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.

  • $\begingroup$ My guess is that because width and height of a plant are more greatly affected by light intensity, the dry mass has less relevance. If the plants were all the same size, and you wanted to know which plant is assimilating the greatest amount of substrate, then biomass would be okay. But, you just want to know which plant is growing the fastest $\endgroup$
    – Bob
    Commented Mar 6, 2017 at 22:37
  • $\begingroup$ Plus, do you really want to be the guy to dry an entire fir tree sapling? I think drying a flower might be easier $\endgroup$
    – Bob
    Commented Mar 6, 2017 at 22:41
  • $\begingroup$ But the dry mass can also be used to determine which plants have had the largest amount of growth. If we start off with cloned plants of the same age (I learnt that this is always the correct way to start these experiments) and I know that I will end the experiment after around two weeks, why not use the dry mass? I am merely starting off with seedlings, so they will have grown to a fairly small size, hence we should be able to obtain their dry mass. Moreover, see my edit to the question. $\endgroup$ Commented Mar 7, 2017 at 11:48
  • $\begingroup$ It might depend on what we mean by "growth" though; actual production, or increase in vertical height ? In the latter case you could have some saplings growing faster than others due to different carbon allocation strategies and "dry mass" as a measure might not pick up on the differences. $\endgroup$
    – Oosaka
    Commented Mar 7, 2017 at 12:23
  • $\begingroup$ @Rozenn Keribin, why do you say that dry mass may not pick up the difference in the rate at which the saplings are incorporating carbon? Isn't that NPP (the production of energy through biomolecules within the plants)? So it should indeed be picked up by dry mass. $\endgroup$ Commented Mar 12, 2017 at 15:02


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