Flora of North America species comparison

Astragalus cyaneus is locally plentiful but uncommon, known only from the eastern side of the Rio Grande in north-central New Mexico.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera ca. 770, species ca. 20,900 (153 genera, 1345 species in the flora).

Fabaceae are the third-largest angiosperm family after Orchidaceae and Asteraceae (M. J. M. Christenhusz et al. 2017). The closest relatives of Fabaceae appear to be Polygalaceae, Quillajaceae D. Don, and Surianaceae (D. E. Soltis et al. 2011). While the family Fabaceae is strongly supported as monophyletic (Angiosperm Phylogeny Group 2016), subfamilial classification has proven to be more complicated. Historically, Fabaceae (often under the alternative name Leguminosae) were considered easily divided into three groups based on morphological features, especially of flowers (A. Cronquist 1981; see morphology discussion below): the caesalpinioids (Caesalpiniaceae R. Brown, or Fabaceae subfam. Caesalpinioideae de Candolle), the mimosoids (Mimosaceae R. Brown, or Fabaceae subfam. Mimosoideae de Candolle), and the papilionoids (Papilionaceae Giseke [Fabaceae in the strict sense], or Fabaceae subfam. Papilionoideae de Candolle [Faboideae Rudd]). Polyphyly of the caesalpinioids has long been suspected, as have affinities to mimosoids, based on morphological data (R. M. Polhill and J. E. Vidal 1981). Phylogenetic studies have confirmed that mimosoids are embedded within a paraphyletic caesalpinioid clade (A. Bruneau et al. 2001, 2008). Most recently, the Legume Phylogeny Working Group (2017) has recognized six subfamilies: Caesalpinioideae (including a monophyletic “mimosoid clade”), Cercidoideae, Detarioideae, Dialioideae LPWG, Duparquetioideae LPWG, and Faboideae [as Papilionoideae]; Dialioideae and Duparquetioideae do not occur in the flora area. Arrangement of genera in this treatment follows the subfamilial classification of Legume Phylogeny Working Group, with the sequence of genera within the subfamilies adapted from G. P. Lewis at al. (2005).

Leaves in Fabaceae are usually compound, often pinnate or twice-pinnate, but also sometimes palmate; leaves with only one blade are likely derived from ancestral compound leaves and are termed unifoliolate (as in Cercis; S. A. Owens 2000). In some taxa, the rachis in a pinnate leaf is extremely reduced (as in Ladeania), making the leaf superficially appear palmate; these leaves are termed pseudopalmate. A characteristic feature of most legume leaves is the pulvinus (plural pulvini), a jointlike thickening of petioles and petiolules that is involved with leaf movement related to changes in light or moisture availability (nyctinasty; T. M. Rodrigues and S. R. Machado 2007) or touch (thigmonasty; J. Braam 2005). Extrafloral nectary glands may be present on leaves (blades, rachises, or petioles) of some taxa, especially in the Detarioideae and Caesalpinioideae, and these may be stalked or sessile, and of various shapes.

Inflorescences in Fabaceae can consist of a single flower, or flowers may be arranged in spikes or heads, which can be grouped in larger paniculate structures, racemes, pseudoracemes (where each node in the racemelike inflorescence bears several flowers), cymes, panicles, or umbels (umbelliform racemes or pseudoracemes with greatly reduced internodes).

