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 thigmo-, 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 roots 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.
