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Introduction

Characteristics Of The Leguminosae

Economic Importance Of The Leguminosae

Nitrogen Fixation

Introduction

Legumes (les l�gumineuses in french) are plants of the pea or bean family, the Leguminosae (Fabaceae in the USA).

The Leguminosae is one of the largest families of flowering plants with 18,000 species classified into around 650 genera (Polhill & Raven, 1981). This is just under a twelfth of all known flowering plants.

The Leguminosae is an extremely diverse family. The major characteristics of the family are given below.

The Leguminosae constitute one of humanity�s most important groups of plants. Legumes are used as crops, forages and green manures. They also synthesise a wide range of natural products such as flavours, drugs, poisons and dyes. Further information on the economic importance of the family is given below.

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Characteristics Of The Leguminosae

Legumes are a significant component of nearly all terrestrial biomes, on all continents (except Antarctica). Some are fresh-water aquatics, but there are no truly marine species.

The species within the family range from dwarf herbs of arctic and alpine vegetation to massive trees of tropical forest.

The principal unifying feature of the family is the fruit, a pod, technically known as a Legume. The Legume is modified in many ways to facilitate dispersal by animals, wind and water.

The family is usually divided into three sub-families: Papilionoideae, Caesalpinioideae and Mimosoideae. These sub-families are sometimes recognised as three separate families: Papilionaceae, Caesalpiniaceae and Mimosaceae. The three subfamilies are generally identifiable by their flowers.

Papilionoideae

The Papilionoideae is the largest of the three subfamilies with about two-thirds of all the genera and species of the family. It is also the most widespread, extending further into temperate regions than the other two subfamilies.

The majority of the species are herbaceous, although there are some trees and shrubs, e.g. Laburnum and Gorse (Ulex).

For the most part the Papilionoideae are easily recognised by their characteristic papilionaceous (butterfly-like) flowers. The flower is irregular (zygomorphic) and is made up of five petals; a �banner� petal, two wing petals, and two petals partially fused together to form a boat-shaped keel. The keel encloses the stamens, which are not visible externally.

The Papilionoideae contains most of the important leguminous crop species such as the Soya Bean (Glycine max), Common Pea (Pisum sativum), Chickpea (Cicer arietinum), French Bean (Phaseolus vulgaris), Lentil (Lens culinaris) and Peanut (Arachis hypogaea).

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Caesalpinioideae

The majority of the Caesalpinioideae are tropical or subtropical trees and shrubs.

The flowers of the Caesalpinioideae are irregular (zygomorphic) with five petals which are not differentiated into standard, wings and keel. The stamens are visible externally.

Several species in this subfamily are well-known tropical ornamentals such as Flamboyant (Delonix regia) and Barbados Pride (Caesalpinia pulcherrima). Alexandrian Senna (Senna alexandrina) is a commercially grown medicinal plant, known for its purgative qualities.

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Mimosoideae

Like the Caesalpinioideae, the majority of the Mimosoideae are tropical or subtropical trees and shrubs.

The Mimosoideae are characterised by their small, regular (actinomorphic) flowers crowded together, generally into spikes or heads which resemble a pom-pom. The stamens have become the most attractive part of the flower, the five petals inconspicuous. The leaves are predominately bipinnate.

Examples of genera within this subfamily are Acacia and Mimosa. Certain Acacia species are extremely important economically. An extract from the bark of the Golden Wattle (Acacia pycnantha) is used in tanning, several species, such as Australian Blackwood (e.g. Acacia melanoxylon) provide useful timbers and some (e.g. Acacia senegal) yield commercial gum arabic, which is used in a wide range of industrial processes.

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Economic Importance Of The Leguminosae

In terms of economic importance the Leguminosae is the most important family in the Dicotyledonae (Harborne, 1994). Legumes are second only to the Grasses (cereals) in providing food crops for world agriculture. In comparison to cereal grains the seeds of Legumes are rich in high quality protein, providing man with a highly nutritional food resource. The major staple foods such as beans, soya, lentils, peas and chickpeas are all legumes. The total world value for leguminous crops is thought to be approximately two billion US dollars per annum. Many more legumes are local food plants.

In addition to those legumes cultivated for human consumption many yield important fodders, green manures and forages, e.g. Lupinus (Lupin), Medicago (Alfalfa) and Trifolium (Clover).

Legumes are utilised for a variety of other purposes including timber, medicine, tannins and gums. Various species of Lonchocarpus and Derris are the source of rotenone, which is used as an insecticide, fish poison or molluscicide. Some Legume trees yield valuable resins, used in varnishes, paints and lacquers, e.g. Copaifera and others are the source of dyes, e.g. Indigofera which is cultivated for a blue dye.

The economic importance of the family is likely to increase as human pressure places greater demand on marginal land. Many Legume species are characteristic of open and disturbed places and are thus well adapted to grow under poor conditions.

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Nitrogen Fixation

Many Legumes are able to convert atmospheric nitrogen into nitrogenous compounds useful to plants. This is achieved by the presence of root nodules (which are visible to the naked eye) containing bacteria of the genus Rhizobium. These bacteria have a symbiotic relationship with Legumes, fixing free nitrogen for the plants. In return legumes supply the bacteria with a source of fixed carbon produced by photosynthesis. This enables many Legumes to survive and compete effectively in nitrogen poor conditions.

Root nodules are general in the Mimosoideae and Papilionoideae, but rarely formed in the Caesalpinioideae.

Information about ILDIS Root Nodule projects can be found on the 'What's New' page.

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Introduction

Characteristics Of The Leguminosae

Papilionoideae

Caesalpinioideae

Mimosoideae

Economic Importance Of The Leguminosae

Nitrogen Fixation

Further Reading

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