DEFINITON OF LEGUME

Definition of Legume is any of the plants that belong to the pea family. They make up the second largest family of flowering plants. The composite family is the largest. Botanists recognize between 14,000 and 17,000 species (kinds) of legumes. The group gets its name from the legumes (seed pods) that the plants bear.

Many legumes are of great economic importance throughout the world. Such legumes as peas, beans, and peanuts are valuable foods. Alfalfa, clover, and vetch are important forage and pasture plants. Other legumes yield medicines, dyes, oils, and timber.

Legumes grow in most parts of the world. They vary widely and may be trees, shrubs, or herbs. Many are climbing plants. The flowers of one large subfamily of legumes look like butterflies. Botanists call this group Papilionoideae, from the Latin word for butterfly. The common sweet pea belongs to this group. The flowers of other legumes may be small and regular. The flowers of still others may be irregular, with spreading petals.

Legumes take nitrogen into their roots from the air. Certain bacteria, called rhizobia, live in nodules (knotlike growths) that form along the roots of the plants. These bacteria take nitrogen from the air and change it into forms that can be used by plants. This characteristic makes leguminous plants valuable in agriculture. Farmers often use them as green manure and as cover crops to improve poor soil.

NITROGEN CYCLE | The circulation of nitrogen among the atmosphere

Nitrogen cycle is the circulation of nitrogen among the atmosphere, the soil and water, and the plants and animals of the earth. All living things require nitrogen, but most organisms cannot use the nitrogen gas that makes up about 78 percent of the atmosphere. They need nitrogen that has combined with certain other elements to form organic compounds. But the supply of this fixed nitrogen is limited, so complex methods of recycling nitrogen have developed in nature.

One part of the nitrogen cycle involves circulation of nitrogen between the soil and living things. After plants and animals die, they undergo decomposition by certain bacteria and fungi. These microorganisms produce ammonia from nitrogen compounds in dead organic matter and in body wastes excreted by animals. Plants absorb some of the ammonia and use it to make proteins and other substances essential to life. The rest of the ammonia is changed into nitrates by nitrifying bacteria. First, nitrifying bacteria called nitrite bacteria convert ammonia into nitrites. Then nitrate bacteria change nitrites into nitrates. Plants absorb most of the nitrates and use them in the same way as ammonia. Animals get nitrogen by eating plants or by feeding on animals that eat plants.

In another part of the cycle, a process called nitrogen fixation constantly puts additional nitrogen into biological circulation. In this process, nitrogen-fixing bacteria in the soil or water, or living within plants such as legumes, convert nitrogen from the atmosphere into nitrogen-containing organic substances.

While nitrogen fixation converts nitrogen from the atmosphere into organic compounds, a series of processes called denitrification returns an approximately equal amount of nitrogen to the atmosphere. Denitrifying bacteria convert nitrates and nitrites in soil into nitrogen gas or into gaseous compounds such as nitrous oxide or nitric oxide. However, fixed nitrogen may circulate many times between organisms and the soil before denitrification returns it to the atmosphere.

Some human activities influence the nitrogen cycle. Industry fixes vast quantities of nitrogen to produce fertilizer, much of which is washed off farmland and into waterways, polluting the water. The combustion of certain fuels produces nitrogen compounds that pollute the air. These compounds may also play a part in the warming of the earth's climate