Having it be ONE plant, or one single gas bag, connected by one single line, seems limiting for the organism. It's not very easy for them to propagate or evolve or anything. Therefore, I am going to go with a model that features lots and lots of these plants.
So, let's say helium is the element you'll be using to float your plants. In this case, you'll have to find a way to have your plants manufacture helium.
Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei. (source)
Since it comes from radioactive materials, these will have to be present in abundance for the plant to process. This may change the entire ecology, and so should be approached with caution.
Helium might not be the way you'll go. There are other options, such as:
- methane, which is far easier to produce, although more often for animals than plants
- hot air, which will need a mechanism for heating, and thus will be difficult for a plant to achieve.
- Hydrogen, which is present in water, and is perhaps the easiest, but your plants will need a mechanism to separate the oxygen from the hydrogen. They will be very flammable. EDIT: This may be a problem, because if there is any kind of fire on this island, many will explode, causing the island to dip lower or fall into the ocean. In fact, this could be part of the natural life cycle, and the island needs to fall in so that the process can begin again. Like pine cones You could also have it so that sections of the island can break off and fall away resulting in holes in the middle of your island or breakaways at the edges. This will depend on growth pattern. (They can grow in rings so that the edges are stronger and the center is empty, filling in as they go, or randomly, or out from a center point).
First, you must decide the mechanism and type of gas you want to use. That determines a lot of other things and factors.
Next, let's look at how the ground will be built.
Are people meant to walk across the top of the plants and that forms the ground of your island? Or are you looking at having the plants grow up THROUGH this "ground" basically as balloons over the ground? If it's the second one, that means the plants have to have a way to form the ground, and once it is formed, a way to poke through it. Forming the ground is a matter of lattice work between plants, and falling leaves from the plant, which will create soil. How stable it will be will depend on how it's built. Once a lattice work of plants is formed, and the ground is too solid, I would expect that it would be difficult for the ones underneath the island to be be able to get above this newly created ground.
So you'd have to find a mechanism for that, and you'd likely have immature ones beneath, that are gas filled but haven't come out yet. EDIT: You could have any mechanism to get this done, but if parts of the plants burn if it's hydrogen, you can have these searching for the "holes" or weak spots created by this.
There's other ways to do this too--the gas bag plants have phases, in which they can grow directly on the created soil (and don't have to have a line down to the ocean).
I would have them start in clusters, over miles of ocean, for sure, so that there's multiple lines down to the ocean.
Edit: Even better, they start as one or a cluster with the line to the ocean, but eventually that falls away and it just floats free in a particular stage of development, such as when the island reaches a certain size. Then, all the plants on it grow from the "soil" produced by the main plants. Additions to the soil will come from birds (their poo, nests, and carcasses) and other plants which grow on that soil. It eventually won't be tethered to the ocean, but the weight will keep it from floating into the atmosphere.