Size-related changes in the primary xylem anatomy of some early tracheophytes

Abstract

A survey of the primary xylem anatomy and dimensions of axes (=“stems”) of Upper Devonian plant petrifaction/compression fossils reveals the following: among genera, the primary xylem strand (1) increasingly deviates from a cylindrical geometry (=haplostele) as both the axis diameter and the nonxylem (“ground”) tissue volume increase and (2) increases in geologically younger specimens. (3) The perimeter length to area ratio of the primary xylem strand (or the primary xylem surface area to volume ratio) is dependent upon axis diameter or the area of the ground tissue. This ratio (4) decreases in evolutionary lineages that involve the appearance of larger plants that maintain a haplostelar anatomy (e.g., the evolution of trimerophytes from a rhyniophyte ancestor) or increases in evolutionary lineages involving the appearance of nonhaplostele anatomy (e.g., the evolution of progymnosperms and pteridophytes from trimerophyte ancestors) and (5) the relative amount of primary xylem to ground tissue increases in stratigraphically younger specimens. These trends are interpreted within the context of (1) correlations between the diameter of axes and the complexity of the primary xylem anatomy reported for some extant pteridophytes and (2) changes associated with the evolution of new primary xylem configurations and organographic relationships. Based on the appearance of new ontogenetic patterns and concomitant changes in the anatomy of the primary xylem strand associated with the evolution of derived plant groups, a stria extrapolation of a size to complexity correlation based on ontogenetic studies to the phyletic level is considered suspect.