A Plant Bends Towards Light Because a Hormone Piles Up on Its Shady Side
Learn what plant growth regulators are and what auxins, gibberellins, cytokinins, abscisic acid and ethylene each do, understand phototropism, geotropism, hydrotropism, thigmotropism and chemotropism with everyday examples, and see how a clinostat proves that gravity steers roots and shoots.
How does a plant respond to its surroundings without nerves or muscles?
A money plant on a windowsill slowly turns its leaves towards the glass. A seed planted upside down still sends its root downwards and its shoot upwards. The tendril of a bitter gourd vine finds a string and coils around it. A plant has no nerves and no muscles, yet it senses light, gravity and touch and responds to them.
It does this with chemicals. Plants produce plant growth regulators, also called plant hormones — substances made in tiny amounts in one part of the plant that travel to other parts and control how cells grow, divide and develop. Changing where a hormone collects changes where the plant grows fastest, and that is how a stem bends or a root turns.
Five main groups do most of the work:
- Auxins — cell elongation, bending towards light, root formation
- Gibberellins — stem elongation, seed germination, larger fruits
- Cytokinins — cell division, keeping leaves green
- Abscisic acid — closing stomata, dormancy, a stress signal
- Ethylene — ripening fruit, shedding leaves
The growth responses are called tropisms when they are directed by the direction of a stimulus — towards light, towards gravity, towards water, around a support, or towards a chemical.
This lesson covers:
- Plant growth regulators, and the effects of auxins and gibberellins
- The effects of cytokinins, abscisic acid and ethylene
- The five tropisms, each with an example
- How a clinostat demonstrates geotropism
These hormones are used on farms every day. Cuttings of rose and hibiscus are dipped in rooting powder containing an auxin; grapes are sprayed with gibberellins to enlarge them; and ethylene is used in ripening chambers for mangoes and bananas. Understanding the hormones explains all three practices.
The link to earlier chapters. Abscisic acid is the hormone that closes stomata under water stress, as the transpiration chapter described, and phototropism is one of the plant adaptations that help photosynthesis. Hormones tie together many responses studied separately.
This page covers the ICSE Class 10 Biology chapter on chemical coordination in plants: plant growth regulators, their effects, tropisms and the clinostat experiment.
It does this with chemicals. Plants produce plant growth regulators, also called plant hormones — substances made in tiny amounts in one part of the plant that travel to other parts and control how cells grow, divide and develop. Changing where a hormone collects changes where the plant grows fastest, and that is how a stem bends or a root turns.
Five main groups do most of the work:
- Auxins — cell elongation, bending towards light, root formation
- Gibberellins — stem elongation, seed germination, larger fruits
- Cytokinins — cell division, keeping leaves green
- Abscisic acid — closing stomata, dormancy, a stress signal
- Ethylene — ripening fruit, shedding leaves
The growth responses are called tropisms when they are directed by the direction of a stimulus — towards light, towards gravity, towards water, around a support, or towards a chemical.
This lesson covers:
- Plant growth regulators, and the effects of auxins and gibberellins
- The effects of cytokinins, abscisic acid and ethylene
- The five tropisms, each with an example
- How a clinostat demonstrates geotropism
These hormones are used on farms every day. Cuttings of rose and hibiscus are dipped in rooting powder containing an auxin; grapes are sprayed with gibberellins to enlarge them; and ethylene is used in ripening chambers for mangoes and bananas. Understanding the hormones explains all three practices.
The link to earlier chapters. Abscisic acid is the hormone that closes stomata under water stress, as the transpiration chapter described, and phototropism is one of the plant adaptations that help photosynthesis. Hormones tie together many responses studied separately.
This page covers the ICSE Class 10 Biology chapter on chemical coordination in plants: plant growth regulators, their effects, tropisms and the clinostat experiment.
What are plant growth regulators, and what do auxins and gibberellins do?
Plant growth regulators are chemical substances produced in small amounts in one part of a plant and carried to other parts, where they control growth and development; auxins mainly cause cell elongation and bending towards light, while gibberellins mainly cause stem elongation and help seeds germinate.
