THE APEX OF STIGMARIA (LYCOPSIDA), ROOTING ORGAN OF LEPIDODENDRALES

An apical segment of Stigmaria ficoides recently has been located among specimens from Pennsylvanian sediments of Iowa, and provides the first indisputable structural evidence for apical organization and development in one of the most enigmatic of all tracheophyte organs. Toward the apex, the specimen tapers to a circular rim. Rootlet scars occur in a helical arrangement from the proximal end of the specimen to the margin of the rim. Within the rim there is a discontinuous groove that surrounds an irregularly concave apex. The specimen compares favorably with the rooting apex of young Nathorstiana specimens, and adds to our knowledge of homologies among lycophyte rhizomorphs with axial elongation. STRUCTURE and homologies of Stigmaria, the rooting organ of Lepidodendrales, have enamoured and perplexed botanists for over 200 years (e.g., Lindley and Hutton,1837). Whereas the steles of most roots and of virtually all other lycophyte axes have an exarch protostele, Stigmaria has an endarch dictyostele. Also, the lateral appendages (= rootlets) of Stigmaria are helically arranged and abscise like leaves (Frankenberg and Eggert, 1969). Many authors have recognized similarities between Stigmaria and the extant quillwort Isoetes (e.g., Stewart, 1947), but only now are we beginning to resolve, with any degree of confidence, the homologies of lycophyte rooting organs. Interest in the topic recently has been stimulated by divergent interpretations of growth in Isoetes proposed by Paolillo (1963, 1982) and Karrfalt (1975,1982,1984; Karrfalt andEggert,1977a,b,1978),andseveralrecent discoveries have also contributed to the process (Jennings, Karrfalt and Rothwell, 1983). The description of embryos in Lepidocarpon (Phillips, Avcin and Schopf, 1975; Phillips, 1979) allows us to interpret Stigmaria as originating from the first dichotomy of the stem. This realization provides confirmation of the hypothesis that Stigmaria is a shoot modified for rooting, and that arborescent lycophytes have secondarily derived bipolar growth (Stubblefield and Rothwell, 1981). ' Received for publication 15 October 1983; revision accepted 12 March 1984. Acknowledgements are gratefully expressed to Dr. Dwayne D. Stone, Marietta College for providing the specimen, to Dr. James E. Mickle, Illinois State Museum for bringing the specimen to the author's attention, to Dr. Gene Mapes and David Klahn, Ohio University for preparing casts of the specimen, and to Dr. E. E. Karrfalt, Fordham University, for his helpful comments. This study was supported in part by NSF Grant BSR83-10576 and an Ohio University Baker Award. More recently, Karrfalt has emphasized that a radial, axially elongating axis like that of Stigmaria characterizes the rhizomorph of Nathorstiana at an early stage of development and that a bilateral, lobed base like that of Isoetes characterizes it at later stages (Magdefrau,1932 Karrfalt,1984). This important discovery suggests that growth in Stigmaria and in the root producing meristem of Isoetes could occur by the modification of a single type of meristem. However, to develop the suggestion further one must demonstrate a correlation between the apical structure of young Nathorstiana and Stigmaria. Indubitable specimens showing the apical structure of Stigmaria have, until now been unknown (Karrfalt,1982). The most welldescribed specimens of putative apices are discussed by Solms-Laubach (1891), and are of two types. The first is described as having attached appendages that diminish in size and spacing toward the apex, and also extend over the apex as if to form a bud (Goeppert, 1841; Steinhauer, 1818; Solms-Laubach, 1891). The second is characterized by appendage scars, and tapers to the apex where there is a domelike tip (Goldenberg, 1855; von Roehl, 1868 Karrfalt,1975). At the present time, more specific features of these specimens, and whether they all represent apical segments, remain in doubt. MATERIAL-The present specimen is a well preserved apex of Stigmaria that conforms closely to the rooting apex of young Nathorstiana. This provides a basis for interpreting homologies among the rooting structures of lycophytes with radial rhizophores. The specimen was collected by Dr. Dwayne Stone, Department of Geology, Marietta College, Ohio, at an abandoned sandstone quarry along White