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.
To the Asplenieæ proper belongs one of our commonest species, the Athyrium Filix-fœmina, or Lady Fern, which, when growing in moist shady places, is one of the most elegant of British ferns. Its bipinnate frond varies from a broad almost ovate outline to a linear lanceolate form. The numerous fronds often spring from the caudex in a vase-like arrangement, to the height of four or five feet, those in the centre nearly erect, but the outer ones, drooping around in all directions, forming a tuft of lovely feathery foliage. The sori have commonly a semilunar outline, as in [fig. 62]; but they are occasionally so much curved as to acquire a horse-shoe form, and more rarely they are linear, and they become at length reflected by the growth of the sporangia. In some varieties they are distinct, in others so close as to become eventually confluent and completely cover the surface. The Filix-fœmina is distinguished by the linear junction of the indusium with the frond. It is one of the most variable of all the known ferns, and is remarkably prolific, so that new forms may be raised from the spores. There are a very large number found in this country.
Fig. 62. Athyrium Filix-fœmina.
All the Asplenieæ have linear sori attached to the side of the vein, so that they are said to be lateral. The indusium opens on its inward side, and is sometimes ciliated. There are many tropical ferns of this group, but the genus Asplenium, which has representatives in all parts of the world, yields eight or nine species in Great Britain.
Fig. 63. Asplenium Ruta-muraria.
The Asplenium Ruta-muraria, or Wall Rue, is an indigenous mountain plant growing in tufts six or eight inches high in the clefts of the rocks, but has diminished much in size in migrating to the plains, where it is found on old brick walls and outhouses. The fronds are deltoid and bipinnate, the pinnules are wedge-shaped and notched, or toothed, on their upper margin; their colour is deep green, but in exposed situations they are always covered with a glaucous secretion. The veins diverge from the footstalk at the base of the pinnules, branch out above, and extend to the teeth or serratures, as in [fig. 63]. The sori, which are produced on the inner side of the veins, are linear, elongated, and eventually become confluent, covering the whole under-surface of each fertile pinnule. The indusium, which is only traceable in the earlier condition of the fructification, is white, and the free inner margin is somewhat jagged.
Changes as remarkable take place in the sori of the Asplenium lanceolatum. At first, when the sori are covered by their white indusium, they are oblong, but they become circular as they enlarge, and eventually often confluent, so as to form a line round the whole under-margin of the sharply toothed pinnule. This fern, which is indigenous in the Atlantic islands, and is found in the Channel Islands, as well as in the maritime counties of England and Wales, grows in the crevices of rocks and old walls, or clothes the sides of wells and deserted mines. It belongs to the section which has a mid-vein.
The genus Ceterach apparently belongs to that section of ferns which have a vertical ring, and no indusium; but its affinities ally it closely with Asplenium. Its want of an indusium is supplied, perhaps occasioned, by the abundance of chaffy scales which cover the back of the fronds. The lateral veins are alternate and irregularly branched, with the branches anastomosing towards the margin. The sori are oblong or linear, attached to the upper side of the anterior branches except the last, which is on the opposite side of the lower branch. The British species, Ceterach officinarum, or Scaly Spleen-wort ([fig. 64]), has deeply pinnatifid, lanceolate fronds, with oblong, obtuse, alternate segments, and linear sori. The whole of the under-surface is densely clothed with brown, pointed, imbricated scales, finely serrated at their margins, the outermost of which extend beyond the edges of the segments. They completely cover the sori, and at first hide them entirely from view. On the unexpanded fronds, the scales are white and silvery. ‘This plant has an obscure indusium, only perceptible at an early stage of fructification, and subsequently as a nearly erect membrane, attached to the back of the vein. It is rendered unnecessary by the arrangement of the scales, which are disposed in regular series along each side of the veins and veinlets, pointing outwards, and concealing the sori with their broad bases, which completely overlap them in their immature condition.’