Definition. Plant growth regulators, or phytohormones, are organic substances that are:
- Produced in very small amounts in particular parts of the plant
- Transported to other parts
- Able to promote, inhibit or modify growth and development
They fall broadly into growth promoters — auxins, gibberellins and cytokinins — and growth inhibitors — abscisic acid, with ethylene often grouped with them for its role in ripening and ageing.
1. Auxins — produced mainly at the tips of shoots.
- Promote cell elongation in shoots, so stems grow longer
- Cause phototropism and geotropism, by collecting unevenly on one side of a stem or root
- Apical dominance — auxin from the shoot tip suppresses the growth of side buds, so the main shoot grows tallest
- Promote root formation in stem cuttings
- Prevent premature dropping of leaves and fruits
- Can produce seedless fruits, such as seedless tomatoes, without fertilisation
- Synthetic auxins are used as weedkillers that destroy broad-leaved weeds in cereal fields
2. Gibberellins.
- Cause stem elongation, making dwarf plants grow taller
- Break seed dormancy and promote germination
- Increase the size of fruits, such as grapes, and lengthen the fruit stalk
- Promote bolting — the rapid lengthening of the flower stalk in plants such as cabbage
- Can also produce seedless fruits
Worked example — which hormone was used? Match each observation to auxin or gibberellin.
- A rose cutting dipped in rooting powder forms roots quickly — auxin
- A dwarf pea plant sprayed with a hormone grows to normal height — gibberellin
- Removing the tip of a shoot makes side branches grow — auxin (removing its source ends apical dominance)
- Grape bunches grow larger berries after spraying — gibberellin
An everyday example. Gardeners and nurseries in India commonly dip the cut ends of hibiscus, rose and bougainvillea cuttings into a rooting hormone powder before planting them. The auxin in the powder makes new roots form at the cut end far faster than they would naturally.
The boundary case — the same hormone can promote or inhibit. A concentration of auxin that speeds up the elongation of stem cells slows down the growth of root cells, because roots are far more sensitive to it. The effect of a hormone depends on its concentration and on the organ it reaches — which is the key to explaining geotropism later in this lesson.
Definition. Plant growth regulators, or phytohormones, are organic substances that are:
- Produced in very small amounts in particular parts of the plant
- Transported to other parts
- Able to promote, inhibit or modify growth and development
They fall broadly into growth promoters — auxins, gibberellins and cytokinins — and growth inhibitors — abscisic acid, with ethylene often grouped with them for its role in ripening and ageing.
1. Auxins — produced mainly at the tips of shoots.
- Promote cell elongation in shoots, so stems grow longer
- Cause phototropism and geotropism, by collecting unevenly on one side of a stem or root
- Apical dominance — auxin from the shoot tip suppresses the growth of side buds, so the main shoot grows tallest
- Promote root formation in stem cuttings
- Prevent premature dropping of leaves and fruits
- Can produce seedless fruits, such as seedless tomatoes, without fertilisation
- Synthetic auxins are used as weedkillers that destroy broad-leaved weeds in cereal fields
2. Gibberellins.
- Cause stem elongation, making dwarf plants grow taller
- Break seed dormancy and promote germination
- Increase the size of fruits, such as grapes, and lengthen the fruit stalk
- Promote bolting — the rapid lengthening of the flower stalk in plants such as cabbage
- Can also produce seedless fruits
Worked example — which hormone was used? Match each observation to auxin or gibberellin.
- A rose cutting dipped in rooting powder forms roots quickly — auxin
- A dwarf pea plant sprayed with a hormone grows to normal height — gibberellin
- Removing the tip of a shoot makes side branches grow — auxin (removing its source ends apical dominance)
- Grape bunches grow larger berries after spraying — gibberellin
An everyday example. Gardeners and nurseries in India commonly dip the cut ends of hibiscus, rose and bougainvillea cuttings into a rooting hormone powder before planting them. The auxin in the powder makes new roots form at the cut end far faster than they would naturally.
The boundary case — the same hormone can promote or inhibit. A concentration of auxin that speeds up the elongation of stem cells slows down the growth of root cells, because roots are far more sensitive to it. The effect of a hormone depends on its concentration and on the organ it reaches — which is the key to explaining geotropism later in this lesson.
What are the effects of cytokinins, abscisic acid and ethylene?
