Ontogenetic Reconstruction of the Carboniferous Seed Plant Lyginopteris oldhamia

Abstract

Two developmental trajectories are hypothesized for growth of the aerial stem and branch system in Lyginopteris oldhamia. They include (1) a determinate single‐phase model, where the primary body decreases in size along the main stem or branches toward the apex, and (2) a less determinate two‐ or three‐phase model, where the primary body of stems and branches increases in size (epidogenesis), possibly maintaining a relatively constant maximal size (menetogenesis), and then diminishes in size (apoxogenesis) toward the apex. Developmental patterns of primary tissues (pith, primary xylem, inner and outer cortex) and secondary tissues (periderm and products of the bifacial vascular cambium, including wood and secondary phloem) are investigated within the context of each model. Primary tissues of inner and outer cortex as well as peridermal tissue initiated in the mid inner cortex show significant changes in response to growth of the bifacial vascular cambium. Patterns of development in both primary and secondary tissues indicate an overall development more consistent with epidogenetic, menetogenetic, and apoxogenetic phases of development. Evidence of epidogenesis is indicated by the presence of small primary bodies with (1) high levels of vascular cambial development and periderm and (2) high levels of compensatory enlargement of the outer primary cortex via tangential and radial deformation in addition to cellular proliferation. Following expansion of the wood cylinder, the outer primary cortex shows extended structural integrity and possible “self‐repair” during extreme straining of the outer cortex. Final development involves sloughing of the entire primary cortex and establishment of an entire layer of periderm enclosing the secondary phloem and wood.