Control and Coordination in Plants and Functions of Plant Hormones

One of the most pressing Biology Topics of our time is the conservation of endangered species and habitats.

What is Control and Coordination in Plants? – Types of Movements and Response of Plants to Their Environment

Plants do not have a nervous system and sense organs like eyes, ears, nose, etc., like animals, but they can still sense things. The plants can sense the presence of stimuli like light, gravity, chemicals, water, touch, etc., and respond to them. Plants can feel things like light, gravity, chemicals, water, touch, etc., by the action of hormones in them. The stimuli like light, gravity, chemicals, water, touch, etc., are called environmental changes. So, we can also say that plants coordinate their behaviour against environmental changes by using hormones. The hormones in plants do not act the same way as in animals. The hormones in plants coordinate their behavior by affecting the growth of a plant. And the effect on the growth of the plant can result in the movement of a part of the plant like a shoot (stem) or root, etc.

The sunflowers always face the sun. Here sunlight is the stimulus and the sunflower plants respond by bending (or moving) towards the sun.

Animals use both the nervous system and hormones for the coordination of their activities. Plants have no nervous system, so plants use only hormones for coordination. Thus, the reaction (or response) of plants to different stimuli like light, gravity, chemical substances, water, touch, etc., is due to the effect of hormones. Please note that animals can respond quickly because they have a nervous system. Plants cannot respond quickly because they have no nervous system. The plants respond to various stimuli very slowly by growing.

So, in most cases, the response of a plant to a stimulus cannot be observed immediately. It usually takes a considerable time to observe the effect of a stimulus on a plant. From the above discussion, we conclude that the function of control and coordination in plants is performed by chemical substances called plant hormones. Please note that the plant hormones are also called phytohormones (‘phyto’ means ‘plant’). Before we discuss the various types of plant hormones, we should know the meanings of ‘dormancy’ and ‘breaking of dormancy’.

A resting, inactive condition in which metabolism almost stops is called dormancy. The seed of a plant is inactive or dormant. It has dormancy. A seed must have certain conditions like water, warmth, air, and hormones to break dormancy and germinate to form a seedling (which then grows into a plant). Another part of a plant having dormancy is the bud. The bud is a young, undeveloped shoot of a plant that on breaking dormancy can form a branch, a leaf, or a flower depending on its position in the plant. The breaking of dormancy of a bud also requires certain plant hormones. Keeping these points in mind, we will now discuss the various types of plant hormones.

Plant Hormones (or Phytohormones)

The control and coordination in plants are done by plant hormones (or phytohormones). The plant hormones coordinate the activities of the plant by controlling one or the other aspect of the growth of the plant. So, plant hormones are also known as plant growth substances. The growth of a plant can be divided into three stages: cell division, cell enlargement, and cell differentiation (or cell specialization), and these stages have particular locations in a plant. These three stages of plant growth, as well as promotion of dormancy, breaking of dormancy, stomata control, falling of leaves, fruit growth, ripening of fruits, and aging in plants, are controlled by the various plant hormones. There are four major types of plant hormones (or phytohormones) that are involved in the control and coordination of plants. These are Auxins, Gibberellins, Cytokinins, and Abscisic acid (ABA).

Auxins, gibberellins, and cytokinins are the plant hormones that promote the growth of plants. On the other hand, abscisic acid is a plant hormone that inhibits (or prevents) the growth. The detailed functions of the various plant hormones are given below.

Auxins are the plant hormones that promote cell enlargement and cell differentiation in plants. Auxins also promote fruit growth. Auxin hormone controls a plant’s response to light and gravity. In other words, the auxin hormone is responsible for the phototropic and geotropic responses of plants. Auxin is made by cells at the tip of stems and roots. Auxin moves away from light and towards gravity. Auxin has the opposite effect on the growth of stems and roots. Auxin speeds up growth in the stem but it slows down growth in roots. Synthetic auxins are applied in agriculture and horticulture.

Gibberellins are plant hormones that promote cell enlargement and cell differentiation in the presence of auxins. Gibberellins help in breaking the dormancy in seeds and buds. They also promote growth in fruits. Gibberellin hormone is involved mainly in shoot extensions. Gibberellin stimulates the elongation of shoots of various plants (see Figure).

