Abstract
The origin of xylem in the Silurian was a major step in plant evolution, leading to diverse growth forms with various mechanical and hydraulic properties. In the fossil record, these properties can only be investigated using modelsbased on extant plant physiology. Regarding hydraulics, previous studies have considered either the properties of a single tracheid or of a set of independent tubes. Here, we use the analogy between the flow of water under tension in a plant and an electrical circuit to develop an extension of Wilson’s single tracheid model to the tissue scale. Upscaling to the tissue-level allows considering wood as a heterogeneous tissue by taking into account differences in tracheid density and the presence of rays. The new model provides a more biologically accurate representation of fossil wood hydraulic properties. The single tracheid and new tissue models are applied to two conspecific specimens of Callixylon (Progymnospermopsida, Archeopteridales) from the Late Devonian of Morocco. Differences are shown at the tissue level that cannot be suspected at the single tracheid level. Callixylon represents the first trees with a conifer-like wood and is a major component of Late Devonian floras worldwide. Our results show that the anatomical disparity of its wood might have led to hydraulic plasticity, allowing growth in various environmental conditions. More generally, the new tissue-model suggests that the various combinations of tracheid and ray sizes present in Palaeozoic plants might have led to a higher variety of ecophysiologies than suspected based solely on the properties of individual tracheids.