TREE-FERN.⁠In old and well-grown specimens of some of the familiar ferns of our temperate climates the wide-spreading crown of fronds may be observed to rise at a distance often of a good many inches above the surface of the ground, and from a stem of considerable thickness. The common male fern Nephrodium (Lastræa) furnishes 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 feet 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 (Maraftiaceæ) may also rise to 2 feet or more. But the tree-ferns proper are practically included within the family Cyatheacese. This includes five genera (Cyathea, Alsophila, Hemitdia, Dicksonia, Balantium) and nearly 200 species, of which a few are herbaceous, but the majority arboreal and palm-like, reaching frequently a height of 50 feet or more, Alsophila excelsa of Norfolk Island having some times measured 60 to 80 feet. The fronds are rarely simple or simply pinnate, but usually tripinnate or decom pound, and may attain a length of 20 feet, thus forming a splendid crown of foliage. The stem may occasionally branch into many crowns. The genera are of wide geo graphical range, mostly of course within the tropics of the Old and New World; 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 fibro-vascular bundles characteristic of all fern stems scattered internal as well as external bundles arising from these are superadded; and in a tree-fern these are of course in greater numbers. The outer bundles give off branches to the descending roots from the region where they pass into the leaves.