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.