The Devonian Period was characterized by major changes in both the terrestrial biosphere, e.g. the evolution of trees and seed plants and the appearance of multi-storied forests, and in the marine biosphere, e.g. an extended biotic crisis that decimated tropical marine benthos, especially the stromatoporoid-tabulate coral reef community. Teleconnections between these terrestrial and marine events are poorly understood, but a key may lie in the role of soils as a geochemical interface between the lithosphere and atmosphere/ hydrosphere, and the role of land plants in mediating weathering processes at this interface. The e¡ective-ness of terrestrial £oras in weathering was signi¢cantly enhanced as a consequence of increases in the size and geographic extent of vascular land plants during the Devonian. In this regard, the most important palaeobotanical innovations were (1) arborescence (tree stature), which increased maximum depths of root penetration and rhizoturbation, and (2) the seed habit, which freed land plants from reproductive dependence on moist lowland habitats and allowed colonization of drier upland and primary successional areas. These developments resulted in a transient intensi¢cation of pedogenesis (soil formation) and to large increases in the thickness and areal extent of soils. Enhanced chemical weathering may have led to increased riverine nutrient £uxes that promoted development of eutrophic conditions in epicontinental seaways, resulting in algal blooms, widespread bottomwater anoxia, and high sedimentary organic carbon £uxes. Long-term e¡ects included drawdown of atmospheric pCO 2 and global cooling, leading to a brief Late Devonian glaciation, which set the stage for icehouse conditions during the Permo-Carboniferous. This model provides a framework for understanding links between early land plant evolution and coeval marine anoxic and biotic events, but further testing of Devonian terrestrial^marine teleconnections is needed.

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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.