Lesson Explainer: Sensitivity in Mimosa Plants Biology

In this explainer, we will learn how to describe the responses a mimosa plant will have to being touched and to light/dark cycles.

While all living organisms are interesting in their own way, Mimosa pudica, also called the sensitive plant or the touch-me-not plant, is a unique and fascinating organism. The species name pudica comes from the Latin word meaning “shy” or “shrinking.” The mimosa plant responds to stimuli like touch and heat by rapidly folding up its leaflets and drooping within just a few seconds! This is shown in the photo below. The mimosa plant also exhibits the same reaction in response to light/dark cycles, folding up and drooping at night and reopening in the daytime. In this explainer, we will be learning the science behind this process and understanding how different stimuli trigger this response.

Folding response

Figure1

Definition: Stimulus

A stimulus is any change in a living organism’s internal or external environment, which can influence the organism’s activity.

Let’s begin by learning about the structure of the mimosa plant.

The leaves of mimosa plants are compound leaves, which means that each leaf is made of multiple leaflets, as you can see in the photo above.

Definition: Compound Leaf

A compound leaf is a leaf made of several leaflets.

The stalk of the leaf, which is called the petiole, branches into slender extensions called rachises. Figure 2 shows the petiole branching into four rachises. Along each rachis, there are several small leaflets called pinnules. Each rachis, together with its pinnules, composes a single pinna. In Figure 2, you can see four pinnae making up the compound leaf. This type of leaf is called a pinnately compound leaf. As shown in the figure, each leaf consists of a single petiole and four pinnae, each of which has multiple pinnules.

Key Term: Rachis

A rachis is a branched extension of the petiole in a compound leaf. Each rachis carries leaflets.

Key Term: Pinna

A pinna is made up of multiple small leaflets or pinnules attached to a central rachis.

Key Term: Pinnately Compound Leaf

A pinnately compound leaf is a leaf in which multiple pinnae are attached to a single stem.

Example 1: Structure of a Mimosa Plant Leaf

What shape do mimosa plant leaves take?

  1. Deltoid
  2. Pinnate
  3. Circular
  4. Rosette
  5. Trifoliate

Answer

The leaves of mimosa plants are compound leaves, which means that each leaf is made of multiple leaflets. The stalk of the leaf, which is called the petiole, branches into slender extensions called rachises. The photo below shows the petiole branching into four rachises. Along each rachis, there are several small leaflets called pinnules. Each rachis, together with its pinnules, composes a single pinna. In the photo below, you can see four pinnae making up the compound leaf. This type of leaf is called a pinnately compound leaf, or a pinnate leaf.

Mimosa leaves therefore have a pinnate shape.

The leaves of mimosa plants have joint-like structures called pulvini. A pulvinus is an enlarged or swollen structure found at the base of a leaf, leaflet, or petiole. The main role of the pulvini is to control the movement of the leaves of a plant in response to certain stimuli, as we will learn.

Each pulvinus has a core of vascular tissue, surrounded by a layer of thick-walled collenchyma cells. This layer is surrounded by flexible parenchyma cells, which are thin-walled cells. Each pulvinus is divided into two sides. The upper side of the pulvinus, on the upper side of the leaflet, is made of cells called extensor cells. The lower side of the pulvinus, which faces downward on the underside of the leaflet, is made of cells called flexor cells.

Key Term: Pulvinus

A pulvinus is a swollen, joint-like structure found at the base of a leaf, leaflet, or petiole and is responsible for the movement of leaves in response to stimuli.

Key Term: Parenchyma

Parenchyma is a plant tissue composed of thin-walled cells. It makes up the bulk of the inside of plant structures such as leaves, stems, and roots.

Key Term: Collenchyma

Collenchyma is a plant tissue made of long cells with thick cell walls. It provides structure and support to the plant.

In mimosa plants, pulvini are found at three different locations, as shown in Figure 3.

The primary pulvini are found at the base of the petiole, where the leafstalk joins the stem of the plant. Secondary pulvini are present at the point where the petiole branches into rachises. Finally, tertiary pulvini are located at the base of each pinnule along the rachises.

Example 2: The Function of the Pulvinus in a Mimosa Plant

What is the main role of the pulvinus in mimosa leaves and leaflets?

  1. To provide essential nutrients to the leaflets
  2. To move the leaflets in response to a stimulus
  3. To release hormones in response to an internal stimulus
  4. To coordinate the nervous responses of the plant

Answer

The mimosa plant responds to stimuli like touch and heat by rapidly folding up its leaflets and drooping. It also exhibits the same reaction in response to light/dark cycles, folding up and drooping at night and reopening in the daytime.

The leaves of mimosa plants have joint-like structures called pulvini. A pulvinus is an enlarged or swollen structure found at the base of a leaf, leaflet, or petiole. Each pulvinus is divided into two sides. The upper side of the pulvinus is made of cells called extensor cells, while the lower side is made of cells called flexor cells. The main role of the pulvini is to control the movement of the leaves of a plant in response to certain stimuli.

The main role of the pulvinus in mimosa leaves and leaflets is therefore to move the leaflets in response to a stimulus.

During the daytime, or in the absence of touch, the leaflets are held open in a horizontal position. At night, or when the plant is touched, warmed, or shaken, the leaflets close or fold up, and the rachises and petiole droop downward, as shown in Figure 4.

Let’s understand how the leaflets of mimosa fold in response to stimuli.

As we know, all plant cells contain water. The pressure exerted by water on the plasma membrane of each cell helps maintain the cell’s shape and rigidity. This is called the turgor pressure.

