In this explainer, we will learn how to describe the tropisms that control movement in plants.
All living organisms are capable of moving and reacting to changes in their internal and external environments. These changes are called stimuli. This is why we say that movement and response to stimuli are two of the characteristic features of living things, and plants are no exception! Although plants might seem to be immobile, they are capable of several different types of movements in response to stimuli like light, touch, heat, and gravity.
A stimulus is any change in a living organism’s internal or external environment that can influence the organism’s activity.
Some stimuli may trigger a directional response in plants. This means that the response may be either toward or away from the stimulus. This type of movement is called tropism.
A tropism is a directional growth or movement response either away from or toward a stimulus.
Do plants have a sense of touch? Some plants are extremely sensitive to touch. When they come into contact with a solid object, they can interpret this as a stimulus and can move in response to it. In fact, some plants are far more sensitive to touch than human beings are! The touch-me-not plant, Mimosa pudica, is a fascinating example of movement in response to a touch stimulus, which is called thigmonasty. The word thigmo comes from the Greek word meaning “touch.” This type of movement is called nondirectional, since the direction of movement does not depend on the direction of the stimulus!
Thigmonasty is the nondirectional movement of a plant in response to a touch stimulus.
The Mimosa plant responds to touch by rapidly folding up its leaflets and drooping within just a few seconds! When the Mimosa plant is undisturbed, the leaflets are held open, in a horizontal position. When the plant is touched or shaken, the leaflets close or fold up, as shown in the photo below.
The opening and closing of the leaflets in a Mimosa plant depends on the changes in turgor pressure in plant cells. Turgor pressure is the pressure exerted by water on the inner walls of a cell, and it helps maintain the cell’s shape and rigidity. In an undisturbed Mimosa plant, the cells are all turgid as they contain water. This turgidity is what enables the plant to remain upright and the leaflets to stay open. When the leaflets are touched, signals are transmitted, triggering them to lose water to the surrounding tissues. Due to the loss of water, the turgor pressure is lowered and the leaflets of the plant close and droop.
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.
While Mimosa plants are capable of moving in response to touch, some other plants can actually exhibit growth in response to touch stimuli. This is called thigmotropism or haptotropism. Let’s take a look at some interesting examples of thigmotropism.
Definition: Thigmotropism (Haptotropism)
Thigmotropism is the directional growth movement of a plant in response to a touch stimulus.
Climbing plants, like the garden pea plant, have specialized structures called tendrils. Tendrils are slender structures that help support climbing plants by twining around objects they come in contact with. The photo below shows the tendrils of a plant twining around the stem of another plant for support.
Key Term: Tendrils
Tendrils are slender specialized structures that help support climbing plants by twining around objects they come in contact with.
The tendrils of such plants extend into the air until they come into contact with a solid object. When this happens, it triggers the release of plant hormones and proteins that cause the tendril to curl around the object. The cells on the side of the tendril that is in contact with the object grow slowly, while the cells on the opposite side are stimulated to grow more quickly, as represented in Figure 3. In this way, the tendril coils or twines closely around the object. The tendril also becomes thickened with mechanical tissue, which provides climber plants with strong, external support, helping them to stay upright.
Example 1: The Response of a Plant to a Touch Stimulus
What is the response of plants to a touch stimulus known as?
All living organisms are capable of moving and reacting to changes in their internal and external environments. Although plants might seem to be immobile, they are capable of several different types of movement in response to stimuli like light, touch, heat, and gravity. Tropism is the word used to describe plant movements toward or away from a stimulus.
The question asks what the response of a plant to a touch stimulus is called. Let’s go through the answers provided and see if we can spot the right one.
If we look at option A, traumatotropism, we can see that this word is made of two word parts: trauma and tropism. As we know, tropism is the movement of a plant toward or away from a stimulus. The word part trauma describes an injury or a wound. Putting the two together, we can see that traumatotropism means the movement of a plant in response to injury, not touch, which means that option A is wrong.
The word part geo- is a prefix that means “ground” or “earth.” You might recognize this prefix from words like geography and geology! Earth exerts a force, gravity, on all objects, pulling them toward its surface. Option B, geotropism, describes the movement of plants in response to gravity and is therefore wrong too.
Let’s move on to option C, chemotropism, and break it down into its word parts. The word chemo is related to chemical compounds. The word chemotropism describes the movement or growth of plants in response to chemicals. For example, when the roots of a plant sense useful nutrients and minerals in the soil, they grow toward them! This option is also wrong because it does not describe a plant’s response to touch.
Option D, hydrotropism, is incorrect too. The word part hydro is used to describe water, as you might know from words like hydration or hydrolysis! Hydrotropism describes the growth response of plants to the presence of water.
The last option is thigmotropism or haptotropism. The word thigmo comes from the Greek word that means touch, and the word part hapto is used to describe attachment in chemistry and biology. Thigmotropism and haptotropism describe a plant’s movement or attachment in response to a touch stimulus.
The response of a plant to a touch stimulus is therefore option E, thigmotropism/haptotropism.
