Legumes include plants like alfalfa, peas, clover and beans as well as ornamental trees like honey locusts, royal poinciana and mimosa. One of the largest plant families, legumes are surpassed in number of worldwide species only by the orchid and daisy families. Legumes serve as foodstuffs for man and animals as well as help improve soil nutrition by fixating nitrogen into low-fertility croplands.
Legumes share only one common feature: from a single flower ovary they develop a fruit (pod) that splits open along two seams to release multiple seeds. Some legumes' pods do not split open but do have the two seams. Many may say that all legumes fixate nitrogen in soils, and while a valid characteristic, not all species of legumes conduct this soil-improving process. Within all the plants in this large family, great variation in leaf structure, plant form, flower shape and nativity exist, and taxonomists strive to further group legumes into smaller groups like subfamilies and genera (plural of genus). Subfamilies and genera place plants with close physical characteristics together to establish shared lineage and relation.
Legumes include plants that are trees, shrubs and herbs (perennial or annual plants) as well as any variety of climbing, rambling or twining vine-like forms. Overall, these types or plant forms have little to do with classifying legumes until the genus or species level. Some legumes are toxic, some grow in dry soils, while others are edible and love moisture--there is huge diversity among the world's legumes.
Known today as Fabaceae and formerly as Leguminosae, the legume family is also called the bean or pea family. Some 18,000 species of plants compose this family in any of the nearly 670 different genera. Members of the family are found growing on all continents except Antarctica, with particularly great diversity of legumes found in the tropical rainforests. Legumes grow in many other habitats, too.
The largest differentiation within the legume family centers on the plant's flower structure. These three subfamilies are Caesalpinioideae, Mimosoideae and Papilionoideae. Flowers in the Caesalpinioideae have one or many axes of symmetry and typically have one petal that is modified to look like a lip or standard. Examples include the orchid tree (Bauhinia spp.), shower tree (Cassia spp.) and the royal poinciana (Delonix regia). Those in Mimosoideae have pompon-like or spidery flowers with many stamens as exemplified by wattles (Acacia spp.) or silk tree (Albizia julibrissin). Lastly, flowers of plants in the Papilionoideae have a "butterfly-like" shape or the traditional shape of a pea blossom. Their flowers are highly modified with five petals: one is an upright standard or banner, two form lateral "ear-like" wings and the bottom two fuse together to create a keel. The classic example is the flower of any pea or bean as well as wisteria or coral-pea vine (Kennedia spp.).
Some taxonomists remove the legume family altogether and elevate the subfamilies each into family status (e.g. the Cronquist System). Thus, any plant formerly simply listed a member of Fabaceae is then placed into the new family based on flower morphology. Caesalpinaceae (caesalpinia family) then includes four tribes named Caesalpinieae, Cassieae, Cercideae and Detarieae. Mimosaceae (mimosa family) is further broken down into four tribes: Acacieae, Ingeae, Mimoseae, and Mimozygantheae. Papilionaceae (pea family) includes numerous tribes, such as Lotiae, Trifoliae, Galegeae, Genisteae, Hedysareae and Phaseoleae.