Classification of Fabaceae

Fabaceae has traditionally been divided into three subfamilies: Caesalpinioideae, Mimosoideae, and Faboideae (or Papilionoideae), each of which have been considered a separate plant family in the past. Classifications based on molecular analyses now separate Caesalpinioideae into several lineages and recognize the tribe Cercideae as a separate and more basal group in the family. The floral types in the legume family are quite variable, with flowers ranging from regular (i.e., actinomorphic, radially symmetric) in Mimosoideae to highly irregular (i.e., zygomorphic, bilaterally symmetric) in Faboideae. The flowers of the tribes Cercideae and Caesalpinioideae are somewhat intermediate between these extremes as regards symmetry.

Cercideae is a small tropical and temperate woody group (e.g., Cercis, Bauhinia) in which the leaves are apparently simple and often bilobed. The flowers of Cercis are only superficially similar to those of Faboideae.

The subfamily Caesalpinioideae (classified as a family, Caesalpiniaceae, by some authorities) is a heterogeneous group of plants with about 160 genera and some 2,000 species. The latest classifications show that this subfamily is the most basal lineage among the legumes and the one from which the other two subfamilies evolved. In that sense it is not a true monophyletic group, and it will undoubtedly be treated taxonomically in a different way in the future. Caesalpinioideae legumes are found throughout the world but are primarily woody plants in the tropics. Their moderate secondary invasion of temperate regions is mostly by herbaceous (nonwoody) evolutionary derivatives. The presence of Gleditsia triacanthos (honey locust) and of the related Gymnocladus dioica (Kentucky coffee tree) in temperate regions is a striking exception to this generalization, however, and they may represent more ancient and relictual lineages in the subfamily.

Caesalpinioideae is more variable than the other three groups. The leaves are usually divided into leaflets (compound), or else the leaflets are again divided into leaflets (bicompound). The flowers also vary in symmetric form, from nearly radial to bilateral to irregular (symmetric in no plane). The sepals are usually separate and imbricate (overlapping in the bud). There are generally five separate imbricate petals, the upper one inside of the lateral petals in the bud. The 10 or fewer stamens are exposed, although not as conspicuously as in many of the members of the subfamily Mimosoideae described below. The fruit conformation is diverse. Bacterial nodulation is much less prevalent than in either of the other two subfamilies. Canavanine is not present. Many Caesalpinioideae species are prized ornamentals in the tropics, such as Delonix regia (royal poinciana), Cassia grandis (pink shower), and Bauhinia (orchid trees). Gleditsia triacanthos (honey locust) is well known in temperate regions.

The subfamily Mimosoideae (classified as a family, Mimosaceae, by some authorities) includes 82 genera and more than 3,200 species. Like Caesalpinioideae, Mimosoideae legumes are primarily woody plants of the tropics, and the few species native to temperate parts of the world are mostly herbaceous. The majority of Mimosoideae have large leaves that are divided into secondary (compound) leaflets, and in many these leaflets are again divided (bicompound) and have a feathery, sometimes fernlike appearance. A striking exception is that of most of the Australian acacias (but not of the American kinds) mentioned above, in which the compound leaves have become modified, losing all their leaflets and appearing to be undivided, or simple. The flowers of the family are radially symmetric and are usually most easily recognized by the long stamens that extend beyond the rest of the flower. The calyx and corolla are both valvate in bud, contrasting with the usual condition in both of the other subfamilies. The petals are small and often not noticed except by close examination. Many of these plants have nodules containing the nitrogen-fixing bacterium Rhizobium on their roots.

Mimosa pudica (sensitive plant) is sometimes grown as a novelty because its leaves quickly fold up when touched. Albizia julibrissin (mimosa, or silk, tree), a widely planted ornamental in the southern United States, folds its leaves together at dusk, decreasing by at least half the amount of leaf surface exposed to the atmosphere. The movement is caused by changes in water pressure in specialized structures at the base of the petioles and leaflets.

