Lesson Video: Movement in Plants Biology

In this video, we will learn how to describe the tropisms that control movement in plants.

12:56

Video Transcript

In this video, we will learn how to describe the directional growth movements, otherwise known as tropisms, that allow plants to react to their environment. We will also look at some examples of nondirectional movements called nastic responses, for example, in response to light–dark cycles and touch.

Although plants may seem immobile, they are actually capable of several different types of movement in response to many different stimuli. Remember, a stimulus is a change in the living organism’s internal or external environment which can influence that organism’s activity. Plants can move in response to stimuli like light, touch, gravity, and even heat. Some of these stimuli, like light or touch, in certain plants trigger directional movement response growing either towards or away from the stimulus.

These directional growth movements are called tropisms. Let’s take a look at an example of a plant tropism in response to touch called thigmotropism, or sometimes known as haptotropism. Climbing plants, like this garden pea plant, have specialized structures called tendrils. Tendrils are slender structures which help support climbing plants by winding around objects that they come into contact with. In the case of the pea plant, this helps to support it, but this can also be a useful response for parasitic plants like this vine to wrap around a host tree and absorb its vital nutrients.

By taking a closer look at one of these tendrils, we can see how this happens. The tendrils of climbing plants extend into the air until they come into contact with a solid object, which triggers the release of certain plant hormones. This causes the cells on the side of the tendril which is in contact with the object to grow slowly, while the cells on the opposite side are stimulated to grow more quickly. This causes the tendril to curl around the object. The tendril also becomes thickened with mechanical tissue, which provides climber plants with strong support, helping them to stay upright.

Thigmotropisms, like most tropisms, can be negative or positive, growing away from or towards a stimulus. In the climbing plants example, the tendrils contacting an external solid object stimulates them to grow towards it, which is called positive thigmotropism. In certain plants, like legumes, which include bean and pea plants, the roots rely on their sense of touch to help them grow into the soil without encountering much 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.

Interestingly, some plants, like this Mimosa pudica plant, are incredibly sensitive to touch. While the examples we just explored are examples of fairly slow directional growth movements, the leaflets of Mimosa pudica, one of which has been circled here, are able to snap shut in a matter of mere seconds in response to a stimulus like being touched. This response is called thigmonasty.

The suffix -nasty refers to the fact that this is a nondirectional response. And the prefix thigma-, like in thigmotropism, refers to touch. This is a nondirectional response, as no matter the direction from which mimosa is touched, the leaflets always fold upwards from their undisturbed horizontal position. And the petiole, which attaches the stem to the leaflets, always droop downwards when the plant is stimulated in this way.

Let’s take a quick look at how mimosa plants can close these leaflets so rapidly. How might this undisturbed leaflet on the left transform into this disturbed leaflet on the right? Let’s find out. This diagram shows a simplified and magnified view of one part of this leaflet. By magnifying our view again of one of the cells in this leaflet, we can see that it contains water inside a structure called a vacuole. Turgor pressure is the pressure exerted by water on the walls of the cells in the base of the leaflets. A lot of water in a cell means that they have a high turgor pressure, which makes the cells turgid and helps them to retain their shape.

When the plant is touched, a signal causes these cells to lose water, and so they develop a low turgor pressure. And the cells’ turgidity decreases, which causes the leaflets to close. It’s been proposed by scientists that this thigmonastic response in mimosa may present an evolutionary advantage to the plant. Being touched or shaken may indicate that a herbivore is nearby or that a herbivorous insect, like this ladybug, has landed on this leaflet and is about to eat it. Closing the leaflets quickly and drooping may make the plants appear smaller and more wilted to large herbivores and may dislodge smaller herbivores like insects, preventing the plant from being eaten.

Mimosa and several other plant species can also move their leaves in light–dark cycles by responding to a different stimulus, light intensity. For example, while mimosa leaflets are open during the day, their leaflets close at night. Photoreceptors, which are sensitive to light, detect the low-light intensity as night approaches and generate an electrical signal which causes the leaflets of mimosa to close. In the daytime, the photoreceptors sense the increase in light intensity and trigger the leaflets to return to their open position. This type of movement is called nyctinasty, or sleep movement.

