Charles B. Beck

The identity of Archaeopteris and Callixylon

Summary and ConclusionsA specimen from the late Devonian Katsberg beds of Delaware County, New York, comprising a pyritized axis determined asCallixylon and leaves determined asArchaeopteris, is described and illustrated. The leaves are probablyA. macilenta Lesquereux and the axisC. zalesskyi Arnold, but because of doubt of the specific identity of the leaves a nomenclature transfer is delayed. Archaeopteris, the valid name for the plant represented by the organ generaArchaeopteris andCallixylon, is included withPitys andArchaeopitys in the Pityales which, together with the orders Aneurophytales and Protopityales, is assigned to the newly recognized class Progymnospermopsida. This class includes woody, pteridophytic plants bearing (where known) large compound leaves or leaf-like branch systems. In numerous characters of both external morphology and internal structure these plants are remarkably similar to two groups of gymnosperms, the Pteridospermales and Cordaitales, which are, respectively, the most primitive groups of the cycadophyte and coniferophyte lines of gymnosperm evolution. Because the Progymnospermopsida are pteridophytic they can-not be logically classified with the ovule-bearing gymnosperms, but it is very likely that they comprise the ancestral complex from which the major groups of gymnosperms evolved. Certain primitive features, especially of the Aneurophytales, suggest that the Progymnospermopsida are descended directly from some psilophyte-like ancestors.The existence of such a group of pteridophytic plants, of possible psilophytic origin, obviously not ferns, showing unmistakeable gymnosperm characters, and which preceded any known gymnosperms in time, eliminates the necessity to consider any group of ferns, known or unknown, as ancestors of the gymnosperms. This supports the separation by Bold (1956) of the Filicineae and Gymnospermae, and in part his abandonment of Pteropsida. On the other hand it suggests that the major groups of gymnosperms (possibly excluding the Gnetales) have a common ancestry, are consequently genetically related, and should, therefore, be retained in a single inclusive taxon.

