Late in the summer, I started seeing little round, ground-huuging plants, with deeply veined oblong green leaves, popping up in a garden that has mainly various kinds of perennials in it.  For a while, I thought these might be something I had planted that just had a late season.  Now, I'm not so sure.  These things have started spreading to other parts of the bed, although so far nowhere else that I can find, and the more mature ones are getting pretty big (10-12 inch diameter in a few cases.)  The big ones are hard to pull, but when I have managed to dig them out, they have a simple fibrous root system that appears to flare out and down into the soil.  No sign of a bulb or rhizome.  I have attached a couple of photos in hopes that someone with more horticultural savvy than I could tell me what these guys are, whether I should be trying to control them, and if so, how..