The prefix nyct- means night. Like thigmonasty, this is a nondirectional response. A similar process is observed in other plant leaves, like those of legumes, drooping at night and then returning to their upright position in the morning. The benefits of sleep movements are not fully understood, but 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 at night, which has led scientists believe that this might, like thigmonasty, be a mechanism to deter herbivores from eating these plants.

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. You can see a single contractile root magnified here, but let’s see how these actually work as the plant grows.

As a stem of the plant is growing upwards, the shrinking of these routes exerts a strong downward pull on the stem, which helps position the plant deeper in the soil. This helps to protect the plant, for example, from damaging light and heat in drought conditions.

Now that we’ve 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.

This is a drawing of what some typical plant cells in the leaf of an aquatic plant might look like under a high-powered microscope. You might notice that some subcellular structures are visible, such as chloroplasts, the cytoplasm, which is a fluid in which organelles like chloroplasts are suspended, and we can also see the vacuole, although this might not actually be visible in a micrograph image.

A key characteristic of the cytoplasm is that it is in a constant rotational flow in one direction within the cell. This is called cytoplasmic streaming, and it moves the organelles and other subcellular structures along with it. Cytoplasmic streaming is responsible for the movement of organelles, nutrients, and metabolites within the cells of multicellular organisms, which cannot move them by simple diffusion.

Let’s see how much we’ve learned by having a go at a couple of practice questions.

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. Or 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 environment. These changes are called stimuli. Plants are capable of movement in response to several different types of stimuli, like light intensity, touch, heat, and gravity. Some plants are incredibly sensitive to touch, and they can move in response to a touch stimulus via a process called thigmotropism. For example, when the roots of some legumes encounter underground rocks in the soil, they are stimulated to change the direction of their growth away from that obstruction.

Some plants, including legumes, have photoreceptors that can sense changes in light intensity. During the day, their leaves are open and upright, but as night approaches and light intensity falls, their leaves may close up and their stems may droop. When light intensity increases again as daylight approaches, their leaves will return to their normal upright position through a sleep-wake cycle sometimes called nyctinasty.

Let’s look at the different statements given in the question. The first statement says that plants are fully immobile and their direction of growth cannot be changed. This is incorrect, as we’ve already seen two of the many examples of how plants can move. The second statement says that plants have no form of sleep-wake cycle to respond to dark–light cycles. However, as we know, light is one of the stimuli that plants can respond to, so this is incorrect too.

The third option 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. Let’s just check that the final option is incorrect before we confirm our answer. The fourth option says that plants communicate using a central nervous system. Plants generally communicate between different plant organs through electrical signals and chemicals like plant hormones, but they do not have a central nervous system. So we can confirm that this option is incorrect.

Therefore, the accurate statement is that plants can respond to stimuli like light and touch by moving.

Let’s try a second practice question together.

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 or haptotropism, negative hydrotropism, positive chemotropism, or negative phototropism.

The directional growth movement of a plant in response to a stimulus is called a tropism. Tropisms can be negative, when something is growing away from a stimulus, or positive, when growing towards a 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 a negative tropism. If we look at the different answers, two of them describe positive tropisms, so these options can be ruled out straight away.

Let’s break down the three remaining tropisms into their word parts so we can work out which one is correct. The prefixes thigmo- and hapto- both mean touch, which explains how the roots of some plants, like legumes, rely on their sense of touch to help them grow into soil without encountering much 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 the roots to find areas of soil that are free to expand in and might increase their ability to take up minerals and water.

This process seems to explain the example in our question pretty perfectly, but let’s have a look at the other tropisms to make sure that this option is correct. The prefix hydro- means water, so this option is not describing a plant’s response to touch. And the prefix photo- means light, so this option is also not referring to a touch stimulus. Therefore, we can deduce that the example in this question is in fact describing negative thigmotropism or haptotropism.

Let’s review some of the key points that we’ve covered in this video. We’ve learned how plants can move in response to a range of different stimuli such as light, touch, and gravity. A plant’s growth movement in response to a touch stimulus is called thigmotropism. Positive thigmotropism is movement towards a stimulus, whereas negative thigmotropism is movement away from a stimulus.

Some plants, like legumes, can move in response to light–dark cycles. This is called nyctinasty or sleep movement. We’ve also learned that plants, like corms, bulbs, and rosettes, have contractile roots that shrink and pull the stem further down to the soil under harsh conditions. Finally, we learned how within plant cells the cytoplasm is in a constant rotational motion in a process called cytoplasmic streaming.

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