This device works much like a pH meter electrode but has a delicate plastic membrane and all operations must be carried out in an air-tight cuvette with proper mixing. You can also measure the oxygen around a leaf sealed in an airtight cuvette with a similar electrode for measuring oxygen gas in air. I'll save both of those projects for the course, Plant Physiology. Alternatively one might remember, from the leaf anatomy exercise, that there is gas space in the leaf and, from General Chemistry I, that gases such as oxygen are less dense than water. Thus a leaf containing gas should float on water.
Leaves produce oxygen gas by photosynthesis.
Punch out ten discs from a green leaf using the hand punch. Apply a vacuum by pulling back on the plunger while blocking the Luer-lok tip with a piece of rubber stopper. Apply and release the vacuum repeatedly while swirling until all of the discs in the syringe are no longer floating. Stand the syringe on its plunger about 15 cm from an exposed light bulb. Now you design an experiment that will test the hypothesis through the prediction.