Water-Conducting Cells in Early Fossil Land Plants: Implications for the Early Evolution of Tracheophytes

Wall structure in the water-conducting cells of Rhynia gwynne-vaughanii and Asteroxylon mackiei from the Lower Devonian Rhynie Chert was examined and compared using thin sections and scanning electron microscopy of etched sections. Although the internal thickenings of these cells are superficially similar in both plants, there are significant differences in other aspects of wall structure. The tracheids of A. mackiei are shown to be of a basic type (G-type) that is common in some early land plant fossils such as zosterophylls and lycopods, and they are comparable to protoxylem elements in some extant `pteridophytes'. The "tracheids" of R. gwynne-vaughanii are more similar to another kind of water-conducting cell (S-type) that combines certain features of tracheids and moss hydroids. The S-type cell is known from two other Lower Devonian sporophytes, Stockmansella langii and Huvenia kleui, supporting recent suggestions that these three taxa form a natural group. S-type cells are also found in the gametophyte Sciadophyton sp. as well as two taxa, Sennicaulis hippocrepiformis and Taeniocrada dubia, for which reproductive structures are unknown. The water-conducting tissues of other early land plants are briefly reviewed and detailed reconstructions of the S-type and G-type cell are provided. A preliminary cladistic analysis focusing on the Rhyniophytina of Banks results in the recognition of a `protracheophyte' grade, as well as a small clade, the Rhyniaceae, comprising Rhynia, Stockmansella, and Huvenia. If the tracheid-like features of S-type and G-type cells are regarded as homologous, then the Rhyniaceae are resolved as the basal clade that forms the sister group to all other tracheophytes. The occurrence of a more or less isomorphic alternation of generations in some Devonian `protracheophytes' and the Rhyniaceae implies that the gametophytes of all extant `pteridophytes' are phylogenetically reduced.