The plants may begin life on the substrate over a source of gas, may begin as a branch of a nearby plant, or may grow downward through the ground until reaching a cavern where it can expand. Especially large plants of this type develop branching hairs on the perimeter of the funnel to capture all bubbles larger than a certain diameter without having to expend all the resources that growing a solid funnel would require. The hairs are designed to direct all rising bubbles into the funnel. Some hairs put down vertical strings that provide surfaces for dissolved gases to adhere to and begin producing small bubbles. When the bubbles are large enough to be captured by the hairs, their buoyancy is strong enough to detach them from the strings. In very old, very large plants of this type, the hairs grow thick for support, and the strings become support columns.
These plants are related to an idea I had long ago for a cave-dwelling plant that gained energy from running water instead of light. In that version, water flowing downwards turned the wheels. Funnels were on the top, but they weren't so elaborate.
In other news, I also thought of stiltsponges, sponges that for whatever reason do not attach to the seafloor, but sit balanced on stilts so that animals can crawl under them and water can circulate.