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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 fifty three 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

Caesalpinioideae subfamily

Probably the basic pattern from which the other two subfamilies evolved. The flowers have five separate, conspicuous petals, ione of which (the banner) is always a different size, shape, or color from the other four. There are ten stamens, as in nearly all legumes; in this subfamily they are separate. All species are woody. Examples include royal poinciana or arbol de fuego (Delonix regia), palo verde (Cercidium and Parkinsonia), bird-of-paradise (Caesalpinia), and cassias (Cassia).

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 fifty 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.