How early ferns became trees

A new anatomically preserved fern, discovered from the basalmost Carbonifer ous of Australia, shows a unique combination of very primitive anatomical c haracters (solid centrarch cauline protostele) with the elaboration of an o riginal model of the arborescent habit. This plant possessed a false trunk composed of a repetitive branching system of very small stems, which establ ished it as the oldest tree-fern known to date. The potential of this primi tive zygopterid fern to produce such an unusual growth form-without real eq uivalent among living plants-is related to the possession of two kinds of r oots that have complementary functional roles: (i) large roots produced by sterns with immediate positive geotropism, strongly adapted to mechanical s upport and water uptake from the soils and (ii) small roots borne either on large roots or on petiole bases for absorbing humidity inside the false tr unk.