Wiktionary
fern seed
n. (context botany English) The asexual, dustlike spores of a fern which resemble seeds
WordNet
fern seed
n. the asexual spore of ferns that resembles dust; once thought to be seeds and to make the possessor invisible
Usage examples of "fern seed".
Shakespeare says, I Henry IV: 'We have the receipt of Fern seed - we walk invisible.
There are more than five hundred known spells to secure the love of another person, and they range from messing around with fern seed at midnight to doing something rather unpleasant with a rhino horn at an unspecified time, but probably not just after a meal.
A spell of inconspicuousness would be great, but it called for fern seed.