These days, smart discussions of food begin and end with Michael Pollan. His Botany of Desire changed the way we look at potatoes and marijuana. The Omnivore's Dilemma gave us the term "locavore." And his mantra "Eat food. Not too much. Mostly plants." is changing dining habits, one foodie at a time. Pollan will speak about his latest book, In Defense of Food, Saturday, May 16, at 7 p.m. in the Central Hall of Enoch Pratt Free Library, central branch, (400 Cathedral St,  396-5430). Seating is limited; doors open at 5:30 p.m. We advise early arrival.