Ontogeny of progymnosperms. II. Shoots of Upper Devonian Archaeopteridales

Abstract

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 would become was determined by its volume and was related to the overall size of its parental axis. On this basis, therefore, one would expect that small axes might have had apices that bore only small, bilaterally symmetrical primordia that were determinate and that grew into leaves. Apices of larger axes might have borne small, determinate primordia (to leaves) and larger, radially symmetrical, and potentially indeterminate primordia which grew into branches. The number and arrangement of branches vary as the diameter of the parental axes increases; small axes have no branches; larger axes bear two orthostichies of branches on opposite surfaces: still larger axe s bear three or more orthostichies of branches in an irregular but dorsiventral pattern. Organotaxy is most regular in small axes and becomes increasingly irregular in larger ones. Orthostichy fractions are mostly non-Fibonacci and fall into an anomolous series. Number and arrangement of sympodia usually correspond to the orthostichies of appendages except in regions of expansion or contraction of the primary xylem where the number of sympodia may temporarily be greater than the orthostichies. Analysis of the histology of the primary xylem tracts of Aneurophytales and Archaeopteridales suggests that the eustele of progymnosperms might have developed in response to a gradual reduction of auxin produced by the shoot promeristems such that the primary vasculature of an axis differentiated increasingly under the influence of auxins from appendage primordia and with greater quantities of internal xylem parenchyma. Trace-bearing sympodia would thus have been isolated into a system like that of Callixylon. The anatomical variation now documented for Archaeopteris suggests that some or all of the criteria used to maintain Svalhardia, Actinopodium, Eddya, Siderella, and Actinoxylon as genera separate from Archaeopteris are suspect.