In Shakespeare's time and long before, people believed that fern seeds or stalks would confer invisibility. E. Cobham Brewer writes in his 1898 Dictionary of Phrase and Fable:
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.
Botanical drawing: "Blätter des Mann=Waldfarn" Nature print from Alois Auer (active 1840s-1850s)Pflanzen Blumen und Blätter Vienna: Imperial Printing Office, 1853. Via New York Botanical Garden
Here's a recipe for making yourself invisible, by Mara Ravensong Bluewater, courtesy of CGHill:
At Dark Moon, in a mortar and pestle, grind together:
* 1 part Fern leaf, dried
* 1 part Poppy seeds
* 2 parts Slippery Elm powder
* 1 part Myrrh
* 1 part Marjoram, dried
* 3 parts Dillweed, fresh if possible
* Grind all together, mixing well.
Add 9 drops almond tincture (almond cooking extract is great.) with enough spring water to make everything barely moist, and mix in well.
Place in a ceramic bowl, spreading as thinly as possible, and dry the mixture over low heat, stirring it occasionally, until it seems lightly browned. Pour back into mortar, and grind again, enchanting:
Things Seen, and Things Not Seen:
Let me walk here in between.
When finely powdered, store in a clear glass container. It will keep its power for years. Sprinkle, just a little bit, on yourself, objects, or in a place to be made invisible.