I'm an A level student (British high school final year) and I was doing an exam question which asked for the design of an experiment to test the effect of light intensity on the growth of a certain type of plant. Here is the paper; it is question 2(c). The marking scheme can be found here.
As my measured dependent variable, I picked dry mass. However, the marking scheme specifically says to 'ignore dry mass' and asks for use of 'mass'. We were always taught that dry mass is the most accurate measure for experiments where we are trying to measure the effect of a certain parameter on the growth of plant. So now I am wondering - when is it unacceptable to use dry mass as a measured dependent variable in such experiments?
As far as my attempts extend, I tried determining what conditions we should use dry mass under, but I could not find any source that talks about detailed use of dry mass (when to use it and when to not use it).
Perhaps a different way to phrase this question is to ask: when do we use mass rather than dry mass? And when do we use dry mass rather than mass? As far as my knowledge extends, dry mass is the most accurate determinant of growth that there is, given that the plants are genetically identical. This is because it ignores the possible fluctuations that may be present due to fluid mass within the plants.