We have the receipt of fern seed, we walk invisible (1 Henry IV., act iv. 4). The seed of certain species of fern is so small as to be invisible to the naked eye, and hence the plant was believed to confer invisibility on those who carried it about their person. It was at one time believed that plants have the power of imparting their own speciality to their wearer. Thus, the herb-dragon was said to cure the poison of serpents, the yellow celandine the jaundice; wood-sorrel, which has a heart-shaped leaf, to cheer the heart; liverwort to be good for the liver, and so on.
Why did you think that you had Gyges' ring, Or the herb that gives invisibility?
Beaumont and Fletcher: Fair Maid of the Inn, i. 1.
The seeds of fern, which, by prolific heat Cheered and unfolded, form a plant so great, Are less a thousand times than what the eye Can unassisted by the tube descry.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894