Fabaceae

Caesalpinioideae

The subfamily Caesalpinioideae (classified as a family, Caesalpiniaceae, by some authorities) is a heterogeneous group of plants with about 160 genera and some 4,680 species. The latest classifications show that this subfamily is the most basal lineage among the legumes, the one from which the other subfamilies evolved. In that sense, it is not a true monophyletic group, and it will no doubt be treated differently taxonomically in the future. Caesalpinioideae legumes are found throughout the world; they are primarily woody plants in the tropics. Their moderate secondary invasion of temperate regions is mostly by herbaceous (nonwoody) evolutionary derivatives. A striking exception to this generalization is the presence in temperate regions of Gleditsia triacanthos (honey locust) and of the related Gymnocladus dioica (Kentucky coffee tree). They may represent more-ancient and relictual lineages in the subfamily.

Caesalpinioideae is more diverse than the other subfamilies. The leaves are usually divided into leaflets (compound), or else the leaflets are again divided into leaflets (bicompound). The flowers vary in symmetric form: nearly radial, bilateral, or irregular (symmetric in no plane). The sepals are usually separate and imbricate (overlapping in the bud). There are generally five separate imbricate petals, the upper one inside the lateral petals in the bud. The 10 or fewer stamens are exposed, although not as conspicuously as in many of the members of the tribe Mimoseae described below. The fruit conformation is diverse. Bacterial nodulation is much less prevalent than in the other subfamilies. Canavanine, a potentially toxic chemical used to deter herbivores, is not present.

Many Caesalpinioideae species are prized ornamentals in the tropics, such as Delonix regia (royal poinciana), Cassia grandis (pink shower), and Bauhinia (orchid trees). G. triacanthos is well known in temperate regions.

Mimoseae

The former subfamily Mimosoideae, now the tribe Mimoseae within Caesalpinioideae, includes 100 genera and more than 3,500 species. The majority of Mimoseae members have large leaves that are divided into secondary (compound) leaflets, and in many members these leaflets are again divided (bicompound) and have a feathery, sometimes fernlike appearance. A striking exception are the Australian acacias (but not the American kinds) mentioned above, in which the compound leaves of most species have become modified, losing all their leaflets and appearing to be undivided, or simple. The flowers of Mimoseae are radially symmetric and are usually most easily recognized by the long stamens that extend beyond the rest of each flower. The calyx and corolla are both valvate in bud, contrasting with the usual condition in other subfamilies. The petals are small and often not noticed except by close examination. Many of these plants have nodules containing the nitrogen-fixing bacterium Rhizobium on their roots.

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.

Faboideae

The subfamily, also called Papilionoideae (classified as a family, Fabaceae or Papilionaceae, by some authorities), is the largest group of legumes, consisting of about 500 genera and nearly 14,000 species grouped in 14 tribes. The name Papilionoideae probably originated because the flowers resemble a butterfly (Latin: papilio). It is the unique bilaterally symmetric (zygomorphic) flowers that especially characterize the group, so that thousands of species can be recognized as a member of Papilionoideae at a glance. The Lathyrus odoratus (sweet pea) flower provides an example. It has a large petal at the top—called the banner or standard—that develops outside the other petals before the flower has opened, two lateral petals called wings, and two lower petals that are usually fused and form a keel enclosing the stamens and pistil. The whole design is adapted for pollination by insects or, in a few members, by hummingbirds. Sweet nectar, to which the insects are cued by colored petals, is the usual pollinator attractant. Various locking and releasing devices of the keel and wing petals control pollination in diverse ways favoring (or enforcing) either outcrossing or self-pollination—e.g., Trifolium (clover), Medicago (alfalfa), and Lotus corniculatus (bird’s-foot trefoil). The most effective kind of obligate self-pollination is that of cleistogamous flowers, which do not open, so they prevent the entry of insects. Lespedeza and many other genera of Papilionoideae legumes bear both kinds of flowers, generally on the same plant. Enforced inbreeding serves to fix and maintain successful strains, whereas outbreeding provides evolutionary diversity that may facilitate habitat or range expansion or provide flexibility to adapt to environmental changes.

The calyx is composed of fused sepals. The stamens are 10 or fewer and are free in a few tribes but are most commonly fused at their filaments (monadelphous) or fused at all filaments but one, which remains free (diadelphous). The ovary has a single carpel and develops into various fruit types.

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 and soybeans. Trifoliolate leaves rarely occur in the other subfamilies. The large genus of Lupinus (lupines) generally has 5 to 11 (occasionally up to 15) palmate leaves. The leaves of clovers 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). 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 legumes produce many kinds of chemical substances—e.g., alkaloids, flavonoids, tannins, and the free amino acid canavanine (the latter found only in legumes). The function of the substances that are physiologically active (i.e., often poisonous) in animals seems usually to be predator defense. The medical potential (especially of the alkaloids) of some of these substances, or of their synthetic derivatives, has been extensively studied. The absence or presence and distribution of these substances in the various groups are used in legume classification. Information about other cryptic features, such as pollen and plant anatomy, contributes to scientific knowledge of legume evolution as well.

Cercideae and other subfamilies

The subfamily Cercidoideae is a small tropical and temperate woody group (e.g., Cercis and Bauhinia) in which the leaves are apparently simple and often bilobed. The flowers of Cercis are only superficially similar to those of Faboideae.

Other small subfamilies of Fabaceae include Detarioideae, Duparquetioideae, and Dialioideae.

Nitrogen fixation

One of the major ecological roles of Fabaceae is in nitrogen fixation, which is almost unique to the family. Nitrogen is an element of all proteins 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; to be metabolized by them, it must be in the form of nitrates or ammonia compounds. Through a mutual benefit arrangement (symbiosis) between legumes 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 bacteria, nitrogen compounds are formed that are used by the 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 nitrogen compounds by eating plants or other animals.

