What are tropisms?
A tropism is a growth response in which a plant grows toward (positive) or away from (negative) a directional stimulus. Because plants respond by growing, tropisms are slow — they take hours or days — but they let a plant place its leaves in the light and its roots in moist soil.
Phototropism
Phototropism is growth in response to light. Shoots are positively phototropic: they bend toward light, which maximises photosynthesis. Light-sensitive proteins called phototropins in the shoot tip detect the direction of blue light.
How auxin works
The plant hormone auxin (indole-3-acetic acid) is made in the shoot tip and moves down the stem. When light comes from one side, auxin is transported toward the shaded side. There, it stimulates cells to elongate more. Faster growth on the shaded side curves the shoot toward the light. In roots, high auxin concentrations have the opposite effect and slow cell elongation.
Gravitropism
Gravitropism (also called geotropism) is growth in response to gravity. Shoots arenegatively gravitropic (they grow up) and roots are positively gravitropic (they grow down). In the root cap, special cells contain dense starch-filled grains called statoliths, which settle to the lowest side of the cell. This tells the root which way is down and leads to auxin being redistributed to the lower side.
Classic experiments
- Charles and Francis Darwin (1880) showed that grass seedlings bent toward light only if their tips were uncovered — the tip detects the light.
- Frits Went (1928) collected a growth substance from shoot tips in agar blocks; placing a block on one side of a cut shoot made it bend. That substance was later identified as auxin.
- Clinostats — slowly rotating holders invented in the 1800s — showed that plants grow without bending when gravity’s direction is constantly changing.
Other tropisms
- Thigmotropism: growth in response to touch, such as tendrils coiling around a support.
- Hydrotropism: roots growing toward moisture.
- Chemotropism: growth toward chemicals, such as pollen tubes growing toward the ovule.
Interesting facts
- Roots sense gravity with statoliths: dense, starch-filled grains in cells of the root cap that settle to the lowest side of the cell, a bit like sand in a tilted jar.
- Climbing plants such as peas and cucumbers coil their tendrils around a support after touching it. This touch response is called thigmotropism.
- Roots also grow towards moisture (hydrotropism), which helps them find water in dry soil.
- Young sunflower heads follow the Sun across the sky during the day and turn back east at night, a growth movement that stops once the flowers open.
- Experiments on space stations, where gravity is almost absent, help scientists separate the effects of light and gravity on how plants grow.
Key terms
- Stimulus
- A change in the environment that an organism can detect.
- Auxin
- A plant hormone that controls cell elongation and is involved in tropisms.
- Statolith
- A dense starch grain that settles under gravity and helps plants sense direction.
- Clinostat
- A device that slowly rotates a plant so that gravity has no consistent direction relative to it.
Real-world applications
- Horticulture: synthetic auxins are used in rooting powders to help cuttings grow roots.
- Agriculture: some synthetic auxins act as selective weedkillers, causing broad-leaved weeds to grow uncontrollably.
- Houseplants: rotating pots on a windowsill keeps plants growing straight.
- Space biology: experiments on the International Space Station study how plants grow without gravity — important for future long missions.
Common misconceptions
- “The plant bends because the lit side grows faster.” It is the shaded side that grows faster.
- “Plants move toward light like animals.” They grow toward it; the bending comes from uneven cell elongation.
- “Tropisms are instant.” They take hours to days because they depend on growth.
Key takeaways
- Shoots grow toward light and away from gravity; roots grow toward gravity.
- Auxin moves to the shaded side of a shoot and makes those cells elongate more.
- Statoliths in root caps sense the direction of gravity.
- Only growing regions bend, so a tilted plant curves back rather than straightening.
Model note: growth is drawn as simple segments that turn toward a weighted target direction; real seedlings respond more gradually and also show nutation (small circling movements).