Mimosa (Gr. a mimic, as some of the plants imitate the movements of animals), a genus of legvminpsce which is so unlike in structure to the majority of the order as to serve as a type of a suborder, the mhnosew. These have small regular flowers in a spike or head, with stamens twice as many as the petals and leaves (sometimes simple phyllodia), twice or thrice pinnate. The genus mimosa was originally very large, but it has been so subdivided that now it includes only about 200 species, which are herbs, under-shrubs, or climbers, very few being erect shrubs or trees. The best known species is the sensitive plant (J/. pudica), noticeable for its irritable leaves; others in the genus possess the same property, but in a less marked degree, and in all the leaves fold and take a sleeping position at night. The sensitive plant is a native of Brazil, and has been in cultivation more than 200 years; it is usually treated as an annual, when it grows only about a foot high, but if kept under glass it will grow 3 ft. high or more, and form a straggling shrub with weak spiny branches which are beset with bristly hairs; the alternate leaves are bipinnate, with usually four pinnce, each bearing numerous small leaflets; the flowers are in small rose-purple heads, and are succeeded by short bristly pods containing the seeds; these retain their germinating power for a long time, in illustration of which it is mentioned that the jardin des plantes has been continuously supplied with sensitive plants by seeds from a bag that was brought there more than 75 years ago.
It is sparingly naturalized in Florida. The sensitiveness of the foliage of this plant, manifested by a peculiar shrinking when touched, is one of the most striking phenomena of plant life; when undisturbed and in a bright light, the leaves stand nearly at right angles to the stem, but a slight touch causes them to fold and droop as if dead. This change in the position of the leaf is completed in three successive movements: first the leaflets close in pairs, bring their faces together, and incline forward; then the secondary petioles or branches of the leaf approach each other; and finally the main leaf Btalk turns directly downward,'bending at its union with the stem; left to itself, the collapsed leaf gradually resumes its former position. The sensitiveness of the leaves is affected by the temperature, being greatest on warm days; if the plants are exposed to the action of'the wind, their irritability is notably diminished. No explanation is given of this phenomenon, but it is regarded as an unusual development of the power of motion which is possessed in a less manifest degree by a large number of other plants. - M. strigillosa, of Florida and the far south, along the banks of rivers, is a prostrate sensitive species with large leaves.
Another of the genus, M. sensitive, not rare in greenhouses, has only one pair of leaflets to each pinna; these are many times larger than those of the sensitive plant, and droop when touched, but much less promptly than the other. Several other mimosas are cultivated as ornamental greenhouse plants, but none of them have any economical importance. - The sensitive plant of the southern states (more properly sensitive brier), which is found from Virginia to Texas, formerly regarded as a single species of mimosa, is now found to be sufficiently distinct to be placed in a separate genus, Schrankia, and two species are distinguished, S. uncinata and S. angustata, differing mainly in the form and reticulation of their leaflets; they are nearly prostrate herbs, with steins 3 or 4 ft. long and armed with hooked prickles; the leaves are bipinnate, and the flowers in small, globular, rose-purple heads; the foliage is sensitive, but only under much rougher handling than is required to affect the sensitive plant.
On the prairies of the far south this plant often covers the ground for wide stretches, and by the closing of its leaves shows for a while the trail of the traveller verv distinctly.
Sensitive Plant (Mimosa pudica).