It always comes as a surprise to my students that all of these vegetables: cabbage, broccoli, cauliflower, kale, brussels sprouts, kohlrabi, are actually just varieties, genetic variants, of one single species, Brassica oleracea, seen here. Here's a really nice article about this to introduce a new botanical blog added to TPP's blogroll, The Botanist in the Kitchen (actually there's two of them). 