Apparatus B is a control to show that the result in apparatus A is due to gravity and not some other factor. By rotating apparatus B, the effect of gravity is applied evenly to all sides of the root, so it does not provide a directional stimulus. Any difference in growth between A and B can therefore be attributed to the directional pull of gravity in A.
In a scientific investigation, a control is used as a baseline for comparison. It is set up to be identical to the main experiment in every way except for the one variable being tested (the independent variable).
In this case, the independent variable is the directional stimulus of gravity.
- In **Apparatus A (Stationary)**, gravity constantly pulls downwards.
- In **Apparatus B (Rotating)**, the device (a clinostat) slowly rotates. This means that over time, the stimulus of gravity is applied equally to all sides of the root. Gravity is still present, but it is not acting in one constant direction relative to the root.
Therefore, apparatus B is needed to show what happens when there is no *directional* gravitational stimulus. By comparing the growth of the root in A (with directional gravity) to the growth in B (without directional gravity), the student can conclude that the downward growth in A is specifically a response to the direction of gravity.
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