Put some green submerged water plants such as Hydrilla in a large beaker filled with water. Add a little soda bicarbonate as a source of carbon dioxide or a small quantity of soda water. Cover the plants under water with a glass funnel, and invert over the funnel under water a test-tube filled with water. Cut the stems and tie up the shoots into a bundle and the cut ends should be projected upwards into the funnel.