adder's tongue fern - Adder's Tongue Fern (Ophioglossum vulgatum); adder's tongue fern - This British fern lacks the typical frond. Each year a single leaf is produced which is divided into two parts: a smooth, leathery, oval blade, 3 ins. or 4 ins. long, and an erect, unbranched spike bearing a double row of capsules which contain the spores. It is much more plentiful than is commonly supposed, growing inconspicuously among grass in damp pastures.