These pictures show the effect of gibberellin plant hormones on the growth of plants. The plants on the left side in the above picture have low levels of gibberellin hormones so their growth is less. When synthetic gibberellin hormones are applied, the plants grow much more rapidly as shown on the right-hand side in the above picture.

Cytokinins are the plant hormones that promote cell division in plants. Cytokinins also help in breaking the dormancy of seeds and buds. They delay the aging of leaves. Cytokinins promote the opening of stomata. They also promote fruit growth.

Abscisic acid is a plant hormone that functions mainly as a growth inhibitor. Abscisic acid promotes dormancy in seeds and buds (this is the opposite of breaking dormancy). It also promotes the closing of stomata. Abscisic acid promotes the wilting and falling of leaves (which is called abscission). It also causes the detachment of flowers and fruits from the plants.

Plant Movements

The plants are fixed at a place with their roots in the ground, so they cannot move from one place to another. That is, plants do not show locomotion (movement of the entire body). However, movements of the individual parts or organs of a plant (like shoots, roots, leaves, etc.) are possible when they are subjected to some external stimuli like light, the force of gravity, chemical substances, water, touch, etc. These movements of the plant part are usually caused by an unequal growth in its two regions by the action of plant hormones, under the influence of the stimulus.

For example, the auxin hormone is made and secreted by the meristematic tissue at the tip of the stem (or tip of the shoot). The auxin hormone speeds up the growth in stems. So, if one side of a stem has more auxin than the other side, then the side of the stem having more auxin hormone will grow faster than the other side (having less auxin hormone). This will cause the stem to bend. And when the stem bends to one side, we say that the stem is showing movement. This movement (or bending) of the stem has been caused by its growth. So, we can say that the bending of a stem (or shoot) (when exposed to light from one side) is a growing movement.

In fact, the movement in any part of a plant is usually a growth movement. Please note that when a plant part shows movement, it remains attached to the main body of the plant. It does not get detached from it. We will now discuss tropism in which the part of a plant shows movement in response to various stimuli.

A young grass seedling bends towards the light (coming from the candle). This bending has been caused by the action of a plant hormone called auxin.

The plant movements made in response to external stimuli fall into two main categories: tropisms and nasties. Though all the tropisms are growth movements nasties may be growth movements or growth-independent movements. In tropisms, the direction of the stimulus determines the direction of movement of the plant part but in nasties, the direction of movement is not determined by the direction of the stimulus.

Tropisms (or Tropic Movements)

A growing movement of a plant part in response to an external stimulus in which the direction of stimulus determines the direction of response is called tropism. Thus, tropism is a directional movement of the part of a plant caused by its growth. The growth of a plant part in response to a stimulus can be towards the stimulus (in the direction of stimulus) or away from the stimulus (against the direction of stimulus) due to which we can have a positive tropism or negative tropism, respectively. So:

• If the growth (or movement) of a plant part is towards the stimulus, it is called positive tropism, and
• If the growth (or movement) of a plant part is away from the stimulus, then it is called negative tropism.

We will now give an example of tropism. When a growing plant is exposed to light from only one side, then it responds by bending its stem (or shoot) towards the light. This is an example of phototropism (which is caused by the ‘light’ acting as ‘stimulus’. ‘Photo’ stands for ‘light’). The bending of the plant stem (or shoot) towards light is actually positive phototropism.

Types of Tropisms

There are five common stimuli in the environment: light, gravity, chemicals, water, and touch (or contact). These five stimuli give us five types of tropisms: phototropism, geotropism, chemotropism, hydrotropism, and thigmotropism. In phototropism, the stimulus is light; in geotropism, the stimulus is gravity, in chemotropism the stimulus is a chemical, in hydrotropism the stimulus is water, and in thigmotropism, the stimulus is touch (of a solid surface). It is obvious that the tropisms are named according to the stimulus. This will become clear from the following table.