Tropisms can either be negative or positive, growing away from or toward a stimulus. Let’s look at our climbing plant example to understand this better. In this type of thigmotropism, the tendrils come into contact with an external solid object and are stimulated to grow toward it. This is called positive thigmotropism.
Negative thigmotropism can also be useful to plants. In certain plants like legumes, the roots of the plant rely on their sense of touch to help them grow into the soil without encountering resistance. When the roots of such plants come into contact with solid objects underground, like rocks or stones, they are stimulated to grow away from them. Since the direction of movement in this case is away from the touch stimulus, this is called negative thigmotropism.
Example 2: Tropism in the Roots of Legumes
If, when growing, the roots of a bean shoot touch an object (like an underground rock), signals are transmitted to encourage the root to grow away from that object. What tropism is being displayed here?
- Positive gravitropism
- Negative thigmotropism/haptotropism
- Negative hydrotropism
- Positive chemotropism
- Negative phototropism
The movement of a plant in response to a stimulus is called tropism. Tropisms can either be positive or negative, growing toward or away from the stimulus. Let’s take a closer look at the question and the options provided.
The question describes the roots of a bean shoot being stimulated to grow away from an underground object it comes into contact with. The growth or movement away from a stimulus is called negative tropism. If we look at the options, we can see that options A and D describe positive tropisms. These options can be ruled out straightaway.
The other three options, B, C and E, all describe some form of negative tropism. Breaking down the different tropisms into their word parts, option B uses the prefixes thigmo- and hapto-, which mean “touch.” Option C uses the prefix hydro-, meaning “water,” and option E uses the prefix photo-, which means “light.”
If we put the different parts of option B together, we can see that negative thigmotropism or haptotropism means the growth of a plant away from a touch stimulus. This perfectly describes the growth of a bean shoot’s roots away from an underground object!
In certain plants like legumes, the roots of the plant rely on their sense of touch to help them grow into the soil without encountering resistance. When the roots of such plants come into contact with solid objects underground, like rocks or stones, signals are transmitted to encourage the root to grow away from them. This helps them find areas of soil that are free to expand in and increases their ability to take up minerals and water.
The type of tropism displayed by the roots of the bean shoot is therefore option B, negative thigmotropism/haptotropism.
Another type of movement exhibited by plants is movement in response to daylight cycles, when the intensity of light available to the plant changes based on the time of day or night. For example, Mimosa plants close their leaflets at night. In certain other plants, like legumes, the leaves droop at night and return to an upright position in the daytime. This type of movement is called nyctinasty or sleep movement. Here, the stimulus is the intensity of daylight.
Definition: Nyctinasty (Sleep Movement)
Nyctinasty, or sleep movement, is the movement of plants in response to daylight cycles.
In the photo below, you can see a prayer plant, Maranta leuconeura, which is well known for exhibiting sleep movement. The image on the left shows what this plant looks like at night, with its leaves drooping, while the image on the right shows the plant with its leaves upright in the daytime.
Plants that show sleep movement have photoreceptors, which are sensitive to light. When the intensity of daylight is low, the photoreceptors generate an electrical signal, causing the leaves of the plant to droop at night. In the daytime, the photoreceptors sense the increase in light intensity and trigger the leaves of the plant to return to their upright position.
The benefits of sleep movements are not fully understood. One theory suggests that the drooping of leaves at night helps reduce the surface area of the plant to prevent excess water loss through transpiration. Sleep movements can make a plant appear smaller or wilted, which has led scientists to believe that this might be a mechanism to deter herbivores from eating these plants.
Example 3: Plant Movements in Response to Light/Dark Cycles
Which of the following best explains how legumes, a type of plant, use movements to utilize light/dark cycles?
- Legumes will move their leaves to close them at night and open them during the day.
- Legumes will move their leaves to ensure that they always grow toward the Sun.
- Legumes will actively move their roots to find areas with more sunlight.
Certain plants, like legumes, are capable of movement in response to light/dark cycles. This type of movement is called nyctinasty or sleep movement. In this type of movement, the stimulus is the intensity of daylight.
If we look through the options in the question, we can straight away see that option C is wrong. This is because the roots of a plant do not need sunlight, as they do not perform photosynthesis! Plants, therefore, have no reason to move their roots to find areas with more sunlight.
Plants that show sleep movement have photoreceptors, which are sensitive to light. These photoreceptors sense the intensity of light and then transmit signals to the plant, causing it to move in response to the light intensity.
The statement in option B says that legumes will move their leaves to ensure that they always grow toward the Sun. This is definitely a useful plant growth mechanism, as it makes sure that the leaves of a plant get as much sunlight for photosynthesis as possible. However, this statement does not correctly answer the question. The question specifically mentions light/dark cycles, which do not come into play in option B.
So, what are light/dark cycles and how does a plant recognize them? In the daytime, the light intensity is high, and at night, the light intensity is very low. This cycle of daylight is what the question is referring to when it mentions light/dark cycles.
