This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
This genus is distinguished amongst hardy Ferns by its globose sori, destitute of an indusium or involucre. Rhizome creeping or tufted; fronds simple, pinnatifid, or pinnate. Nearly 400 species of widely different habit are collected under this name, which is derived from many, and a foot, probably in allusion to the numerous creeping rhizomes of P. vulgare,
1. P. vulgare. - This Fern is readily distinguished from all other native species by its creeping densely scaly aboveground rhizomes and alternate pinnatifid glabrous stipitate fronds with oblong obtuse pinnules and conspicuous yellow eventually reddish-brown naked sori. It flourishes best on stumps of trees, etc., by the side of brooks or moist places, but it may frequently be seen on old walls, etc. The variety Cambricum has the pinnules finely divided.
2. P. Dryopteris. Oak Fern. - An elegant species with slender creeping rootstocks and alternate bipinnate deltoid membranous fronds of a pale green colour, rarely more than 6 to 9 inches high. Stipes slender, scaly at the base. Frond divided into three nearly equal branches, forming a triangle; pinnules obtuse, obscurely toothed. This is found in shady mountainous districts in Britain, and is widely distributed in the northern hemisphere.
P. Robertianum, syn. P. calcareum, is very near the last, and perhaps only a variety of it. The fronds are more coriaceous and glandular, and the lateral branches of the frond are smaller than the central one. It is a rare Fern in Britain, growing on limestone rocks.
3. P. Phegopteris. Beech Fern. - A delicate small-growing species with pale-green pinnate triangular fronds from 6 inches to a foot high. Pinnae pinnatifid, the lower pair much smaller than the others and deflexed. Pinnules obtuse, ciliate; stipes very slender and brittle, exceeding the leafy portion of the frond, slightly scaly at the base. Moist shady woods and rocks throughout Britain. Distribution general in the north temperate zone.
4. P. alpestre, syn. Pseudathyrium alpestre. - This is strictly an alpine species, resembling the Lady Fern in general appearance. It has a stout rhizome and lanceolate bipinnate fronds varying from 6 inches to 2 or even 3 feet in height. Pinnae pinnatifid; pinnules toothed. Stipes 4 to 6 inches high, scaly at the base. Sori arranged in a single row on each side of the midrib. In Britain only on the lofty mountains of Scotland, and in arctic and alpine Europe, North America, and Western Asia.
P. flexile or humile is a variety with looser narrower fronds, and more distant deflexed pinnae.
Polyp odium or Niphobolus Lingua has coriaceous entire strap-shaped fronds, rusty tomentose beneath. It is a native of Japan and other Eastern countries.