Stelar morphology and the primary vascular system of seed plants

This paper deals primarily with the morphology, anatomy, and evolution of the eustele in seed plants. Introductory sections treat stelar terminology, problems of representation and interpretation of stelar diagrams, and the history of studies on the stele. Also included is a classification of stelar types. A significant part of the paper consists of descriptions and illustrations of the primary vascular systems of the stems of all major seed plant taxa (and their progymnosperm precursors) for which data were available. In a critical analysis of recent studies, the stele of monocotyledons is interpreted as a eustele that has become modified in relation to the distinctive morphology and modes of development of this group. Our viewpoint contrasts with that of Zimmermann and Tomlinson who consider the monocotyledon stele to be fundamentally different from that of dicotyledons. In a section on nodal anatomy the emphasis by some systematists on characters of nodal structure is decried because, as is demonstrated, taxa with similar nodal anatomy may differ significantly in their internodal structure. An original statistical study, based on characters of the primary vascular systems of 102 species of dicotyledons and data from other sources, provides the basis for a model of the primitive eustele in seed plants, for a discussion of the adaptive value of certain characteristics of the eustele, and for recognizing probable trends of specialization in the eustele. The primitive eustele is characterized as an open primary vascular system with helical trace departure, and consisting of five sympodia. It is suggested that during the course of evolution of the eustele there has been an increase in the number of vascular bundles in the system. This, apparently, has been accomplished in the gymnosperms (as reflected in the conifers) by an increase in the number of axial bundles, but in the angiosperms by an increase in the number of traces per leaf and an increase in the number of internodes traversed by leaf traces prior to their entry into leaves. There seems to have been a concomitant establishment of connection between the sympodia in the vascular system. Both the increase in number of vascular bundles and their interconnection seem to be adaptive because they probably enhance the survivability of individuals whose vascular systems are damaged by herbivores or other biotic or physical agents. Because diversity among stelar types is relatively limited, stelar morphology seems to have systematic significance primarily at or above the ordinal level. The paper closes with a set of recommendations designed to encourage the future production of comparable, useful data on the stele.PrécisCe travail traite surtout de la morphologie, de l’anatomie, et de l’évolution de la stèle des plantes à graines. Les parties initiales introduisent la terminologie stélaire, les problèmes de représentation et d’interprétation des diagrammes stélaires, et l’histoire des études faites sur la stèle. Aussi inclue est une classification des types stélaires. Une très grande partie de ce travail est consacrée aux descriptions et aux illustrations des systèmes vasculaires primaires des tiges de tous les groupes majeurs des plantes à graines (aussi bien que les progymnospermes qui les ont précédées) pour lesquels les donées soient disponibles. Dans une analyse critique des études récentes, la stèle de monocotylédones est considérée comme une eustèle modifiée vis-à-vis de la morphologie distinctive et des modes de développement de ce groupe. Notre point de vue diffère de celui de Zimmermann et Tomlinson qui considèrent la stèle monocotylédoné différente fondamentalement ce celle des dicotylédones. Dans une partie sur l’anatomie nodale, on décrie l’emphase mise sur des aspects de structure nodale par quelques taxonomistes car, comme constaté, des taxons d’une anatomie nodale similaire peuvent se différer d’une manière significative en ce qui concerne leur structure internodale. Une étude statistique originale, basée sur les caractères de systèmes vasculaires primaires de 102 espèces de dicotylédones, aussi bien que des donées d’autres sources, fournit la base d’un modèle de l’eustèle primitive des plantes à graines, d’une discussion des valeurs adaptives de certains caractères de l’eustèle, et d’une reconnaissance des tendances probables de spécialisation dans l’eustèle. L’eustèle primitive se caractérise en système vasculaire primaire ouvert qui comprend un écart de trace hélicoidale et cinq sympodies. On suggère qu’à travers l’évolution de l’eustèle il y avait une augmentation de nombre de faisceaux vasculaires dans le système. Ceci a été apparamment accompli dans les gymnospermes (par exemple, parmi les conifères) par une augmentation du nombre de faisceaux axiaux, maix dans les angiospermes par une augmentation du nombre des traces dans chaque feuille et par une augmentation du nombre d’internodules traversés par les traces de feuille avant leur entrée dans les feuilles. Il semble avoir été un établissement concomitant de liaison entre les sympodies du système vasculaire. Tant l’augmentation du nombre de faisceaux vasculaires que leur liaison semblent adaptives à cause de leur capacité probable d’améliorer la capacité de survivre chez les individus dont les systèmes vasculaires sont endommagés par des herbivores ou par d’autres agents biotiques ou physiques. Parce que la diversité des types stélaires est relativement limitée, la morphologie stélaire semble avoir de la signification systématique au niveau ou au-dessus de niveau ordinal. On termine le travail en présentant une série de recommandations destinées à encourager la future production des données utile, d’un ordre scientifiquement comparable sur la stèle.Kurze ÜbersichtDiese Arbeit befasst sich hauptsächlich mit der Morphologie, Anatomie und Evolution der Eustele in Samenpflanzen. Einleitende Abschnitte behandeln Stelenterminologie, Probleme der Veranschaulichung und Interpretation von Stelendiagrammen, und die Geschichte der Forschung über die Stele. Auch enthalten ist eine Klassifizierung von Stelentypen. Ein bedeutender Teil dieser Arbeit besteht aus Beschreibungen und Abbildungen der Primärgefässsysteme von den Stämmen aller wichtigen Samenpflanzengruppen (und ihrer Progymnosperm Vorläufer) für welche Daten vorhanden waren. In einer kritischen Analyse der vor kurzem entstandenen Studien, wird die Stele der Monocotyledonen als eine Eustele interpretiert, die in Beziehung zu der eigentümlichen Morphologie und den Arten der Entwicklung dieser Gruppe modifiziert worden ist. Unser Gesichtspunkt kontrastiert mit dem von Zimmermann und Tomlinson, die die Stele der Monocotyledonen als grundsätzlich verschieden von der Stele der Dicotyledonen ansehen. In einem Abschnitt über Knotenanatomie wird die von manchen Systematikern auf Charaktere der Knotenstruktur aufgelegte Betonung abgelehnt, weil, wie gezeigt wird, Gruppen mit ähnlicher Knotenanatomie in ihrer inwendigen Knotenstruktur wesentlich verschieden sein können. Eine statistische Originalstudie, die sich auf Merkmale der Primärgefässsysteme von 102 Arten Dicotyledonen und auf Daten von anderen Quellen basiert, ergibt die Grundlagen für ein Modell der ursprünglichen Eustele in Samenpflanzen, für eine Diskussion über den Anpassungswert von bestimmten Merkmalen der Eustele, und für das Erkennen von vermutlichen Tendenzen der Spezialisierung in der Eustele. Die ursprüngliche Eustele wird als ein offenes Primärgefässsystem mit schneckenförmigem Spurabgang charakterisiert, welches aus fünf Scheinachsen besteht. Es wird behauptet, dass während der evolutionären Entwicklung Eustele eine Erhöhung in der Anzahl der Gefässbündel stattgefunden hat. Diese Erhöhung wurde anscheinend in den Gymnospermen vollzogen (wie bei spielsweise bei den Koniferen), und zwar mit einer Erhöhung in der Anzahl von achsenförmigen Bündeln, in den Angiospermen jedoch mit einer Erhöhung in der Anzahl der Spuren pro Blatt und einer Erhöhung in der Anzahl von inwendigen Knoten, die vor ihrem Eingang in die Blätter von Blattspuren durchquert worden sind. Es scheint dort eine begleitende, gegenseitige Verbindung der Scheinachsen gegeben zu haben. Sowohl die Erhöhung in der Anzahl der Gefässbündel als auch deren gegenseitige Verbindung scheinen anpassungsfähig zu sein, weil sie wahrscheinlich die Überlebensfahigkeit der Individuen, deren Gefässsysteme von Pflanzenfressern oder anderen biotischen oder physischen Agenten beschädigt worden sind, steigern. Weil Mannigfaltigkeit unter Stelentypen relativ beschränkt ist, scheint die systematische Bedeutung der Stelenmorphologie hauptsächlich an oder über der Ordnungstufe zu liegen. Die Arbeit schliesst mit einer Reihe von Vorschlägen, die es beabsichtigen, die kunftige Hervorbringung von vergleichbaren, nützlichen Daten über die Stele anzuregen.

