Phototropism and Geotropism, Grade 12: The 25-Mark Rubric, Auxins and Four Corrections

There is a formally assessed practical in this topic worth 25 marks, and the task does not say how the 25 marks are split.

It gives four instructions: state your hypothesis, plan and conduct the investigation, record your results, write a report. Then a bracketed 25. Every teacher in the country has to invent the rubric, and every teacher invents a different one.

We have written one. It is below, it adds to 25, and it is free. The rest of the page gives the plant hormones, the two tropisms, and four places the printed account is wrong or unusable.

Where this fits in the curriculum

Subject Life Sciences
Grade 12
Term 3
Topic Topic 9, Responding to the environment, plants
Formal assessment Practical task, design an investigation to show phototropism, 25 marks
Marks Our worksheet is 60

The 25-mark rubric the task does not give you

Use this, change it, or hand it to the class before they start. Learners who know where the marks are write far better reports, and that is not cheating, it is teaching.

Section What earns the mark Marks
Hypothesis Written as a testable statement, not a question. Names the independent and dependent variable. Predicts a direction 3
Variables Independent variable named. Dependent variable named. At least two controlled variables named, with how each is kept constant 4
Method Numbered steps another group could follow. A control included. More than one seedling per condition. Enough days allowed for a visible response 6
Results Diagrams or photographs, dated. Labelled. The direction of the light shown. Changes described, not just pictured 5
Conclusion Refers back to the hypothesis in words. States whether it is supported. Explains the result using auxins 3
Evaluation One honest limitation, and one thing that would be done differently 2
Report presentation Headings in order, legible, work of the group's own 2
Total 25

The control is the mark most groups lose. A phototropism investigation needs seedlings that get light from all sides, or none, to compare against the ones lit from one side. Without it there is nothing to say the bending was caused by the light.

What the investigation costs

Almost nothing, and we would rather say so. Bean or maize seeds from a supermarket, cotton wool, a shoebox with a hole cut in one side, and a windowsill. That is the whole apparatus.

Item Price In stock Why you might want it
Petri dish, borosilicate glass, 90 mm R35,00 615 Germinating seedlings on damp cotton wool, where the class can see the roots as well as the shoots. Reusable for years
Petri dish, plastic, pack of 10 R13,50 149 The cheap option if you want one per group and do not need them back

We do not stock seeds and we do not stock a growth lamp. Seeds are a supermarket item and a classroom window is a better light source than anything we could sell you, because it is free and it is already pointing one way.

Unit 1: the plant hormones

Hormone What it does
Auxins Keep cell walls elastic so cells elongate. Coordinate the response to light and gravity. Stimulate fruit formation. Prevent side shoots from developing. Stop leaves and fruit falling early
Gibberellins Stimulate cell division and cell elongation, so plants grow tall. Stimulate seed germination, flowering, and growth of dormant buds
Abscisic acid Slows growth, which helps a plant survive harsh conditions. Stops seeds germinating too early. Closes stomata in dry conditions

Where hormones are made: dividing cells, chiefly at the root tip and the shoot tip.

The one with a commercial use worth knowing: 2,4-D is a synthetic auxin used as a herbicide to control broadleaf weeds in fields of cereals or grass. It works by making the weeds grow themselves to death while the narrow-leaved crop is largely unaffected.

Finding 1: one line in the auxin list cannot be learnt

You will read that auxins "stop flowering in some plants and stimulate flowering in others".

As written, that covers every possible outcome, which means a learner cannot use it, cannot apply it and cannot be examined on it. It is not wrong. It just does nothing.

Learn the four auxin effects that can be stated definitely: cell elongation, response to light and gravity, fruit formation, and suppression of side shoots. Those are the ones a question can be built on.

Unit 2: the two tropisms

A tropism is a growth response in which the plant responds to the direction of a stimulus. The direction is the whole point, and it is where the marks are.

Organ Gravity Light
Root Positively geotropic. Grows towards gravity, so downwards Grows away from light
Shoot Negatively geotropic. Grows away from gravity, so upwards Positively phototropic. Grows towards light

How auxin produces the bend, in both organs

Step Shoot lit from one side Root laid horizontally
Where auxin collects The shaded side, because auxin moves away from light The lower side, because auxin moves with gravity
What high auxin does there Speeds elongation up Inhibits elongation
Which side grows faster The shaded side The upper side, where auxin is at a normal level
Result Shoot curves towards the light Root curves downwards

The sentence that gets the mark: high auxin speeds elongation in stems and inhibits it in roots. Same hormone, same side, opposite effect, because the two tissues respond differently to the same concentration. Learners who memorise "auxin makes things grow" cannot explain the root.

Finding 2: the summary contradicts itself three bullets apart

One line says "roots are positively geotropic; shoots are negatively geotropic". Three lines later: "the root or shoot appears to bend towards the stimulus".

Negatively geotropic means growing away from the stimulus. A shoot bends away from gravity. It is the definition of the word negative in that sentence, and the later line contradicts it.

Case Towards or away
Root and gravity Towards
Shoot and gravity Away
Shoot and light Towards
Root and light Away. The summary never states this case at all

Three of the four cases are towards, which is probably how the mistake happened. Teach the four cases as a table and the problem disappears.

Finding 3: the bending is real, not apparent

You will read that the root or shoot "appears to bend", with bend in inverted commas.

It does bend. The curvature is physically there and can be measured with a protractor. The inverted commas seem meant to signal that the plant is not bending the way an arm bends, with a joint, but that is not what they communicate to a learner, who reads them as a hint that the bending is some kind of illusion.

The accurate version: the organ curves, because one side elongates faster than the other. No hedging needed.

Finding 4: the people who did this work are not named

You will read that growth responses "were discovered by scientists who did experiments on the coleoptiles of germinating grasses and cereals".

Those scientists have names, and one of them is already in this course.

Who What they did
Charles Darwin and his son Francis The original coleoptile experiments. They covered the tip of a grass seedling and found it stopped bending towards light, which showed the tip detects the light while the bending happens lower down
Later workers Showed that something chemical carries the signal from the tip downwards, which is what was eventually identified as auxin

Darwin is named in the evolution topic of this same course and left anonymous here, for work he actually did. It is worth a minute of class time, because it shows a learner that one person's curiosity ran across two topics they are studying in the same year.

Unit 3: plant defence

Defence How it works
Chemical Plants produce chemicals in response to damage by insects or herbivores. These may be toxic, or may make the plant less nutritious
Structural, thorns Some plants produce more and longer thorns in response to browsing
Structural, leaves Some produce smaller, denser leaves after browsing

This is the easiest South African example in the grade. Acacia trees browsed by kudu raise the tannin level in their leaves, and the response spreads to neighbouring trees. A class that has seen a thorn tree has seen the topic.

What you should be able to do by the end

  • Define a tropism, with direction in the definition
  • Give all four cases of root and shoot against gravity and light, towards or away
  • Explain phototropism using auxin, from where it collects to which side elongates
  • Explain geotropism in a root, including why high auxin inhibits there
  • Name three plant hormones and give the main effects of each
  • Design a valid investigation with a hypothesis, a control and named controlled variables
  • Give two plant defence mechanisms

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

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