References:

Plants and People, pages 208-220.
On Food and Cooking, pages 249-261

What is a legume?
Nitrogen fixation by Rhizobium.
Old World Legumes

lentils
peas
broad beans
cick peas

Legumes from the far east

Soybeans
Mung beans (black eyed peas)

Legumes from the new world

peanuts
common bean
lima bean

Secondary compounds in legumes

favism

Legumes and flatulence
Legumes as forage crops

alfalfa

Leucaena

We eat the fruits of legumes, which is called a legume. As we have seen before, a legume is a pod with a row of seeds inside. The leaves of the seeds, the cotyledons, are where the seed stores nutrient for the embryo. Such nutrient is not stored in an endosperm, as in corn (for example).

The flowers of legumes are irregular, sometimes called butterfly like, or boat like. Legume flowers are adapted for specialized insect pollination, usually by bees. In the case of legumes like alfalfa, only certain bees can get into the flower.

Legumes are important because of their high protein content. In fact, they rival meat in protein content. Legumes contain a lot of protein because of a SYMBIOTIC association between the roots of legumes and NITROGEN FIXING BACTERIA in the genus Rhizobium. The Rhizobium live in ROOT NODULES, which can be found on the roots of many legumes. Because they contain a lot of nitrogen, and because nitrogen is one of the most limiting nutrients in soils, many farmers plat legumes when fields are lying fallow to regenerate soil nitrogen levels, often plowing under the legumes. A common crop grown in this manner is alfalfa. Alfalfa is a common forage crop, and a very high quality of forage crop, and has been used as such for thousands of years in the middle east. George Washington Carver developed the idea of planting crops like soybeans and peanuts as a means of replenishing soils depleted of nutrient by cotton farming.

Recall that many prominent Roman families, like Cicero (chick pea) were named after legumes.

Legumes also contain large amounts of B vitamins and iron. Legume seeds green in the pod contain a lot of vitamin A and vitamin C (but less protein).

The first legumes known to be cultivated were lentils, and remains of the earliest agricultural settlements in the middle east include evidence of lentil cultivation. Lens is the Latin word for lentil, and is used to describe lenses because doubly convex lenses are shaped like lentils.

Other legumes cultivated in the middle east region are (and were) chick peas (garbanzo beans), peas and the broad bean. Hummus is made from Chick Peas. In India chick peas form a substantial part of the diet. The broad bean, or Faba bean, was the  only bean cultivated in Europe until the discovery of the new world. It was believed by ancient Greeks and Romans that Faba beans contained the souls of the dead. Faba beans contain a toxin called VICINE that causes a disease called FAVISM in people who lack a functional gene for an enzyme that processes vicine. If people lacking the enzyme come eat undercooked broad beans, or inhale the pollen, vicine causes oxidative damage and rupturing of red blood cells (hemolysis). Peas have been an accessory dish in Europe and the middle east for thousands of years. Johann Mendel performed insightful genetics experiments using peas.

Legumes domesticated in the far east are mung beans and soybeans. Black eyed peas are a type of mung bean. Soybeans are one of the most nutrient rich of all plant products, with about 13-25% oil content, and about 30-50% protein content. Soybean oil is highly unsaturated, and very unstable, and thus was not used much until the process of hydrogenation was developed. Today soybeans are the largest cash crop in the US, which produces 75% of the total world soybean crop. Soybeans were (and are) cultivated by Buddhists, who are vegetarian, and the spread of soybeans throughout the far east was related to the spread of Buddhism. Soy oil is the main oil used to make margarine, and the remains of the soybean following oil extraction is used to make a high protein flour that, among other things, can be used to feed people in areas of famine. Each of us consumes about 6 gallons of soy oil per year. Soy oil is also used to make paints, inks, soaps, insecticides and cosmetics. The food additive lecithin is a lipid extracted from soybeans. In 1940, Henry Ford used plastic made from soybeans to make a car! Soybeans are probably our best means of improving protein deficient diets in the third world.

The common bean and the lima bean originated in the new world. The lima bean was introduced into Africa as a means of feeding slaves during the slave trading era of western Europe, and has remained as a major food product in Africa. There are now hundreds of varieties of the common bean, like kidney beans, navy beans, black beans, field beans and pinto beans, to name a few.

Peanuts are also highly nutritious, with 45-50% oil and 25-30% protein. The paste made from peanuts (peanut butter) is much tastier than it's soy counterpart (tofu). Peanut oil is used for the same things as soy oil, with the remains of the peanut following oil extraction used to feed animals. Peanut flowers grow down into the ground following pollination, with 2 seeds in each pod developing underground.

Flowering plants are noted for the secondary chemical compounds the produce as a defense against herbivory. Legumes are no different in this regard, as we have seen with vicine. Legume seeds produce mainly protease inhibitors and cyanogens that limit digestibility of food in general. Heat destroys these compounds, however raw legume seeds may have such low nutritional value that energy is lost in processing them when they are eaten. Lima beans in particular produce a lot of cyanogens, and has caused cyanide poisoning in some people. Low cyanogen varieties of lima beans have been developed to try to eliminate the problem of cyanide poisoning.

Like a variety of food products, legumes have been associated with flatulence. Little was known about the specific causes of flatulence until the planners involved in the space program became concerned that astronauts could conceivably poison themselves with flatulence, as there was originally no place to get rid of methane and hydrogen sulfide gases produced by flatulence. In the study of flatulence, it was learned that flatulence is about 1/2 nitrogen and 40% carbon dioxide. The nitrogen comes from air which is swallowed while eating and drinking. The carbon dioxide is the product of aerobic respiration by intestinal bacteria. The other 10% of flatulence is composed of hydrogen, methane and hydrogen sulfide gases produced by anaerobic intestinal bacteria. All bacteria produce ammonia and the highly odorous indoles and skatoles. The amount and composition of flatulence varies among individuals, and astronauts were once (maybe still are!) subject to criteria as to the amount of flatulence they produced and the percent of flatulence that was composed of methane and hydrogen. Legumes and other foods products cause flatulence when they are digested, by leaving some OLIGOSACCHARIDES (short chains of sugar molecules) that our intestines cannot digest. These oligosaccharides become food for intestinal bacteria, who respond to increased food supply with increased metabolic activity, and increased gas production, resulting in increased flatulence. It is generally thought that components of "fiber", the indigestible celluloses and hemicelluloses are the main contributors to flatulence. Test have shown that navy beans and lima beans are the worst offenders in the case of flatulence. Boiling in water and sprouting of legume seeds decreases oligosaccharide content, and thus reduces flatulence.

A legume is becoming an important source of wood (for firewood) in developing countries where wood is the main source of fuel. The Leucaena tree grows very rapidly, up to 30 feet per year per plant. Like other legumes, Leucaena forms a symbiotic association with Rhizobium, resulting in rapid assimilation of large amounts of nitrogen into organic forms such that 6 bag of dried leaves have as much nitrogen as one bag of ammonia nitrate fertilizer. Pre Colombian indian societies planted Leucaena trees in their  crop fields to provide nutrient for their crops. The leaves are used as animal feed and the fruits are highly nutritious for humans.