Given below is an apparatus which was set up to investigate a physiological process in plants. The setup (a potted plant under a bell jar, the pot sealed in a rubber sheet, standing on a glass plate with vaseline) was placed in bright sunlight. Answer the questions that follow: Given below is an apparatus which was set up to investigate a physiological process in plants. The setup (a potted plant under a bell jar, the pot sealed in a rubber sheet, standing on a glass plate with vaseline) was placed in bright sunlight. Answer the questions that follow:
Why was the pot enclosed in a rubber sheet?
Topic: Purpose of the rubber sheet
Answer
To PREVENT EVAPORATION of water from the soil and from the surface of the pot. This ensures that the water droplets collecting inside the bell jar have come ONLY from the plant, by transpiration, and not from the soil.
This is the CONTROL feature of the experiment, and the reason must be stated in those terms: without it the result would prove nothing, because moisture from the wet soil would condense on the jar and be mistaken for transpired water.
The VASELINE around the glass plate serves the same purpose from the other side — sealing the join so that no moisture enters or escapes.
THE HABIT TO FORM in every experimental question: ask what each part of the apparatus RULES OUT. Examiners award marks for understanding the control, not merely the result.