Video Transcript
In this video, we will learn how to
describe the shape of a mimosa plant leaf and identify the different structures that
it consists of. We will discover how to describe
the responses of a mimosa plant, for example, to being touched and explain how cell
turgidity might control these responses. Finally, we will investigate the
response of a mimosa plant to light–dark cycles and the potential evolutionary
advantages that it might give the mimosa plant.
This image is of a fascinating and
incredibly sensitive plant called Mimosa pudica. The mimosa plant has been used
traditionally for hundreds, if not thousands of years, for its pharmacological
properties. For example, it’s been found to
have antibacterial, antidepressant, antivenom, and even antiasthmatic properties in
treating numerous disorders and ailments. In this video, however, we’ll be
focusing on the impressive way this plant can respond to stimuli, which are changes
in an organism’s internal or external environment, showing a fantastic example of
sensitivity in plants.
The name for the genus Mimosa
originates from the latin word for mime, and the species name pudica is a latin word
that means shy or shrinking. Its scientific name reflects Mimosa
pudica’s more common name “the touch-me-not plant,” which aptly describes how it can
respond to stimuli such as being touched or shaken or even to stimuli like heat by
folding up and drooping in a matter of mere seconds. Even night can cause the leaflets
of a Mimosa pudica to close up periodically in a day–night cycle of opening and
closing. Interestingly, in Japan, Mimosa
pudica is called bowing grass, and some believe that this plant can predict natural
disasters as they’ve been observed to close their leaflets just before major
earthquakes.
In this image, we can see some of
the lovely pink flowers that Mimosa pudica can produce. But let’s start off by looking at
the distinct structure of Mimosa pudica leaves, so we can understand better how
they’re able to respond to these stimuli. The leaves of mimosa plants are
compound leaves. This means that each leaf, one of
which you can see in this drawing, is made of several distinct leaflets. The stalk of the leaf is called the
petiole, which branches into multiple extensions called rachises each of which
carries a distinct leaflet.
In this leaf, you can see that the
petiole branches into four rachises, which individually are each called a
rachis. The multiple smaller leaf-like
structures that you can see attached to each rachis are called pinnules. Each rachis together with its
pinnules composes a single pinna, sometimes called a leaflet. In this compound leaf, therefore,
there are four pinnae or leaflets attached to a single petiole. So this type of leaf is called a
pinnately compound leaf.
Each mimosa leaf then, in summary,
consists of a single petiole and four pinnae, each of which has multiple pinnules,
attached to a rachis. The leaves of mimosa plants have
joint-like structures called pulvini. And you can see one such pulvinus
labeled here, where the petiole branches into each rachis. The specific classification of
these pulvini depends on where in the plant they are located. So let’s see a wider view of the
whole plant to picture this more clearly.
A pulvinus is a swollen, joint-like
structure that’s found at the base of a leaf, leaflet, or petiole and is responsible
for the movement of leaves in response to stimuli. Some of the key structures that we
have reviewed previously are labeled on this diagram. Primary pulvini are found where
each petiole branches from the stem. Secondary pulvini are found where
each petiole branches into rachises, one of which we saw labeled in the previous
diagram. By zooming in on a section of this
leaflet, we can see a part of the rachis and two of the pinnules more closely as a
cross section. We can also see the swollen
appearance of the pulvini more distinctly, and we can see that the tertiary pulvini
are where the pinnules branch from each rachis.
All of the pulvini in the mimosa
plant, primary, secondary, and tertiary, are organized in the same way, with each
divided into two halves. The region on the upper half of the
leaflet is made of cells called extensor cells, labeled here in pink. The lower half of the pulvinus,
which faces downwards on the underside of the leaflet, is made of cells called
flexor cells. But how exactly do these pulvini
allow the leaflets to close and the petioles to droop so rapidly in response to
certain stimuli?
