A whorl is the ring of branches radiating from one level of a conifer’s trunk, “making a springy resting place for fresh snow”. Lenticels are the pores bark breathes through; a gemel is two trees grown together by natural grafting. Poet Hannah Fries gives each of 61 such words a short essay, part science and part memoir, so the book reads less like a glossary than like a walk with someone who knows what everything is called.