order of dicotyledonous flowering plants in the Rosid I group among the core eudicots. The order comprises 4 families ( Fabaceae, Polygalaceae, Quillajaceae, and Surianaceae), 754 genera, and more than 20,000 species. However, more than 95 percent of the genera and species belong to Fabaceae, the legume family. Fabaceae is the third largest family of angiosperms (angiosperm), exceeded only by Asteraceae (aster or sunflower family) and Orchidaceae ( orchid family). Along with Poaceae (the grass family), Fabaceae is the most important plant family in the production of food for humans and livestock, as well as in the production of industrial products.

Because they develop bacteria-harbouring root nodules that maintain the nitrogen balance in the soil, which is necessary for plant growth, the legumes are also an essential element in nature and in agriculture. Legumes are perhaps best known by their more common cultivated names, such as peas (pea), beans (bean), soybeans (soybean), peanuts (peanut) (groundnuts), alfalfa (lucerne), and clover. The characteristic fruit of most legumes is a pod (legume) consisting, in essence, of an ovary that is a tightly folded leaf, as in a pea pod. The pod normally splits into two halves when mature.

Polygalaceae, the milkwort family, is the second largest family in the order, with about 21 genera and some 1,000 species. Its members are distributed worldwide, except for the Arctic and New Zealand. The genus Polygala contains about a third of the species in the family.

Most of these floral features, however, also can be found in other plant families. It is the pistil, or gynoecium, of Fabales that is unique. The single carpel develops into a fruit (the pod, or legume) that generally splits open (dehisces) along one or both edges (sutures) at maturity, releasing the seeds that have developed from the ovules. This basic legume type is idealized in a pea or bean pod, which bears two rows of marginally placed ovules along the upper suture. But evolution within the family has variously modified many legume fruits, and they bear but scant resemblance to that of a bean or pea. Some retain the form of the basic type but do not split open when ripe (indehiscent), as with Robinia (locusts) and Cercis (redbud). In many Fabaceae—for instance, Melilotus (sweet clover)—the fruit has been reduced to a single-seeded indehiscent structure that resembles a tiny nutlet. In others, it is several-seeded and indehiscent but is divided transversely into single-seeded segments that break apart at maturity (e.g., Desmodium). In another variant, the fruit coat becomes fleshy and plumlike as in the tropical Andira inermis (angelin tree). There are species in which the fruit is flattened and winged, facilitating wind transport. A few legumes have fruits that are produced or that mature underground; the peanut (Arachis hypogaea) is the best-known example. The peanut flower is actually produced above ground but assumes a position close to the soil surface as it ages. The ovary elongates and develops as a subterranean pod. All of these and other modifications are derivative and can be traced back to the basic, dehiscent pod of the pea or bean.

Seeds (seed and fruit) within the legumes are also variable, ranging from the size of a pinhead to that of a baseball. Legume seeds are sometimes quite colourful; the Abrus precatorius (jequirity bean) ( jequirity bean) and Ormosia species, for example, produce striking black and red seeds. These seeds have been used as currency by native peoples and in the production of beads and handbags, especially in the more tropical regions. They may be quite poisonous if eaten, however.

The subfamily Caesalpinioideae (classified as a family, Caesalpiniaceae, by some authorities) is a heterogeneous group of plants with about 160 genera and some 2,000 species. The latest classifications show that this subfamily is the most basal lineage among the legumes and the one from which the other two subfamilies evolved. In that sense it is not a true monophyletic group, and it will undoubtedly be treated taxonomically in a different way in the future. Caesalpinioideae legumes are found throughout the world but are primarily woody plants in the tropics. Their moderate secondary invasion of temperate regions is mostly by herbaceous (nonwoody) evolutionary derivatives. The presence of Gleditsia triacanthos ( honey locust) and of the related Gymnocladus dioica (Kentucky coffee tree) in temperate regions is a striking exception to this generalization, however, and they may represent more ancient and relictual lineages in the subfamily.

Mimosa pudica (sensitive plant) is sometimes grown as a novelty because its leaves quickly fold up when touched. Albizia julibrissin (mimosa, or silk, tree), a widely planted ornamental in the southern United States, folds its leaves together at dusk, decreasing by at least half the amount of leaf surface exposed to the atmosphere. The movement is caused by changes in water pressure in specialized structures at the base of the petioles and leaflets.

Like the other subfamilies, members of Papilionoideae have their origins in the tropics, but their occupation of the arid and temperate parts of the world, mostly as herbaceous plants, is far more extensive. In the forests, prairies, and deserts, they are among the most common plants. The largest genus of legumes, Astragalus (2,400 to 3,300 species, known as locoweed), is mainly western North American but also occurs in Eurasia, India, Africa, and South America. These temperate legumes have mostly pinnate leaves among which those with three leaflets (trifoliolate) are common—e.g., beans (bean) and soybeans (soybean). Trifoliolate leaves rarely occur in the other subfamilies. The large genus of Lupinus (lupines (lupine)) generally has 5 to 11 (occasionally up to 15) palmate leaves. The leaves of clovers (clover) are most commonly palmately trifoliolate, as are those of Baptisia. In one tribe the leaf axis terminates in a tendril, which facilitates climbing; members include the sweet pea and Vicia (vetches (vetch)). The symbiotic relationship between Rhizobium and the plant, which takes place in root nodules and “fixes” atmospheric nitrogen into compounds useful to the plant, is most strongly developed in Papilionoideae legumes.