Cytokinins promote cell division and keep leaves green, abscisic acid slows growth and helps plants survive stress by closing stomata and causing dormancy, and ethylene ripens fruits and makes leaves and fruits fall.
1. Cytokinins.
- Promote cell division
- Delay ageing of leaves — cut leaves treated with cytokinin stay green longer
- Promote the growth of side buds, acting against apical dominance
- Help break dormancy of seeds and buds
- Together with auxin, control whether roots or shoots form in plant tissue culture
2. Abscisic acid (ABA) — a growth inhibitor and stress hormone.
- Closes the stomata when the plant is short of water, reducing transpiration
- Induces dormancy in seeds and buds, so they do not grow in unsuitable seasons
- Inhibits seed germination and slows growth
- Promotes the falling of leaves, flowers and fruits in some plants
3. Ethylene — a gaseous hormone.
- Promotes ripening of fruits
- Promotes the falling of leaves, flowers and fruits
- Speeds up ageing of leaves and flowers
- Can influence flowering, for example increasing female flowers in cucumber
Comparing the hormones by effect:
- Cell elongation — auxin, gibberellin
- Cell division — cytokinin
- Seed germination — promoted by gibberellin, inhibited by abscisic acid
- Side-bud growth — suppressed by auxin, promoted by cytokinin
- Stomatal closure — abscisic acid
- Fruit ripening — ethylene
Worked example — pairs with opposite effects. Name the hormone that works against each of these.
- Gibberellin promotes germination — abscisic acid inhibits it
- Auxin suppresses side buds — cytokinin promotes them
- Cytokinin delays leaf ageing — ethylene speeds it up
Plants control growth by balancing hormones with opposite effects, not by switching single hormones on and off.
An everyday example. Putting a ripe banana in a closed bag with unripe mangoes or green bananas makes them ripen faster. The ripe fruit gives off ethylene gas, which spreads through the bag and triggers ripening in the rest — the same principle used, under careful control, in commercial ripening chambers.
The boundary case — ethylene is a gas. Unlike the other hormones, ethylene travels through the air as well as within the plant. One ripening fruit can affect its neighbours, which is why a single overripe fruit in a basket can hasten the spoiling of the whole batch.
1. Cytokinins.
- Promote cell division
- Delay ageing of leaves — cut leaves treated with cytokinin stay green longer
- Promote the growth of side buds, acting against apical dominance
- Help break dormancy of seeds and buds
- Together with auxin, control whether roots or shoots form in plant tissue culture
2. Abscisic acid (ABA) — a growth inhibitor and stress hormone.
- Closes the stomata when the plant is short of water, reducing transpiration
- Induces dormancy in seeds and buds, so they do not grow in unsuitable seasons
- Inhibits seed germination and slows growth
- Promotes the falling of leaves, flowers and fruits in some plants
3. Ethylene — a gaseous hormone.
- Promotes ripening of fruits
- Promotes the falling of leaves, flowers and fruits
- Speeds up ageing of leaves and flowers
- Can influence flowering, for example increasing female flowers in cucumber
Comparing the hormones by effect:
- Cell elongation — auxin, gibberellin
- Cell division — cytokinin
- Seed germination — promoted by gibberellin, inhibited by abscisic acid
- Side-bud growth — suppressed by auxin, promoted by cytokinin
- Stomatal closure — abscisic acid
- Fruit ripening — ethylene
Worked example — pairs with opposite effects. Name the hormone that works against each of these.
- Gibberellin promotes germination — abscisic acid inhibits it
- Auxin suppresses side buds — cytokinin promotes them
- Cytokinin delays leaf ageing — ethylene speeds it up
Plants control growth by balancing hormones with opposite effects, not by switching single hormones on and off.
An everyday example. Putting a ripe banana in a closed bag with unripe mangoes or green bananas makes them ripen faster. The ripe fruit gives off ethylene gas, which spreads through the bag and triggers ripening in the rest — the same principle used, under careful control, in commercial ripening chambers.
The boundary case — ethylene is a gas. Unlike the other hormones, ethylene travels through the air as well as within the plant. One ripening fruit can affect its neighbours, which is why a single overripe fruit in a basket can hasten the spoiling of the whole batch.
What are tropisms, and how do phototropism, geotropism, hydrotropism, thigmotropism and chemotropism work?
