TREE-FERN. In old and well-grown specimens of some of the familiar ferns of temperate climates the wide-spreading crown of fronds may be observed to rise at a distance often of a good many inches above the ground, and from a stem of considerable thickness. The common male fern Lastraea (Filix-mas) affords the commonest instance of this; higher and thicker trunks are, however, occasionally presented by the royal fern (Osmunda regalis), in which a height of 2 ft. may be attained, and this with very considerable apparent thickness, due, however, to the origin and descent of a new series of adventitious roots from the bases of each annual set of fronds. Some tropical members and allies of these genera become more distinctly tree-like, e.g. Todea; Pteris also has some sub-arboreal forms. Oleandra is branched and shrub-like, while Angiopteris and Marattia may also rise to 2 ft. or more. But the tree-ferns proper are practically included within the family Cyatheaceae. This includes seven genera (Cyathea, Alsophila, Hernitelia, Dicksonia, Thyrsopteris, Cibotium and Balantium) and nearly 300 species, of which a few are herbaceous, but the majority arboreal and palm-like, reaching frequently a height of 50 ft. or more, Alsophila excelsa of Norfolk Island having sometimes measured 60 to 80 ft. The fronds are rarely simple or simply pinnate, but usually tripinnate or decompound, and may attain a length of 20 ft., thus forming a splendid crown of foliage. The stem may occasionally branch into many crowns.
The genera are of wide geographical range, mostly within the tropics; but South Australia, New Zealand, and the southern Pacific islands all possess their tree-ferns. In Tasmania Alsophila australis has been found up to the snow-level, and in the humid and mountainous regions of the tropics tree-ferns are also found to range up to a considerable altitude. The fronds may either contribute to the apparent thickness of the stem by leaving more or less of their bases, which become hardened and persistent, or they may be articulated to the stem and fall off, leaving characteristic scars in spiral series upon the stem. The stem is frequently much increased in apparent thickness by the downgrowth of aerial roots, forming a black coating several inches or even a foot in thickness, but its essential structure differs little in principle from that familiar in the rhizome of the common bracken (Pteris). To the ring or rather netted cylinder of fibrovascular bundles characteristic of all fernstems scattered internal as well as external bundles arising from these are superadded and in a tree-fern the outer bundles give off branches to the descending roots from the region where they pass into the leaves.
Tree-ferns are cultivated for their beauty alone; a few, however, are of some economic applications, chiefly as sources of starch. Thus the beautiful Alsophila excelsa of Norfolk Island is sail to be threatened with extinction for the sake of its sago-like pith, which is greedily eaten by hogs; Cyathea medullaris also furnishes a kind of sago to the natives of New Zealand, Queensland and the Pacific islands. A Javanese species of Dicksonia (D. chrysotricha) furnishes silky hairs, which have been imported as a styptic, and the long silky or rather woolly hairs, so abundant on the stem and frond-leaves in the various species of Cibotium have not only been put to a similar use, but in the Sandwich Islands furnish wool for stuffing mattresses and cushions, which was formerly an article of export. The "Tartarian lamb," or Agnus scythicus of old travellers' tales in China and Tartary, is simply the woolly stock of Cibotium Barometz, which, when dried and inverted, with all save four of its frond-stalks cut away, has a droll resemblance to a toy sheep.
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