The ‘seed’ of the fern. Before the mode of reproduction of ferns was understood, they were popularly supposed to produce an invisible seed, which was capable of communicating its invisibility to any person who possessed it.

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1596.  Shaks., 1 Hen. IV., II. i. 96. We steale as in a Castle, cocksure: we haue the receit of Fernseede, we walke inuisible.

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1630.  B. Jonson, New Inn, I. Wks. (Rtldg.), 411/1.

                    I had
No medicine, sir, to go invisible:
No fern-seed in my pocket; nor an opal
Wrapt in bay-leaf, in my left fist, to charm
Their eyes with.

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1756.  Smart, The Horatian Canons of Friendship, 75.

        And ask thy heart, if Custom, Nature’s heir,
Hath sown no undiscover’d fern-seed there.

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1815.  Scott, Guy Mann., xlv. ‘They say she has gathered the fern-seed and can gang ony gate she likes.’

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1859.  Sala, Tw. round Clock (1861), 266. We … are in the receipt of fern-seed, and can walk invisible without incommoding ourselves or anybody else.

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