The Encyclopedia Americana (1920)/Leguminosæ
LEGUMINOSÆ, a group of plants formerly considered a family, but now generally divided into three families. The term, however, is a convenient one and is retained for the group as a whole. The plants are herbs, shrubs and trees widely distributed in all climates but most numerous in tropical and subtropical regions; growing upon all kinds of soil; exhibiting a great range of habit from creeping annual to climbing shrub; useful for a great variety of purposes — ornament, food, timber, fodder and in the arts; and constituting one of the largest groups of plants, about 7,000 species distributed among about 450 genera. The species are characterized by alternate, stipulate, usually compound leaves; papilionaceous or sometimes regular flowers commonly arranged in racemes; monadelphous, diadelphous or occasionally distinct stamens, typically 10, surrounding a single simple pistil which generally becomes a pod or legume containing one to many seeds.
The species naturally fall into three families: (1) Fabaceæ, with flowers resembling a butterfly; (2) Cæsalpiniaceæ, with imperfectly or not at all papilionaceous corollas, which may sometimes be nearly regular; (3) Mimosacecæ, with small, regular flowers. The first group contains more than two-thirds of the species. Its members are adapted for insect fertilization, especially by bees, which alight upon the lower petals, brush against the pistil which is thrust out by the insect's weight, then come in contact with the stamens and finally carry the pollen, which has been discharged, to other flowers. Thus the pistils receive pollen from stamens not in the same flower with them. In some instances they may also obtain pollen from these stamens, thus having a double chance to be fertilized. See Flowers And Insects.
Perhaps the most interesting trait found in the group is the power possessed by the species of obtaining free nitrogen from the air by means of the tubercles or wart-like excrescences upon their roots. These tubercles are the homes of bacteria which have gained entrance to the plant's tissue through the root-hairs and are thus the result of irritation. The plants, it is believed, furnish the bacteria with carbohydrate food in return for the nitrogenous material prepared by them, thus exhibiting excellent examples of symbiosis (q.v.). Largely owing to this reciprocal action, the legumes are valued as green manures, a fact long acted upon but unexplained until the last quarter of the 19th century, when Hellriegel and other investigators proved it, and even went further to show that soils poor in the bacteria could be inoculated profitably. See Clover; Nitrogen; Root-Tubercles; Green Manuring; Vetch; Cowpea; Bean; Lupine; Acacia; Licorice; Indigo; Sweet Pea; Medic; Laburnum; Tamarind; Mimosa.