Christian Science Monitor: Some 27 percent of the flowers around Walden Pond in Concord, Mass. have vanished since the mid 19th century, and another 36 percent are on the brink of disappearing. We know this because we have meticulous records of Walden`s plant species and birds, taken by none other than Henry David Thoreau. On nearly every spring morning from 1851 to 1858, the transcendentalist writer explored the woods around the pond, noting the first seasonal blooms of 465 species of flowers. Since …