The New International Encyclopædia/Leguminosæ

LEGUMINOSÆ (Neo-Lat. nom. pl., from Lat. legumen, bean). A great order of dicotyledonous plants, the second of flowering plants in number of species, containing herbs, shrubs, and trees, growing in all kinds of soil and climates, usually erect, but sometimes trailing, twining, or climbing, many of them of great size. The leaves are alternate, usually compound, and have two early deciduous stipules at the base of the leaf-stalk. The inflorescence is commonly racemose. The calyx is inferior, 5-parted, toothed or cleft, the segments often unequal. The petals are 5, or by abortion fewer, inserted into the base of the calyx, usually unequal, often papilionaceous. The stamens are typically 10, free or united into a tube, in which case 9 are joined by their filaments, the tenth free, otherwise they are few or many, distinct or variously united. The ovary is 1-celled, in some cases 2-celled by a sort of false partition, generally of a single carpel; the style simple, proceeding from the upper margin, the stigma simple. The fruit is usually a pod or legume. The seeds are solitary or numerous, occasionally with an aril, often curved; the cotyledons large and well supplied with reserve material for the young plant. There are three suborders: (1) Papilionaceæ, with papilionaceous flowers; (2) Cæsalpineæ, with irregular flowers and spreading petals; (3) Mimoseæ, with small regular flowers. This order contains about 450 genera and 7000 species, of which about 5000 belong to the suborder Papilionaceæ. They are spread over all parts of the world, from the equator to the limits of vegetation, but their number is greatest in tropical and subtropical regions. They are applied to a great variety of purposes, and some of them are of great importance in domestic economy, the arts, medicine, etc. Many species are interesting on account of their beauty of form, foliage, or flowers. The structure of the flower indicates that the Papilionaceæ at least are designed for insect fertilization, bees being the agents. When a bee lights upon the wings and keel of the flowers, as the side and lower petals are called, the weight thrusts the stigma out of the flower in contact with the pollen-laden body of the bee. When relieved of the weight, the stigma returns to its normal position, and cross-pollination is usually effected. In some cases the returning stigmas may receive pollen from the stamens directly, and thus he close pollenized. In this way the flower has two chances of fertilization. The Leguminosæ are of added interest on account of their ability to assimilate free atmospheric nitrogen through the small tubercles on their roots. These tubercles are the dwelling-places of myriads of peculiar bacteria which enter the roots through root-hairs and set up an irritation resulting in the formation of galls. In these the bacteria multiply rapidly and are supplied with their necessary carbohydrates by the plant, and in turn give nitrogen to their hosts. This form of life is known as symbiosis (q.v.), and it is one of the chief reasons why plants of this order are so valuable in enriching the soil. In the seeds of many is found a nitrogenous substance called legumin (q.v.) or ‘vegetable casein.’

The discovery that leguminous plants assimilate the free nitrogen of the air through their root-tubercles was made by Hellriegel (q.v.), who also observed the presence of bacteria in the tubercles. Other investigators established the fact of the symbiosis of the bacteria and the root-tubercles and worked out practical methods for the inoculation of the soil with bacterial cultures to promote the growth of legumes even on a large scale. (See the articles Clover; Green Manuring; Root-Tubercles.) The principal genera of the order are: Mimoseæ, Acacia, Mimosa; Cæsalpineæ, Bauhinia, Cereis, Cæsalpinia, Hæmatoxylon, Cassia, Ceratonia, and Tamarindus; Papilionaceæ, Onobrychis, Desmodium, Arachis, Robinia, Lupinus, Astragalus, Cytisus, Ulex, Lotus, Anthyllis, Medicago, Trifolium, Vicia, Lathyrus, Phaseolus, Indigofera, Glycyrrhiza, Amorpha, Crotalaria, Dalbergia, Pterocarpus, etc.

Fossil representatives of this family are common in the Cretaceous rocks of Greenland and in the Tertiary deposits of Europe and America.

USEFUL LEGUMES

1. VETCH (Vicia villosa).	4. PEANUT (Arachis hypogæa).
2. GARDEN PEA (Pisum sativum).	5. COWPEA (Vigna catjang).
3. STRING BEAN (Phaseolus vulgaris).	6. LIMA BEAN (Phaseolus lunatus).