The unique ecological role of Fabaceae is in nitrogen fixation. nitrogen is an element of all proteins (protein) and is an essential component in both plant and animal metabolism. Although elemental nitrogen makes up about 80 percent of the atmosphere, it is not directly available to living organisms; nitrogen that can be metabolized by living organisms must be in the form of nitrates (nitrate) or ammonia compounds. Through a mutual benefit arrangement ( symbiosis) between legumes (legume) and Rhizobium bacteria, nitrogen gas (N₂) is fixed into a compound and then becomes available to the biotic world. The legume plant furnishes a home and subsistence for the bacteria in root nodules. In a complex biosynthetic interaction between the host plant and the bacterium, nitrogen compounds are formed that are used by the host plant. These compounds are also available to other plants after decayed roots (and other plant parts) of the host plant have allowed these nitrogen products to be released into the soil. Animals obtain compound nitrogen by eating plants or other animals.

Legume nitrogen fixation is of prime importance in agriculture. Before the use of synthetic fertilizers (fertilizer) in the industrial countries, the cultivation of crop plants, with the exception of rice, was dependent on legumes and plant and animal wastes (as manure) for nitrogen fertilization. A common procedure was the use of crop rotation, usually the alternation of a cash grain crop such as corn (maize) with a legume, often alfalfa (Medicago sativa), in the temperate world. Apart from the nitrogen contribution, the legume in this case furnishes animal forage ( hay or silage). Pastures or other grazing areas must have legume components, such as a clover (Trifolium), as well as a grass component.

Forage legumes (which concentrate their vitamins and proteins in their young growing parts) also are grown as animal feed. Their role as such is especially common in countries that can afford the luxury of meat (luxury because livestock typically yield fewer calories than the plants they are fed). Some major forage legumes of the temperate world include clovers (clover), alfalfa, bird's-foot trefoil (Lotus corniculatus), and vetches (vetch). In the tropics or arid regions, some of the important elements of the habitat are species of Glycine (soybean), Stylosanthes, and Desmodium (tick trefoil).

Various forms of leucaena (such as Leucaena leucocephala) have been developed for animal forage, firewood, and construction, as well as for the high production of nitrogen that enriches impoverished soils, especially in the Asiatic tropics. Other important plants are acacia, used for animal food (both pods and leaf forage), for soil improvement and revegetation, and as a source of tannin and pulpwood; Cordeauxia edulis (yeheb), an uncultivated desert shrub of North Africa that has been so extensively exploited for food (seeds) that it is in danger of extinction; Ceratonia siliqua ( carob), a Mediterranean plant whose fruits are used as animal and human food and in the manufacture of industrial gums; and Tamarindus indica ( tamarind) of Africa, now primarily grown in India, which has food and medicinal uses and is also used as an industrial gum.

The soybean is a bushy annual whose seeds are an important source of oil and protein. An edible oil pressed from the seeds is used to make margarine and as a stabilizing agent in the processing of food and the manufacture of cosmetics and pharmaceuticals. The oil is employed in such industrial products as paint, varnish, printing ink, soaps, insecticides, and disinfectants. Oil cakes pressed from the seeds are used as protein concentrate in the mixed-feeds industry. The soybean is a good source of vitamin B (vitamin B complex) and is dried to produce soy milk, which is used in infant formulas. Fermented pods are used in making soy sauce, a flavouring common in Asian cooking.

The peanut, a native of South America, is high in vitamin B complex, proteins, and minerals. The peanut is eaten raw or roasted or is processed into peanut butter. An edible oil is pressed from the seed and is used as a cooking oil and in processing margarine, soap, and lubricants. The oil also is employed by the pharmaceutical industry in making medications. Pressed oil cake is fed to livestock. Peanuts are commercially grown in the United States, Asia, Africa, and Central and South America.

As mentioned above, the legumes produce secondary compounds of an irritating or poisonous (poison) nature that provide protection against predators. Some of these secondary compounds are being studied for their pharmacological potential. They are found in the leaves and fruiting parts and include flavonoids (flavonoid), alkaloids (alkaloid), terpenoids, nonprotein amino acids (amino acid), and others. Some of these—for example, the amino acid canavanine—may comprise up to 5 percent of the dry weight of seeds. The chemical compound rotenone, which is toxic to a number of organisms, is sufficiently abundant in the roots and stems of certain species belonging to the Papilionoideae that primitive peoples often used these plants to poison fish. More recently it has been shown that serious bone and neural diseases afflicting humans (e.g., lathyrism) and livestock may be caused by the ingestion of unusually large amounts of certain free amino acids (amino acid). In sheep, ingestion of large quantities of the amino acid mimosine, found in Leucaena glauca and some other species of the Mimosoideae, apparently halts the growth of hair or wool, and in certain cases the fleece itself has been observed to shed. A wide variety of alkaloids (alkaloid) are found in the order, most of them restricted to Fabaceae, however. Some alkaloids occur in sufficient concentration in range plants to be poisonous to livestock, especially in species belonging to the large genus Astragalus. Species of Astragalus are commonly referred to as locoweed in North America because, following excessive consumption of these plants, cattle seem to become unmanageable and “go crazy” or “loco.” Astragalus is poisonous in any of three ways: by promoting selenium accumulation, through locoine, and through several nitrogen-containing toxins. In the early 20th century, several African species of Crotalaria were brought to the United States for use as soil-improvement plants. Their poisonous qualities were discovered in connection with animal stock loss, and development was then halted, but several persist as common noxious weeds.

The subfamilies Caesalpinioideae and Mimosoideae do not contain many food crops and are perhaps best known for their shade and ornamental species, such as Cercis siliquastrum (the Judas tree, or redbud), Bauhinia bartlettii (orchid tree), and Acacia farnesiana (sweet acacia), although some of the more rapid-growing weedy species—for example, Leucaena leucocephala (white popinac) and Albizia species—are widely employed as green manure and fodder crops. Acacia species are used extensively in the production of gum exudates and wood, especially in South Africa and Australia, where the species are known as wattle trees.