Fancy a mimosa, anyone? Pictured here is Mimosa microphylla, or littleleaf sensitive-briar, growing as it typically does–low to the ground. It is one of many Mimosa species with intriguing properties.
Known also by the names of little-leaf mimosa, catclaw sensitive briar, and smooth-leaf sensitive briar, this herbaceous perennial is found throughout much of the southeastern United States in dry, sunny situations such as prairies, ravines, and open woods. A vine-like plant, it is a flimsy grower with sprawling stems that can reach nearly 1m (3 ft.) in height. Each stem is lined with hooked prickles, hence one of its names, “catclaw”. The leaves are small and bi-pinnately compound, and are the source of the plant’s “sensitivity”; the leaflets will immediately fold against one another upon being touched (the phenomenon of plants or fungi moving in response to touch is called thigmonasty). The fragrant inflorescences are clusters of many small pink flowers arranged in a globular shape, while the fruits are slender pods covered in small prickles.
Mimosa contains roughly 400 species. However, with a tortuous taxonomic history, the genus has accumulated over 3000 taxon names–quite a few synoynms! The etymology of Mimosa stems from its nastic movements as well; the Greek “mimos-” (“mime”) and “-osa” (“to resemble”) refer to the sensitive leaves that mimic the more typical organisms capable of movement. Upon learning the name of this genus, one perhaps wonders how the similarly-named alcoholic drink was coined–surely it can’t be the moving leaves again? References suggest the alcoholic drink is named for the widely cultivated Acacia dealbata, which also bears the common name of mimosa. The connection between the two is the yellow colour of Acacia dealbata‘s flowers is similar to that of the mimosa composed of equal parts champagne and orange juice. Both Acacia and Albizia (yet another common name of mimosa!) have similar morphologies to species of true Mimosa, particularly the inflorescences and leaf arrangements. The true species of Mimosa are distinguishable from these two genera by having flowers with fewer than 10 stamens.
Mimosa species are recognized as some of the few that are capable of quick nastic movements. However, not all of these movements are in response to physical stimuli! Some species, including Mimosa microphylla, close their leaves at night and on cloudy days. This movement reaction to light is, perhaps unsurprisingly, called photonasty. While photonasty is exhibited in many plants, Mimosa leaves exhibiting this behaviour became the first described examples of biological clocks. In 1729, Jacques d’Ortous de Mairan published evidence of this internal circadian clock after documenting the continuous rhythmic movement of Mimosa leaves in the absence of any light. Mimosa pudica was the test subject in this experiment, and is the most popular example of these nastic movements. That species was featured on Botany Photo of the Day a few years ago, in an entry which also includes an explanation of the mechanisms of thigmonasty. Some additional reading on circadian clocks can be found here: The Basics of Chronobiology.