Liverworts and hornworts are a successful group of non-flowering, rootless lower plants, grouped as bryophytes. They possess a polykiohydric nature, namely their water content tends to adjust to the moisture conditions of their environment (Deltoro et al 1998). This condition is markedly different from that of the tracheophytes, which are homohydric, where water supply (from roots) and water status are maintained by stomatal apparatus which prevent the desiccation of the photosynthetic tissue. Unusually though, an ancient living group of bryophytes named hornworts possess stomata in the sporophyte but not the gametophyte (REF).
Polykiohydry in land plants appears to be a much more ancient state than homoihydry, with Mega-fossil evidence of the early land plants shown to be liverwort-like in ultra structure (Niklas 1997; Edwards et al 1998). However within the range of surviving orders of bryophytes, progressive specialisation of their morphology show evidence of a movetowards greater control over water relations; from simplesolid thallus (Pellia spp.) to the more complex differentiated structures with pores (Marchantia spp.) that show more control over water status.
In terms of the origin and evolution of early land plants, modern day bryophytes quite possibly resemble the characteristics of the earliest plants on land. The first good evidence for the existence of bryophyte-like land plants (Eoembryophytes) is seen in spore tetrads (comprising four membrane-bound spores), found over a broad geographic area in the mid-Ordovician period, 476 Myrs (Gray 1993). The combination of decay resistant walls (implying the presence of sporopollenin) and tetrahedral configuration (implying haploid meiotic products) are further diagnostics of land plants. Further evidence lay in spore wall ultra structure and the structure of fossil cuticles from the Late Silurian and Devonian mega fossils, leading Kendrick and Crane (1997) to suggest the above palaeobotanical evidence would support previous arguments that land flora during these times was liverwort-like.
According to Kendrick and Crane (1997), land plants (embrophytes) are most closely related to the Charophyceae, a small group of predominantly freshwater green algae. Within this group, either Coleochaetales (15 living species) or Charales (400 living species) or a group containing both, is a sister group to land plants. Land plant monophyly is supported by comparative morphology and gene sequences (18S cox iii).
Relationships among the major basal living groups are uncertain. But the best supported hypothesis resolves liverworts as basal and either mosses or hornworts as the living sister group to vascular plants (tracheophytes).