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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.