Consequently, the vegetation of the forests, prairies, and deserts of most of the world is primarily dependent on the legume component of their vegetation and could not exist without it. Only in a few ecosystems—those that include few legume species—have alternative biological nitrogen-fixing arrangements evolved. These include symbiotic relationships between miscellaneous woody species other than legumes and certain actinomycetes or bacteria, and they are limited mostly to boreal evergreen forests, certain coastal areas, and acid bogs. Nitrogen fixation by free-living cyanobacteria seems to be important in aquatic ecosystems. On a worldwide scale, however, these alternative arrangements of nitrogen fixation are relatively minor compared with those supported by legumes.

Legume nitrogen fixation is of prime importance in agriculture. Before the use of synthetic fertilizers in industrialized 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.

The 20th-century substitution of petroleum-derived synthetic nitrogen fertilizers is partly a consequence of economics, in that a cash grain, such as corn or wheat, planted every year provides a higher fiscal return than alternating it with a legume crop. In addition, legume-Rhizobium nitrogen fixation is inhibited when the level of nitrogen in the soil is high and is insufficient for maximum yields of a grass crop. Therefore, in developed countries, chemical fertilizers have largely replaced biological fixation in row-crop culture. On a worldwide basis, however, dependence on legumes is still preeminent. Even in the United States, when rangeland and pasture agriculture are included, it has been estimated that nitrogen production by biological fertilizers still exceeds chemical application.

Other benefits accrue from the use of legumes to maintain soil nitrogen. Weed control is facilitated by a crop sequence that alternately changes the growing environment. Such legumes as alfalfa may be harvested for forage (hay or silage) or grazed by livestock. As cover crops, legumes prevent or reduce soil erosion and may be plowed under as green manure. Even though starch-producing grasses, such as corn, are more efficient under favorable conditions in producing energy foods, grain legumes are commonly grown in the tropics, because they are more successful in depleted, nitrogen-deficient soils.

Food and fodder crops

Legume seeds constitute a part of the diet of nearly all humans. Their most vital role is that of supplying most of the protein in regions of high population density and in balancing the deficiencies of protein from cereals (Poaceae). Except for the soybean and peanut, the order is not noted for the oil content of the seeds, since most seeds have only about 10 percent oil content by weight. The legume seeds generally are highest in carbohydrate compounds, followed by protein and fat. Legumes are thus considered to be energy foods. Most legumes that are used for foods are multipurpose plants, serving for animal forage and soil improvement as well. Some, notably the soybean, are also important industrial crops. Fabaceae contains the more important crop plants, such as soybeans, beans, cowpeas (Vigna), pigeon peas (Cajanus cajan), chickpeas (Cicer arietinum), lentils (Lens culinaris), peas (Pisum sativum), and peanuts.

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 as an ingredient in 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, and it is dried to produce soy milk, which is used in infant formulas and as a nondairy milk substitute. Fermented pods are used in making soy sauce, a flavoring 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 is also 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.

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

Apart from the legume plants of worldwide importance, the following are examples of locally significant legume species that are cultivated or gathered from the wild. Some would plainly have substantial potential were they subject to genetic evaluation and development through modern breeding techniques. Some are still in the same stage as teosinte (the ancestor of corn) or einkorn and emmer (the ancestors of the modern varieties of cultivated wheats) in yield and potential for practical use.

Notable among the locally useful plants of the legume family is Vigna subterranea (Bambara groundnut), a plant that develops underground fruits in the arid lands of Africa. Important too are the seeds of Bauhinia esculenta; they are gathered for the high-protein tubers and seeds. V. aconitifolia (moth bean) and V. umbellata (rice bean) are much used in the tropics for forage and soil improvement, and their seeds are palatable and rich in protein. Psophocarpus tetragonolobus (winged bean) is collected in Southeast Asia for the edible fruits and protein-rich tubers. Pachyrhizus (yam bean) is a high-yield root crop of Central America.

Various forms of Leucaena (such as L. leucocephala [white popinac]) 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 subfamily Caesalpinioideae, including Mimoseae, does not contain many food crops. It is perhaps best known for its 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, L. leucocephala 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.

Biochemicals

As mentioned above, many legumes produce secondary compounds of an irritating or poisonous nature that provide protection against predators. Some of these secondary compounds are being studied for their pharmacological potential. Found in the leaves and fruiting parts, they include flavonoids, alkaloids, terpenoids, and nonprotein amino acids. Some of these—for example, the amino acid canavanine—may constitute 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 of Papilionoideae that some cultures have 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. In sheep, ingestion of large quantities of the amino acid mimosine, found in Leucaena glauca and some other species of Mimoseae, apparently halts the growth of hair or wool, and, in certain cases, the fleece itself has been observed to shed.

Some alkaloids occur in sufficient concentration in range plants to be poisonous to livestock, especially in plants 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 some species persist as common noxious weeds.

An interesting biochemical component of the legume seed is phytohemagglutinin, a large protein molecule that is specific in its capacity to agglutinate certain human blood types. Approximately 60 percent of the several thousand seeds belonging to the order Fabales tested to date contain the compound. Phytohemagglutinin is particularly abundant in the common bean (Phaseolus vulgaris) and has been extracted in a relatively pure state on a commercial scale from species belonging to the genus Phaseolus. In addition to its agglutination properties, the compound has been of interest because of its other biological effects. It is toxic to rats, inactivates some human tumor cells, and has beneficial effects in the treatment of aplastic anemia, the shortage of blood cells in humans because of the destruction of blood-forming tissues.