THE EVOLUTION OF TRACHEID DIAMETER IN EARLY VASCULAR PLANTS AND ITS IMPLICATIONS ON THE HYDRAULIC CONDUCTANCE OF THE PRIMARY XYLEM STRAND.

A cumulative correlation analysis of the maximum diameter of primary xylem tracheids recorded for 41 tracheophyte fossils, plotted against their ages (ranging from the Upper Silurian to the Lower Devonian), yields a Spearman rank coefficient (rsp ) of 0.696 (P < 0.01). Data for specimens taxonomically referable to zosterophyllophytes and lycopods reveal an increase in the range and maximum diameter of tracheids from the Siegenian to the Emsian. Correlation analysis of these data yields an rsp value of 0.95 (P < 0.02). The mean and maximum tracheid diameters recorded for rhyniophytes, when correlated against stratigraphic occurrence, yield rsp values of 0.81 (P < 0.05) and 0.85 (P < 0.02), respectively. A correlation analysis of the data for rhyniophytes, trimerophytes and progymnosperms yields an rsp value of 0.87 (P < 0.01). Therefore, despite a relatively small sample size of early Paleozoic plants, the available data show a surprising level of statistical robustness. The data are interpreted to indicate that during the early evolution of tracheophytes, both the range and maximum tracheid diameter of the primary xylem increased, while in some plant lineages (zosterophyllophytes) there is evidence for a plateauing of maximum tracheid diameters by the Middle Devonian. The statistical trends in the data are interpreted within the context of the evolution of the hydraulic conductance of tracheophyte primary xylem. © 1985 The Society for the Study of Evolution.