How do plants respond without a nervous system?
Plants have no nerves and no muscles. They respond to stimuli in two ways:
- Movement not linked to growth: fast, happens in seconds, like the folding of touch-me-not leaves.
- Movement due to growth: slow, takes hours or days, and has a direction, like a shoot bending towards light.
Plant cells pass information from cell to cell electrically and chemically, but there are no special tissues for carrying it.
Immediate response: the touch-me-not plant
In the sensitive plant (Mimosa pudica, chhui-mui), touch at one point is sensed and the message spreads to other leaflets. At the leaf base, cells quickly move water out. They lose their stiffness (turgor), change shape, and the leaflets fold and droop. There is no growth here, and the direction of folding does not depend on where you touched. Such non-directional movements are called nastic movements.
Tropic (growth) movements
A tropism is a directional growth movement in response to a stimulus. Growing towards the stimulus is positive; growing away is negative.
Phototropism (light)
Shoots bend towards light (positive); roots bend away (negative).
Geotropism (gravity)
Roots grow downwards (positive); shoots grow upwards (negative).
Hydrotropism (water)
Roots grow towards moist soil or a water source.
Chemotropism (chemicals)
The pollen tube grows towards the ovule, guided by chemicals, during fertilisation.
Tendrils of climbers such as pea curl around a support: the part away from the support grows faster (a touch response, thigmotropism).
Plant hormones (phytohormones)
- Auxin: made at the shoot tip; helps cells grow longer. When light comes from one side, auxin moves to the shaded side, cells there elongate more, and the shoot bends towards light.
- Gibberellins: help the stem grow taller; also used to enlarge fruits and break seed dormancy.
- Cytokinins: promote cell division; found in high amounts where cells divide fast, such as fruits and seeds.
- Abscisic acid (ABA): a growth-stopping hormone; causes wilting and falling of leaves and helps plants survive stress such as drought.
The first three promote growth; abscisic acid inhibits it. Ethylene (fruit ripening) is another plant hormone you may meet later.
Board exam tip: expect "name the hormone" questions, the phototropism experiment and the difference between tropic and nastic movements.
Key formulas and definitions
- Tropism = directional growth towards (+) or away from (−) a stimulus
- Phototropism: shoot + , root −
- Geotropism: root + , shoot −
- Hydrotropism: root + towards water
- Chemotropism: pollen tube + towards ovule
- Nastic movement: not directional, not growth (touch-me-not)
- Auxin → cell elongation, bending to light
- Gibberellin → stem growth; Cytokinin → cell division; Abscisic acid → stops growth, wilting
Worked examples
1. A plant kept in a room with one window bends towards the window. Explain.
Light comes from one side. Auxin at the shoot tip moves to the darker side, cells there grow longer than on the lit side, so the shoot curves towards the window. This is positive phototropism.
2. A potted plant is laid on its side for two days. What happens to the root and the shoot?
The root tip bends to grow downwards (positive geotropism) and the shoot tip bends to grow upwards (negative geotropism).
3. How is the folding of touch-me-not leaves different from a shoot bending to light?
Touch-me-not folding is fast, not due to growth and not directional (a nastic movement caused by water leaving cells). Bending to light is slow, due to unequal growth and directional (a tropism).
4. Which hormone would you choose to make a dwarf pea plant grow taller, and which to make a plant shed old leaves during drought?
Gibberellin to increase stem length; abscisic acid to cause wilting and fall of leaves under stress.
5. Why does a pollen tube reach the ovule even though the ovule is deep inside the pistil?
The ovule releases chemicals. The pollen tube grows towards them (chemotropism) and so is guided down to the ovule.
Common mistakes
- Calling touch-me-not folding a tropism. It is not growth and not directional; it is a nastic movement.
- Saying auxin collects on the lit side. It moves to the shaded side, which grows more.
- Mixing up positive and negative: roots are positively geotropic but negatively phototropic.
- Saying abscisic acid promotes growth. It is the growth inhibitor.