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Use newly germinated seedlings of a suitable species and stage. A light-direction comparison and a gravity-orientation comparison address different stimuli. Change the chosen factor while keeping water, temperature, starting stage and other relevant conditions comparable. Define the observation period before looking for a result.
Video by Freesciencelessons. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Design a controlled seedling-response comparison
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why label the light direction on a drawing?
To interpret growth orientation relative to the stimulus. The relationship depends on the external direction.
After watching 2: Which quantities should not be treated as interchangeable?
Curved organ length and straight-line height. Define the measurement consistently.
What you'll be able to do
Use newly germinated seedlings of a suitable species and stage. A light-direction comparison and a gravity-orientation comparison address different stimuli. Change the chosen factor while keeping water, temperature, starting stage and other relevant conditions comparable. Define the observation period before looking for a result.
A suitable control helps distinguish directional growth from normal development. Exposing one group to light and another to darkness may also alter other developmental features, so interpret the specific design rather than assuming all differences demonstrate phototropism. Rotate or position material only as the approved method specifies.
Measure clearly defined root or shoot lengths with appropriate units and make accurate labelled drawings showing orientation and the stimulus direction. A curved shoot’s length and its straight-line height are different measurements. Retain a consistent method across observations.
Record starting and final observations, use suitable independent seedlings or containers and summarise variation. Many seedlings in one shared container may experience the same treatment variation. A single striking curved example is not enough to establish a consistent response in the population.
A shoot bending toward directional light is consistent with positive phototropism. Roots and shoots can show different gravity responses. At this level, connect differential growth to the appropriate plant-hormone explanation without assuming that a photograph directly measures hormone concentration.
Elongation, curvature and the fraction germinating are distinct outcomes. A taller shoot has not necessarily germinated more successfully, and a curved organ has not necessarily grown a greater total length. Identify the measured response and discuss plausible confounding factors or limits.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A drawing shows a shoot bending toward a lamp, but no length measurements were taken. What can it support and what is missing?
Evidence of direction in that specimen; a length-change rate still needs measurements and time.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.