Alexander Dyce, A General Glossary to Shakespeare's Works

fern-seed — “The receipt of,” 1 HENRY IV., ii. 1. 84. “The ancients, who often paid more attention to received opinions than to the evidence of their senses, believed that fern bore no seed. Our ancestors imagined that this plant produced seed which was invisible. Hence, from an extraordinary mode of reasoning, founded on the fantastic doctrine of signatures, they concluded that they who possessed the secret of wearing this seed about them would become invisible” (HOLT WHITE) .