Tropic Responses
Senior 1 · Biology
Tropic Responses
Lesson 1 of 5
20 min
In a Senior One classroom in Huye, a class plants bean seeds in old tins and leaves them on the windowsill. A week later every seedling leans towards the glass, as if looking outside. The teacher turns the tins round so that the seedlings lean into the room. Two days later they have bent back towards the window.
A bean plant has no eyes, no nerves and no muscles, and it cannot walk to a sunnier place. Yet it clearly detected where the light was coming from and changed its shape. In Unit 1 you met sensitivity as one of the seven characteristics of living things. This unit is about how plants show it.
You need two tins of damp soil, each with five bean seedlings about 3 cm tall.
What you will see. In tin A the shoots curve towards the window, and after the tin is turned they curve back towards it. In tin B, which had no light at all, the shoot tips turn to point upwards, and if you gently tip out the soil you will find the root tips turned downwards.
Tin A shows a response to light. Tin B cannot be responding to light, because it was dark: it shows a response to gravity.
Warning
Safety. Seed bought for planting is often coated with a fungicide or insecticide, which may be coloured pink or blue. Never eat or taste seeds used in experiments, and wash your hands with soap after handling seeds and soil.
Definition
Stimulus (plural stimuli): a change in the surroundings that an organism can detect, for example light coming from one side, gravity, water, a chemical or a touch.
Response: the change in an organism, or in part of it, caused by a stimulus. Example: the shoot in tin A bending towards the window.
Sensitivity (also called irritability): the ability to detect stimuli and respond to them.
Definition
Tropism (a tropic response): a growth movement of part of a plant in which the direction of the response is decided by the direction of the stimulus.
Positive tropism: growth towards the stimulus (a shoot growing towards light). Negative tropism: growth away from the stimulus (a shoot growing upwards, away from the pull of gravity).
The name of a tropism is the stimulus + "tropism": photo (light), geo (the Earth, meaning gravity), hydro (water), chemo (chemicals), thigmo (touch). Geotropism is also called gravitropism.
Figure 1: A single bean seedling responds to three stimuli at once: light, gravity and water. Each response is growth in a direction set by the stimulus.
The same organ can respond to more than one stimulus. The bean shoot in Figure 1 is positively phototropic (it grows towards light) and negatively geotropic (it grows away from the pull of gravity). The main root is positively geotropic, and its side root is positively hydrotropic. All five types of tropism you will study are summarised below.
| Tropism | Stimulus | Response | Example |
|---|---|---|---|
| Phototropism | light from one direction | shoots grow towards light (positive) | bean seedlings bending towards a window |
| Geotropism (gravitropism) | gravity | roots grow down (positive); shoots grow up (negative) | a maize grain planted upside down still sends its root downwards |
| Hydrotropism | water in the soil | roots grow towards wetter soil (positive) | tree roots growing towards a leaking water pipe |
| Chemotropism | chemicals | pollen tube grows towards the ovule (positive) | the pollen tube growing down the style of a bean flower |
| Thigmotropism | touch (contact with a solid object) | tendril or stem coils round a support (positive) | a passion fruit tendril coiling round a wire |
When you touch a hot saucepan, nerves carry a message to your arm muscles and your hand moves away in a fraction of a second. A plant has no nerves or muscles. It detects light with light-sensitive proteins in its shoot tip, and gravity with heavy grains of starch that settle to the bottom of special cells in the root cap. The message is then carried by a plant hormone, a chemical that makes cells on one side of the organ grow more than cells on the other side, so the organ bends.
| Feature | Animal response | Plant tropism |
|---|---|---|
| How the message travels | nerves (and hormones) | plant hormones only |
| What causes the movement | muscles contract | uneven growth of cells |
| Speed | fractions of a second | hours to days |
| Can it be undone? | yes: the arm can move back at once | only by new growth in the other direction |
Because tropisms are growth, they happen only in the parts of a plant that are still growing: the region just behind a shoot tip or a root tip, and young tendrils. An old, woody stem cannot bend towards light.
Warning
Common mistakes. (1) Writing that the plant bends "because it wants light". A plant has no brain and no wishes; write what happens: cells on the shaded side grow longer, so the shoot bends towards the light. (2) Calling every plant movement a tropism. The leaves of the sensitive plant (Mimosa pudica, "touch-me-not") fold up whichever side you touch them from; the direction of the stimulus does not matter and no growth is involved, so it is a nastic response, not a tropism. (3) Forgetting "positive" or "negative". "The root shows geotropism" is incomplete: say "positive geotropism".
Worked example 1: naming the tropisms
A maize grain germinates in a glass jar of damp sawdust. When the root is 2 cm long, the jar is turned upside down. Two days later the root tip has bent round to point downwards and the young shoot has bent round to point upwards. Name each response.
Answer: the root shows positive geotropism and the shoot shows negative geotropism.
Worked example 2: tropism or not?
Which of these are tropisms? (a) A passion fruit tendril coils round a wire it touches. (b) The leaflets of a Mimosa fold when touched from any side. (c) The roots of a eucalyptus tree grow into a cracked, leaking drain. (d) A bean seed swells when it is soaked in water.
Ask two questions of each: Is it growth of part of the plant? and Does the direction of the stimulus decide the direction of the response?
Answer: (a) and (c).
Plant Hormones: Tropisms (FuseSchool). Watch for which part of the shoot detects light, which part bends, and the name of the hormone involved; you will study it in the next lesson.
Tip
Memory hook. The first half of the name tells you the stimulus: photo = light (as in photograph), geo = Earth (as in geography), hydro = water (as in hydroelectric), chemo = chemical, thigmo = touch. Then add positive (towards) or negative (away from). "Shoots seek the sun; roots go down and find water."
Practice: plant responses to stimuli
Key points
Sources and credits
Written by Ganzaa: the observation, Figure 1, both tables, the worked examples and the practice questions.
