Observations on anatomy of Adoketophyton subverticillatum from the Posongchong Formation (Pragian, Lower Devonian) of Yunnan, China

Abstract

C.-S. Li and Edwards [Palaeontology 35 (1992) 257–272)] revised Zosterophyllum subverticillatum X.-X. Li and C.-Y. Cai (1977) as Adoketophyton subverticillatum. This distinctive plant, of Pragian age (Lower Devonian) from the Posongchong Formation at Zhichang village of Gumu in the Wenshan district of Yunnan Province, China, has terminal strobili composed of decussately arranged, fan-shaped sporophylls, each of which bears a single sporangium attached to the adaxial surface at its base. Three outcrops yielding permineralized parts of this plant, and revealing anatomical details, have been discovered in the Wenshan and Guangnan districts of Yunnan. Observations with optical and scanning electron microscopes reveal that the stem contains a columnar centrarch primary xylem. Metaxylem tracheids are characterized by annular and helical secondary wall thickenings between which are simple perforations. The sporophylls contain parenchyma cells, polygonal or hexagonal in transverse section, and possible tracheids in veins. This combination of anatomy and morphology (a centrarch xylem composed of tracheids of perforated wall structure, and a strobilus of fan-shaped sporophylls bearing adaxial sporangia) illustrates the complexity of land plants that lived during the Early Devonian.