They do, but they may not absorb enough to keep your tank at stable levels for fish, and they can't remove debris like a filter can.
_Filters:_If you only have plants, you don't really need one. If you're adding animals, you do. A common question is, "Do I really need a filter if I have aquarium plants? I thought they absorbed toxins." They do, but they may not absorb enough to keep your tank at stable levels for fish, and they can't remove debris like a filter can. It depends on your plant to fish ratio. Plus, filters give more surface area for good bacteria to grow.