A tropism is a directional growth movement of a plant part in response to a stimulus coming from one direction; it is positive when the part grows towards the stimulus and negative when it grows away.
1. Phototropism — response to light.
- Shoots are positively phototropic — they bend towards light
- Roots are negatively phototropic — they grow away from light
- Example: a potted plant on a windowsill bends towards the window
- How it happens: auxin collects on the shaded side of the shoot, so cells there elongate more, bending the shoot towards the light
2. Geotropism — response to gravity.
- Roots are positively geotropic — they grow downwards
- Shoots are negatively geotropic — they grow upwards
- Example: a seedling laid on its side turns its root down and its shoot up
- How it happens: auxin collects on the lower side of both root and shoot. In the shoot it speeds elongation, so the shoot bends up; in the more sensitive root it slows elongation, so the root bends down
3. Hydrotropism — response to water.
- Roots are positively hydrotropic — they grow towards water
- Example: roots of trees growing towards a leaking water pipe or a drain
4. Thigmotropism — response to touch or contact.
- Tendrils are positively thigmotropic — they coil around a support they touch
- Example: tendrils of pea, bitter gourd and cucumber winding round a stick or string
- How it happens: cells on the side away from the contact grow faster than those touching the support, so the tendril curls around it
5. Chemotropism — response to chemicals.
- Pollen tubes are positively chemotropic — they grow towards the ovule
- Example: after pollination, the pollen tube grows down the style towards chemicals released by the ovule
Worked example — positive or negative? State the tropism and whether it is positive or negative.
- A mango seedling's root grows downwards in a pot — geotropism, positive
- A sunflower seedling's stem bends towards a lamp — phototropism, positive
- A root of the same seedling grows away from the lamp — phototropism, negative
- A pumpkin tendril curls round a fence wire — thigmotropism, positive
An everyday example. Farmers growing bitter gourd and bottle gourd put up frames of bamboo and string beside the plants. The tendrils find the strings and coil round them, lifting the vines off the ground where the fruits would rot.
The boundary case — tropism versus nastic movement. The folding of the leaves of the touch-me-not plant (Mimosa) when touched is not a tropism. It happens in the same way whichever direction the touch comes from, and it is a quick change in water pressure rather than growth. A tropism is always directional growth.
1. Phototropism — response to light.
- Shoots are positively phototropic — they bend towards light
- Roots are negatively phototropic — they grow away from light
- Example: a potted plant on a windowsill bends towards the window
- How it happens: auxin collects on the shaded side of the shoot, so cells there elongate more, bending the shoot towards the light
2. Geotropism — response to gravity.
- Roots are positively geotropic — they grow downwards
- Shoots are negatively geotropic — they grow upwards
- Example: a seedling laid on its side turns its root down and its shoot up
- How it happens: auxin collects on the lower side of both root and shoot. In the shoot it speeds elongation, so the shoot bends up; in the more sensitive root it slows elongation, so the root bends down
3. Hydrotropism — response to water.
- Roots are positively hydrotropic — they grow towards water
- Example: roots of trees growing towards a leaking water pipe or a drain
4. Thigmotropism — response to touch or contact.
- Tendrils are positively thigmotropic — they coil around a support they touch
- Example: tendrils of pea, bitter gourd and cucumber winding round a stick or string
- How it happens: cells on the side away from the contact grow faster than those touching the support, so the tendril curls around it
5. Chemotropism — response to chemicals.
- Pollen tubes are positively chemotropic — they grow towards the ovule
- Example: after pollination, the pollen tube grows down the style towards chemicals released by the ovule
Worked example — positive or negative? State the tropism and whether it is positive or negative.
- A mango seedling's root grows downwards in a pot — geotropism, positive
- A sunflower seedling's stem bends towards a lamp — phototropism, positive
- A root of the same seedling grows away from the lamp — phototropism, negative
- A pumpkin tendril curls round a fence wire — thigmotropism, positive
An everyday example. Farmers growing bitter gourd and bottle gourd put up frames of bamboo and string beside the plants. The tendrils find the strings and coil round them, lifting the vines off the ground where the fruits would rot.
