Fabaceae

Growth Form and Habit

The Fabaceae family encompasses an extraordinary diversity of growth forms, including annual and perennial herbs, shrubs, woody vines (lianas), and trees of various sizes from small understory species to emergent canopy giants. This diversity reflects the family's evolutionary adaptability to different ecological niches. Many herbaceous legumes have extensive root systems with nitrogen-fixing nodules, while woody species often develop deep taproots. Some genera, like Acacia in dry environments, have evolved specialized adaptations such as phyllodes (modified leaf stalks) instead of true leaves to reduce water loss.

Leaves

Leaves in the Fabaceae family are typically alternate and stipulate, with the stipules varying from small and inconspicuous to large and leaf-like. The leaves are most commonly compound, either pinnate (feather-like with leaflets arranged along a central axis) or palmate (with leaflets radiating from a single point). Some species have bipinnate leaves (twice compound), while others have simple leaves or phyllodes. Many legumes exhibit nyctinastic movements, with leaflets folding together at night or when touched (as in the sensitive plant, Mimosa pudica).

Leaf modifications are common in the family and often reflect adaptations to specific environments. These include:

• Tendrils derived from leaflet modifications (as in Pisum and Vicia)
• Phyllodes, which are flattened leaf stalks functioning as leaves (common in some Acacia species)
• Reduced or absent leaves in some desert-adapted species
• Spines derived from stipule modifications (as in Robinia)

Flowers

Flowers in the Fabaceae family are typically bisexual and pentamerous (based on a plan of five), with a distinctive structure that varies among the three traditionally recognized subfamilies. The flowers are usually arranged in racemes, panicles, spikes, or heads, though solitary flowers occur in some species.

The three main flower types in Fabaceae correspond to the traditional subfamilies:

• Papilionoid flowers (subfamily Faboideae): These have a distinctive "pea-like" or "butterfly-like" zygomorphic structure with five petals arranged in a standard (banner), two wings, and two lower petals fused to form a keel that encloses the stamens and pistil. The calyx is typically tubular with five lobes. Most papilionoid flowers have ten stamens, either all fused into a tube (monadelphous) or with nine fused and one free (diadelphous).
• Mimosoid flowers (subfamily Mimosoideae): These are small, actinomorphic (radially symmetrical) flowers typically arranged in dense globular heads or spikes. They have a small calyx and corolla, but the numerous stamens with their long, colorful filaments are the showy part of the flower. The petals are often fused at the base.
• Caesalpinioid flowers (subfamily Caesalpinioideae): These show intermediate characteristics, typically with five separate petals arranged in a bilateral symmetry that is less pronounced than in papilionoid flowers. The uppermost petal is innermost in the bud (unlike in papilionoid flowers where it is outermost). Stamens are usually ten or fewer and may be of unequal lengths.

The gynoecium in all Fabaceae consists of a single carpel that develops into the characteristic legume fruit. The ovary is superior, with a terminal style and stigma, and contains one to many ovules arranged along the ventral suture.

Fruits and Seeds

The defining fruit type of the Fabaceae family is the legume (also called a pod), a dry dehiscent fruit derived from a single carpel that typically splits along two sutures to release the seeds. However, the family exhibits remarkable diversity in fruit morphology, with numerous modifications of the basic legume structure:

• Typical dehiscent legumes that split along both sutures (e.g., garden pea, beans)
• Indehiscent legumes that do not open at maturity (e.g., peanut)
• Loments, which break transversely into one-seeded segments (e.g., Desmodium)
• Samaras, with wing-like extensions for wind dispersal (e.g., Dalbergia)
• Drupes, with fleshy or fibrous outer layers (e.g., Andira)
• Follicles, which split along a single suture (e.g., some Astragalus)

Seeds of Fabaceae are typically oval to round, sometimes flattened, and vary greatly in size from tiny (as in clover) to very large (as in some tropical species). They often have a hard seed coat (testa) that provides protection and may enforce dormancy. Many legume seeds have a distinctive hilum, the scar marking the point of attachment to the fruit wall. Some species also have an aril, a fleshy outgrowth from the funiculus that may aid in seed dispersal by attracting animals.

Seed dispersal mechanisms in the family are diverse and include:

• Explosive dehiscence, where the pod suddenly splits and forcibly ejects seeds (e.g., Cytisus)
• Wind dispersal via winged or hairy fruits or seeds
• Animal dispersal through edible fruits or seeds with hooks or barbs
• Water dispersal in some riparian or coastal species

Root Nodules and Nitrogen Fixation

One of the most ecologically significant characteristics of the Fabaceae family is the ability of many species to form symbiotic relationships with nitrogen-fixing bacteria, primarily of the genus Rhizobium. These bacteria inhabit specialized structures called nodules that develop on the roots of host plants. Within these nodules, the bacteria convert atmospheric nitrogen (N₂) into ammonia (NH₃), which can be used by the plant. In exchange, the plant provides carbohydrates and other nutrients to the bacteria.

This symbiotic relationship allows legumes to thrive in nitrogen-poor soils and contributes significantly to global nitrogen cycling. It also makes legumes valuable in agriculture for improving soil fertility through crop rotation, intercropping, and green manuring practices. However, not all legumes form these symbiotic relationships; the ability is most common in the Papilionoideae subfamily and less frequent in the other groups.

Chemical Characteristics

The Fabaceae family is known for producing a diverse array of secondary metabolites, many of which have ecological functions in defense against herbivores and pathogens or serve as signaling molecules. These compounds also contribute to the medicinal, toxic, or nutritional properties of many legumes. Key chemical groups include:

• Alkaloids: Diverse nitrogen-containing compounds found in many genera, including the quinolizidine alkaloids in Lupinus and Cytisus, and the pyridine alkaloids in Sophora
• Isoflavonoids: Compounds with structural similarity to estrogens, particularly abundant in Glycine (soybean) and Trifolium (clover)
• Non-protein amino acids: Unusual amino acids that can be toxic to herbivores, such as canavanine in Canavalia and mimosine in Leucaena
• Lectins: Proteins that bind to specific carbohydrates, found in high concentrations in many legume seeds
• Protease inhibitors: Compounds that inhibit protein-digesting enzymes, serving as defense against insects
• Tannins: Complex polyphenolic compounds that bind to proteins, common in many woody legumes