Family Leguminosae (Fabaceae)

Family Leguminosae (also called Fabaceae), the pea family, ranges in habit from small herbs to large trees and is one of the most economically vital plant families, both for direct food value and for its distinctive symbiotic relationship with nitrogen-fixing bacteria.

Vegetative characters. Leaves are typically compound (pinnately or palmately), with a swollen, cushion-like base to the petiole called a pulvinus, which in some species (such as the touch-me-not-related plants) allows rapid leaf movement; stipules are commonly present, and venation is reticulate. Root nodules, formed through infection by the nitrogen-fixing bacterium Rhizobium, are frequently visible on the root system of many members, an easily observed field character linking this family directly back to the root-modification content covered earlier in this chapter.

Floral characters. The inflorescence is usually a raceme. The flower is zygomorphic and bisexual. The calyx has five sepals fused together (gamosepalous), sometimes distinctly two-lipped (bilabiate). The corolla is the family's single most recognisable feature: five petals of markedly unequal size and shape, arranged in a form called papilionaceous (butterfly-like). The largest petal, positioned at the back of the flower, is called the standard (or vexillum) and is outermost in the bud; it overlaps two lateral petals called the wings; these in turn overlap two smaller anterior petals that are fused together along their lower margins into a boat-shaped structure called the keel, which encloses the stamens and style within it. This specific, consistently ordered overlap -- standard over wings over keel -- is called vexillary aestivation, distinguishing the pea flower's corolla arrangement from the four other aestivation patterns described earlier in this chapter. The androecium usually has ten stamens; in the most common arrangement, nine of these have their filaments fused together into a tube, while the tenth remains free -- a condition called diadelphous -- and this slight gap allows the free filament to move, aiding both self- and cross-pollination depending on the species. The gynoecium consists of a single carpel (monocarpellary), folded lengthwise upon itself so that its two edges meet along a ventral suture; the ovary formed this way is superior, and the ovules develop in two rows along this ventral suture, a placentation type called marginal.

Fruit. The single folded carpel, with its marginal row of developing seeds, matures directly into the family's namesake fruit, a legume (or pod): a dry, dehiscent fruit that splits open along both of its sutures at maturity to release the seeds. …