The boundary case — tropism versus nastic movement. The folding of the leaves of the touch-me-not plant (Mimosa) when touched is not a tropism. It happens in the same way whichever direction the touch comes from, and it is a quick change in water pressure rather than growth. A tropism is always directional growth.
How does a clinostat demonstrate geotropism?
A clinostat slowly rotates a seedling fixed horizontally on its axis, so that gravity acts equally on all sides; a rotating seedling grows straight while a stationary one bends its root down and shoot up, proving that one-sided gravity causes geotropism.
What a clinostat is. A clinostat is an apparatus with a horizontal axis that is turned slowly and steadily by a motor or clockwork mechanism. A plant or seedling can be fixed to the axis so that it rotates with it.
The experiment:
- Setup: take two similar germinating seedlings with straight roots and shoots. Fix one to a clinostat that is kept rotating, and the other to a clinostat that is kept stationary, both with their roots and shoots horizontal
- Conditions: keep both in darkness, to remove any effect of light, and in moist air, so they do not dry out
- Leave them for a day or two
Observations:
- Stationary clinostat — the root bends downwards and the shoot bends upwards
- Rotating clinostat — the root and shoot continue to grow straight and horizontal
Inference.
- On the stationary clinostat, gravity acts from one direction, so auxin gathers on the lower side of root and shoot, and each bends
- On the rotating clinostat, each side of the root and shoot faces downwards in turn, so the effect of gravity is spread equally around them and no side receives more auxin than another
- Therefore geotropism is caused by gravity acting from one side
Why the seedlings are kept in darkness. If light came from one direction, phototropism would also bend the shoot, and the result could not be traced to gravity alone. Darkness removes that second factor.
Worked example — predicting a variation. A seedling is fixed vertically on a clinostat, root pointing down, and rotated about a vertical axis. Will its root and shoot bend?
No. Gravity still acts downwards along the length of the root and shoot, exactly as it would for a seedling growing normally, and rotation about a vertical axis does not change which side faces down. The root keeps growing down and the shoot up. The clinostat cancels geotropism only when it turns the plant about a horizontal axis.
An everyday example. Seeds scattered by hand into a field land every which way, yet the young plants all end up with roots going down and shoots going up. Geotropism corrects each seedling's direction, which is exactly what the stationary clinostat shows happening in a single seedling.
The boundary case — the clinostat does not remove gravity. Gravity still acts on the rotating seedling at every moment. The clinostat only stops it from acting on the same side for long enough to cause bending — which is why the result proves that it is the direction of gravity, not gravity itself, that steers growth.
What a clinostat is. A clinostat is an apparatus with a horizontal axis that is turned slowly and steadily by a motor or clockwork mechanism. A plant or seedling can be fixed to the axis so that it rotates with it.
The experiment:
- Setup: take two similar germinating seedlings with straight roots and shoots. Fix one to a clinostat that is kept rotating, and the other to a clinostat that is kept stationary, both with their roots and shoots horizontal
- Conditions: keep both in darkness, to remove any effect of light, and in moist air, so they do not dry out
- Leave them for a day or two
Observations:
- Stationary clinostat — the root bends downwards and the shoot bends upwards
- Rotating clinostat — the root and shoot continue to grow straight and horizontal
Inference.
- On the stationary clinostat, gravity acts from one direction, so auxin gathers on the lower side of root and shoot, and each bends
- On the rotating clinostat, each side of the root and shoot faces downwards in turn, so the effect of gravity is spread equally around them and no side receives more auxin than another
- Therefore geotropism is caused by gravity acting from one side
Why the seedlings are kept in darkness. If light came from one direction, phototropism would also bend the shoot, and the result could not be traced to gravity alone. Darkness removes that second factor.
Worked example — predicting a variation. A seedling is fixed vertically on a clinostat, root pointing down, and rotated about a vertical axis. Will its root and shoot bend?
No. Gravity still acts downwards along the length of the root and shoot, exactly as it would for a seedling growing normally, and rotation about a vertical axis does not change which side faces down. The root keeps growing down and the shoot up. The clinostat cancels geotropism only when it turns the plant about a horizontal axis.
An everyday example. Seeds scattered by hand into a field land every which way, yet the young plants all end up with roots going down and shoots going up. Geotropism corrects each seedling's direction, which is exactly what the stationary clinostat shows happening in a single seedling.
