the family Julianiaceae, the Mexican huachalata (Amphipterygium adstringens) has a bark that can be applied medicinally as an astringent, for treating malaria, and for toughening the gums. It also contains tannins and a red dye. Cneorum tricoccon, a western and central Mediterranean species of the family Cneoraceae, has leaves and fruits that are used locally as a purgative. Neither of the single species of the families Akaniaceae or Bretschneideraceae appears to have any economic uses.

Characteristic morphological features

The order Sapindales is overwhelmingly composed of woody plants, mostly trees, large shrubs, or woody climbers. The latter are particularly common in the Sapindaceae (Paullinia, Serjania, and Urvillea) and Anacardiaceae (Toxicodendron). Small subshrubs and herbs are rare, although they are common in the family Zygophyllaceae. Many beautiful forest trees belong to this order. The largest trees are in the family Sapindaceae; they include the Ceylon oak (to 60 metres) in Myanmar, Pometia pinnata (to 50 metres) in Malaysia, and Tristiropsis canarioides (to 45 metres) in New Guinea. Several species of the Anacardiaceae, such as the Old World tropical Campnosperma auriculata, Mangifera lagenifera, and Buchanania lucida, may reach heights of 33 metres. In North America, the sugar maple may grow to 35 metres high. At the other extreme are a few genera of the Zygophyllaceae (Kallstroemia, Tribulopis, and Tribulus) with species that are prostrate annual herbs.

Most species of Sapindales have pinnately compound leaves (with the leaflets of each compound leaf arranged along both sides of a central axis). They are rarely palmately compound (the leaflets radiating from the petiole apex) or simple. Leaves are generally alternately arranged along a stem and are only rarely opposite or whorled. An interesting form of leaf is found in Citrus, where the simple blade is separated from the petiole by a joint. Because this type of leaf occurs in other genera of the Rutaceae with compound leaves, it is believed that this is reduced from a compound form. Reduction goes still further in Citrus, in which the first one or two leaves of an axillary bud (a bud located in an axil, or upper angle, between a stem and a branch or leaf) are reduced to spines. Leaves of the Asian genus Chisocheton (Meliaceae) show indeterminate or continuous growth. Stipules are usually absent from the leaves of members of the Sapindales, but the Melianthaceae, Staphyleaceae, and Zygophyllaceae rarely lack them.

The flowers may be single in the axils of the leaves, or they may be arranged in racemes (spikelike clusters that bloom from the base upward), cymose spikes (clusters blooming from the top downward), or panicles (many-branched clusters). Flowers of most Sapindales appear to be pollinated by insects. The coloured petals and sometimes stamens of many of these flowers make them conspicuous; fragrance makes others attractive to insects. The disc at the base of the pistil often secretes an abundance of nectar attractive to insects as well. In some species, cross-pollination is favoured by protandry (maturation of the stamens before the stigma of the same flower is receptive to pollen) or protogyny (the stigma matures first). In Kallstroemia (Zygophyllaceae) the petals and stamens spread horizontally from the pistil when the flower opens in the morning. The stigma is receptive to pollen carried in by insects (bees, wasps, butterflies, and flies) visiting the open flower for its nectar. By early afternoon the flowers begin to close, and the petals and stamens bend back upward, causing appression of the stamens, and what pollen they may still contain is placed onto the stigma, effecting self-pollination. This is a remarkable instance in which seed formation is ensured by self-pollination if necessary, but cross-pollination is first attempted. This is an important adaptation in a genus of plants growing in arid areas where their pollinators might not be present or abundant.

In a number of families and genera throughout the order, there is a strong tendency toward obligate cross-pollination through the progressive development away from the typical floral bisexuality, in which both sexes are present and functional in the same flower. This culminates in dioecism, a condition where male and female flowers in the species are borne on separate plants. Dioecism has evolved independently in most families. Many members of the order have bisexual and unisexual flowers on the same plant (polygamous). In some inflorescences, for example, the terminal flower is bisexual, while the others are all male or female. This combines the advantages of maximum fertility through inbreeding with increased variability through outcrossing. Many structurally bisexual flowers, as in many maples for example, are functionally unisexual, as either the stamens do not produce fertile pollen, the stigmas are not receptive, or the pistils do not contain ovules. Some species (e.g., Acer platanoides, Norway maple; A. saccharinum, silver maple; and A. spicatum, mountain maple) are self-sterile and must be outcrossed. Most unisexual flowers in the order have become that way after either the pistil or the stamens fail to mature, as the flowers often retain the rudimentary remains of the nonfunctional organ. Thus, although many species of the Sapindales have insured cross-pollination and outbreeding, most have not achieved complete anatomical dioecism.

Flowers in the Sapindales are either radially or bilaterally symmetrical and are typically small, although there are a number of exceptions to the latter. Generally, both sepals and petals are present. They are usually free, but in some genera sepals may be fused into a calyx or petals into a corolla (e.g., the tropical American genera of Burseraceae, Tetragastris and Trattinnickia). Petals may be lacking in some (e.g., Acer negundo, box elder).

