XVI International Botanical Congess

Three kinds of water conducting cells characterising major lineages of early tracheophytes are reviewed. A further novel strand is described from sterile, naked axes from Lower Devonian rocks. The central cells have homogenised walls surrounded by cells with irregular secondary thickenings, some traversing the lumen. The latter is lined by a loose microporate layer or one with minute globules. A comparative survey of the development, (ultra)structure and functioning of moss hydroids and liverwort water-conducting elements weakens homology between these and the tracheids in extant tracheophytes. Conducting elements have not yet been recovered from Lower Devonian axial sporophytes with terminal sporangia containing tetrads and dyads similar to those found in Ordovician rocks and thought to derive from plants at a bryophyte grade.