Morphometric analysis of Rhynia and Asteroxylon: testing functional aspects of early land plant evolution

New morphometric data gathered from cross-sections of two Lower Devonian la nd plants (Rhynia gwynne-vaughanii and Asteroxylon mackiei) are interpreted in terms of the evolution of the function of vascular bundles in early lan d plants. The following conclusions can be drawn from these new data: (1) T he ratio of the cross-sectional area of the xylem (representing the conduct ing volume supplying the axis with water) to the xylem perimeter (represent ing the "contact area" between xylem and parenchyma through which water lea ves the xylem and enters the parenchyma) is not constant for Rhynia axes, a lmost constant for Asteroxylon axes, and different between Rhynia and Aster oxylon. Thus, Bowers hypothesis that the ratio of cross-sectional area of t he xylem to xylem perimeter is constant during ontogenetic development is t rue for Asteroxylon. That this ratio is constant during phylogeny, however, is not supported by our data. (2) The ratio between cross-sectional area o f xylem to parenchyma is higher in Asteroxylon than in Rhynia. (3) As predi cted by previous computer simulations, the ratio of the xylem perimeter to the axis perimeter plays a major role in determining water transport perfor mance of the transpiring axis. This ratio is constant within ontogeny but i s different in Asteroxylon and Rhynia. In Asteroxylon axes, this ratio is a bout twice as large as in Rhynia axes. (4) Contrary to the expectations, th e distance between the outermost layer of the xylem and the transpiring sur face, which represents the low-conductivity pathway through the parenchyma, appears not to be a limiting factor for the water transport in axes of Rhy nia and Asteroxylon. (5) From the analysis of the geometric parameters, it is evident that Rhynia and Asteroxylon with their distinct stelar geometrie s represent two different constructional types for which no transitional st ages are known.