DUCKETT, JEFFREY G.*, ROBERTO LIGRONE, NEIL ANDREWS, AND KAREN SUE RENZAGLIA. School of Biological Sciences, Queen Mary & Westfield College, London, E1 4NS, UK. - The enigma of pegged and smooth rhizoids in marchantialean hepatics; a functional explanation.
Though long cited as a key feature of complex thalloid liverworts,
the functional significance of their dimorphic rhizoids has never been
investigated experimentally. Cryo-scanning electron microscopy of
hydrated, dehydrated and rehydrated thalli now provides the likely
answer. Pegged rhizoids recover their structural and functional
integrity after dehydration, whereas smooth rhizoids tend to remain
completely flattened. The grooves in the carpocephala of
Marchantiales may be regarded as an ‘internalized’ external water
conducting system with the pegged rhizoids therein being the
functional equivalent of hydroids and tracheids. In contrast smooth
rhizoids anchor thalli to their substrata and act as gateways to the
inner thallus cells for endophytic fungi. Some marchantialean taxa
(eg Monoclea, Neohodgsonia) have only smooth rhizoids but these
fall into two size classes. The smaller are interpreted as
derivatives of pegged rhizoids that have lost their wall thickenings
and desiccation-resistant properties due to existence in perpetually
wet habitats.
Key words: cryo-SEM, dehydration, liverworts, rhizoids, water-conduction