Close-up View of the Mimosa
by Brian Johnston (Canada)
A Sensitive Plant in a garden grew,
And the young winds fed it with
And it opened its fan-like leaves
to the light.
And closed them beneath the kisses
"The Sensitive Plant"
I have seen (and touched) many
Mimosa plants over the years, but I must admit that this was the first
one that was in bloom. Its striking and unusual flowers prompted
me to write this article. Strangely, I have not seen another
flowering Mimosa since!
pudica is a member of the Pea family (Leguminosae or Fabaceae), and is native to
Brazil. Its common names Touch-Me-Not, Tickle-Me-Plant, Sleeping
Grass, Shame Plant, and Prayer Plant all allude to the fact that when
touched, the leaflets fold flat and the entire leaf droops, an action
referred to as a seismonastic
movement. The same action occurs as the sun sets, and the
original positions are regained at sunrise, an action referred to as a nyctinastic movement. Stranger
still, even heat can cause the Mimosa to react!
The closing and opening actions of
the Mimosa (nyctinasty) are a
form of circadian rhythm. In fact the nyctinasty displayed by the
Mimosa was the very first circadian rhythm to be observed, way back in
the early seventeen hundreds. Even Charles Darwin studied the
The plant’s scientific name, Mimosa pudica is derived from both
Greek and Latin. The genus name Mimosa
in Greek means ‘a mimic’, and refers to the sensitivity and movement of
the leaves, while the species name pudica
has a Latin derivation and means bashful, retiring, or shrinking.
The first image in the article
shows what looks like a Mimosa flower, but in reality it is a composite
flower-head composed of many tiny flowers. This 1.5 centimetre
diameter inflorescence is commonly called a capitulum, and it looks like a tiny,
fluffy pompom with a multitude of hair-like stamens projecting from its
centre. The composite buds that can be seen below, are
ellipsoidal in shape, and have a pineapple-like texture. Each of
the tiny green sections is of course, an immature bud-let.
Closer views show the hairy pink
stalk, and the hairs that grow out from between the bud-lets.
Mimosa’s leaves are described as bipinnate. This means that they are
compound leaves consisting of four main leaflets called pinnae, which in turn are composed
of many even smaller leaflets called pinnules.
(Large leaflets composed of smaller leaflets – bipinnate.) The
many tiny leaflets give the plant a distinctly fern-like
appearance. In the two images that follow, the new leaves have a
light, yellowish-green colour, while the older ones are a darker green.
A Mimosa bloom is a pale lilac
pink, and it grows on a slender stalk from a leaf axil (the point where the leaf stalk
meets the main stem).
The Mimosa plant contains many
chemical compounds, but one in particular, mimosine, is of interest to
pharmaceutical companies, because of its potential medical uses.
The structure of this compound can be seen below. (Hyperchem Pro was used to produce
In the image that follows, most of
what you see is composed of the hundreds of long, pink filaments of the
flowers hidden in the off-white sphere that is barely discernable at
the flower-head’s centre.
Higher magnification reveals the
small white anther at the tip of each filament.
An extremely high magnification
macro-photograph shows the two ridged lobes that constitute each anther.
As we move closer to the
flower-head, the tiny white petals of individual flowers become
I have removed the stamens of a
single flower in the two images that follow. This allows us to
see the flower’s extremely small, red and white petals, and its single
bright red pistil.
The petals of two additional
flowers can be seen in the images that follow. In these, the
pistil is not visible.
After blooming, the flower-head has
a drastically altered appearance. The red filaments have turned
brown, and some of the enlarged anthers have a purple hue.
A closer view allows the anthers to
be seen more clearly.
Notice the light-green leaf in the
upper left corner of the following images. Before taking the
second photograph, I gently touched the uppermost pinnules.
Notice how two of the four pinnae have started to fold.
Here is an entire Mimosa
leaf. What did it look like when it first began to grow?
It’s remarkable how something so
complex can develop from such a small beginning.
The secret as to how the Mimosa’s
tiny leaflets move has to do with the small white dot at the base of
each leaflet. These dots are fluid filled, sac-like structures
called pulvini that are tiny
actuators powered by turgor pressure.
The image on the left below shows
the reverse side of a leaf. If you look carefully, you can see
that both the leaflet stalks, and main stem, are covered by
hairs. Many of these hairs are prickly, and this supports one
hypothesis as to why the leaflets fold when touched. Perhaps this
is a defense mechanism that exposes the unpleasant spines to an animal
that desires a mouthful of the plant tissue.
A more mature leaf is shown below
with its pulvini.
The young leaf on the left
below was touched a moment before the photograph, and the image on the
right shows the beginning of the folding process.
For some reason, the strength of
the initiating force determines the speed of the Mimosa’s
reaction. In order to give myself as much time as possible to get
the two photographs below, I touched the leaflets very gently. If
I had hit them hard, the folding process would have happened almost
instantly. Notice how neatly the small leaflets are packed in the
If a grazing animal were to
take a bite of the lush plant matter, it would expose the interior of
its mouth to the prickly hairs seen on the left below. The image
on the right shows a side view of the packed leaflets.
Botanists are still not certain of
the exact mechanism of the touch-respond behaviour. It is
believed that a touch somehow induces an electrical signal that travels
at high speed to the motor-actuator – the pulvinus at the base of a
leaflet. The pulvinus responds by flushing out water and
potassium ions, and this lowers the turgor pressure in the cells,
causing the leaflet to droop. Mimosa is of course not unique in
its ability to react to a touch stimulus; the Venus fly Trap displays a
The low magnification, (to 1:1),
macro-photographs were taken using a 13 megapixel Canon 5D full frame
DSLR, using a Canon EF 180 mm 1:3.5 L Macro lens.
An 8 megapixel Canon 20D DSLR,
equipped with a specialized high magnification (1x to 5x) Canon macro
lens, the MP-E 65 mm 1:2.8, was used to take the remainder of the
Why does the
Mimosa close its leaves?
A Flower Garden of
A complete graphical index of all
of my flower articles can be found here.
The Colourful World of
A complete graphical index of all
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the
October 2009 edition of Micscape.
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