What is the boiling tube now filled with and what is put into the boiling tube after this?

Now fill the boiling tube with sodium hydrogen carbonate solution which releases carbon dioxide needed for photosynthesis. Next put a piece of pond-weed into the tube with the cut end at the top, leave for 5 minutes to acclimatise to the tube. At the top of the pond-weed bubbles of oxygen are produced by photosynthesis should be visible.