POLYPODIUM  , in

botany, a large genus of true ferns (q.v.), widely distributed throughout the world, but specially developed in the tropics . The name is derived from Gr. arokvs, many, and arb&ov, a little foot, on account of the foot-like appearance of the rhizome and its branches . The species differ greatly in size and general appearance and in the character of the frond; the sori or groups of spore-cases (sporangia) are borne on the back of the leaf, are globose and naked, that is, are not covered with a membrane (indusium) (see fig . 1) . The common polypody (fig . 2) (P. vulgare) is widely diffused in the British Isles, where it is found on walls, banks, trees, &c.; the creeping, densely-scaly rootstock bears deeply pinnately cut fronds, the fertile ones bearing on the back the bright yellow naked groups of sporangia (sori) . It is also known as adder's foot, golden maidenhair and wood-fern, and is the oak-fern of the old herbals . back of leaf . There are a large number of varieties, differing chiefly in the form and division of the pinnae; var. cambricum (origin-ally found in Wales) has the pinnae themselves deeply cut into narrow segments; var. cornubiense is a very elegant plant with finely-divided fronds; var. cristatum is a handsome variety with fronds forking at the apex and the tips of all the pinnae crested and curled . P. dryopteris, generally known as oak-fern, is a very graceful plant with delicate fronds, 6 to 12 in. long, the three main branches of which are themselves pinnately divided; it is found in dry, shady places in mountain districts in Great Britain, but is very rare in Ireland . P. phegopteris (beechfern) is a graceful species with a black, slender root-stock, from which the pinnate fronds rise on long stalks, generally about 12 in. long, including the stalk; it is characterized by having the lower pinnae of the frond deflexed; it is generally distributed in Britain, though not common . Many other species from different parts of the world are known in green-house cultivation .