Let’s look more closely at a
tertiary pulvinus to understand the mechanism that enables these movements. During the daytime or in the
absence of touch or heat, the pinnules are held open at their maximum angle as close
to horizontal as they can be as we can see in the diagram here. At night or when the plant is
touched, warmed, or shaken, the pinnules close up or fold up as we can see in this
diagram on the right, and the rachises and petiole will also droop downwards.
Let’s find out what happens in the
cells of the pulvini in more detail to mediate this movement. The simple diagram in the center of
the screen shows a typical plant cell. Like all plant cells, mimosa
pulvini cells contain water in large central vacuoles, which here has been colored
blue. This water gives the cells their
shape and structure. And altering the volume of water
within plant cells is what allows them to change this shape. For example, this cell in the
center is turgid, as it is full of water, which exact turgor pressure on the cell
membrane. This makes the cell appear large or
swollen.
When turgid cells lose water, the
cell membrane pulls away from the cell wall, and the cells shrinks. When this occurs on a large scale
in a plant, their tissues can be referred to as flaccid, which causes the plant to
wilt. When the cells in the pulvini are
undisturbed, like those in the plant on the left, the flexor cells are less turgid
than the extensor cells. In this form, the pinnules stay
open.
But the mimosa plant can alter the
level of turgidity in the flexor and extensor cells. There is a commonly accepted theory
that might explain how the pinnules of mimosa close in response to touch. When a pinnule is touched, this
mechanical stimulation is recognized by the plant and converted into an electrical
signal that propagates. This signal spreads to the
pulvini.
Let’s take a closer look at the
flexor and extensor cells of the pulvini to see what is thought to happen there as a
result of this electrical signal by covering it step by step. The electrical signal stimulates
ions like potassium and chloride to flow out of extensor cells via channels which
are shown here in orange. This decreases the ion
concentration in these extensor cells, which in turn increases the water potential
in the extensor cells compared to the flexor cells, which means that these extensor
cells have more free water molecules that can move away by osmosis.
This causes water molecules, which
are shown in blue, to flow quickly by osmosis out of the extensor cells, which have
a higher water potential, and into the flexor cells, which have a lower water
potential. As the extensor cells have lost
water to the flexor cells in the disturbed plant, the water potential of the
extensor cells decreases. So the extensor cells lose some of
their turgor pressure, and the turgidity decreases. In contrast, the flexor cells now
have a higher water potential, and so water exerts more turgor pressure on their
cell membranes than in the extensor cells. You can see this change in
turgidity of the two different cell types in the diagram here.
We can also see how this change
into turgidity of the extensor and flexor cells causes the pulvinus to act as a
joint-like structure and the leaflets to fold up. As the signal is propagated, the
extensors in the secondary pulvini, and eventually the primary pulvini, also lose
turgor pressure, which causes the petiole to droop downwards.
This movement that we’ve just
described is an example of a nastic movement in a plant. A nastic movement is the general
term to describe a nondirectional response to a stimulus, which is independent of
the direction of the stimulus. This means that in the case of
mimosa, regardless of the direction of the touch stimulus, mimosa’s pinnules always
fold up in the same way, and its petioles always droop downwards. More specifically, this
nondirectional movement of a plant in response to touch that we’ve just explored is
called thigmonasty, so named as thigmo- derives from the Greek word for touch.
Another nastic movement that is
carried out by mimosa plants is called nyctinasty, or sleep movement, which derives
from the Greek word for night as this is a movement of the leaves in response to
light–dark cycles.
Mimosa leaves are sensitive to the
onset of night. During daylight hours, the pinnules
are open. At night, the low light intensity
and reduction in temperature is detected by the plant and converted into an
electrical signal. This is sent along the plant’s
pinnules causing them to close and along the petioles, causing them to droop,
probably by the same mechanism of water loss as in the thigmonastic response.
When the sun rises again, the light
intensity and temperature tend to increase once more, and the pinnules will
open. This is why this process is called
a light–dark cycle.
