in Thoreau’s Journal:

The gentian (Andrewsii) now generally in prime, on low, moist, shady banks. Its transcendent blue shows best in the shade and suggests coolness; contrasts there with the fresh green; a splendid blue, light in the shade, turning to purple with age. They are particularly abundant under the north side of the willow row in Merrick’s pasture. I count fifteen in a single cluster there, and afterward twenty in Gentian Lane near Flint’s Bridge, and there were other clusters below; bluer than the bluest sky they lurk in the moist and shady recesses of the banks.