Arizona-Sonora Desert Museum Digital Library

Legumes are a very large family of 16,000 species in nearly all of the world's habitats. Champion drought tolerators, they are most abundant in the arid tropics. Their prevalence in the Sonoran Desert flora (for example, there are 53 legume species in the Tucson Mountains, 8% of its plants) reflects this desert's tropical origin. North of the Mexican border most of the common Sonoran Desert trees are legumes.

Description

The family was named Leguminosae for its fruit, which in most species is a legume (the technical term for bean pod, a single-chambered capsule enclosing what appears to be a single row of seeds that is actually two rows — alternate seeds are attached to opposite halves of the pod). There are three subfamilies with flowers that look very different from one another at first glance, but arose from a common pattern: Caesalpinioideae, Faboideae, and Mimosoideae

Faboideae (pea) subfamily

This group is characterized by three upper petals and two lower fused petals, and nine of the ten filaments are fused with the tenth being separate. This subfamily's flowers diverged from Caesalpinoids in another direction from the mimosoids. Again, visualize a palo verde flower. Enlarge the banner (top) petal until it's the largest part of the flower. Fold the two adjacent petals forward until they touch at their tips; these are called the wing petals. Finally, reduce the remaining lower two petals and fuse them along their bottom edges to form a boat-shaped structure called the keel. Conceal most of the keel between the wings, and hide the stamens and pistil inside the keel. The result is a sweet pea-shaped flower. This subfamily includes many herbaceous as well as woody species. Examples are desert ironwood (Olneya tesota), all cultivated beans (Phaseolus, etc.), lupines (Lupinus), and of course, sweet peas (Lathyrus). In a few species, such as New World coral beans (Erythrina), the banner is reduced rather than enlarged.

Notes

Plants require large quantities of three minerals: nitrogen, phosphorus, and potassium. The latter two elements are present in soil, but nitrogen is an atmospheric gas that plants cannot use directly. Some soil bacteria and cyanobacteria (blue-green algae) can fix nitrogen (convert it into nitrate or other compound) into a form which plants can use. Another major source of nitrogen is the decomposition of dead plants and animals. In arid soils especially, where decomposition of organic material is slow, plant growth is often limited by the available amount of soil nitrogen. Many legumes harbor colonies of nitrogen-fixing bacteria in their roots. The plant provides favorable habitat and carbon for the bacteria, and the bacteria in turn provide surplus nitrate to the plants. Nitrogen-fixing legumes have higher concentrations of nitrogen compounds in their tissues than non-fixing plants. When legume leaves decompose they release the nitrogen and enrich the soil. Nitrogen is an essential element in proteins, so nitrogen-fixing plants can make large crops of seeds with high protein contents (more than 50 percent in some species).

The typically large, nutritious, and abundant seeds of legumes are an important food source for many wildlife species, including insects such as bruchid beetles. Adult bruchids are flower beetles, while the larvae of most species are seed predators. Bruchids are not restricted to legumes, but there is a myriad of species that specialize on legume seeds. Some species are very host-specific, while others feed on a wide range of seeds. Decades of intensive study of the bruchid-seed relationship would likely not reveal all aspects of this tiny part of the ecological web.