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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

Mimosoideae subfamily

The petals are fused in this group, but they're so tiny that they are not noticeable. What one sees is a powder puff of stamens. It's easy to visualize the derivation of flowers of this group from the above subfamily. Start with a caesalpinoid flower such as a Palo Verde blossom. Reduce the petals until they nearly disappear, greatly elongate the filaments of the stamens, and combine several to many flowers into a tight cluster. The visual result is a ball or cylinder of stamens (powder-puffs or catkins, respectively). All species are woody. Examples include acacias (Acacia), mesquite (Prosopis), fairy duster (Calliandra), and mimosa (Albizia).

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.