Fabaceae
Faboideae
Caesalpinioideae
Mimosoideae
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The Family
Fabaceae (formerly
Leguminosae) is a grouping of
plants in the Order
Fabales, and one of the largest families of
flowering plants with 650 genera and over 18,000 species. These plants are commonly called
legumes and the
family contains some of our most valuable
food crops, such as
beans,
peas,
peanuts,
soybeans, and
lentils. Other members of the family are important sources of animal feed or green manure, such as
lupins,
clover,
alfalfa, and soybean. Some genera such as
Laburnum,
Robinia,
Gleditsia,
Acacia,
Mimosa, and
Delonix are ornamental
trees and
shrubs. Still other members of the family have medicinal or insecticidal properties (for instance
Derris) or yield important substances like
gum arabic,
tannin,
dyes, or
resins. Then there is
kudzu, an east
Asian species originally planted in the U.S. southeast for soil improvement and as a cattle feed, that has become a notorious
invasive weed that tends to grow over everything.
All members of this family have five-petaled flowers in which the superior ovary ripens to form a "pod", technically called a
legume, whose two sides split apart, releasing the seeds which are attached to one or both seams. The legumous plants are classified into three subfamilies, sometimes raised to the rank of family in the order Fabales, on the basis of
flower morphology (specifically,
petal shape):
- Faboideae (Fabaceae), previously called Papilionoideae: One petal is large and has a crease in it, the two adjacent petals are on the sides, and the two bottom petals are joined together at the bottom, forming a boat-like structure.
- Caesalpinioideae (Caesalpiniaceae): The five petals are equal in size and large.
- Mimosoideae (Mimosaceae): The petals are small, and the stamens are the most showy part of the flower.
Nitrogen fixation
A significant characteristic of legumes is that they host bacteria in their roots, within nodules called root nodules. These Rhizobium bacteria have the ability to take nitrogen gas (N2) out of the air and convert it to a form of nitrogen that is usable to the host plant (NO3-). This process is called nitrogen fixation. The legume, acting as a host, and the Rhizobium bacteria, acting as a provider of usable nitrate, form a symbiotic relationship.