It’s not entirely clear to
scientists why mimosa plants close their leaves at night, but let’s consider the
possible evolutionary advantage of nyctinasty by thinking about the main function of
leaves. By opening pinnules during daylight
hours, the photosynthetic parts of the plant can capture the maximum amount of light
as possible. This means that they can carry out
photosynthesis at a high enough rate to survive and thrive. Photosynthesis cannot occur without
light, so there is no reason to keep the pinnules open at night, but why does Mimosa
pudica bother spending energy closing them?
Closing pinnules at night may
prevent damage to the plant, as open panels are more exposed to hungry herbivores
and extreme weather. It may even help mimosa plant to
conserve water. The daily cycle of opening and
closing pinnules may be energy demanding, but it could help the plant to survive in
the long run. Preventing herbivory is also the
most likely advantage that explains the evolution of the thigmonastic response in
mimosa. Pinnules snapping shut at the touch
of a potential herbivore or one nearby touching other plants may make the mimosa
look smaller, more wilted, and less appealing as a meal to these herbivores, thereby
discouraging them from consuming the plant and, as a result, killing it.
The triggering of the thigmonastic
movement may also serve as a useful method of dislodging insects attempting to sit
on the leaves, which may also want to consume them.
Let’s see how much we’ve learned
about sensitivity in mimosa by having a go at a practice question.
Complete the following: The folding
and opening of mimosa leaflets are dependent on changes in turgor.
Let’s approach this question by
addressing what we already know about the word turgor and how it is involved in the
folding and opening of mimosa leaflets. All plant cells, like the one
that’s shown in the simple diagram here, contain water. The pressure that is exerted by
water on the plasma membrane of each cell helps to maintain cell shape and
rigidity. This enables the plant to remain
upright and the leaflets on a mimosa plant to stay open. The pressure that is exerted on the
cell membrane by water is called turgor pressure, and it allows certain cells in
this leaflet to remain turgid.
When a leaflet is touched, this
mechanical stimulation is recognized by the plants and converted into an electrical
signal. Each distinct, smaller, leaf-like
structure on a leaflet is called a pinnule. Let’s magnify two pinnules and
where they connect to see how turgor pressure in certain cells might change to cause
these leaflets to close. In the diagram on the left, you can
see two pinnules from the base of the leaflets and pulvini, which form swollen,
joint-like structures at the base of each pinnule. Changes in the turgor pressure of
certain cells at the pulvini are what allow the leaflets to open and close.
So let’s erase some of these labels
so we can look at this more closely. The cells in the upper half of each
pulvinus are called extensor cells, labeled here in pink. And the cells of the bottom of each
pulvinus are called flexor cells, which have been labeled here in blue. You might have noticed that the
flexor cells appear less turgid than the extensor cells. They have a slightly lower water
potential, and so a slightly lower turgor pressure. This holds the leaflets open.
When an electrical signal arrives
at the pulvini, however, ions flow out of the extensor cells. This increases the water potential
of the extensor cells, so water then flows out of the extensor cells and into the
flexor cells, which have a comparatively lower water potential. This increases the turgor pressure
in the flexor cells but decreases the turgor pressure in the extensor cells, as they
contain less water. The changes in the turgor pressure
of the flexor cells and of the extensor cells causes their shape to change. So the flexor cells become larger
and more turgid, while the extensor cells shrink and become less turgid. This causes the pinnules to fold
upwards and the leaflet to temporarily close.
Therefore, the folding and opening
of mimosa leaflets are dependent on changes in turgor pressure.
Let’s summarize what we’ve learned
about sensitivity in mimosa by reviewing the key points from this video. The leaves of mimosa plants are
described as pinnately compound as they consist of multiple distinct leaflets called
pinnae, branching from a single petiole. We’ve learned how the Mimosa pudica
plant can fold up its leaflets and droop when stimulated by touch, heat, or even by
night falling. The nondirectional movement in
response to touch is called thigmonasty and to light–dark cycles is called
nyctinasty or sleep movement. These responses that mimosa
exhibits are a result of changes in the turgor pressure of cells in the joint-like
structures called pulvini at the base of each of their leaflets and of their
petioles.