Stimulus	Type of Tropism
Light	Phototropism
Gravity	Geotropism
Chemical	Chemotropism
Water	Hydrotropism
Touch	Thigmotropism

We will now give the definitions of all the five types of tropisms.
The movement of a plant part in response to light is called phototropism. In other words, the response of a plant to light is called phototropism. If the plant part moves towards the light, it is called positive phototropism. On the other hand, if the plant part moves away from light, then it is called negative phototropism. The stem (or shoot) of a growing plant bends towards the light, so the stem (or shoot) of a plant shows positive phototropism (see Figure). On the other hand, the roots of a plant move away from light, so the roots of a plant show negative phototropism.

The shoots of this potted plant kept near the window of a room bend towards sunlight coming from outside the window (on the left side). This is positive phototropism.

The movement of a plant part in response to gravity is called geotropism. In other words, the response of a plant to gravity is called geotropism. If the plant part moves in the direction of gravity, it is called positive geotropism. On the other hand, if the plant part moves against the direction of gravity, it is negative geotropism (Please note that the force of gravity acts in the downward direction). Now, the roots of a plant move downwards in the direction of gravity, so the roots of a plant show positive geotropism (see Figure). On the other hand, the stem (or shoot) of a plant moves upwards against the direction of gravity, so the stem (or shoot) of a plant shows negative geotropism (see Figure).

The roots of this potted plant bend downward in the direction of the force of gravity. They show positive geotropism. On the other hand, the shoot (or stem) of this plant bends upwards, showing negative geotropism.

The movement of a plant part in response to a chemical stimulus is called chemotropism. In other words, the response of a plant to chemical stimulus is called chemotropism. If the plant part shows movement (or growth) towards the chemical, it is called positive chemotropism. On the other hand, if the plant part shows movement (or growth) away from the chemical, then it is called negative chemotropism. The growth of the pollen tube towards the ovule during the process of fertilization in a flower is an example of chemotropism (It is actually positive chemotropism). In this case, the pollen tube grows towards the sugary substance (chemical) secreted by the ripe stigma of the carpel in the flower.

The movement of a plant part in response to water is called hydrotropism. In other words, the response of a plant part to water is called hydrotropism. If the plant part moves towards the water, it is called positive hydrotropism. On the other hand, if the plant part moves away from water, then it is called negative hydrotropism. The roots of a plant always go towards water, so roots are positively hydrotropic (see Figure).

The roots of a plant always go toward the water. They show positive hydrotropism.

The directional growth movement of a plant part in response to the touch of an object is called thigmotropism. The climbing parts of the plants such as tendrils grow towards any support which they happen to touch and wind around that support. So, tendrils of plants are positively thigmotropic (see Figures). We will now describe a plant’s response to light, gravity, chemicals, water, and touch with the help of diagrams.

The tendrils of a plant always grow towards any support which they happen to touch. They show positive thigmotropism.

This cucumber plant is climbing on a support with the help of its climbing organs called tendrils.

Response of Plants to Light: Phototropism

Plants need sunlight, so the stems (or shoots) respond to sunlight by growing towards it. The plants also turn their leaves to face the sun. This makes sure that the leaves get as much sunlight as possible. When a plant is grown in the open ground with the sunlight ‘coming from above, then the stem of the plant grows straight up. If, however, the plant is grown with sunlight coming from one side, then the stem bends towards the direction from which the sunlight comes. The root of the plant, however, bends away from the direction from which the sunlight comes. We will now describe an experiment to show the response of plant parts to light.

We take a potted plant growing in a transparent glass jar. When this potted plant is kept in the open space, the sunlight falls from above due to which the stem of the plant grows straight up towards the source of light ‘the sun’ [see Figure]. The root of the plant also grows straight but in a downward direction.

Diagrams to show the response of a plant to light (or phototropism).

