A close up view of the mimosa

A Sensitive Plant in a garden grew,
And the young winds fed it with silver dew,
And it opened its fan-like leaves to the light.
And closed them beneath the kisses of Night.
.......

Percy Bysshe Shelley
"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!

Mimosa 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 Mimosa’s rhythm!

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 the illustration.)

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 resolvable.

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 folded structure.

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 similar reaction.

Photographic Equipment

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 images.

Further Information

The Sensitive Plant
http://www.bbc.co.uk/dna/h2g2/A8571963

Mimosa pudica
http://www.bio.miami.edu/mimosa/mimosa.html

Why does the Mimosa close its leaves?
http://www.thehindu.com/thehindu/seta/2001/12/06/stories/2001120600130400.htm

A Flower Garden of Macroscopic Delights

A complete graphical index of all of my flower articles can be found here.

The Colourful World of Chemical Crystals

A complete graphical index of all of my crystal articles can be found here.

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