The group of Asplenieæ is a very comprehensive one, including, besides the extensive typical genus Asplenium, the cognate subdivisions of Diplazieæ and Scolopendrieæ. The latter is represented by Scolopendrium vulgare, the Hart’s-tongue fern, in which the sori are linear, and situated in pairs on two parallel veins, so closely approximate that, though really double, they seem to form but one straight line; they are not covered with one indusium splitting down the middle, but each has its own indusium opening in opposite directions. On the under-surface of the long strap-shaped entire fronds of the common Hart’s-tongue, which is a type of the genus, the double sori with their opposite indusia form parallel equidistant straight lines, diverging on each side from the mid-vein of the frond, as in fig. 61. The fronds are cordate at the base; sometimes they are forked, and occasionally crisped and wavy, and the leaf-stem is shaggy with narrow membranaceous scales. The caudex is very compact and deep-rooted; it does not much elongate, but increases slowly by the formation of new crowns round the older, attaining considerable bulk. From this the fronds rise in circular tufts to the height of a foot or more, and attain from two to three feet, in favourable shady localities. At first the tufts are straight, but ultimately they radiate and bend outwards. This plant is exceedingly variable; hundreds of varieties have been found or raised in Great Britain. Though twin sori are characteristic of the Scolopendrieæ, they diverge somewhat in Camptosorus, a North American genus, sometimes called the Walking fern, from its habit of throwing forward a bud on a thread-like prolongation of the point of the frond, which becomes established as a new growing centre, and thus carries the plant onwards. The group of ferns called Diplazieæ, another section of the Asplenieæ, typified by the genus Diplazium, is altogether tropical. The sori are bilateral or double, but placed back to back on the vein, exactly the opposite of Scolopendrieæ; so that the indusia, instead of opening face to face, open in opposite directions. The species are rather numerous, and embrace much variety of form and character.