1911 Encyclopædia Britannica/Mimosa

MIMOSA (so named from the movements of the leaves in many species which “mimic” animal sensibility), a genus of the natural order Leguminosae, which gives its name to the large sub-order Mimoseae (characterized by usually small regular 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 ṭalḥ (M. ferruginea) of Arabia and Central Africa, are important trees. Most are herbs or undershrubs, but some South American species are tall woody climbers. They are often prickly. The roots of some Brazilian species are poisonous, and that of M. pudica, has irritating properties. The mimosas, however, owe their interest and their extensive cultivation, partly to Branch and leaves of the sensitive plant (Mimosa pudica), showing the petiole in its erect state, a, and in its depressed state, b; also the leaflets closed (c), and the leaflets expanded (d); p, pulvinus. the beauty of their usually bipinnate foliage, but still more to the remarkable development in some species of the sleep movements manifested to some extent by most of the pinnate Leguminosae, as well as many other (especially seedling) plants. In the so-called “sensitive plants” these movements not only take place under the influence of light and darkness, but can be easily excited by mechanical 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 movement being propagated centripetally. 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 (p in figure) or swollen base of the leafstalk. When shaken in any way, the leaves close and droop simultaneously, but if the agitation be continued, they reopen as if they had become accustomed to the shocks. The common sensitive plant of hot-houses is M. pudica, a native of tropical America, but now naturalized in corresponding latitudes of Asia and Africa, but the hardly distinguishable M. sensitiva and others are also cultivated. Species of the closely allied genus Schrankia are known as sensitive-briar in the southern United States.