Plant hormones give exams a small, fixed set of questions. Which hormone makes plants grow taller? Which one ripens fruit? Which one helps a plant survive drought? The answers never change. This post explains the five plant hormones in simple words, tells you what each one does, and lists the tropisms and reproduction terms that exams ask about. At the end there are 15 previous year questions.
Key Points (Quick Revision)
- Auxin makes stems grow longer. It is also called the growth hormone.
- Gibberellin makes plants tall and helps seeds sprout.
- Cytokinin helps cells divide and delays ageing of leaves.
- Ethylene is a gas. It ripens fruit.
- Abscisic acid (ABA) is the stress hormone. It closes leaf pores in dry weather.
1. What plant hormones are
Plants cannot move or speak. But they still react to light, water and touch. They do this with the help of chemicals called plant hormones or phytohormones.
These chemicals are made in very small amounts. They travel from one part of the plant to another. There they tell the plant what to do - grow taller, drop a leaf, ripen a fruit, or open and close pores.
There are five main plant hormones. Exams ask about all five. Learn each one with just one main job attached to it. That is enough for almost every question.
2. The five hormones and their jobs
| Hormone | Main job | Also remember |
|---|---|---|
| Auxin | Makes the stem grow longer | Discovered from the work of Charles Darwin; used in weedkillers |
| Gibberellin | Makes plants tall; helps seeds sprout | Found first in a fungus called Gibberella |
| Cytokinin | Helps cell division | Keeps leaves green for longer |
| Ethylene | Ripens fruit | The only hormone that is a gas |
| Abscisic acid | Works during stress like drought | Closes the pores of leaves; also called the stress hormone |
Two of these are asked more than the rest.
Ethylene is asked because it is a gas, and because farmers use it. A ripe fruit gives out ethylene. That is why one ripe banana makes the other bananas in the basket ripen faster. Fruit sellers also use it to ripen mangoes and bananas before selling them.
Abscisic acid is asked because it works in the opposite way from the others. Auxin, gibberellin and cytokinin help the plant grow. Abscisic acid slows growth down. It shuts the tiny pores on leaves called stomata so the plant does not lose water. It also makes leaves and fruit fall off.
3. Tropism - how plants move towards things
Tropism means the movement of a plant part in the direction of something outside it. The movement is slow, but it is real. Auxin is the hormone behind most of it.
| Type | Plant moves towards | Example |
|---|---|---|
| Phototropism | Light | A shoot bends towards a window |
| Geotropism | Gravity | Roots grow downwards, shoots grow upwards |
| Hydrotropism | Water | Roots grow towards a water source |
| Thigmotropism | Touch | A pea plant's tendril curls around a stick |
| Chemotropism | Chemicals | A pollen tube grows towards the ovule |
Here is the reason a shoot bends towards light, in simple words. Auxin moves away from the bright side and collects on the shady side. That side then grows faster and becomes longer. So the shoot bends towards the light. This is a favourite question.
In roots it works the other way. Extra auxin slows root growth, so the lower side of a root grows less and the root turns downwards.
One more term goes with this topic. The movement of a Touch-me-not plant, whose leaves fold when you touch them, is not tropism, because the plant does not move in the direction of the touch. It is a simple response to touch, and it is fast.
4. How plants make new plants
Plants make new plants in two ways.
Asexual reproduction needs only one parent. There are no seeds and no flowers. The new plant is an exact copy of the parent.
- Vegetative propagation - a new plant grows from a root, stem or leaf. Potato grows from its eyes, sugarcane from stem cuttings, and Bryophyllum from the edge of a leaf.
- Budding - a small bud grows on the parent and then separates. Yeast does this.
- Spore formation - fungi like bread mould spread through tiny spores.
- Fragmentation - the body breaks into pieces and each piece becomes a new plant, as in Spirogyra.
- Tissue culture - a few cells are grown in a lab to make many plants. It is used for orchids and banana.
Sexual reproduction needs a flower. Pollen from the male part joins the egg in the female part, and a seed is formed.
The parts of a flower are asked directly, so learn them once:
| Part | What it does |
|---|---|
| Stamen | The male part. It makes pollen. |
| Pistil or carpel | The female part. It holds the ovules. |
| Petals | Coloured leaves that attract insects and birds |
| Sepals | Green cover that protects the bud |
| Ovary | Becomes the fruit after fertilisation |
| Ovule | Becomes the seed after fertilisation |
Two words often get mixed up. Pollination is only the movement of pollen to the female part. Fertilisation is what happens later, when the male and female cells join. Pollination comes first, fertilisation after.
Pollination happens with the help of wind, water, insects, birds and even bats. If the pollen lands on the same flower or the same plant, it is called self-pollination. If it goes to a different plant of the same kind, it is cross-pollination.
5. Previous year questions
- Which hormone is called the growth hormone of plants? — Auxin
- Which plant hormone makes stems grow longer? — Auxin
- Which hormone helps seeds to sprout? — Gibberellin
- Which plant hormone is a gas? — Ethylene
- Which hormone is used to ripen fruit? — Ethylene
- Which hormone helps in cell division in plants? — Cytokinin
- Which plant hormone is called the stress hormone? — Abscisic acid
- Which hormone closes the stomata of leaves? — Abscisic acid
- The bending of a shoot towards light is called what? — Phototropism
- The growth of roots towards gravity is called what? — Geotropism
- The growth of roots towards water is called what? — Hydrotropism
- Potato is grown by which method? — Vegetative propagation
- Which part of a flower becomes the fruit? — Ovary
- Which part of a flower becomes the seed? — Ovule
- What is the male part of a flower called? — Stamen
6. How to revise this topic
Learn the five hormones with one job each. Say them out loud: auxin grows the stem, gibberellin makes it tall, cytokinin divides cells, ethylene ripens fruit, abscisic acid handles stress. Most questions only need this one line.
Next, keep the two odd ones ready. Ethylene is the only gas. Abscisic acid is the only one that slows growth down. Options in this chapter usually test these two.
For tropism, learn the word ending and its meaning: photo is light, geo is gravity, hydro is water, thigmo is touch, chemo is chemical. Then remember one example for each.
For reproduction, keep two pairs clear. Ovary becomes fruit, ovule becomes seed. And pollination comes first, fertilisation comes after. These four facts cover most of the questions from this part.
7. Frequently Asked Questions
Which plant hormone ripens fruit?
Ethylene. It is the only plant hormone that is a gas. A ripe fruit gives out ethylene, which is why one ripe banana makes the rest of the bunch ripen faster. Traders also use it to ripen mangoes and bananas.
Why does a plant shoot bend towards light?
Because auxin moves to the shady side of the shoot. That side grows faster and becomes longer, so the shoot bends towards the light. This movement is called phototropism.
Which hormone is the stress hormone in plants?
Abscisic acid, often written as ABA. In dry weather it closes the tiny pores on leaves called stomata so the plant does not lose water. It also makes leaves and fruit fall off.
What is the difference between pollination and fertilisation?
Pollination is only the movement of pollen from the male part of a flower to the female part. Fertilisation happens after that, when the male and female cells join to form a seed. Pollination comes first.
Which part of a flower becomes the fruit and which becomes the seed?
The ovary becomes the fruit and the ovule becomes the seed. Exams swap these two in options very often, so learn them as one pair.
For more General Science topics in simple language, see our Photosynthesis and Plant Nutrition and Cell Structure and Functions posts, or browse the General Science section. Preparing for a specific exam? Start with the SSC CGL guide or join the 100 Hour GS Course.