Alevel biology practical question help!Watch
Potometers tend to be used to measure the rate of transpiration in a plant, and this is done by looking at the rate of water uptake. However, this presumes that all the water the plant takes up evaporates from the surface of its leaves and ignores the fact that some of it is used during processes in the plant such as photosynthesis.
Otherwise, the answer would just be because we want the rate of transpiration, not overall water uptake.
Hope this helped