Flowers in Fabaceae are generally five-merous, with more or less distinct petals (which are sometimes differentiated into blade and claw) and often with connate sepals. Floral structure conforms in general to three main types: papilionaceous, caesalpinioid, or mimosoid. Papilionaceous flowers (from Latin papilio, butterfly) are usually bilateral and somewhat perigynous. Sepals are connate, at least at the base, into a tube that sometimes completely encloses the floral bud, with or without lobes at the distal end. The usually five petals are arranged into a keel formed from the innermost (abaxial) pair, the wings, the lateral pair positioned outside the keel, and a banner (sometimes called the standard), which is positioned outermost in the bud, surrounding the other petals. The keel petals may be completely distinct from each other or may be connate at the distal end, while the wing petals are usually distinct from each other. The banner petal, which is often the largest of the petals, may be straight (as in Erythrina), or reflexed at a joint or callous or from its base (as in Maackia). The stamens in a papilionaceous flower are usually ten and may be monadelphous (as in Lupinus) or diadelphous (in a 9 + 1 arrangement, with nine filaments connate into a closed tube or at least partially open sheath surrounding the pistil and one stamen distinct from the others, as in Trifolium) and are not usually long-exserted or the showy part of the flower. The pollen is in monads. Caesalpinioid (or pseudopapilionaceous) flowers are often bilateral (zygomorphic) and perigynous (rarely hypogynous). The sepals in caesalpinioid flowers are distinct or nearly so, imbricate or valvate in bud, or connate into a five-lobed cup or even nearly absent. The petals are usually less obviously organized into keel, wings, and banner. The keel and wings may be positioned as in papilionaceous flowers (as in Cercis), or they may be more widely open and more similar to each other. The banner petal is situated in the bud to the inside of the wing petals and is sometimes smaller than the other petals (as in Caesalpinia). Stamens in caesalpinioid flowers range from one to ten, sometimes more, and are often distinct. They are often shorter than the corolla, though sometimes much longer and showy, and are sometimes heteromorphic, of differing sizes or shapes, sometimes with a mix of fertile and sterile (staminodial) anthers. The pollen is usually in monads. Mimosoid flowers are regular (radially symmetric) and hypogynous or slightly perigynous, and the most distinctive of the floral types, usually small and individually inconspicuous. They are not organized as noted above but instead have corollas that are often connate into a tube, rarely distinct, and valvate (rarely imbricate) in the bud. The sepals are usually connate at the base, usually regular, and often lobed distally. The petals may be shorter than or longer than the calyx. Stamens are distinct from one another or proximally connate, range in number from twice as many as the petals to many, and are usually longer or even very much longer than the perianth and thus are the showiest part of the flower. The pollen is usually in tetrads or polyads (Legume Phylogeny Working Group).

Fruits of Fabaceae are generally legumes, usually 1-carpellate and 1-locular, with seeds positioned marginally, and often dehiscent into two valves at maturity. Legumes can vary greatly in morphology (C. R. Gunn 1984, 1991; J. H. Kirkbride et al. 2003), ranging from flattened laterally (as in Pisum) to inflated at maturity (as in Astragalus crassicarpus), and from a few millimeters to 60 centimeters. They are often dry at maturity but may be fleshy (as in Styphnolobium japonicum), with valves (the two halves of the fruit wall) ranging in texture from thin and fragile to thick and woody. Fruits of some Astragalus species may be partially or completely bilocular, due to intrusion of tissue (the replum) from the dorsal (abaxial) suture (Kirkbride et al.; S. L. Welsh 2007). Some legumes are indehiscent (as in Arachis), while others may open along one or both sutures; some taxa have fruits which are explosively dehiscent (as in Zapoteca); valves may remain flat post-dehiscence or may be twisted. A modified form of legume, the loment, is divided into one-seeded indehiscent sections (sometimes called joints) that break apart from each other at maturity (as in Desmodium); a craspedium is similar, but the one-seeded segments leave behind the replum as they fall away (as in Mimosa pudica). Andira is unusual among North American Fabaceae for its drupelike fruit. A few taxa produce samaroid fruits, such as Dalbergia ecastaphyllum. In addition, some taxa in the flora area have geocarpic fruits (Arachis species; Trifolium amphianthum has geocarpic cleistogamous fruits, in addition to the chasmogamous fruits produced on long, erect peduncles).

