MODELING FLUID FLOW IN AN EXTINCT SEED PLANT

Recent advances in whole-plant physiology, when combined with new techniques for microchemical analysis, allow the functional significance of fossil terrestrial plant structures to be understood quantitatively, even in taxa with no close living relatives or analogues. Modeling fluid flow in the Paleozoic seed fern Medullosa illustrates this approach. Medullosa has commanded attention for over 100 years because of its unusual anatomy and morphology, with anomalous secondary development modifying a eustele into a ring of discrete vascular strands that anastomose throughout the plant. Medullosan fronds have large surface areas relative to the dimensions of the stems that support them; furthermore, medullosan tracheids are among the widest and longest found in the terrestrial plant record. Modeling fluid flow through tracheids shows that the wide, long, and extensively pitted tracheids of Medullosa could conduct 2-30 times more water than those of cordaites and conifers, even at comparable xylem cell widths. However, medullosan xylem likely functioned near the physical limit of tracheid fluid flow, risking lethal implosion at modest pressure gradients, and could not have been a source of structural support for the plant. The unusual cambium found in Medullosa created an intimate relationship between tracheids and parenchyma cells, potentially maximizing the ability of the plant to repair embolisms. This anatomical arrangement also places the secondary xylem much closer to primary xylem strands, minimizing resistance to flow to the actively transpiring leaf surface. Our results indicate that medullosan anatomy is strongly adapted to provide a constant volume of water to large leaves with high evapotranspiration rates.