We discover loads of new species every year – nearly a hundred were found in 2011. Of course, it is hard to know at first glance if a species is undiscovered – so you then have to go back and compare it to other plants that it might be similar to.
Plants are always continuously evolving and adapting, whilst others go extinct. Although we have probably discovered most plants in the world, we will probably never be able to discover them all. There might be some up on an unvisited mountaintop somewhere that we never find!