TRACHEID ULTRASTRUCTURE OF HSÜA DEFLEXA FROM THE LOWER DEVONIAN XUJIACHONG FORMATION OF YUNNAN, CHINA.

We describe the tracheid ultrastructure of Hsüa deflexa from the Lower Devonian (upper Pragian-lower Emsian) Xujiachong Formation, Qujing District, Yunnan Province, China. Scanning electron microscopy study of fusainized and permineralized tracheids shows that they are of G-type, bearing many irregularly distributed perforations. The cell wall consists of a decay-resistant inner part lining the cell lumen and a nonresistant outer part. The resistant inner part, i.e., the assumed heavily lignified secondary wall, is continuous over the poorly lignified outer part that comprises the primary wall and discontinuous secondary wall. It forms the annular thickening over the discontinuous secondary wall. The heavily lignified secondary wall may be divided into three layers. Minute perforations occur on the entire heavily lignified secondary wall, i.e., between and on the thickenings. Concerning the distribution, density, and diameter of perforations, G-type tracheids of H. deflexa share some characters of S-type tracheids. Based on observations of casts, cell wall perforation is cylindrical, with a smooth enclosing wall and two slightly enlarged ends. Lumen casts of tracheids have been formed in several stages. The tracheid ultrastructure of H. deflexa supports the theory of two depositional processes for the secondary cell wall. It indicates that at the genus level of Devonian plants, G-type tracheids differentiate in wall perforation characters; the chemical diversification of the secondary cell wall might have occurred by the Early Devonian. Moreover, this work adds to our knowledge about the evolution of tracheids and early land vascular plants. [ABSTRACT FROM AUTHOR]