Biomechanics of hollow stemmed sphenopsids: II.: Calamites — to have or not to have secondary xylem

Abstract

Biomechanical properties of the Palaeozoic genus Calamites are investigated from 18 anatomically preserved segments of axes from varying ontogenetic stages, a range of parameters based on morphometric measurements of compression fossils and mechanical properties of extant plant tissues. The anatomical sections from isolated fossils (general data) were used to reconstruct a range of possible side-branch lengths (hypothetical positional data) under certain assumptions of tapering, based on the constraints caused by their own weight or by wind loads. Side branch maximal length was then used to estimate the sail-area of a Calamites and the maximal height of the Calamites stem depending on wind speeds and wind profile. The estimate is compared with the Euler buckling length calculated for the stem. Analysed solely on the basis of mechanical constraints, the maximal height of a solitary Calamites would have been limited by wind loads, at least if wind speeds larger than 25 m/s were experienced. In a dense surrounding vegetation canopy, the height of the stem is more likely to have been limited by Euler buckling. The cylinder of secondary xylem would have enabled the plant to survive mechanical failure during periods of water stress when the parenchyma of the stem and branches would not have been fully turgescent and thus conferred certain mechanical advantages over the organisation observed in one of the largest-bodied extant sphenopsids Equisetum giganteum, which tends not to be self-supporting.