Fabaceae (Leguminosae) Legume or Pea family


  1. Herbs,shrubs, or trees; extremely versatile in morphology and ecology; roots often with nitrogen-fixing bacterial nodules (Rhizobium spp.).
  2. Leaves – mostly alternate; pinnately or bipinnately compound; sometimes palmately compound; sometimes simple by suppression of leaflets (Acacia), reduced to one phyllode.
    1. Phyllode – flattened leaf-like petiole (with no blade) that functions a s leaf, as the primary photosynthetic surface.
    2. Pulvinus – enlargement of the petiole of petiolule base at its point of attachment to the stem, or of the petiole apex at its point of attachment to the blade. Turgor pressure in adaxial and abaxial surfaces cause the petiole to collapse or remain outstretched. This is associated with leaf movement. Leaflets of many legumes open in the day and close at night; in some spp., like Mimosa, leaflets fold up in response to touch.
    3. Stipulate – (well-defined at base of petiole). Stipules of some species of Acacia, Prosopis (mesquite), Robinia (locust) develop into spines.
  3. Flowers – zygomorphic or actinomorphic (Mimosoideae), usually perfect, complete.
    1. Calyx – 5 sepals fused.
    2. Corolla – 5 petals, either distinct or 2 or more petals may be fused.
    3. Androecium – usually 10 stamens, filaments may be distinct or all united into a tube (Monadelphous), or 9 united into a strap with one filament free (diadelphous).
    4. Gynoecium – 1 pistil, simple, 1 carpel, ovary superior, usually 1-locular; ovules 2-many.
  4. Fruit – legume; dehiscent or not dehiscent (mesquite); sometimes a loment breaking transversely into 1-seeded segments. It may be that the primitive pod (legume) type was one that dehisced explosively when ripe, scattering seeds (beans) away from the parent plant. In riparian species, the ejected seeds are buoyant and further dispersed by streams or river currents. There are many variants of the basic legume pod – inflated buoyant pods, pods that disarticulate in 1-seede segments, pods that adhere to animals. The effectiveness of these adaptations is evident from the wide ranges of many legume genera, which span different continents and remote islands.
  5. Seeds – often with hard seed coats (testa) and capable of long dormancy in soil. Germination often requires some sort of scarification, either physical “nicking” when tumbling down a wash, or chemical erosion while passing thru the alimentary canal (digestive tract) of various animals. Some sees are bright red to attract birds; others are enclosed within tasty and nutritious pods (carob bean, and mesquite). Legumes are uniquely adapted to colonize infertile soil by their special relationship with Rhizobium bacteria, which inhabit root nodules and convert abundant, inert atmospheric nitrogen to compounds plants can absorb (ammonia). Lavish use of nitrogenous compounds is especially obvious in legume seeds. These usually lack endosperm, but have rich stores of protein in the cotyledons. This is precious food for herbivores. Legume seeds are also commonly loaded with alkaloids and other toxic nitrogenous compounds (Astragalus, Sophora, Cassia).
  6. Distribution – cosmopolitan. Fabaceae is the 3rd largest family of flowering plants, behind the Asteracea and Orchidaceae.
  7. Economic plants – Second only to Poaceae (grass family) in economic importance. It is valuable for its restoration or maintenance of soil fertility; rotation of leguminous plants with crops that deplete the soil of nitrogen reserves is an alternative to expensive chemical fertilizers in many regions.
    1. Food:

i.         Arachis hypogea (peanut)

j.        Cicer arietinim (garbanzo bean)

k.      Glycine max (soybean)

l.         Lens culinaris (lentil)

m.     Phaseolus spp. (bean)

n.       Pisum sativum (pea)

o.      Glycyrrhiza sp. (the source of liquorice)

p.      Pachyrrhizus sp. (jicama)

    1. Forage

i.         Medicago sativa (alfalfa)

j.        Melilotus spp. (sweet clover)

k.      Trifolium (clover)Vicia (vetch)

l.         Lupinus (lupine)

    1. Toxic

i.         Astragulus (locoweed) toxic levels of selenium

j.        Cassia and Senna (alkaloids)

k.      The insecticide Rotenone comes from Derris and Lonchocarpus

    1. Wood

i.         Dalbergia (rosewood)

j.        Prospis (mesquite)

k.      Robinia (locust)


  1. Fabaceae is a large, heterogeneous taxon which is traditionally separated into 3 sub-families: Papilionideae, Caesalpinoideae, Mimosoideae.


    1. Papilionoideae (or Faboideae) – the largest of the sub-families.

i.         the perianth is composed of 4 parts.

j.        zygomorphic corolla, with the banner enclosing the other petals in bud (aestivation).

k.      leaves are pinnately compound, trifoliate, occasionally reduced to a single leaflet.

l.         androecium of 10 distinct or fused or diadelphous stamens.


    1. Caesalpinoideae

i.         Corolla of 5 parts,  slightly zygomorphic, but uppermost petal (banner) is enclosed with the lateral wings in bud.

j.        Leaves are pinnately, sometimes bipinnately compound.

k.      Androecium of 10 stamens or less.

l.         Primarily tropical, with few temperate species.

m.     Flowers generally large with conspicuous corollas.

    1. Mimosoideae

i.         flowers actinomorphic and usually small.

j.        Leaves usually bipinnately compound.

k.      Androecium of 10-many stamens, often exserted well-beyond the corolla.


The unifying feature of the family is the unicarpellate gynoecium (which develops into a  legume) with a double row of ovules