The origin and early evolution of tracheids in vascular plants: integration of palaeobotanical and neobotanical data

Although there is clear evidence for the establishment of terrestrial plant life by the end of the Ordovician, the fossil record indicates that land p lants remained extremely small and structurally simple until the Late Silur ian. Among the events associated with this first major radiation of land pl ants is the evolution of tracheids, complex water-conducting cells defined by the presence of lignified secondary cell wall thickenings. Recent palaeo botanical analyses indicate that Early Devonian tracheids appear to possess secondary cell wall thickenings composed of two distinct layers: a degrada tion-prone layer adjacent to the primary cell wall and a degradation-resist ant (possibly lignified) layer next to the cell lumen. In order to understa nd better the early evolution of tracheids, developmental and comparative s tudies of key basal (and potentially plesiomorphic) extant vascular plants have been initiated. Ultrastructural analysis and enzyme degradation studie s of wall structure (to approximate diagenetic alterations of fossil trache id structure) have been conducted on basal members of each of the two major clades of extant vascular plants: Huperzia (Lycophytina) and Equisetum (Eu phyllophytina). This research demonstrates that secondary cell walls of ext ant basal vascular plants include a degradation-prone layer ('template laye r') and a degradation-resistant layer ('resistant layer'). This pattern of secondary cell wall formation in the water-conducting cells of extant vascu lar plants matches the pattern of wall thickenings in the tracheids of earl y fossil vascular plants and provides a key evolutionary link between trach eids of living vascular plants and those of their earliest fossil ancestors . Further studies of tracheid development and structure among basal extant vascular plants will lead to a more precise reconstruction of the early evo lution of water-conducting tissues in land plants, and will add to the curr ent limited knowledge of spatial, temporal and cytochemical aspects of cell wall formation in tracheary elements of vascular plants.