All of the air had to be removed from the leaf discs before placing them in the beaker. Suggest one reason why.

Leaves naturally have air spaces within their spongy mesophyll layer, which allows for gas exchange. The air trapped in these spaces makes the leaf discs buoyant, so they would normally float on the surface of the solution. The aim of the experiment is to measure the time it takes for them to rise, which is only possible if they start at the bottom. By removing the air (often done by placing the discs in a syringe with the solution and pulling the plunger), the discs become denser than the solution and will sink. They will only rise again once they have produced enough oxygen from photosynthesis to make them buoyant.