Xylem in early tracheophytes

The architecture of the presumed water-conducting cells of the major lineages of early tracheophytes recorded in Silurian and Devonian rocks is reviewed, together with descriptions of further diverse types whose derivation remains uncertain. Evidence has been obtained from a wide gamut of fossils including coalified compressions, silicified, pyritized and calcium carbonate perimineralizations. Most of the cells, only a few having been unequivocally demonstrated to be tracheidal, have walls with two layers, the inner sometimes broadly similar to annular, spiral and scalariform secondary thickenings of extant xylem. There are, however, very few cases of identical construction and the fossil representatives show greater complexity and variety. Their walls are presumed to have been lignified, but the polymer has not yet been directly identified in the fossils. The implications of these wall architectures on the functioning of the cells in water conduction are briefly considered, as is their relevance to hypotheses on inter-relationships of early tracheophytes and on tracheid ontogeny.