“I am on the alert for the first signs of spring,” wrote Henry David Thoreau ... and when the ice melted on Walden Pond. Some 160 years later, Richard Primack, a biologist at Boston University ...

"A single gentle rain," Thoreau observes in his chapter on spring, "makes the grass many ... During his first year at Walden Pond, Thoreau cultivated about two and a half acres of Emerson's ...