Why the pot is enclosed in a pastic bag?

A potted flower is placed over a top pan balance and only the pot including the soil, not the flower, is enclosed in a plastic bag. As time increases, mass at the top pan balance decreases. why do u think the pot is enclosed in a plastic bag?

describe how , using asimilar appparatus to that in the example, you could compare the transpiration rates of two different plants?



If anybody has O' level Biology Paper 6 of November 2003 than it would be easier to answer this.

Why the pot is enclosed in a pastic bag?
I think the plastic bag is to prevent water from evaporating from the soil, if you're trying to measure transpiration rates.



For the second part of the question: you could use a two-pan balance to compare two different plants.
Reply:Tactic: look at the next question for the answer to the current one. Transpiration is the answer.



Plants waste water constantly. Its an inevitable consequence of porous leaves that need to let oxygen out and CO2 in; quite a lot of H20 is lost. In dry climates plants develop elaborate schemes to conserve water, but your average cultivated flower doesn't have that evolutionary pressure and acts like a lantern wick only for water.



The plastic bag is there so you know its not just evaporation from the soil that's causing the decrease in mass.



To compare transpiration rates of two different plants, put them both on balances and cover the bases with plastic bags and watch how fast they lose mass.