The boundary case — the clinostat does not remove gravity. Gravity still acts on the rotating seedling at every moment. The clinostat only stops it from acting on the same side for long enough to cause bending — which is why the result proves that it is the direction of gravity, not gravity itself, that steers growth.
Exam tip
What earns full marks on plant hormones and tropisms?
Define each term precisely, give two or three effects for every hormone, name each tropism with its stimulus, direction and example, and describe the clinostat experiment with its control and conditions.
- Define plant growth regulators: produced in small amounts, transported, regulate growth
- Give auxin effects: cell elongation, phototropism, apical dominance, rooting of cuttings, preventing fruit drop, seedless fruits
- Give gibberellin effects: stem elongation, germination, larger fruits, bolting
- Give cytokinin effects: cell division, delaying leaf ageing, side-bud growth
- Give abscisic acid effects: closing stomata, dormancy, inhibiting germination
- Give ethylene effects: fruit ripening, leaf and fruit fall
- Define tropism as directional growth in response to a stimulus from one direction
- For each tropism, name the stimulus, say positive or negative, and give an example
- Explain phototropism with auxin on the shaded side
- For the clinostat, state horizontal placement, rotating versus stationary, darkness, and the inference
The misconception to name. Plants do not bend towards light because the lit side grows faster. It is the shaded side, with more auxin, that elongates more, pushing the tip towards the light. Reversing the sides is the most common error in phototropism answers.
A second trap. Calling the folding of Mimosa leaves a tropism. It is a nastic movement — non-directional and not caused by growth.
- Define plant growth regulators: produced in small amounts, transported, regulate growth
- Give auxin effects: cell elongation, phototropism, apical dominance, rooting of cuttings, preventing fruit drop, seedless fruits
- Give gibberellin effects: stem elongation, germination, larger fruits, bolting
- Give cytokinin effects: cell division, delaying leaf ageing, side-bud growth
- Give abscisic acid effects: closing stomata, dormancy, inhibiting germination
- Give ethylene effects: fruit ripening, leaf and fruit fall
- Define tropism as directional growth in response to a stimulus from one direction
- For each tropism, name the stimulus, say positive or negative, and give an example
- Explain phototropism with auxin on the shaded side
- For the clinostat, state horizontal placement, rotating versus stationary, darkness, and the inference
The misconception to name. Plants do not bend towards light because the lit side grows faster. It is the shaded side, with more auxin, that elongates more, pushing the tip towards the light. Reversing the sides is the most common error in phototropism answers.
A second trap. Calling the folding of Mimosa leaves a tropism. It is a nastic movement — non-directional and not caused by growth.
Did you know
Why does pruning a tea bush make it grow bushier instead of taller?
Travel through the tea gardens of Assam, Darjeeling or the Nilgiris and the tea plants stand in neat, flat-topped rows at about waist height, each one dense and bushy. Left alone, a tea plant would grow into a small tree. The workers keep it low and bushy by regularly plucking and pruning the tips — and the reason this works is apical dominance, controlled by auxin.
How apical dominance works.
- The growing tip of a shoot produces auxin
- The auxin moves down the stem and suppresses the side buds below it
- So the main shoot grows upwards strongly, while the side buds stay dormant
What pruning does.
- Cutting off the tip removes the main source of auxin
- The side buds are released from suppression and begin to grow
- Several new shoots develop where there was one, and each new tip is itself pruned in turn
The result is a broad, low bush with many young shoots — exactly what tea growers want, because the tender young leaves and buds at the tips are what is plucked to make tea.
Gardeners use the same trick everywhere.
- Pinching out the tips of tulsi, mint and coriander makes the plants branch and produce more leaves
- Trimming a hedge turns a few upright stems into a dense green wall
- Pruning rose bushes encourages many flowering side branches
Cytokinins play the opposite role. They promote side-bud growth, so the shape of any plant depends on the balance between auxin from the tips and cytokinin in the side buds. Pruning shifts that balance in favour of branching.
So a neatly trimmed tea garden is a landscape shaped by plant hormones — every flat-topped bush a demonstration of what happens when the auxin from a shoot tip is taken away.
How apical dominance works.