Let us now keep the potted plant having a straight stem and straight root near the window in a dark room so that sunlight falls on it from the right side (through the window) only. After some days we will see that the stem of the plant bends towards the right side from where the light is coming [see Figure]. This observation shows that the stem of the plant responds to light and bends towards it. Even the leaves of the plant turn towards the sun so as to obtain the maximum sunlight. Thus, the stem (and leaves) of a plant are positively phototropic. Now, if we look at the root of the plant in Figure, we find that the root bends to the left side away from the light. This observation shows that the root of the plant responds to light by growing away from it. Thus, the root of the plant is negatively phototropic. We will now explain the bending of a plant stem towards sunlight. The plant stem responds to light and bends towards it due to the action of ‘the auxin hormone’. This happens as follows:

When sunlight comes from above, then the auxin hormone present in the tip of the stem spreads uniformly down the stem [see Figure]. Due to the equal presence of auxin, both sides of the stem (A and B) grow equally rapidly [see Figure]. And the stem grows straight up.

Diagrams to explain the bending of a plant stem (or shoot) towards light by the action of the ‘auxin’ hormone.

When the light falls only on the right side of the stem [side B in Figure], then the auxin hormone collects in the left side (shady side A) of the stem, away from light. This is because the auxin hormone prefers to stay in the shade.

Now, more auxin hormone is present on the left side of the stem but not on its right side. Due to more auxin hormone, the left side (A) of the stem grows faster than its right side (B) where there is no auxin. Since the left side of the stem grows faster and becomes longer than its right side, therefore, the stem bends towards the right side (in the direction of light) [see Figure].

We can also explain the bending of a plant root away from light by the action of the auxin hormone. For this, we have to remember that the effect of auxin on the growth of a root is exactly opposite to that on a stem. Thus, though auxin hormone increases the rate of growth in a stem but it decreases the rate of growth in a root. Now, the side of a root away from light will have all the auxin concentrated in it. Due to this, the side of the root which is away from light will grow slower than the other side and make the root bend away from light. Please draw the diagram to show the bending of plant roots away from light yourself.

The Response of Plants to Gravity: Geotropism

The force with which the earth pulls all the things towards it is called gravity. The force of gravity always acts in a downward direction. The response of plants to gravity is called geotropism. Geotropism is also known as gravitropism.

• The roots of plants always grow downward in response to gravity. This makes sure that they will find soil and water.
• The stems (or shoots) of plants always grow up, away from the pull of gravity. This makes sure that they will get light.

The movement of plant roots toward the earth and that of stem away from the earth, both are cases of geotropism. Since the roots grow down towards the pull of gravity, so the downward growth (or downward movement) of roots is called positive geotropism. The stem (or shoot) grows upwards, away from the pull of gravity, so the upward growth (or upward movement) of the stem or shoot is called negative geotropism. The response of plants to gravity (or geotropism) will become more clear from the following experiment.

We take a potted plant growing in a transparent glass jar. When this potted plant is kept in the normal position, we can see that its roots are growing downwards and its stem is growing upwards [see Figure].

Experiment to show the response of a plant to gravity (geotropism).

Let us now tilt the potted plant and keep the pot horizontally on its side as shown in Figure. In this position, the roots and stem both are parallel to the ground (or earth). Allow the plant to remain in this position for a few days.

After a few days, we will find that the roots of the potted plant bend downwards towards the earth and the stem of the plant bends upwards, away from the earth [see Figure], The roots of the plant grow downwards in response to the pull of gravity. The plant’s stem responds to gravity in the opposite way, by growing upwards (away from the pull of gravity).

Response of Plants to Chemicals: Chemotropism

The growth (or movement) of a plant part due to chemical stimulus is known as chemotropism. The growth (or movement) of a pollen tube towards the ovule induced by a sugary substance as stimulus, is an example of chemotropism. This can be explained as follows: The ripe stigma in the carpel of a flower secretes a chemical substance (which is a sugary substance) into the style towards the ovary (see Figure).

The diagram shows the response of a plant part ‘pollen’ to a chemical secreted by stigma (or chemotropism).

This sugary substance acts as a stimulus for the pollen grains which fall on the stigma of the carpel. The pollen grain responds to this stimulus by growing a pollen tube in the downward direction into the style of the carpel and reaching the ovule in the ovary of the flower for carrying out fertilization. This growth of the pollen tube in response to a chemical substance secreted by the stigma of a flower is an example of chemotropism.

