MIMOSA, in botany, a genus of the Polygamia Monoecia class and order. Natural order of Lomentacere ; Legumi nom, Jussieu. Essential character: her maphrodite, calyx five-toothed ; corolla five-cleft; stamina five or more ; pistillum one ; legume : male, calyx five-toothed; corolla five-cleft; stamina five,ten,or more. There are eighty-five species ; among which the M. sensitive, sensitive plant, rises with a slender woody stalk seven or eight feet in height, armed with short recurved thorns ; the leaves grow upon long foot-stalks, which are prickly, each sustaining two pair of wings ; from the place where these are inserted come out small branches, having three or four globular heads of pale purplish flow ers, coming out from the side. on short peduncles ; the principal stalk has many of those heads of flowers on the tipper part, for more than a foot in length ; this, as also the branches, is terminated by like heads of flowers . the leaves move but slowly when touched, but the foot stalks fall, when they are pressed pretty hard. It is a native of Brazil.. M. pudi ca, humble plant, has the roots composed of many hairy fibres, which mat closely together; from these come out several woody stalks, declining towards the ground, unless supported ; they are arm ed with short recurved spines, having winged or pinnate' leaves ; flowers from the arils, on short peduncles, collected in small globular heads, of a yellow colour. "Naturalists," says Dr. Darwin, "have not explained the immediate cause of the collapsing of the sensitive plant; the leaves meet and close in the night during the sleep of the plant, or when exposed to much cold in the day-time, in the same manner as when they are affected by ex ternal violence, folding their upper sur faces together, and in part over each other, like scales, or tiles, so as to expose as little of the upper surface as may be to the air ; but do not, indeed, collapse quite so far ; for when touched in the night during their sleep, they fall still further; especially when touched on the foot-stalks between the stems and the leaflets, which seems to be their most sensitive, or irritable part. Now as their
situation after being exposed to external violence resembles their sleep, but with a greater degree of collapse, may it not be owing to a numbness or paralysis con sequent to too violent irritation, like the faintings of animals from pain or fatigue A sensitive plant being kept in a dark room till some hours after day-break, its leaves and leaf-stalks were collapsed as in its most profound sleep ; and on expos ing it to the light, above twenty minutes passed before the plant was thoroughly awake, and had quite expanded itself. During the night the upper surfaces of the leaves are oppressed ; this would seem to show that the office of this sur face of the leaf was, to expose the fluids of the plant to the light as well es to the air.".