Xylem network architecture in basal tracheophytes

Plant hydraulic networks are tightly coupled to carbon assimilation, as well as botanical form and function. Plant water transport must also respond to the concurrent needs for hydraulic efficiency and protection against cavitation-induced hydraulic failure.  How plants balance transport efficiency with safety is functionally well understood but it's only recently that hydraulic network models have supported these requirements.  A recent refinement of the 'WBE' model now incorporates adjustments in conduit size with respect to branch diameter (taper) and frequency (conduit number/area) along the path length, and explicitly predicts scaling exponents for several physiological and anatomical variables of xylem structure and function.  This flexible model was validated for ring-, diffuse- and coniferous xylem, but how well does it predict the relevant scaling components in more basal tracheophytic lineages such as the Lycophytes and Pteridophytes?  Here, we subject the hydraulic architecture of Huperzia sp., Equisetum, Selaginella, and the frond and trunk xylem of the tree fern Dicksonia antarctica, to the same scrutiny, and show that across these basal lineages, the scaling exponents for taper (0.24-0.59) and conduit frequency (^-1.65, ^-1.94) are within the range of predictions for woody plants, at 0.08 to 0.50, and ^-3.35 to ^-0.97, respectively.  Interestingly, the conduit frequency scaling exponent in our two Huperzia species is identical to that of conifers (-1.64). This suggests that selection has acted on xylem network architecture in tracheid-bearing in a convergent manner that is independent of phylogenetic lineage.