Sacks traveled with a group from the American Fern Society (yes, there is such a thing) to Oaxaca for a one week ferning trip. The group had world-class botanists, gifted amateurs, at least one mathematician, an aeronautics engineer - all of them knowledgeable and passionate about ferns. This seems strange to me, but Sacks’ journal leaves little doubt that passion and ferns are two words that can go together.
Besides ferning, the group toured the area around Oaxaca, including Monte Alban, Mixtla, Tule, etc., and Sacks provides fine descriptions, background, and reflections on each place. He tells us how the Zapotecs were able to make elastic rubber from latex, using the acid and sulphur compounds in morning glory to transform it into the kind of elastic rubber that we are accustomed to - some thousand years before Charles Goodyear (re)discovered the process. And we learn about the immense effort required to make a pound of cochineal red - 70,000 cochineal beetles harvested from cactus, and processed with acid and alum. And he tells us about the defense systems of bracken: squirting acid when bitten into by ants; containing a neurotoxin that in small doses causes disequilibrium, and death from convulsions in large doses.
The book is full of interesting tidbits, and well worth a read.