Write any two limitations of using a Ganong’s Potometer to demonstrate the uptake of water.

1. It measures the rate of WATER UPTAKE, not the rate of TRANSPIRATION. Not all the water absorbed is transpired — some is used in photosynthesis and retained in the tissues — so the reading is only an approximation.
2. It is very difficult to make the apparatus AIRTIGHT and to insert the cut shoot without letting an air bubble enter the xylem, which would block the water column and stop the movement altogether.

Two limitations, one mark each.

THE FIRST IS THE FUNDAMENTAL ONE and should always be given: a potometer measures ABSORPTION and we use it as a substitute for TRANSPIRATION. The two are close but not equal, because a small fraction of the water taken up is used by the plant rather than lost.

Other acceptable limitations: a cut shoot is not a whole plant and behaves differently, since the natural root system has been removed; the shoot may die during a long experiment; and it is difficult to reset the air bubble accurately for repeat readings.

Questions on apparatus usually reward the candidate who knows what the instrument ACTUALLY measures as opposed to what it is used for.