This was a laboratory test we did at the university.
It's sense was to find out whether plants generate oxygen O via photosynthesis or not. As everyone know they will - this way it was an how-to-show-something-test, which was still interesting to look at.
This picture shows two of four Erlenmeyer flasks. There was a liquid (indigocarmine) in it which indicates whether there is oxigen in it (blue / dark colour) or not (yellow colour). To prevent oxygen / air from around getting in the flasks there's a paraffin-oil layer on top of the liquid (clear phase).
As you can see the flask with the plant (Elodea canadensis) in it indicates oxygen, the other without does not ;)
Two other flasks included one burnt, dead plant and one plant which was given no light.