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Description

This species is a small subshrub with fuzzy gray-green leaves. S. covesii has four or six leaflets. Foliage is shed during dry seasons. Rains trigger short spikes of yellow caesalpinoid flowers, mostly in spring and late summer.

Range

The species is widespread in the Sonoran Desert and into the arid tropics of Mexico. Its range overlaps with S. bauhinioides in Arizona.

Notes

Sennas are buzz-pollinated by large carpenter bees and bumblebees. The anthers of buzz-pollinated flowers don't split open lengthwise to expose their pollen as in most flowers. Instead each anther has a small pore at one end that is too small for even the smallest insects to enter. The flower is so oriented that the bee lands on it hanging upside down with the anther pores facing downward. The bee then vibrates its wing muscles, making an audible buzzing sound different from that of the bee in flight. The pollen is thus shaken out of the anthers onto the hanging bee. The pollen grains are tiny and nonsticky to prevent them from clogging the anther pores. Carpenter bees have especially fine hairs on their bodies for catching this pollen. As the bees visit other flowers, some of the pollen on their bodies gets stuck on the stigmas and effects pollination.

In addition to the sennas, a number of plants in the nightshade family (Solanaceae) are buzz-pollinated, including tomatoes. Though they are self-fertile, tomato flowers must be buzzed — ordinarily by wild bees— to dislodge pollen so that it falls onto the stigmas. As recently as the 1980s, greenhouse growers hired workers to vibrate the flowers with electric toothbrushes to ensure fruit set. Today they purchase commercially-produced bumblebee hives. The bees are not only more efficient pollinators than humans with toothbrushes, they are also less expensive.

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.