Adders-tongues are plants of the genus Ophioglossum, which means "snake-tongue". Ophioglossum is in the family Ophioglossaceae, in the order Ophioglossales, a small group of vascular plants. The family includes another genus, Cheiroglossa (hand ferns).

Adders-tongues are so-called because the spore stalks are thought to resember snakes' tongues. Each plant typically sends up a small, undivided leaf blade with netted venation, and the spore stalk forks from the leaf stalk, terminating in sporangia which are partially concealed within a sructure with slitted sides. The plant grows from a central, budding, fleshy structure with fleshy, radiating roots. When the leaf blade is present, there is not always a spore stalk present, and the plants do not always send up a leaf, sometimes going for a year to a period of years living only under the soil, nourished by association with soil fungi.

Adders-tongues have the highest chromosome counts of any known plant.

Also see:

• Grape-fern
• Moonwort