If Marco Polo were alive today, he might be content to just hang out at his neighborhood Greenmarket and have the world come to him. This morning I was introduced to something I would never in 700 years have imagined encountering just two and a half blocks from home: sesame seeds on the vine. Jeff Bialas had brought in a plant to show us clueless urbanites how one of the crops he’s experimenting with grows. I’ve done my share of sesame stories but couldn’t visualize how they cluster in pods that look something like okra and are harvested something like beans, left to ripen until they can be shaken loose and easily gathered. As he said, “open sesame” comes from the sign they give of being ready to be tipped out. Just as revelatory was my next stop, at Ray Bradley’s stand, where his right-hand man Hardeep told me he never saw a lemon until he emigrated from India back in the Seventies. In his birthplace, everyone had a lime tree to pull from to add acid to things like tomato-onion salads. And 40 years ago, limes were not easy to find in America. Even with global warming, I somehow doubt I will ever see citrus at NYC Greenmarkets in my lifetime. No matter how many times I overhear shoppers asking where the lemons are.