Definition: Turgor Pressure

Turgor pressure is the pressure exerted by water, pushing the plasma membrane against the cell wall and maintaining the cell’s shape and rigidity. This is called the turgor pressure.

There are several different theories that explain the mechanism of how the mimosa plant closes its leaflets when stimulated by touch. Let’s look at a commonly accepted explanation of this process.

When a leaflet is touched, this mechanical stimulation is recognized by the plant and is converted into an electrical signal. This signal is passed on to the pulvini, causing ions like potassium and chloride to flow out of the extensor cells. This changes the ion concentration in the extensor cells and increases their osmotic potential. The flexor cells in the lower side of the pulvini, in contrast, have a lower osmotic potential. The difference between the volume change of flexor cells and that of extensor cells is due to the transport of ions along with the osmotic transport of water. This causes water to flow rapidly out of the extensor cells and into the flexor cells by osmosis, as shown in Figure 5.

Key Term: Osmosis

Osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration.

Key Term: Osmotic Potential

Osmotic potential is the potential of water to move from a region of low solute concentration to a region of high solute concentration.

Osmotic potential is caused by solutes that are dissolved in the cell sap, where it is directly proportional to the solute concentration and inversely proportional to the water volume.

Because of the loss of water, the extensor cells rapidly lose their turgor pressure. The flexor cells, on the other hand, retain turgor pressure, becoming swollen from the influx of water. Due to this, the pulvinus acts as a joint-like structure, causing the leaflets to fold, as shown in Figure 6.

As the signal is propagated, the extensors in the secondary pulvini and eventually the primary pulvini also lose turgor pressure. This causes the petiole to droop downward, as we saw in Figure 4. This type of plant movement, in response to being touched or shaken, is called thigmonasty. Thigmonasty is a type of nastic movement, which means that the direction in which the plant moves is independent of the direction of the stimulus. No matter in which direction the mimosa plant is touched or shaken, its leaflets will always fold up in the same way and droop downward! This is why we say that thigmonasty is a nondirectional response.

Definition: Thigmonasty

Thigmonasty is the nondirectional movement of a plant in response to a touch stimulus.

Example 3: How Leaflets in Mimosa Open and Close

Complete the following: The folding and opening of mimosa leaflets are dependent on changes in turgor .

  1. flow
  2. transport
  3. pressure
  4. ions

Answer

All plant cells contain water. The pressure exerted by water on the plasma membrane of each cell helps maintain the cell’s shape and rigidity. This enables the plant to remain upright and the leaflets to stay open.

When a leaflet is touched, this mechanical stimulation is recognized by the plant and is converted into an electrical signal. This signal is passed on to the pulvini, causing ions like potassium and chloride to flow out of the extensor cells. This changes the ion concentration in the extensor cells and increases their osmotic potential. The flexor cells in the lower side of the pulvini, in contrast, have a lower osmotic potential. This causes water to flow rapidly out of the extensor cells and into the flexor cells by osmosis.

Due to the loss of water, the extensor cells become flaccid. This is because there is no longer any water exerting pressure on the plasma membrane of the cells. We say that these cells have lost their turgor pressure. The flexor cells, on the other hand, remain turgid, as they still have water within them, exerting turgor pressure.

The folding and opening of mimosa leaflets therefore are dependent on changes in the turgor pressure.

Aside from its response to touch, mimosa plants also exhibit the same reaction in response to light/dark cycles. This type of movement is called nyctinasty or sleep movement: the plant folds up its leaflets and droops at night and reopens them in the daylight.

Definition: Nyctinasty

Nyctinasty, or sleep movement, is the movement of leaves in response to light/dark cycles.

The mimosa plants’ unique movements also give them interesting evolutionary advantages. One theory is that these plants fold up their leaflets in response to being touched in order to appear smaller or wilted, to discourage herbivores from consuming them! The rapid folding movement may also help to dislodge insects that attempt to sit on the leaflets.

Example 4: The Evolutionary Advantage of Mimosa Plants

When touched, mimosa leaflets close up. What is a possible evolutionary advantage of this?

  1. The surface area for light to be captured on is increased.
  2. It prevents the entry of pathogens and parasites.
  3. More carbon dioxide is taken in while the plant is not photosynthesizing.
  4. It makes the plant look wilted and deters herbivores from eating it.

Answer

The mimosa plant responds to stimuli like touch and heat by rapidly folding up its leaflets and drooping. It also exhibits the same reaction in response to light/dark cycles, folding up and drooping at night and reopening in the daytime.

The mimosa plants’ unique movements also give them interesting evolutionary advantages. One theory is that these plants fold up their leaflets in response to being touched in order to appear smaller or wilted, to discourage herbivores from consuming them! The rapid folding movement may also help to dislodge insects that attempt to sit on the leaflets.

A possible evolutionary advantage of the mimosa plant is therefore that the folding of its leaves makes the plant look wilted and deters herbivores from eating it.

Let’s summarize everything we’ve learned about sensitivity in mimosa plants.

Key Points

  • Mimosa pudica is a plant that can fold up its leaflets and droop in response to certain stimuli.
  • The mimosa plant has pinnately compound leaves, made of multiple leaflets called pinnae.
  • Joint-like, swollen structures, called pulvini, are responsible for moving the leaflets of the plant in response to stimuli.
  • The folding and opening of the leaflets of a mimosa plant depend on changes in turgor pressure.
  • Mimosa plants fold their leaflets in response to being touched or shaken, called thigmonasty, and in response to light/dark cycles, called nyctinasty.

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