Legume
| Legumes | ||||||||||
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 Kudzu - larger image | ||||||||||
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Faboideae | ||||||||||
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| GRIN-CA 2002-09-01 |
Legumes are members of the Fabaceae (also called Leguminosae) family of flowering plants. They are one of the largest plant families and include beans, peas, peanuts, lupines, alfalfa, clover, acacia, and many others.
The word "legume" can also refer to the type of fruit which is characteristic of most of them. A legume in this sense is a simple dry fruit which develops from a simple carpel and usually dehisces (opens along a seam) on two sides. A common name for this type of fruit is a "pod", although pod is also applied to a few other fruit types.
The term is derived from the French word "légume" (which, however, has a wider meaning and refers to any kind of vegetable). The French word comes from the Latin word legumen, which means "anything that can be gathered."
The legumes were among the first plants cultivated by humans and are still among the most important agricultural crops.
The Sub-Families
The legumes are traditionally classified into three subfamilies, sometimes raised to the rank of family in the order Fabales, based on the shape of their flowers:
- Caesalpinioideae: The flowers are zygomorphic, not perfectly symmetrical, but are very variable, e.g. closely resembling Faboideae flowers in Cercis, while symmetrical with five equal petals in Bauhinia. Most of the Caesalpiniodieae are shrubs and trees. Among them are Brazilwood, Royal Poinciana, Honey locust, and the Redbuds.
- Mimosoideae: The petals are small and the stamens are the most showy part of the flower. Among the Mimosoideae are the Acacia and Mesquite.
- Faboideae or Papilionoideae (Fabaceae sensu strictu or Papilionaceae): One petal is large and has a crease in it, the two adjacent petals are on the sides, and the two bottom petals are joined together at the bottom, forming a boat-like structure. Most of the legumes, including those well-known as food crops, are in this sub-family.
Nitrogen fixation
Most legumes, through a symbiotic relationship with certain bacteria called rhizobia (from the Greek words riza = root and bios = life), are able to take nitrogen from the air and "fix" it, that is change it into chemical compounds which can be used by plants. The nitrogen is then available to be used by animals which eat the legume plants and by other plants when it is absorbed by the soil after the legumes' death.
This is tremendously important since nitrogen is an essential part of amino acids and nucleic acids both of which are essential to all life.
Nitrogen fixing legumes form nodules on their roots in which the bacteria live, being fed carbohydrates by the plant, and in which nitrogen is produced and stored.
Nitrogen is also fixed by bacteria living in the soil itself and in the atmosphere due to the action of lightning.
Legumes in nature
There are around 18,000 species of legumes. They grow world-wide in many different environments. Some are annual herbs, some are vines, and some are bushes and trees. They play important roles in many natural ecosystems because their nitrogen fixation enriches the soil. They also provide food for many animals. Many have beautiful flowers, such as lupine and albizia.
Acacia
Acacias, genus Acacia, are one of the largest tree and shrub genera in the world. They mostly live in dry tropical and subtropical environments, with Australia having the most species. Some species form symbiotic relationships with ants in which the trees provide the ants with food from sap and special fruits and with hollow thorns as nesting places and the ants protect the trees from grazing animals, other insects and competing plants.
Acacia senegal of North Africa provides gum arabic, which is an important additive in many foods. Many other Acacias provide valuable hardwoods, including the Koa tree, Acacia koa, of Hawaii and Acacia seyal from which it is said that the Ark of the Covenant was made. Acacia seeds are also used as food. In Australia where seeds gathered from about 120 species are an important part of the traditional diet of the native people (Armstrong 1998).
Kudzu
Kudzu, Pueraria lobata, is another well-known legume. It is native to eastern Asia and was introduced from Japan to the United States as livestock fodder and as a ground cover to control erosion. Because of its rapid growth and great size, it can grow as much as 30 cm (12 inches) a day in the right conditions, it has become an harmful invasive species in much of the Southeast. It replaces native vegetation, even killing trees by covering them and cutting off their access to sunlight.
Cultivation
Legumes were among the first plants cultivated by humans. One reason is the the large seeds of most species are easy to gather and store and provide good nutritional value.
Cultivated legumes fall into two classes: grain and forage.
- Grain legumes are cultivated for their seeds, and are also called pulses. The seeds are used for human and animal consumption or for the production of oils for industrial uses. Grain legumes include beans, lentils, lupines, peas and peanuts.
- Forage legumes, like alfalfa, clover and vetch, are sown in pasture and grazed by livestock or harvested as hay.
Grain legumes were first cultivated in the Middle East, perhaps as much as 10,000 years ago. The first five species cultivated are considered "Neolithic founder crops" (the very first cultivated plants), along with flax, barley, and two species of wheat. These legume species are the lentil Lens culinaris, the pea Pisum sativum, the chick pea Cicer arietinum, the bitter vetch Vicia ervilia, and the broad bean Vicia faba.
The soybean, Glycine max was first cultivated in China, perhaps 5,000 years ago. It is now the most world's most important legume crop.
The common bean, Phaseolus vulgaris and the peanut, Arachis hypogaea, were first cultivated in the New World.
One reason grain legumes are so important as crops is that they have high protein content. They are an important source of protein, as well as B vitamins and iron, for people living where meat is scarce or expensive and for vegetarians. Today India is the largest producer and consumer of grain legumes for human consumption (AEP 2006).
Alfalfa, Medicago sativa, was one of the first forage legumes cultivated. It is native to Iran (ancient Persia) and was cultivated there soon after the introduction of the horse, around 3,000 to 4,000 years ago. It was spread to Europe when Persian Empire invaded Greece around 490 B.C.E. Alfalfa is now an important crop in the United States being harvested as hay to feed cattle and horses.
Clovers, members of the genus Trifolium with about 300 species, are also important forage legumes. They are most often planted along with grasses to provide forage for grazing animals, although sometimes they are harvested as hay. They are also edible by humans and are sometimes boiled and eaten in times of famine and emergency.
Besides the valuable products they produce another important reason legumes are grown is to restore the nitrogen to soil that has been depleted of it by other crops. As early as ancient Rome writers on agriculture, including Pliny and Virgil, understood the value of using legumes like peas, beans, and vetches to to enrich the soil. In some cases the plants are plowed under before another crop, such as wheat, is planted. This is known as "green manure." Besides adding nitrogen it also loosens the soil and enables it to hold more water.
In the 1700s and 1800s in Europe and North America crop rotation began to become widely practiced. Crops of legumes, such as alfalfa and beans, were grown in alternate years with grains, such as wheat and corn. This greatly increased the yields while reducing the need for added fertilization. Sometimes a three or four year cycle of different crops was planted. The system of corn and soybeans grown in rotation has become one of the most important and productive farming patterns in the United States today (UM 2003).
ReferencesISBN links support NWE through referral fees
- Armstrong, W.P., 1998, "The Unforgettable Acacias" [1]
- European association for grain legume research (AEP), Website [2]
- International Legume Database & Information Service (ILDIS), World Database of Legumes [3]
- University of Minnestota (UM), 2003, "Forage Legumes"[4]
- Watson, R., 2004 "A Closer Look at Legumes" Mediterrasian.com[5]
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