Article: Novel ultrastructure in water-conducting cells of the Lower Devonian plant Sennicaulis hippocrepiformis

A description of the ultrastructure of the water-conducting cells in Sennicaulis hippocrepiformis Edwards (1981), an early land plant of uncertain affinity, is based on pyrite and limonite permineralizations from two Lower Devonian localities in Dyfed and Powys, Wales. The ultrastructure of the two-layered cell wall is unique among land plants, although a simple large helical thickening suggests affinity with the Tracheophyta. The lumen of each cell is lined with a thin microporate layer that overlies the bulk of the wall, including the simple helical thickening, which has a spongy texture. Tapering end walls like those seen in tracheids have not been observed. The reconstruction of the cell wall is based on an analysis of the mineral and coalified material using polished thick sections and scanning electron microscopy; the interpretation relies on comparative morphology and recent advances in knowledge of the process of sedimentary pyrite formation. This novel cell type is shown to be very different from that recently described in a similarly preserved plant, Gosslingia breconensis Heard, and comparisons are also made with presumed water-conducting cells of other early land plants. The microporate layer resembles that found in some extant hepatics, although a convincing argument for a close phylogenetic relationship requires more information on the chemical structure of the wall layers and the morphology of the whole plant.