Seeds of legumes range in number from one to more than 75, and may be positioned in the fruit parallel, obliquely, or transversely to the length of the fruit (C. R. Gunn, 1991; J. H. Kirkbride et al. 2003). They may possess a true aril (as in Pithecellobium), an aril-like, enlarged funiculus, or these may be absent. The hilum (scar of attachment to the funiculus) is sometimes used in identification, and may vary in outline from V-shaped, to linear or circular, or other shapes, and can be variable in size or conformation. A pleurogram (U-shaped or elliptic line) is often present in mimosoid legumes but is usually absent in other groups. The embryo radicle is usually curved in subfam. Faboideae, and is usually straight in the rest of the subfamilies.

Roots of many Fabaceae bear nodules containing nitrogen-fixing bacteria (rhizobia, in numerous genera) that have coevolved with their hosts, which allow the plants to occupy marginal, nitrogen-poor habitats (D. P. S. Verma and J. Stanley 1989; J. P. W. Young and K. E. Haukka 1996; M. Andrews and M. E. Andrews 2017). Nitrogen fixation by rhizobial symbionts of Fabaceae is ecologically important in natural systems and in agricultural systems.

The economic importance of legumes to humans is second only to that of grasses (J. A. Duke 1981; P. H. Graham and C. P. Vance 2003). Among important legume food crops are the grain legume (pulse) genera Arachis, Cicer, Faba Miller, Glycine, Glycyrrhiza, Lens, Phaseolus, Pisum, Tamarindus, and Vigna, which provide a large portion of the dietary protein requirements and other products to people worldwide. In addition, legume forage crops, including Medicago, Trifolium, and Vicia, are important for production of grazing animals and as nectar sources for bees. Many other genera, such as Albizia, Cassia, Cercis, Delonix, Gleditsia, Laburnum, Lupinus, Robinia, Senna, and Wisteria are grown as ornamental plants. In addition, important lumber trees, such as species of Acacia and Dalbergia, are members of Fabaceae. Natural gums, such as gum Arabic (often from Acacia or Senegalia species) and guar gum [from Cyamopsis tetragonoloba (Linnaeus) Taubert], as well as dyes such as true indigo (from Indigofera tinctoria) and haematoxylin (from Haematoxylum campechianum Linnaeus), are derived from species of Fabaceae.

In the key to genera of Fabaceae, characters and character states are sometimes used that are included in descriptions of taxa within the relevant genera but not in the generic descriptions themselves. Such characters are not usually found as part of a normal genus description (for example, measurements), but they are included in the key because they are useful for identifying some genera within the family.

The following taxa are excluded from the flora due to lack of documentation, because they are waifs that did not become established, or because they are only known from cultivation in the flora area: Carmichaelia R. Brown (D. Isely 1998); Chesneya nubigena (D. Don) Ali (Isely); Cojoba graciliflora (S. F. Blake) Britton & Rose (as Pithecellobium graciliflorum S. F. Blake; R. W. Long and O. Lakela 1971; D. B. Ward 1972; R. P. Wunderlin 1998); Cullen americanum (Linnaeus) Rydberg (P. A. Rydberg 1919–1920; J. K. Small 1933); C. corylifolium (Linnaeus) Medikus (as Psoralea corylifolia Linnaeus; R. R. Tatnall 1946); Hippocrepis comosa Linnaeus (E. T. Wherry et al. 1979; A. F. Rhoads and W. M. Klein 1993); Lotononis bainesii Baker (Wunderlin); Otholobium fruticans (Linnaeus) C. H. Stirton (A. Kellogg 1877; Isely); and Scorpiurus muricatus Linnaeus (Tatnall; Rhoads and Klein; M. D. Cullina et al. 2011).

Recent additions to the flora area that are not treated here include: Callerya reticulata (Bentham) Schot (Florida; http://florida.plantatlas.usf.edu/Plant.aspx?id=4377); Dorycnium hirsutum Seringe (California; E. Dean et al. 2008); Lonchocarpus punctatus Kunth (Florida; R. P. Wunderlin 1998); and Psophocarpus tetragonolobus (Linnaeus) de Candolle (Florida; http://florida.plantatlas.usf.edu/Plant.aspx?id=4403).

(Discussion copyrighted by Flora of North America; reprinted with permission.)