Tropic Responses
Senior 1 · Biology
Tropic Responses
Lesson 2 of 5
25 min
In the last lesson the bean seedlings on the windowsill bent towards the glass. But which part of the seedling detects the light, and which part does the bending? In 1880 Charles Darwin and his son Francis set out to answer exactly this question, using the young shoots of canary grass. Their experiments, and the ones that followed, are still the clearest way to understand how a plant responds to light.
Definition
Phototropism: a growth movement of a plant part in response to light coming from one direction. Shoots show positive phototropism.
Coleoptile: the protective sheath that covers the first leaves of a grass or cereal seedling (maize, wheat, sorghum) as it pushes up through the soil. It is a simple, straight shoot, which makes it ideal for experiments.
Etiolation: the growth of a plant kept in darkness: a long, thin, pale stem with small yellow leaves.
Aim: to find out how the direction of light affects the growth of shoots.
You need: three shoe boxes, three small pots of bean or maize seedlings of the same age (about 2 cm tall), scissors and a lamp or a sunny window.
Figure 1: Results of Experiment 1. Only the seedlings lit from one side (A) bend. In the dark (B) they grow tall and pale but stay straight; lit evenly from above (C) they grow straight.
Conclusions. Box A shows that shoots bend towards light from one side. Box C, the control, shows that light itself does not cause bending: it is one-sided light that does. Box B shows that seedlings in the dark grow faster in length but become etiolated.
Warning
Safety. Cut the slit with scissors, or with a craft knife on a cutting board, always cutting away from your hand. Do not use a hot lamp close to the box: cardboard can scorch, and heat would add a second stimulus to the experiment.
Darwin grew grass seedlings in the dark, then treated their coleoptiles in different ways and lit them from one side. Figure 2 shows five of the treatments and what happened.
Figure 2: Coleoptile experiments. Covering or removing the tip stops the bending; covering the base does not. So the tip detects the light, but the bending happens lower down.
A later scientist, Peter Boysen-Jensen (1913), cut off coleoptile tips and put them back with a thin layer of jelly (gelatin) in between: the shoots still bent. When he used a thin sheet of mica, which no chemical can pass through, they did not. So the "message" from the tip is a chemical that moves down the shoot. It was later named auxin.
A class repeated Experiment 2 with 20 maize coleoptiles in each group. The table shows their results. These are illustrative figures, made up for this lesson to show the kind of results a class obtains; your own will differ.
| Group | Treatment | Coleoptiles that bent towards the light | Percentage that bent |
|---|---|---|---|
| A | intact (control) | 19 out of 20 | 95% |
| B | tip cut off | 0 out of 20 | ? |
| C | tip in foil cap | 1 out of 20 | ? |
| D | tip in clear cap | 18 out of 20 | ? |
| E | base in foil | 18 out of 20 | ? |
Worked example 1: interpreting the results table
Complete the last column and state what the results show.
Conclusion: light is detected by the tip of the coleoptile. The single "odd" coleoptile in group C (5%) is probably an error, for example a cap that slipped; one result in twenty does not change the conclusion, which is why each group used 20 coleoptiles, not one.
Auxin is a plant hormone made in the tip of the shoot. It moves down into the region just behind the tip, where it makes the young cells elongate (grow longer).
Figure 3: How auxin causes positive phototropism. The shoot bends because its shaded side grows longer than its lit side.
Warning
Common mistakes. (1) "Light kills the auxin, so the lit side stops growing." Both sides keep growing; auxin is moved to the shaded side, which simply grows more. (2) "Auxin is made where the shoot bends." It is made in the tip and travels down to the growing region, which is why a coleoptile without its tip does not bend. (3) Mixing up the sides: the shaded side has more auxin and grows faster.
Worked example 2: predicting from a diagram
In the dark, the tip of a coleoptile is cut off and stood on a small block of jelly (agar) for an hour. The tip is thrown away and the block is placed on the left half of the cut stump. The seedling is kept in the dark. Predict which way the stump will bend, and explain.
Answer: it bends to the right, even in total darkness. This experiment (by Frits Went, 1928) proved that uneven auxin alone, without any light, is enough to cause bending.
Every rainy season, nurseries across Rwanda raise hundreds of thousands of tree seedlings, such as Grevillea, avocado and coffee, in small plastic pots for farmers and for agroforestry. Seedlings raised under a shelter that is open on one side lean towards the open side, and crowded seedlings on the edge of a bed lean outwards. A leaning seedling planted out grows a crooked trunk. Nursery workers prevent this by giving the beds even light, spacing the pots, and moving or turning pots every week or two, so that no side stays in the shade for long.
GCSE Biology: Plant Hormones, Tropisms and Auxins (Cognito). Watch for how auxin moves to the shaded side of a shoot. The video also explains roots, which you will study in the next lesson.
Tip
Memory hook: "Shaded Side Stretches." Auxin moves to the shaded side, the shaded side stretches (its cells elongate), and the shoot bows towards the light. In an exam, always name all three: where the auxin goes, what it does to the cells, and which way the shoot bends.
Practice: phototropism and auxin
Key points
Sources and credits
Written by Ganzaa: the experiments as set out here, Figures 1 to 3, the results table (illustrative figures), the worked examples, the nursery application and the practice questions.