Stamens are usually double in number of petals, with an outer whorl opposite the sepals and an inner whorl opposite the petals. In some they are equal to the number of petals and alternate with them, while in others there may be three whorls of stamens or there may be a larger number with whorls that are difficult to distinguish. Filaments are usually free, but they may be fused into a tube in most Meliaceae and some Rutaceae. In many Zygophyllaceae, filaments have a basal appendage. Anthers have two locules and open lengthwise. In species with unisexual flowers, staminodes (sterile stamens) are usually present in female flowers. There is almost always a nectar-producing disc between the stamens and the pistil.

The pistil is usually compound, but in a few genera (e.g., Zanthoxylum, Rutaceae) there are separate simple pistils in the flower. The ovary is almost always superior and is composed of 1 to 20 carpels. The ovary is usually multilocular, but it may be multilocular at the base and unilocular above in several members of the Meliaceae. It is unilocular in simple pistils. Ovules are typically two per locule, although they may be one or several. Styles are usually united and the same number as the carpels. Functionally male flowers often contain a pistillode (sterile pistil).

The fruit in Sapindales is basically a capsule that splits open between the internal partitions, although it has been modified into a number of other types in most families. For example, in the Rutaceae one finds capsules (e.g., Ruta, rue), follicles (Zanthoxylum, prickly ash), drupes (Amyris, torchwood), berries (Triphasia, limeberry), samaras (hop tree), and schizocarps (Helietta, barreta). The fruit of Citrus is a modified berry with a thick rind called a hesperidium (after the golden apples of the Hesperides). In the myth of the Greek hero Heracles, one of Heracles' 12 Labours was the fetching of the golden apples kept by the Hesperides. These mythical apples may well have been based upon oranges, which were not grown in the West in ancient times, but which the Greeks may have been aware of owing to their trade with the East.

When mature, the seeds of Sapindales may be with or without endosperm, the starchy nutrient tissue that supplies energy for the developing embryo. There are commonly one or two seeds per locule. Some genera of Rutaceae (e.g., Citrus and Zanthoxylum) and Anacardiaceae (Mangifera) contain many embryos in a single seed; these embryos may come from different parts of the ovule besides the fertilized egg. (The formation of an embryo from tissue other than the fertilized egg is called apomixis.) The seeds of Meliaceae are often winged and dispersed by wind. In many Sapindaceae and Burseraceae, the seed is surrounded by a colourful fleshy aril that is attractive to birds, which disperse these seeds. Most families have fleshy fruits attractive to mammals and birds and hard seeds that will pass through their alimentary canals without harm.

Evolution

The evolutionary relationships among the families placed in the Sapindales, and between them and other orders, are still somewhat obscure. No two authors of contemporary systems of classification seem to be able to agree on the circumscription of the order and its close relatives. Many of the tropical genera, in particular, are still poorly known. In spite of this, the families have many morphological and anatomical features in common. New data from such research areas as comparative biochemistry, palynology, paleobotany, embryology, and the natural history of sexuality, pollination, and seed dispersal are helping to establish their relationships.

The Sapindales consists of basically two groups of related families that have been placed together historically because of their morphological resemblances. One group is clustered around the Sapindaceae (Staphyleaceae, Melianthaceae, Bretschneideraceae, Akaniaceae, Hippocastanaceae, and Aceraceae), the other around the Rutaceae (Burseraceae, Anacardiaceae, Julianiaceae, Simaroubaceae, Cneoraceae, Meliaceae, and Zygophyllaceae).

The Staphyleaceae is the most primitive family in the Sapindales, with its unspecialized wood structure, mostly stipulate leaves, sometimes unfused carpels, usually several ovules per ovary locule, and well-developed endosperm. These are very primitive features for the Sapindales. Evidence from leaf and stem anatomy and flower morphology and anatomy indicate that the Staphyleaceae also shows close relationships with the Cunoniaceae (the cunonia family) and Saxifragaceae (the saxifrage family) of the order Rosales. Thus, the ancestry of the Sapindales may lie close to these families. In the Sapindales, the Staphyleaceae is most closely related to the Sapindaceae complex of families.

Morphologically, anatomically, and chemically, the Melianthaceae, Bretschneideraceae, Akaniaceae, Sapindaceae, Hippocastanaceae, and Aceraceae form a close-knit group of families. Indeed, there is evidence that the Aceraceae and Hippocastanaceae are so similar to the Sapindaceae that they should not be recognized as separate families but be placed in it. Akania and Bretschneidera also are placed by some in the Sapindaceae.

Morphologically and anatomically, the remaining families form another more or less cohesive complex. Chemically, however, they appear to form two groups, with the Burseraceae, Anacardiaceae, Julianiaceae (often put in the Anacardiaceae), and perhaps Zygophyllaceae in one group, and the Simaroubaceae, Cneoraceae, Meliaceae, and Rutaceae in another. In spite of this, the overall evidence indicates that the Zygophyllaceae is most closely related to the Rutaceae. The Zygophyllaceae has a number of herbaceous species and because of this and other derived characteristics, it is clearly the most advanced family of the Sapindales.