The stem, rachides, and pinnae of Archaeopteris macilenta, formerly considered to be a fern of Devonian age, comprise a branch system in which the ultimate divisions heretofore referred to as pinnules are the leaves. The primary vascular system of the "frond" is a lobed siphonostele from which leaf traces arise in a,spiral sequence. The anatomy of the "rachis" and of the "pinnae" is shown to be similar to that of the stem, Callixylon, which bore these "fronds." Branching, epidermal pattern and stomates are described for the spirally arranged leaves (fertile pinnules). Attachment and dehiscence of sporangia as well as their stomates are reported. Archaeopteris is retained in the Class Progymnospermopsida which includes plants with gymnospermous anatomy and pteridophytic reproduction. It is suggested that Actinopodium, Svalbardia and Siderella are related closely to Archaeopteris and that this group of genera shows evolutionary stages in webbing of leaves and planation of branch systems. The opportunities for ontogenetic studies of the arborescent genus Archaeopteris are pointed out.

~ Z P u Yosk 14853, U.S..I. f h r p n n i $~y r i c~ colnphy1lu.i Grirrson & Banks is transferred to flizskinsia Grierson & Banks on the Ixisis of the rnorpliology of its leaves and its anatomical structure. Slender, dichotomizing axes b a r helically arfiiiigrd I c~v r s that iirr falcatr, petiolate, simple, Ianceolate and entire. A solid cylinder of metaxylein is surrountlrd lq ridges of protoxylern. Pitting on tracheid w;ills is annular, helical, reticulate, sc;il;trifi)nn t o circiil;ir-l)ortlrred. 'l'his lycopod extends from early Givetian into early Frasnian time in the Catskill strata of rastrrn N r w York State. The d e r t of clravage of fossiliferous rock on patterns ohsened 011 ttir rxposed fossils mid the Ixiring of Ipcrnt rrsrarch on the Lbssil histor) of lycopods ;irr disc iissrd. m24 4074/8:3/010081 +21$03.00/0 (') 1983 The Linnean Society of Londona4

Ibyka gen. n. is described from late Middle Devonian compressions and petrifactions collected in eastern New York State. It is a robust plant of which three orders of branching and ultimate appendages (leaves) are known. The latter dichotomize up to five times, are arranged spirally on all orders of branching, are three‐dimensional, and all orders are terete in cross section. Fertile appendages, homologous with the sterile, are terminated by sporangia. The protostele has five or six arms, the maturation is mesarch, and the protoxylem disintegrates leaving lacunae at the tips of the arms. Traces to appendages are terete and arise spirally from the tips of the arms. The primary xylem consists of tracheids only, the phloem of thin‐walled cells and probable tanniniferous cells. The cortex consists of parenchyma and groups of sclereids. Secondary xylem is lacking. Ibyka is placed in a new order, Ibykales, close to the Hyeniales (protoarticulates) and to the Pseudosporochnales all three of which probably evolved from Trimerophytina.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. International Association for Plant Taxonomy (IAPT) is collaborating with JSTOR to digitize, preserve and extend access to Taxon. SummaryThe historical reasons for the failure to abandon the category Psilophyta until long after it had become a catchall group for unrelated taxa are reviewed. Recent advances in the understanding of the major types of early land plants are summarized in order to test the usefulness of the three Subdivisions of Tracheophyta that were proposed by Banks in 1968; Rhyniophytina, Zosterophyllophytina, Trimerophytina. HISTORICALThe taxon Psilophyta was retained long after its usefulness had passed. The primary reasons were the creation of an image of early Devonian plants as the simplest imaginable vascular land plants and the impact of such plants on morphological thought. The fact that nearly a century elapsed between the first fragmentary descriptions of some taxa and the final discovery of their critical features such as sporangial position and stelar anatomy permitted unrelated plants to be classified together. I begin with a brief historical account which will be neither complete nor wholly chronological. Past and current concepts, descriptions, and nomenclature will be intermixed in an effort to demonstrate why Psilophyta became a concept so difficult to dislodge.Dawson (1859) recognized that his leafless but spiny, dichotomously branching, vascular plant fossils from the Devonian of Gaspd Peninsula, Canada resembled modern Psilotum. Hence he named the fossils Psilophyton. His first species was P. princeps Dawson I859 for which a type specimen was not selected until 1967 (Hueber and Banks, 1967) and for which a suitable description appeared only in 1968 (Hueber, 1968). Drepanophycus spinaeformis, a second notable Devonian vascular plant, had been described from Germany by G6ppert in 1852. Similar specimens from the Gaspe Peninsula were given the name Arthrostigma gracile by Dawson (1971). Many years later Arthrostigma was recognized to be a synonym of Drepanophycus. Psilophyton and Drepanophycus (Arthrostigma), were long regarded as distinct although they were the two best known Psilophyta. Psilophyton bore spiny emergences, Drepanophycus was clothed with leaves. Fructifications assigned to Psilophyton by Dawson (1859) neither resembled sporangia nor were they organically attached. None were known for Drepanophycus. AUGUST 1975 401 Dawson (1871) subsequently described P. princeps var. ornatum. It bore stout spines. Associated with it, but never attached to the main axis, were groups of paired, terminal, recurved sporangia. Despite the warnings of Solms-Laubach (I895) and Halle (1916) that the terminal sporangia were not proven to be organically...