The genus Pteris is the type of another group, the Pterideæ, in which the sori form a continuous marginal line covered by the attenuated edge of the frond folded over it, and forming an indusium. This is the structure in most of the species of Pteris, a large family abounding in the tropics, and very widely distributed in almost every part of the globe. In the Pteris aquilina, or Bracken (fig. 66), however, in which the lateral veins of the leaf are divided two or three times before they reach the margin, and the extremities of the branches anastomose and form a vein at the exterior or extreme margin of the leaf, the sporangia are produced on the upper-surface of the marginal vein, and are enclosed by an extension of the skin from both surfaces of the leaf; so that the fructification, which is folded back on the under-surface of the leaf, is in the earliest stage of its development enclosed between two thin membranes, both of which have their margins ciliated with jointed hairs, while, under the microscope, their cellular structure will be found to differ in accordance with that of the upper and under epidermis, from which they are individually extensions. This fern is so universal in Britain, as to exclude in some places almost every other kind of vegetation, and is so well known, that it may appear superfluous to mention that the fronds are deltoid, with an elongated, stem-like petiole two or three times divided, the primary pinnæ opposite, the ultimate segments oblong, obtuse, and confluent. There are exotic forms of Pteris in every part of the world, which are mere varieties of the Pteris aquilina, so that the Bracken may be said to rank as the most universally distributed of all vegetable productions, extending from West to East, over both continents and islands, in a zone reaching from Northern Europe and Siberia to New Zealand, where it is represented by, and perhaps identical with, the well known Pteris esculenta. The Allosorus crispus, Parsley fern, or Curled Rock Bracken, which may be taken as a type of its genus, is by some authors referred to the same group of Pterideæ, though others regard it as one of the Polypodieæ. It is a pretty little fern, growing in tufts in sheltered crevices of mountain rocks, from Lapland to the Mediterranean. The fronds are deltoid, twice or three times pinnate, and of two kinds. The ultimate divisions of the barren fronds are wedge-shaped, cut, and toothed; those of the fertile fronds are linear, oblong, and entire. Although the sori are at first circular, and situated near the extremities of the lateral veins, they become confluent in maturity, and being covered by the reflexed margin of the pinnules of the contracted but scarcely altered fertile frond, instead of by an indusium, they bear some resemblance in fructification to Pteris aquilina.