(THE LEGUME FAMILY)
called Leguminosae after the type of fruit most of its members have -- a legume
(bean pod) -- leguminous trees, along with columnar cactuses (e.g., Saguaro)
characterize the Sonoran Desert. In fact, north of the Mexican border, most
of the common trees in the Sonoran Desert are legumes (all the trees spotlit
here are legumes). Legumes occur as forbs and shrubs as well as trees and were
found to comprise 8% of the 660+ plants
in the Tucson Mountains (Rondeau, R.J., et al. 1996). They have their origin
and highest prevalence in arid tropical environments, attesting to the tropical
influence we have in our Tucson flora (and fauna).
are critical to the desert ecosystem in many ways. Some of these are listed
Food: many organisms (including humans) eat their pods and other parts.
Shelter: many organisms use legumes (especially shrubs and trees) for nesting,
for escaping predation, for ambushing prey, and the escaping the elements.
Leguminous trees serve as nurse plants, providing
protection from trampling, the hot sun, and maybe most importantly, from frost
at night to other plants (e.g., the saguaro).
Soil Enhancer: legumes indirectly add nitrogen, a critical nutrient often
missing in desert soils, to the soil around them. They do this by providing
root nodules for special, nitrogen-fixing bacteria. The bacteria take unusable
(to most organisms) nitrogen from the air and convert it into usable nitrogen
that then gets into the soil for other plants. Farmers will plant legumes
(e.g., alfalfa, soybeans, etc.) in their fields to recharge the soil's nitrogen.
are three subfamilies, each with different flowers.
Subfamily (includes Blue and Foothills
Paloverdes): Flower has 5 separate petals, one of which is called the banner
petal because it is different in size, shape, or color from the rest of the
petals. Each flower has 10 separate stamens. All members of this family are
woody. The two other subfamilies are descended from this basic pattern.
Subfamily (includes Catclaw and Whitethorn
Acacias, Velvet Mesquite, and Fairy
Duster): The petals are fused and so tiny as not to be easily seen. But
the stamens are long. Many flowers cluster tightly to form a ball (powder-puff)
or cylinder (catkin). As in Caesalpinia, all
species are woody.
(Pea) Subfamily (includes Desert Ironwood): Flowers
have 3 upper petals (banner and wings) and 2 lower petals fused along the
bottom (keel). Nine of the 10 stamen filaments are fused and the 10th is separate
and they (and the stigma) are housed in the keel. In short, these are the
typical sweet-pea flowers.