Michael Pollan is at it again: taking drugs and spinning wild, mesmerizing yarns about being human in a plant world. In his new book, “This Is Your Mind on Plants,” he grows opium poppies and mescaline cacti in his garden, heroically (and miserably) tries to avoid caffeine, and, in general, participates with gusto in our species’ age-old labor of helping the cleverest of plants propagate their genes. He weaves together botany, brain sciences, evolutionary biology and political economy into a sort of grand unified theory of drugs. And, of course, he experiments on himself, with intellectually delectable results. While not as revelatory as Pollan’s major works, this is a wonderful and compelling read that will leave you thinking long after you set it down.