Tree fern - any of a group of relatively primitive ferns in the order Cyatheales, most of them characterized by ascending trunklike stems and an arborescent (treelike) habit. Tree ferns are conspicuous plants of humid tropical forests around the world.
They have a large, single trunk with an extensive root system in which the tangled mass of roots may be as much as 30 ft (9 meters) deep and from 2 to 3 ft (60 to 90 cm) across. The base of this trunk is often protected by the persistent leaf bases, forming a kind of water-bearing cup. The trunk is also protected by dense masses of tough, lanceolate scales (leaves) along its length. Except in Cyathea, which lacks roots and has very large spreading fronds (leaves), the trunk does not branch; instead, it terminates in a crown of foliage. In the genera Cyathea, Alsophila, and Sphaeropteris, the foliage includes a single large leaf—often 30 ft (9 meters) in length in some species of Cyathea—which is borne at the apex of the trunk.
Among these ferns is Cyathea, which consists of about 380 species, distributed throughout the world. The best-known representative of the group is probably the Australian tree fern ( C . australis ), with trunks up to 20 meters (65 feet) tall; this species was formerly classified in the genus Phlebodium.
Some tree ferns, such as Dicksonia antarctica, have a single trunk that is topped with many spreading crowns of large lacy fronds. Others, such as the Australian Lophosoria quadripinnata, develop several trunks from a single rootstock to form a cluster of separate stems. Notable among the group is Cyathea brownii, the only fern to grow extensively on New Zealand's North Island.
Many species of tree ferns, such as Elaeocarpus joga and Dicksonia squarrosa, are cultivated as ornamentals. Australian tree ferns (Cyathea australis) have been used as houseplants for over 100 years, and the Japanese species C . tamariscina has become popular since its first introduction to Europe in 1859.
Tree ferns produce spores rather than flowers; they are therefore classified with seedless vascular plants (gymnosperms) instead of flowering plants (angiosperms). Sori, the sites of spore production, develop on the undersides of young fronds.
In Cyathea and Dicksonia, two kinds of sori are produced: bottlebrush types, in which a central stalk supports a cluster of individual sporangia (spore cases), and scattered types, in which each sporangium is borne on a narrow stalk arising from the tissues of the rhizome.
Cyathea subg. Colysis contains only bottlebrush-sorus types, but these are relatively large and showy. The most widely cultivated species is probably C. dealbata, known as the silver tree fern, which grows in tropical and subtropical regions around the world. The Australian tree fern (Cyathea australis) is much shorter when mature, having a single unbranched trunk topped with many spreading crowns of large lacy fronds.