PHLOEM ANATOMY IN STAUROPTERIS BISERIATA FROM THE PENNSYLVANIAN OF NORTH AMERICA

Phloem anatomy in the coenopterid fern Stauropteris biseriata is detailed from Lower-Middle Pennsylvanian coal ball specimens from eastern Kentucky. Axes exhibit a cruciate-shaped xylem trace in transverse section. Phloem tissue completely surrounds the xylem, but is more extensively developed in the embayments between the xylem arms. Phloem is composed of elongate conducting elements with a few scattered parenchyma cells. Large and small sieve cells are present, with larger ones occurring in the embayments within the primary plane of symmetry of the axes. Large elements are approximately twice the diameter of the smaller sieve elements. Oval sieve areas and pores have been observed on lateral and oblique end walls of both large and small elements. The structure and composition of Stauropteris phloem is discussed in relationship to the available information on phloem anatomy in other fossil cryptogams. THE GENUS Stauropteris is a relatively poorly understood taxon that has traditionally been included within the coenopterid ferns based upon the anatomy of the axis in cross section (Eggert, 1964; Taylor, 1981). Although reproductive structures have been described (e.g., Scott, 1908; Surange, 1952), the exact morphological nature of the main axis (i.e., whether stem or petiole) is still uncertain. In addition, the presence of both homosporous (S. oldhamia) and heterosporous (S. burntislandica) species within the genus indicates that the natural affinities of this plant are still uncertain. The taxon was originally described from anatomically preserved specimens from the Upper Carboniferous of Europe (Binney, 1872; Bertrand, 1907, 1909), but has also been found in the Lower-Middle Pennsylvanian Breathitt Formation of Kentucky (Cichan and Taylor, 1982). The genus is characterized by axes that exhibit a four-lobed stele with four protoxylem areas in transverse section. Branches are produced in a four-ranked (quadriseriate) pattern in the European species and a two-ranked pattern in Stauropteris biseriata, the North American species. Sporangia are borne terminally on ultimate brances, and vary from globose to elongate. Stauropteris biseriata was described from coal ball specimens collected in eastern Kentucky and included three orders of branching, each I Received for publication 3 October 1985; revision accepted 17 February 1986. Special appreciation is extended to Dr. Thomas N. Taylor, The Ohio State University, for his generous loan of specimens used in this study. Supported in part by the National Science Foundation (BSR-8500541 and BSR8500395) and by the College of Natural Sciences, Hope College. subtended by branched aphlebiae (Cichan and Taylor, 1982). The species is distinguished from the others in the genus (S. burntislandica, S. berwickensis and S. oldhamia) on the basis of the arrangement of the laterals. Fertile parts are unknown to date. Previous studies on the phloem of Carboniferous vascular cryptogams from coal balls have shown that a fairly broad range of anatomical detail is present. The simplest type of phloem anatomy is found in genera such as Etapteris, Botryopteris, Lepidodendron, Stigmaria and Ankyropteris (Stewart, 1940; Eggert and Kanemoto, 1977; Smoot and Taylor, 1978; Smoot, 1979, 1985), all of which contain sieve cells that are similar to parenchyma cells in their overall size and shape. In addition, these genera include phloem tissue that is poorly organized with respect to the arrangement of conducting elements versus phloem parenchyma cells. Contrasting with these relatively simple types, more complex phloem organization has been described from other Paleozoic vascular cryptogams, including Sphenophyllum (Eggert and Gaunt, 1973), Astromyelon (Wilson and Eggert, 1974), Psaronius (Smoot and Taylor, 1981; Smoot, 1984), and Anachoropteris (Smoot, 1985). Specimens of Sphenophyllum and Astromyelon have secondary phloem consisting of sieve elements, axial parenchyma and ray parenchyma. Psaronius and Anachoropteris contain only primary phloem, but both include conducting elements of two distinct sizes with phloem parenchyma organized into definite bands rather than being randomly arranged. The general size and shape of sieve cells that have been described from other fossil cryptogams are very similar, regardless of the