It is, of course, a powerhouse of the cell and therefore must have enzymes which release the energy stored in organic molecules. Perhaps we could stain for the special enzymes of the energy pathway. This might add to our theory about their living state. The respiration pathway includes enzymes such as succinate dehydrogenase which liberate electrons and hydrogen ions. Thus methylene blue might reveal mitochondria to our microscopic probe.
Plant cells lack mitochondria.
If plant cells lack mitochondria, then applied methylene blue should not stain the mitochondria and thereby reveal their location. Your instructor will give you several other instructions to observe some other cell types, or will focus your attention upon the types you have already observed, depending upon time remaining. Other activities you can do with any remaining time would be to think of ways to investigate cells and tissues of your mystery plant. By now you may have some seedlings to look at. Maybe you can spare a seed to look at under the microscope, or test its storage molecules.