The subfamily Faboideae, also called Papilionoideae (classified as a family, Fabaceae or Papilionaceae, by some authorities), is the largest group of legumes, consisting of about 475 genera and nearly 14,000 species grouped in 14 tribes. The name of the group probably originated because of the flower’s resemblance to a butterfly (Latin: papilio). It is the unique bilaterally symmetric (zygomorphic) flowers that especially characterize the group, so that thousands of species can be recognized as a member of Papilionoideae at a glance. The Lathyrus odoratus (sweet pea) flower provides an example. It has a large petal at the top, called the banner, or standard, that develops outside of the others before the flower has opened, two lateral petals called wings, and two lower petals that are usually fused and form a keel that encloses the stamens and pistil. The whole design is adapted for pollination by insects or, in a few members, by hummingbirds. Sweet nectar, to which the insects are cued by coloured petals, is the usual pollinator attractant. Various locking and releasing devices of the keel and wing petals control pollination in diverse ways favouring (or enforcing) either outcrossing or self-pollination—e.g., Trifolium (clover), Medicago (alfalfa), and Lotus corniculatus (bird’s-foot trefoil). The most effective kind of obligate self-pollination, however, is that of cleistogamous flowers, which do not open and thus prevent the entry of insects. Lespedeza and many other genera of Papilionoideae legumes bear both kinds of flowers, generally on the same plant. Enforced inbreeding serves to fix and maintain successful strains; outbreeding provides evolutionary diversity that may facilitate habitat or range expansion or may serve to provide flexibility for environmental changes.

The calyx is composed of fused sepals. The stamens are 10 or fewer and are free in a few tribes but are most commonly fused at their filaments (monadelphous) or fused at all filaments but one, which remains free (diadelphous). The ovary has a single carpel and develops into various fruit types.

Like the other subfamilies, members of Papilionoideae have their origins in the tropics, but their occupation of the arid and temperate parts of the world, mostly as herbaceous plants, is far more extensive. In the forests, prairies, and deserts, they are among the most common plants. The largest genus of legumes, Astragalus (2,400 to 3,300 species, known as locoweed), is mainly western North American but also occurs in Eurasia, India, Africa, and South America. These temperate legumes have mostly pinnate leaves among which those with three leaflets (trifoliolate) are common—e.g., beans and soybeans. Trifoliolate leaves rarely occur in the other subfamilies. The large genus of Lupinus (lupines) generally has 5 to 11 (occasionally up to 15) palmate leaves. The leaves of clovers are most commonly palmately trifoliolate, as are those of Baptisia. In one tribe the leaf axis terminates in a tendril, which facilitates climbing; members include the sweet pea and Vicia (vetches). The symbiotic relationship between Rhizobium and the plant, which takes place in root nodules and “fixes” atmospheric nitrogen into compounds useful to the plant, is most strongly developed in Papilionoideae legumes.

The legumes produce many kinds of chemical substances—e.g., alkaloids, flavonoids, tannins, and the free amino acid canavanine (the latter found only in legumes). The function of those that are physiologically active (i.e., often poisonous) in animals seems usually to be that of predator defense. The medical potential (especially of the alkaloids) of some of these substances, or of their synthetic derivatives, has been extensively studied. The absence or presence and distribution of these substances in the various groups are also used in legume classification. Information about other cryptic features, such as pollen and plant anatomy, contributes to scientific knowledge of legume evolution as well.

Evolution

The origin of Fabales and its relationship to other plant families and orders are now becoming clearer. The order is closely related to a group of Rosid orders that also contain nitrogen-fixing plants: Rosales, Cucurbitales, and Fagales. Members of these orders which do fix nitrogen, however, use root-dwelling actinomycetes, typically Frankia, rather than Rhizobium and relatives used in legumes. There are at least six independent origins of the symbiotic relationships of Frankia and host plants. Within the order Fabales, the family Polygalaceae is most distant with the three other families, Surianaceae, Quillajaceae, and Fabaceae, possessing stipules and separate carpels. Of the latter three families, Quillajaceae and Fabaceae are most closely related and share the feature of clawed petals.

Molecular evidence confirms the hypothesis that Caesalpinioideae includes the earliest diverging lineages among the legumes. This was also the prevailing theory prior to molecular studies, based on the group’s high diversity in the tropics, an extended fossil record, and the wide variation of floral and vegetative structures beyond the specializations in the other two subfamilies. The unique Rhizobium nitrogen-fixation symbiosis is much less developed in Caesalpinioideae than in the other groups; indeed, it seems to have originated in this subfamily. What is becoming more clear, however, is that Caesalpinioideae legumes are more diverse than previously thought and that the other two subfamilies, Mimosoideae and Papilionoideae, were derived from particular lineages among the diverse Caesalpinioideae legumes. This strengthens the idea that legumes form a single family; however, the phylogenetic relationships within the family are more complex than the former simple division into three subfamilies. A clearer picture is expected to develop in the future as further molecular analysis is obtained.