Response of Plants to Water: Hydrotropism

The roots of plants always go towards water, even if it means going against the pull of gravity. Though roots normally grow downwards in order to reach water, they can grow sideways or even upwards! The roots grow in the direction of the source of water so as to obtain water for the developing plant. Since roots always grow (or move) towards the water, therefore, roots are positively hydrotropic. When the roots bend by growing toward the water, it appears that they move toward the water. We will now describe an experiment to demonstrate hydrotropism. This will show us the response of roots to water.

We take two glass troughs A and B and fill each one of them two-thirds with soil (see Figure). In trough A we plant a tiny seedling [see Figure], In trough B we plant a similar seedling and also place a small ‘clay pot’ inside the soil [see Figure]. Water the soil in trough A daily and uniformly. Do not water the soil in trough B but put some water in the clay pot buried in the soil. Leave both the troughs for a few days.

Experiment to show the response of a plant to water (hydrotropism).

Now, dig up the seedlings carefully from both troughs without damaging their roots. We will find that the root of the seedling in trough A is straight. On the other hand, the ‘root of the seedling in trough B is found to be bent to the right side (towards the clay pot containing water) [see Figure]. This can be explained as follows.

In trough A, the root of the seedling gets water from both sides (because the soil is watered uniformly). But in trough B, the root gets water oozing out from the clay pot which is kept on the right side. So, the root of the seedling in trough B grows and bends towards the source of water to the right side. This experiment shows that the root of a plant grows in water. In other words, the root of a plant is positively hydrotropic.

Directional Response of Plants to the Touch of an Object: Thigmotropism

There are some plants called ‘climbing plants’ which have weak stems and hence cannot stand upright (or erect) on their own. The climbing plants have climbing organs called tendrils. Tendrils are the thin, thread-like growths on the stems or leaves of climbing plants. Thus, there are two types of tendrils: stem tendrils and leaf tendrils. Tendrils are sensitive to the touch (or contact) of other objects. That is, tendrils have cells that can sense their contact with a nearby solid object like a bamboo stick, or the stem of another plant.

So, when a tendril touches an object, then the side of the tendril in contact with the object grows more slowly than its other side. This causes the tendril to bend towards the object by growing towards it, wind around the object, and cling to it (see Figure 15). The winding movement of the tendril of a climbing plant is an example of thigmotropism. The stimulus in thigmotropism is the touch (or contact) of an object. The winding movement of the tendril of a plant around a nearby object gives support to the plant having a weak stem.

Thigmotropism is often seen in plants having tendrils. Tendrils are positively thigmotropic which means that they grow towards things they happen to touch. The plants having stem tendrils or leaf tendrils which are positively thigmotropic climb up artificial supports, other plants, or fences very easily. The plants such as bitter gourd (karela), bottle gourd (lauki), grapevine, and passion flower have stem tendrils that are positively thigmotropic and make these plants climb up by winding around various types of supports [see Figure], The plants such as peas and glory lily have leaf tendrils which are positively thigmotropic.

Diagrams to show the response of a plant part ‘tendril’ to the touch of an object (here a bamboo stick).

These leaf tendrils also make their plants climb up by winding around various types of nearby supports [see Figure]. From the above discussion, we conclude that tendrils are the climbing organs of plants that are positively thigmotropic.

The Usefulness of Tropic Movements

The various types of tropic movements help the plants to survive. For example, even if a seed is planted upside down, its root will still grow downwards into the earth because it is positively geotropic (see Figure). The root will also grow towards the water because it is positively hydrotropic. Similarly, the shoot of such a seed will grow upwards because it is negatively geotropic and towards light because it is positively phototropic. These tropic movements help plants to obtain water and nutrients from soil and light from the sun, which are necessary for their growth and survival.

Whichever way up a seed is planted, its root always grows downwards into the soil. This is positive geotropism in seeds.

Nasties (or Nastic Movements)

We have just studied that in tropism, a plant part either moves towards the stimulus or away from the stimulus. However, in some plants, the movement of the plant part is neither towards the stimulus nor away from the stimulus. That is, the movement of plant parts in some plants is not in a particular direction with respect to the stimulus. The movement of a plant part in response to an external stimulus in which the direction of response is not determined by the direction of stimulus is called nastic movement. Nastic movements of plants are also called nasties.

