The glowing splint
The bubbles prove a gas is being made. One burning splinter of wood proves which gas it is.
The observation
It is seen that gas bubbles are released from these aquatic plants, and that gas collects at the upper end of the boiling tube.
Testing the gas
To test whether the gas released here is oxygen, wait until gas has collected in about three quarters of the volume of the boiling tube. Then slowly remove the water in it and insert a glowing splint into the tube through its open end.
The conclusion
Since the glowing splint burns brightly, it can be concluded that oxygen is produced in photosynthesis.
You have done this test before
The glowing splint is the standard test for oxygen, and grade 10 used it on oxygen made in a chemistry lesson rather than by a plant. A glowing splint relighting means the gas supports combustion better than air does. The plant has produced the same gas as the chemical reaction did.
Beyond the syllabus — who showed this first
It was the scientist Lavoisier who first showed that the gas released by green plants in the presence of light is oxygen.
A splint that relights means oxygen
Gas bubbles from an aquatic plant in sunlight collect in the boiling tube. A glowing splint burns brightly in that gas, so oxygen is produced in photosynthesis.