The Collaborative International Dictionary of English v.0.48:

Winter's bark \Win"ter's bark`\ (Bot.)
   The aromatic bark of tree (Drimys Winteri syn. Drymis
   Winteri) of the Magnolia family, which is found in Southern
   Chile. It was first used as a cure for scurvy by its
   discoverer, Captain John Winter, vice admiral to sir Francis
   Drake, in 1577.
   [1913 Webster]