The nastic movements of plants are induced by stimuli such as heat, light, touch (or contact), etc. The main difference between tropic and nastic movements is that tropic movement is a directional movement of a plant part but nastic movement is not a directional movement of the plant part with respect to the stimulus. The direction of nastic movement is not determined by the direction from which the stimulus is applied. In nastic movement, from whichever direction the stimulus is applied, it affects all the parts of the organ of a plant equally and they always move in the same direction. Nastic movements are mostly exhibited by the flat organs of the plants like ‘leaves’ and ‘petals of flowers’.

Some examples of the nastic movements of plants (or nasties) are given below:

• The folding up of the leaves of a sensitive plant (Mimosa pudica) on touching is an example of nastic movement. Here the stimulus is touch.
• The opening up of the petals of dandelion flowers in the morning in bright light and closing in the evening when the light fades is an example of nastic movement. In this case, the stimulus is light.
• The closing of the petals of the moonflower in the morning in bright light and opening at dark, when the light fades, is also an example of nastic movement. In this case, also the stimulus is light.

Please note that though all tropisms are growth movements all nasties (or nastic movements) are not growth movements. Nastic movements may or may not be growth movements. For example, the folding up of the leaves of a sensitive plant on touching is not a growth movement but the opening and closing of petals of flowers by the action of sunlight is a growth movement.

We have just said that most of the movements of the plant parts are caused by their growth. Now, since the growth of a plant part is usually a slow process, therefore, most of the movements of plant parts are very slow. There are, however, some exceptions. We will now describe the movement of a plant part (leaves) which is unusually fast and takes place almost immediately. It is the folding up of the leaves of a sensitive plant when touched with a finger (or any other object). This is discussed below under the topic of thigmonasty.

Thigmonasty

The non-directional movement of a plant part in response to the touch of an object is called thigmonasty. In other words, thigmonasty is the nastic movement of a plant part in response to touch. Thus, the stimulus in thigmonasty is the ‘touch’. An example of the nastic movement in plants caused by touch (or thigmonasty) is provided by the sensitive plant (Mimosa pudica) which is also known as a touch-me-not plant. It is called chhui-mui in Hindi. If we touch the leaves (or rather leaflets) of the sensitive plant with our fingers, then its leaves fold up and droop almost immediately. The folding up of the leaves of sensitive plants on touching is an example of nastic movements in plants (in which the stimulus is the ‘touch’ of our fingers).

Diagrams to show the nastic movements in the leaves of the sensitive plant (Mimosa pudica) caused by ‘touch’.

The figure shows the open leaves of a sensitive plant. When we touch the leaves of this sensitive plant with our fingers, then these leaves of sensitive plant fold up at once as shown in Figure. In this case, the ‘touch’ of our fingers is the stimulus and the leaves respond by ‘folding up’. Please note that the folding of leaves of a sensitive plant is not a case of tropism (like thigmotropism) because in this case, the direction of movement of leaves does not depend on the direction of stimulus (touch). We will now describe how the leaves of a sensitive plant fold up when touched.

The sensitive plant has pad-like swellings called ‘pulvini’ at the base of each leaf [see Figure] (The singular of pulvini is pulvinus). The pulvini contain a lot of water in their cells. Due to the internal ‘water pressure’ in them (called turgor), all the pulvini are very firm and hold the leaves above them upright [see Figure], The pulvini have also large intercellular spaces (empty spaces) between their cells. The folding up of the leaves of a sensitive plant on touching is due to the sudden loss of water from pad-like swellings called ‘pulvini’ present at the base of all leaves of the sensitive plant which makes the pulvini lose their firmness causing the leaves to droop and fall. This happens as follows.

The leaves of sensitive plant fold due to the loss of water from the pulvinus at their base.

When the leaves of sensitive plants (having pulvini at their base) are touched with a finger, then an electrical impulse is generated which travels through ordinary cells (because there are no nerve cells in sensitive plants or other plants). This electrical impulse acts on a plant hormone. The plant hormone makes the water migrate from the cells of one half of a pulvinus to the intercellular spaces in the other half of the pulvinus. This loss of water from half of the pulvinus causes the pulvinus to lose its firmness making the leaf fold [see Figure]. Similarly, all the pulvini lose firmness and become limp due to which all the leaves above them collapse and fold up. At a gap of 15 to 30 minutes after the leaves have folded, water usually diffuses back into the same cells of the pulvinus from which it left, and the leaf returns to its original position.

