All internal water-conducting elements in land plants have three common features; they are dead at maturity, have specialized cell walls and conduct water preferentially as a consequence of transpiration. In bryophytes these elements range from short, thin-walled cells with small plasmodesma-derived holes in Calobryales and Takakia, elongate, thick-walled cells with large perforations in a few metzgerialean taxa (Pallavicinia, Symphyogyna) to very highly elongate imperforate cells (hydroids) in endohydric mosses. Developmental cytological studies indicate that, contrary to long established dogma, none of these are the same as tracheary elements in vascular plants but appear to have closer affinities with elements found in Devonian fossils. Cryo-scanning electron microscopy reveals that the “hydrolysed” appearance of the end walls of hydroids, as seen by transmission electron microscopy is an artefact of aldehyde fixation. The unique ability of hydroids to retain their functional integrity through periods of dehydration is related to a protein moiety in their end walls that renders the cells highly resistant to cavitation.

Key words: Bryophytes, dehydration cryo-SEM, hydroids, water-conduction