Osmund the Waterman

In his description of the *Flowering Fern (Osmunda regalis)*, Mr. Newman observes, that "the rhizoma [root-stock], when cut through, has a whitish centre or core, called by old Gerarde in his *Herbal*, 'the heart of Osmund the waterman.' My lore is insufficient to furnish my readers with the history of the said Osmund." (*History of British Ferns*, by Ed. Newman, 2nd ed., p. 334.) Can any of *your* readers supply this deficiency?