sentences:  So while Siluro-Devonian plants such as "Rhynia" and "Horneophyton" possessed the physiological equivalent of roots, roots – defined as organs differentiated from stems – did not arrive until later.Unfortunately, roots are rarely preserved in the fossil record, and our understanding of their evolutionary origin is sparse.Rhizoids – small structures performing the same role as roots, usually a cell in diameter – probably evolved very early, perhaps even before plants colonised the land; they are recognised in the Characeae, an algal sister group to land plants.