Family Fabaceae

Family Fabaceae: Classification and Vegetative Features

Family Fabaceae, traditionally also called Papilionaceae because of its butterfly-shaped corolla, is placed in the subclass Polypetalae of Bentham and Hooker's classification, since its petals remain free rather than fusing together. The family shows considerable variety of habit, including herbs, shrubs, trees, and climbers, so habit by itself is not a reliable identifying feature and the floral structure carries most of the diagnostic weight. The root system is generally a tap root, and one of the family's most distinctive vegetative features is the common occurrence of root nodules — small swellings that house nitrogen-fixing bacteria in a symbiotic partnership with the plant, which is the reason several Fabaceae members are valued as soil-enriching green-manure crops. The stem may stand erect, as in most pulse crops, or may climb or twine, as in a number of wild and ornamental members. The leaves are typically compound — commonly pinnately compound or trifoliate — and usually bear a pair of stipules at the leaf base.

Floral Characters

The inflorescence is usually a raceme. The flower is bisexual and clearly zygomorphic, bilaterally rather than radially symmetrical, distinguishing it immediately from the actinomorphic flowers of the other two families in this chapter. The calyx has five sepals fused into a short tube (gamosepalous). The corolla shows the family's signature papilionaceous arrangement: five free petals of unequal size, consisting of one large, usually posterior standard (vexillum) that is the most conspicuous petal and often wraps around the others in bud, two lateral wing petals (alae), and two lower, anterior petals fused along their edges into a boat-shaped keel (carina) that encloses the reproductive parts.

The androecium normally has ten stamens in the characteristic diadelphous condition, with nine stamens fused by their filaments into a single sheath and the tenth remaining free — described as the "nine-plus-one" arrangement — and this staminal sheath, together with the style, sits enclosed within the keel. The gynoecium is monocarpellary, formed from a single carpel, with a superior ovary and marginal placentation, the ovules attached along the fused seam of that one carpel, exactly as visible in a split-open pea pod.

Pollination Mechanism

Pollination in Fabaceae is mostly carried out by insects. A visiting bee lands on the wing petals and, in probing for nectar, presses down on the keel; this depresses the keel and exposes the previously hidden anthers and stigma, so that pollen is transferred to or picked up from the insect's body — a mechanism often described as the keel "tripping" open like a spring-loaded trigger, ensuring reliable pollen transfer as the insect moves between flowers.

Fruit, Floral Diagram/Formula, and Economic Importance

The fruit is a legume (pod), a dry fruit that splits open along two seams to release its seeds. The floral diagram displays this zygomorphic arrangement, with the standard petal posterior, the wings lateral, and the keel anterior, enclosing the diadelphous stamens and the single carpel; the floral formula represents the flower as zygomorphic and bisexual, with five fused sepals, five free petals in the papilionaceous pattern, ten diadelphous stamens, and a monocarpellary superior ovary. …