LIGRONE, ROBERTO*, KAREN SUE RENZAGLIA, NEIL ANDREWS, AND JEFFREY G DUCKETT. Facolta di Scienze Ambientali, Secondo Ateneo Napoletano, Via Arena 22, 81100 Caserta, Italy. - Multiple evolution of water-conducting systems in bryophytes.
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