Tracheids in an early vascular plant: a tale of two branches

Permineralized xylem strands in an otherwise coalified pseudomonopodially branching axis from the Lower Devonian of Röragen, Norway, allow the description of the architecture of the presumed tracheids using scanning electron microscopy, and a discussion on the roles of the various types of pitting in the functioning of the water‐conducting cells. Tracheid construction differs in the main axis and lateral branch. The former has G‐type tracheids in which close‐set annular secondary thickenings are connected by a further microperforated secondary layer lining the imperforate presumed primary wall. Tracheids in the lateral branch possess bordered pits that may be circular, elliptical or scalariform, with rare examples of reticulate pitting. The secondary wall structures of the tracheids are discussed in relation to their major roles, namely conduction, mechanical strength and safety (i.e. prevention of implosion and embolism) and to different needs in different parts of the plant. It is suggested that in the main axis, tracheids are adapted for longitudinal transport of water with their abundant (presumed) lignification providing mechanical strength and resistance to embolism. In the lateral branch, by contrast, they are apparently adapted for rapid lateral transport at the expense of their structural and safety roles. © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 115–130.