FABALES

The Fabales is sometimes treated as one family, the Fabaceae (old name of Leguminosae). Here, we follow Cronquist and treat it as three families.

Mimosaceae - Flowers actinomorphic

Caesalpiniaceae - Zygomorphic flowers, standard (banner) positioned within the wings and petals

Fabaceae - Zygomorphic flowers, standard positioned outside the wing.

One important characteristic of legumes is their ability to fix nitrogen. This is accomplished by bacteria that are members of the genus Rhizobia that infect the roots and form nodules on the roots. These nodules contain a hemoglobin that functions in the same manner as hemoglobin in animals except that it binds oxygen more tightly. The hemoglobin keeps the free oxygen level in the nodule low but high enough for the bacteria to survive, but low enough that the oxygen senstitive enzyme (nitrogenase) that is responsible for nitrogen fixation remains functional. Humans have known for a long time that including a legume in crop rotations resulted in soil enrichment. This type of farming largely disappeared with the widespread use of nitrogen fertilizer. This shift began after World War I when chemical companies producing explosives shifted to producing fertilizer in the form of ammonium nitrate, which is itself a potent explosive. This change in agricultural practices has resulted in higher yields but has been accompanied by increased soil erosion and surface and ground water contamination and it has become obvious that the higher yields are not sustainable. In addition, domestic terrorists have brought ammonium nitrate full circle in using it to blow up buildings. These problems have renewed interest in the use of crop rotation with legumes as an important component in restoring soil fertility.