Recently I’ve been trying to imagine a world without leaves. It’s not easy to do at this time of year, when the trees around my house turn my windows into green walls. But a paper published on-line today at the Proceedings of the National Academy of Science inspires some effort. A team of English scientists offer a look back at Earth some 400 million years ago, at a time before leaves had evolved. Plants had been growing on dry land for at least 75 million years, but they were little more than mosses and liverworts growing on damp ground, along with some primitive vascular plants with stems a few inches high. True leaves–flat blades of tissue that acted like