A Brassica of some type - maybe Mustard? What do the leaves smell like when you crush them? Do the flowers have any scent?
Thanks for the response. The leaves smell like chlorophyll to me, and while the flowers do have a distinctive smell, I can't quite describe it. They certainly do not smell anything like the mustard I have in my refrigerator (Grey Poupon). I did notice a Cabbage White butterfly laying eggs on some of the plants a few weeks ago, if that helps narrow it down.
I do not believe that these butterflies are so specific for their hosts. Pieris brassicae and Pieris napi are known to lay their eggs on some other Brassicaceae as well. The plant reminds me of Rorippa sylvestris but this might be way off the correct name. Fruits (even immature fruits) might be great and an image on which the whole plant is shown would be helpful as well. It is difficult to select the correct name if you do not know the Brassicaceae of your area anyway and more so when you have got only two closeup photographs.