Photonasty

The non-directional movement of a plant part (usually petals of flowers) in response to light is called photonasty. In other words, photo nasty is the nastic movement of a plant part (like petals of flowers) in response to light. Thus, the stimulus in photo nasty is light. A dandelion flower opens up in the morning in bright light but closes in the evening when the light fades and it gets dark (see Figure).

(a) Dandelion flower opens the petals in bright light during the daytime

(b) Dandelion flower closes the petals at dusk (or night) when it gets dark

Nastic movements of petals of a dandelion flower in response to light. This is an example of photo nasty.

The opening and closing of petals of dandelion flowers in response to the intensity of light is an example of nastic movement in which the stimulus is light. In other words, it is an example of photo nasty. The moonflower behaves exactly opposite to that of dandelion flowers in respect of response to light. The petals of moonflower close during the day when there is bright light but open up at night when it is dark and there is no light (see Figure). This is also an example of photo nasty.

(a) Moonflower closes the petals during the daytime when there is a bright light

(b) Moonflower opens the petals at dusk (or night) when it gets dark and there is no light

Nastic movements of petals of moonflower in response to light. Another example of photo nasty.

Please note, that the opening and closing of flowers in response to light (or photo nasty) are growth movements. Petals open when their inner surfaces grow more than their outer surfaces. On the other hand, petals close when their outer surfaces grow more than their inner surfaces. Before we end this discussion, we would like to give the functions of plant hormones.

Functions of Plant Hormones (or Phytohormones)

Plant hormones (or phytohormones) regulate many functions in plants. The various functions in plants that are regulated by the plant hormones (or phytohormones) are:

• Germination of seeds (or Breaking the dormancy of seeds),
• Growth of root, stem, and leaves,
• Movement of stomata (or stomatal movement) in leaves,
• The flowering of plants,
• Ripening of fruits, and
• Phototropism, geotropism, chemotropism, hydrotropism, thigmotropism, and nastic movements.

Let Us Answer Some Questions Now.

Sample Problem 1.
Which of the following is a plant hormone? (NCERT Book Question)
(a) Insulin
(b) Thyroxine
(c) Oestrogen
(d) Cytokinin
Answer:
(d) Cytokinin.

Sample Problem 2.
How do auxins promote the growth of a tendril around a support? (NCERT Book Question)
Answer:
When the tip of a tendril touches a support, then the auxins (plant hormones) present in its tip move to that side of the tip which is away from the support. Auxins promote growth. So, due to more auxins in it, the side of the tendril away from the support grows faster (and becomes longer) than the side which is in contact with the support and makes the tendril curve (or bend) towards the support. This ‘curving’ tendril can then encircle the support and wind around it.

Sample Problem 3.
How is the movement of the leaves of the sensitive plant different from the movement of a shoot toward light? (NCERT Book Question)
Answer:
The main differences between the movement of the leaves of a sensitive plant and the movement of a shoot toward light are as follows:

Movement of leaves of sensitive plant	Movement of a shoot towards light
1. It is a nastic movement that does not depend on the direction of the stimulus applied.	1. It is a tropic movement that depends on the direction of the stimulus applied.
2. The stimulus is ‘touch’.	2. The stimulus is ‘light’.
3. It is caused by the sudden loss of water from the swellings at the base of leaves.	3. It is caused by the unequal growth on the two sides of the shoot.
4. It is not a growth movement.	4. It is a growing movement.

Sample Problem 4.
What is the difference between the manner in which movement takes place in a sensitive plant and the movement in our legs? (NCERT Book Question)
Answer:
The movement in the leaves of a sensitive plant takes place due to the sudden loss of water in the pad-like swellings (called pulvini) at the base of all the leaves. The loss of water makes the pulvini limp leading to the drooping and folding of leaves. On the other hand, the movement in our legs takes place when the leg muscles pull on the leg bones.