fern seed

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The dustlike spores of ferns, formerly believed to be seeds and once thought to have the power of making their possessor invisible.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany The asexual, dustlike spores of a fern which resemble seeds

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the asexual spore of ferns that resembles dust; once thought to be seeds and to make the possessor invisible