- The growing tip of a shoot produces auxin
- The auxin moves down the stem and suppresses the side buds below it
- So the main shoot grows upwards strongly, while the side buds stay dormant
What pruning does.
- Cutting off the tip removes the main source of auxin
- The side buds are released from suppression and begin to grow
- Several new shoots develop where there was one, and each new tip is itself pruned in turn
The result is a broad, low bush with many young shoots — exactly what tea growers want, because the tender young leaves and buds at the tips are what is plucked to make tea.
Gardeners use the same trick everywhere.
- Pinching out the tips of tulsi, mint and coriander makes the plants branch and produce more leaves
- Trimming a hedge turns a few upright stems into a dense green wall
- Pruning rose bushes encourages many flowering side branches
Cytokinins play the opposite role. They promote side-bud growth, so the shape of any plant depends on the balance between auxin from the tips and cytokinin in the side buds. Pruning shifts that balance in favour of branching.
So a neatly trimmed tea garden is a landscape shaped by plant hormones — every flat-topped bush a demonstration of what happens when the auxin from a shoot tip is taken away.
Exam relevance
How are plant hormones and tropisms tested in NEET Biology?
This is foundation work for Class 11 Plant Growth and Development in NEET Biology, with links to Morphology of Flowering Plants and Class 12 Sexual Reproduction in Flowering Plants.
Where the hormones lead. Class 11 Plant Growth and Development covers the five major groups of plant growth regulators in detail — auxins, gibberellins, cytokinins, ethylene and abscisic acid — with their physiological effects and agricultural uses. Match-the-column questions pairing a hormone with its effect are among the most common NEET items in plant physiology, and the effects listed in this lesson are exactly the ones matched.
Where specific effects lead. NEET frequently asks about apical dominance, bolting, parthenocarpy (seedless fruit), delay of senescence, stomatal closure under stress, fruit ripening and breaking of dormancy. Remembering which hormone causes each — and which pairs have opposite effects — is the skill tested.
Where the antagonism leads. The chapter describes abscisic acid as working against gibberellins in dormancy and germination, and auxin and cytokinin together controlling root and shoot formation in tissue culture. Assertion-reason questions often test these opposing actions.
Where tendrils lead. Class 11 Morphology of Flowering Plants describes tendrils as modified stems or leaves, and their coiling as a response to contact.
Where chemotropism leads. Class 12 Sexual Reproduction in Flowering Plants describes the pollen tube growing through the style to the ovule — the chemotropic growth introduced here.
Question types to expect. At this level: definitions, hormone effects, tropisms with examples, and the clinostat experiment. In NEET: hormone-effect matching, agricultural uses of hormones, antagonistic pairs, and statement-based questions on growth responses.
The single trap that costs marks. Mixing up auxin and cytokinin in apical dominance. Auxin from the tip suppresses side buds; cytokinin promotes their growth — and options that swap them are common.
A second trap. Confusing abscisic acid and ethylene. Abscisic acid closes stomata and causes dormancy; ethylene ripens fruits — though both can promote abscission, their main roles differ.
Board versus competitive emphasis. The ICSE paper marks definitions, lists of effects and a described experiment; NEET marks precise hormone-effect pairs and uses. The transferable habit is organising the hormones by what they promote and what they oppose — a table in the mind that answers most questions on this chapter.
Where the hormones lead. Class 11 Plant Growth and Development covers the five major groups of plant growth regulators in detail — auxins, gibberellins, cytokinins, ethylene and abscisic acid — with their physiological effects and agricultural uses. Match-the-column questions pairing a hormone with its effect are among the most common NEET items in plant physiology, and the effects listed in this lesson are exactly the ones matched.
Where specific effects lead. NEET frequently asks about apical dominance, bolting, parthenocarpy (seedless fruit), delay of senescence, stomatal closure under stress, fruit ripening and breaking of dormancy. Remembering which hormone causes each — and which pairs have opposite effects — is the skill tested.
Where the antagonism leads. The chapter describes abscisic acid as working against gibberellins in dormancy and germination, and auxin and cytokinin together controlling root and shoot formation in tissue culture. Assertion-reason questions often test these opposing actions.
Where tendrils lead. Class 11 Morphology of Flowering Plants describes tendrils as modified stems or leaves, and their coiling as a response to contact.
