MIMOSA, so named from the movements of the leaves in many species which "mimic" animal sensibility, a genus of 400 species of the family Leguminosae, which gives its name to the large sub-order Mimoseae (characterized by usually small regu lar flowers with valvate corolla), to which belongs also the nearly allied genus Acacia. They are distributed throughout almost all tropical and subtropical regions, the acacias preponderating in Australia and the true mimosas in America. The former are of considerable importance as sources of timber, gum and tannin, but the latter are of much less economic value, though a few, like the talh (M. ferruginea) of Arabia and Central Africa, are im portant trees. Most are herbs or undershrubs, but some South American species are tall woody climbers. They are often prickly.
The roots of some Brazilian spe cies are poisonous, and that of M. pudica has irritating proper ties. The mimosas, however, owe their interest and their exten sive cultivation, partly to the beauty of their usually bipin nate foliage, but still more to the remarkable development in some species of the sleep move ments manifested to some ex tent by most of the pinnate Leguminosae, as well as many other (especially s e e d l i n g) plants. In the so-called "sensi
tive plants" these movements not only take place under the in fluence of light and darkness, but can be easily excited by me chanical and other stimuli. When stimulated—say, at the axis of one of the secondary petioles— the leaflets move upwards on each side until they meet, the move ment being propagated centri petally. It may then be communicated to the leaflets of the other secondary petioles, which close (the petioles, too, converging), and thence to the main petiole, which sinks rapidly downwards towards the stem, the bending taking place at the pulvinus (swollen base of the leafstalk). When shaken in any way, the leaves close and droop simultaneously, but if the agitation be continued they soon cease to respond to the shock. The common sensitive plant of hot-houses is M. pudica, a native of tropical America, now naturalized in corresponding latitudes of Asia and Africa, but the hardly distinguishable M. sensitive and others are also culti vated. Species of the closely allied genus Sclzrankia are known as sensitive-briar in the southern United States.