When the intensity of daylight is low, the photoreceptors generate an electrical signal, causing the leaves of the plant to fold at night. In the daytime, the photoreceptors sense the increase in light intensity and trigger the reopening of the leaves. This is called sleep movement or nyctinasty.
If we look at option A, this perfectly fits the description of sleep movement: legumes will move their leaves to close them at night and open them during the day.
The correct answer is, therefore, option A.
Some plants, like corms, bulbs, or rosettes, have specialized roots called contractile roots. Contractile roots are thickened root structures that are capable of shrinking under harsh environmental conditions, like seasonal drought. This type of movement is also called pull movement. The photo below shows a hyacinth bulb with contractile roots.
As the plant grows, the shrinking of these roots exerts a strong downward pull on the stem, which helps position the plant deeper in the soil. This protects the plant from the damaging light and heat in drought conditions.
Key Term: Contractile Roots
Contractile roots are thickened, specialized root structures in corms, bulbs, and rosettes that serve to pull the stem of the plant deeper in the soil.
Now that we have understood some of the different types of plant movements that involve entire plant organs, let’s take a closer look at movement at the cellular level.
Every living cell contains cytoplasm, the fluid in which all the organelles of the cell are suspended. A key characteristic of the cytoplasm is that it is in constant motion. This can be seen clearly when the cells of some aquatic plants, like Elodea canadensis or Hydrilla verticillata, are observed under a microscope. The cytoplasm appears to be in constant rotational flow, in one direction, within the cell. This constant motion is called cytoplasmic streaming.
Key Term: Cytoplasmic Streaming
Cytoplasmic streaming is the constant, unidirectional flow of the cytoplasm within a cell.
The photo below shows the surface leaf cells of Hydrilla verticillata under a high-power microscope. The green structures you can see are the chloroplasts in these cells. When observed under the microscope, the chloroplasts in each cell appear to be moving along the cell wall, either clockwise or counterclockwise. We can conclude that the chloroplasts move in this way because they are being swept along by the constant rotational flow of the cytoplasm within each cell.
In smaller cells, such as bacteria, nutrients move within the cell by diffusion through the cytoplasm. These cells are usually less than 5 μm in size, which is only 0.005 μm! However, in higher organisms, like plants, the cells can be between 10 μm and 100 μm in size, which means that diffusion of nutrients would take too long. Instead, cytoplasmic streaming is responsible for the movement of nutrients, metabolites, and organelles within these cells. By keeping the fluid in constant motion, cytoplasmic streaming ensures that the organelles of the cell are supplied with the molecules they need to function.
Example 4: Understanding Movement in Plants
Which of the following statements about plants is correct?
- Plants are fully immobile organisms, and their direction of growth cannot be changed.
- Plants have no form of sleep/wake cycle to respond to dark/light cycles.
- Plants can respond to stimuli, like light and touch, by moving.
- Plants communicate between their own structures using a central nervous system.
All living organisms are capable of moving and reacting to changes in their internal and external environments. Plants are capable of several different types of movement in response to stimuli like light, touch, heat, and gravity.
Let’s look at the different statements given in the question.
The first statement says that plants are fully immobile organisms and their direction of growth cannot be changed. This is incorrect, as plants are capable of changing the direction of their growth in response to changes in their internal or external environment, which are called stimuli. Some plants are extremely sensitive to touch, and they can move in response to a touch stimulus. This is called thigmotropism. For example, when roots of some legumes encounter underground rocks in the soil, they are stimulated to change the direction of their growth.
The second statement says that plants have no form of sleep/wake cycles to respond to light/dark cycles. However, one of the stimuli that plants can respond to is light. Some plants, including legumes, have photoreceptors that can sense changes in light intensity. These plants fold up or close their leaves at night and reopen them in the day. This type of movement is called nyctinasty, or sleep movement. This statement is, therefore, incorrect.
The third statement says that plants can respond to stimuli, like light and touch, by moving. This statement is correct: plants are capable of responding to stimuli, or changes in their environment, by moving.
The fourth statement says that plants communicate using a central nervous system. This statement is also incorrect, as plants do not have a centralized nervous system. Plants generally communicate between different plant organs through electrical signals and chemicals like plant hormones. For example, when the tendril of a pea plant encounters a solid object, plant hormones stimulate the tendril to twine around the object.
The correct statement about plants is, therefore, that plants can respond to stimuli, like light and touch, by moving.
Let’s go over the key points from this explainer.
- Plants are capable of moving in response to stimuli like light, touch, and gravity.
- The response of plants to a touch stimulus is called thigmotropism. Positive thigmotropism is movement toward the stimulus, and negative thigmotropism is movement away from the stimulus.
- Some plants, like legumes, move in response to light/dark cycles. This is called nyctinasty or sleep movement.
- Plants like corms, bulbs, and rosettes have contractile roots that shrink and pull the stem further down into the soil under harsh conditions.
- Within plant cells, the cytoplasm is in constant motion, called cytoplasmic streaming.