Biomechanical reconstruction of the carboniferous seed fern Lyginopteris oldhamia: implications for growth form reconstruction and habit

The mechanical architecture of the Carboniferous seed fern Lyginopteris oldhamia is investigated from development stages including naturally decorticated stems. The growth form shows a relatively long semi-self-supporting phase of growth, with prolonged retention of the primary outer "dictyoxylon" cortex contributing significantly during early and mature growth. The outer cortex is retained on the stem despite significant secondary vascular growth via radial and tangential expansion and proliferation of interstitial parenchyma and longitudinal division of the radial bands of fibers. In the final stages of development, the outer "mechanical dictyoxylon" cortex is sloughed from the stem after periderm development in the inner cortex. Loss of the outer cortex results in a decrease in calculated bending stiffness. Calculations of mechanical stem properties based on the overall development indicate for a climber a comparatively stiff mechanical architecture that is comparable with certain types of extant lianas. Mechanisms that allow L. oldhamia to retain and then shed the outer cortex show developmental and mechanical strategies analogous to those of extant climbing plants. The study indicates that relatively sophisticated climbing stem architectures had already evolved by the Late Carboniferous.