Where chemotropism leads. Class 12 Sexual Reproduction in Flowering Plants describes the pollen tube growing through the style to the ovule — the chemotropic growth introduced here.
Question types to expect. At this level: definitions, hormone effects, tropisms with examples, and the clinostat experiment. In NEET: hormone-effect matching, agricultural uses of hormones, antagonistic pairs, and statement-based questions on growth responses.
The single trap that costs marks. Mixing up auxin and cytokinin in apical dominance. Auxin from the tip suppresses side buds; cytokinin promotes their growth — and options that swap them are common.
A second trap. Confusing abscisic acid and ethylene. Abscisic acid closes stomata and causes dormancy; ethylene ripens fruits — though both can promote abscission, their main roles differ.
Board versus competitive emphasis. The ICSE paper marks definitions, lists of effects and a described experiment; NEET marks precise hormone-effect pairs and uses. The transferable habit is organising the hormones by what they promote and what they oppose — a table in the mind that answers most questions on this chapter.
Key takeaways
What must you be able to do from this part?
Five hormones, five tropisms and one experiment.
- Plant growth regulators: made in small amounts, transported, regulate growth and development
- Auxins: cell elongation, phototropism and geotropism, apical dominance, rooting of cuttings, prevent fruit drop, seedless fruits, synthetic weedkillers
- Gibberellins: stem elongation, break dormancy and promote germination, larger fruits, bolting
- Cytokinins: cell division, delay leaf ageing, promote side buds, break dormancy
- Abscisic acid: closes stomata under water stress, induces dormancy, inhibits germination
- Ethylene: gaseous; ripens fruits, causes leaf and fruit fall, speeds ageing
- Opposing pairs: gibberellin and abscisic acid in germination; auxin and cytokinin in side-bud growth
- Tropism: directional growth in response to a stimulus from one direction — positive towards, negative away
- Phototropism: shoots towards light, roots away; auxin collects on the shaded side
- Geotropism: roots down, shoots up; auxin on the lower side speeds shoot growth but slows root growth
- Hydrotropism: roots towards water
- Thigmotropism: tendrils of pea, bitter gourd and cucumber coil round supports
- Chemotropism: pollen tube grows towards the ovule
- Mimosa leaf folding is a nastic movement, not a tropism
- Clinostat: horizontal seedlings in darkness; stationary one bends root down and shoot up; rotating one grows straight — one-sided gravity causes geotropism
The sharpest self-test is five garden jobs. Explain which hormone or tropism is at work when a cutting is dipped in powder, a hedge is trimmed, green mangoes ripen beside a ripe banana, a gourd vine climbs a string and a plant leans towards a window.
- Plant growth regulators: made in small amounts, transported, regulate growth and development
- Auxins: cell elongation, phototropism and geotropism, apical dominance, rooting of cuttings, prevent fruit drop, seedless fruits, synthetic weedkillers
- Gibberellins: stem elongation, break dormancy and promote germination, larger fruits, bolting
- Cytokinins: cell division, delay leaf ageing, promote side buds, break dormancy
- Abscisic acid: closes stomata under water stress, induces dormancy, inhibits germination
- Ethylene: gaseous; ripens fruits, causes leaf and fruit fall, speeds ageing
- Opposing pairs: gibberellin and abscisic acid in germination; auxin and cytokinin in side-bud growth
- Tropism: directional growth in response to a stimulus from one direction — positive towards, negative away
- Phototropism: shoots towards light, roots away; auxin collects on the shaded side
- Geotropism: roots down, shoots up; auxin on the lower side speeds shoot growth but slows root growth
- Hydrotropism: roots towards water
- Thigmotropism: tendrils of pea, bitter gourd and cucumber coil round supports
- Chemotropism: pollen tube grows towards the ovule
- Mimosa leaf folding is a nastic movement, not a tropism
- Clinostat: horizontal seedlings in darkness; stationary one bends root down and shoot up; rotating one grows straight — one-sided gravity causes geotropism
The sharpest self-test is five garden jobs. Explain which hormone or tropism is at work when a cutting is dipped in powder, a hedge is trimmed, green mangoes ripen beside a ripe banana, a gourd vine climbs a string and a plant leans towards a window.