Tropic Responses
Senior 1 · Biology
Tropic Responses
Lesson 5 of 5
25 min
Imagine a maize grain whose root could not sense gravity. Planted upside down, its root would grow up into the air and dry out, and its shoot would push down into the dark soil and never reach the light. The seedling would die within days. Every tropism you have studied solves a problem like this one. A plant cannot walk to water, light or a mate, so it grows towards them instead.
Tropisms matter to people too. Rwanda's farmers grow climbing beans on stakes and passion fruit on wire trellises precisely because these plants climb by thigmotropism.
Definition
Adaptation: a feature of an organism that helps it survive and reproduce in its surroundings. Tropisms are adaptations: each one places an organ where it can do its job.
Staking: pushing a pole (stake) into the ground beside a climbing plant so that its stem can twine up it. A trellis is a frame of poles and wires or strings that climbing plants grow along.
Figure 1: The steps shared by every tropism, shown for phototropism. The last box, the benefit, is why tropisms are important.
Whatever the stimulus, a tropism follows the same steps: the stimulus is detected by a growing tip, a messenger (usually auxin) is shared unevenly, the two sides of the organ grow unequally, and the organ bends. The result helps the plant survive. The table shows the benefit of each tropism, and how people make use of it.
| Tropism | Importance to the plant | How people use it |
|---|---|---|
| Positive phototropism of shoots | leaves are held where they receive the most light for photosynthesis, so the plant makes more food | nursery workers turn pots so seedlings grow straight; seedlings under shade grow towards gaps |
| Positive geotropism of roots | roots grow down into the soil, anchoring the plant and reaching water and mineral salts | seeds can be sown any way up |
| Negative geotropism of shoots | a seedling underground grows up towards the surface and the light, even in darkness | crops flattened by wind or rain lift their stems up again |
| Positive hydrotropism of roots | roots find water in dry soil, so the plant survives the dry season | deep watering of young trees gives them deep roots |
| Positive chemotropism of pollen tubes | the tube reaches an ovule, so fertilisation happens and seeds form | seed and fruit yield depend on it |
| Positive thigmotropism of tendrils and twining stems | a weak stem climbs a support to reach light without building a thick trunk | climbing beans on stakes; passion fruit on trellises |
Beans are one of Rwanda's most important foods. Since the 1980s, researchers now at the Rwanda Agriculture Board (RAB) have worked with farmers to grow climbing beans in place of bush beans in the highlands. A climbing bean's stem twines up a stake about two metres tall, so the plant uses height instead of ground area. Researchers at World Agroforestry report that climbing beans occupy less space and yield two to three times more than bush beans, which matters in a country where farms are small.
The stakes must be in place early, while the stems are still young and growing: only growing stems can twine. Plants staked late sprawl on the ground, where the pods touching damp soil can rot. Because good stakes are expensive, many farmers now grow fast-growing trees and shrubs on their land to provide them.
Worked example 1: interpreting an investigation of seed position
A class planted 20 maize grains in each of three positions and counted the seedlings that reached the surface. The figures are illustrative, made up for this lesson.
| Position of grain | Seedlings that emerged | Mean days to emerge |
|---|---|---|
| embryo facing up | 19 | 5 |
| on its side | 18 | 6 |
| embryo facing down | 17 | 7 |
Conclusion: geotropism lets seeds planted in any position survive, which is why farmers do not need to place each seed the right way up. The small differences between 85% and 95% could be chance; more grains would be needed to be sure.
Worked example 2: evaluating a student's plan
Keza wants to show that bean roots grow towards water. She plans to put damp cotton wool on the left of one box and dry cotton wool on the right, pin one seedling in it, and leave it on the windowsill for three days. Evaluate her plan.
Improved plan: five or more seedlings in each box, a damp-both-sides control, both boxes in the same dark place, and results recorded as the number and percentage of roots that curve towards the damp side.
Warning
Common mistakes. (1) Saying a tropism happens "so that the plant can get light", as if the plant plans it. Say what the response does: "the shoot grows towards light, so its leaves receive more light for photosynthesis". (2) Testing two stimuli at once, for example light and gravity together, so that no conclusion can be drawn. (3) Forgetting that tropisms need growth: an old tendril or a mature stem will not respond.
Staking climbing beans (Access Agriculture, an organisation that makes training videos for African farmers). Watch when and how the stakes are put in, and how the young bean stems twine up them.
Tip
Exam technique. For "explain the importance of" a tropism, use three linked steps: the organ grows towards or away from the stimulus → so it reaches light, water, soil, a support or an ovule → so the plant can photosynthesise, absorb water, stay anchored, climb or make seeds.
Practice: importance of tropisms
Key points
Further reading for this unit
Sources and credits
Written by Ganzaa: Figure 1, the importance table, the worked examples (illustrative figures), and the practice questions.
