Ontogeny of progymnosperms. II. Shoots of Upper Devonian Archaeopteridales

A consistent pattern of variation of the primary xylem has been documented for most levels of the shoot system of Archaeopteris and for smaller pieces of Callixylon (Archaeopteridales). In general, there is a decrease in diameter of primary xylem from major to minor units of branching. Variation, however, exists in each category of branching. Primary xylem of ultimate and penultimate branches of Archaeopteris is smallest in diameter and number of sympodia (or ridges of the stele) proximally and distally and is largest in midregions. Leaves have much smaller traces with the greatest diameter of their xylem at the base; the traces then contracting with each more distal forking. Thus branches show epidogenetic, menetogenetic, and apoxogenetic changes in xylem and seem to be indeterminate at first but eventually become determinate. Leaves show only apoxogenesis and are clearly both fully determinate and appendicular. Judging from the diameter of its trace, one may speculate that the type of organ a primordium...