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