Legume Family (Fabaceae)

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Legume Family (Fabaceae)

Third Largest Plant Family On Earth

A sun-bleached "mape" or Tahitian chestnut (Inocarpus fagiferus) washed ashore on the island of Tetiaroa. The specific epithet is also spelled fagifer. This tree is widespread in the South Pacific, from Java and Papua New Guinea throughout numerous Melanesian and Polynesian islands. It grows in coastal forests and along the edges of mangrove swamps. The distinctive, massive trunk is buttressed and irregularly fluted. The cooked seeds are an important food source for indigenous people of this large tropical region. The seeds reportedly have a flavor similar to chestnuts. Grated seeds are used for making cakes, breads and puddings.

The legume family (Fabaceae) is the third largest family of flowering plants with more than 18,000 described species. It is surpassed in size only by the orchid family (Orchidaceae) with about 20,000 species and the sunflower family (Asteraceae) with about 24,000 species. The family includes herbs, shrubs, trees and vines distributed throughout the world, especially the tropical rain forest. The fruit is technically called a legume or pod. It is composed of a single seed-bearing carpel that splits open along two seams. Legume fruits come in an enormous variety of shapes and sizes, including indehiscent pods that do not split open. Of all the legumes, the peanut is especially fascinating because it develops below the ground.

The world's largest legume fruits (bean pods) are produced by the tropical liana Entada. The longest pods of the Central American E. gigas may be up to 5 feet long (1.5 m). This gigantic woody vine is truly like Jack's fabulous bean stalk. In Costa Rica it is called "monkey ladder" or "escalera de mono." The woody seeds of E. gigas are called "sea hearts" and are often washed down streams to the sea where they drift across the ocean to distant continents. Familiar edible legume pods in the background include green beans, peanuts, soybeans and snow peas.

Diversity of Flowering Plants
The World's Longest Bean Pod

The South American tipu tree (Tipuana tipu). Unlike the fruits of most members of the legume family, the unusual fruits have a distinctive wing that causes the legume to spin as it falls from the rain forest canopy. The tipu tree is commonly planted as a shade tree along streets in San Diego County. In fact, it lines the major thoroughfares at California State University, San Marcos.

Seeds and Fruits Dispersed By Wind

The peanut (Arachis hypogaea) is native to central South America. By the time Columbus reached the New World, peanuts were already cultivated throughout warmer regions of the Americas. Peanuts have been introduced into Africa and Asian countries where they have become an important food crop. Like clover and alfalfa, the peanut root system contains nodules of nitrogen-fixing bacteria that convert inert atmospheric nitrogen into ammonia. Other bacteria convert the ammonia into nitrites and nitrates that enrich the soil with a usable form of nitrogen.

Read About Nitrogen Fixation In Plants

Like other members of the subfamily Papilionoideae, the peanut flower is papilionaceous, typical of a pea blossom. The peanut flower is produced on a slender stalk (pedicel) near the base of the plant. Each flower consists of five petals: a large banner, two lateral wings, and a keel formed by two fused petals. The keel petals enclose the 9 stamens (androecium) and pistil (gynoecium).

More Information & Images About Peanuts

Three Subfamilies of the Legume Family

1. Family Fabaceae--Subfamily Papilionoideae:

Members of the subfamily Papilionoideae have true papilionaceous flowers in which the upper petal is outside the lateral petals in the bud. This subfamily includes most members of the Fabaceae with typical pea-like flowrs, including Dalea (smoke tree), Lupinus (lupine), Lathyrus (sweet pea), Erythrina (coral tree), Robinia (black locust) and Astragalus (locoweed).

A dissected papilionaceous flower of Erythrina crista-galli showing all the major perianth segments removed from their attachment inside the calyx. The five petals consist of one large, oval banner or standard, two elongate keel petals that are fused together enclosing the stamens, and two reduced wings. Nine stamen filaments are united into a sheath that surrounds the pistil. One stamen filament is separate from the fused nine, a condition referred to as diadelphous. A drop of nectar is secreted at the base of the petals inside the calyx. At maturity the banner is widely separated from the keel petals, thus making the nectar fluid at the base of the petals (within the calyx) readily available to short-billed perching birds. Erythrina species pollinated by hummingbirds have elongate, tubular blossoms.

See The Dissected Flower Of A Peanut
The Pollination & Dispersal Of Coral Trees

Moreton Bay chestnut (Castanospermum australe), a beautiful rain forest tree native to northeastern Australia and New Caledonia.

Moreton Bay chestnut (Castanospermum australe), an Australian rain forest tree.

Developing pod of Moreton Bay chestnut (Castanospermum australe)

Immature pods of the Moreton Bay chestnut (Castanospermum australe).

2. Family Fabaceae--Subfamily Caesalpinioideae:

Members of the subfamily Caesalpinioideae have flowers that are bilateral, typically with five distinct petals, upper petal (banner) enveloped in the bud by the lateral wings. This subfamily includes Cassia (senna), Cercis (redbud), Bauhinia (orchid tree), Cercidium (palo verde), Parkinsonia (Jerusalem thorn), Caesalpinia (brazilwood), Haematoxylum (logwood), Ceratonia (carob), Tamarindus (tamarind) and Delonix (royal poinciana).

The Hong Kong orchid tree (Bauhinia blakeana) native to souhern China. Although it is truly a legume, the flower is not papilionaceous. The upper petal is enveloped in the bud by the lateral wings.

Bauhinia galpinii, a shrubby species native to South Africa. The beautiful blossoms are brick red to orange. Although it is truly a legume, the flower is not papilionaceous. The upper petal is enveloped in the bud by the lateral wings.

A. Jerusalem thorn or Mexican palo verde (Parkinsonia aculeata), a tree native to deserts of Arizona and Baja California. B. Brazilwood (Caesalpinia echinata), a tree native to Brazil and one of the important dyewoods of the 1600s. The flowers are bilateral (irregular) but not truly papilionaceous.

See Logwood and Brazilwood

3. Family Fabaceae--Subfamily Mimosoideae:

Members of the subfamily Mimosoideae have flowers with radial symmetry, small, inconspicuous corollas and numerous, showy stamens. The flowers are typically in many-flowered heads or spikes. This subfamily includes Acacia (wattle), Albizia (silk tree), Samanea (monkeypod), Prosopis (mesquite) and Calliandra (powder puff).

Red powder puff (Calliandra haematocephala) native to Bolivia.

Plume albizia (Albizia distachya) native to Australia.

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