THE APPEARANCE OF GYMNOSPERMOUS STRUCTURE

During Devonian time, as diversification of psilophytes, lycopsids, sphenopsids and ferns was taking place and these groups were evolving into clearly definable taxa, another major group, the progymnosperms (Beck, 1960u, b, 1962a), was also becoming established as a dominant element of the flora. This group first becomes clearly recognizable in Middle Devonian strata. By Upper Devonian time it had acquired many of the important characteristics of gymnosperms. The progymnosperms are of great significance because they seem to be the immediate ancestors of seed plants (Beck, 1960b, 1962a, 1964a, 1966; Meeuse, 1963; Banks, 1968). In this paper I shall consider the appearance, primarily within this group of plants, of three major aspects of gymnospermous vegetative structure: the secondary wood, the leaf, and the primary vascular system.

Current status of the Progymnospermopsida

Abstract The three progymnosperm orders, Aneurophytales, Protopityales, and Archaeopteridales, are characterized and analyzed. The possibility that some of the apparent taxonomic diversity of the Aneurophytales and Archaeopteridales might reflect developmental variation is considered as well as the probability that much of it reflects evolutionary change. A detailed discussion of the phylogenetic position of the Progymnospermopsida concludes the paper.

Connection between Archaeopteris and Callixylon

Characters of two Upper Devonian genera, Archaeopteris, often considered to be a fern, and Callixylon, classified with the gymnosperms, have been recognized in a single specimen.

Techniques for preparation of pyrite and limonite permineralizations

Stein, W.E., Jr., Wight, D.C. and Beck, C.B., 1982. Techniques for preparation of pyrite and limonite permineralizations. Rev. Palaeobot. Palynol., 36: 185--194. Procedures are presented for the preparation of fine-polished surfaces of pyrite permineralizations, and demineralized thin-sections of limonite permineralizations. A major new feature in both procedures involves the re-embedding in plastic of individual cut sections in order to promote stability of the often crumbly material during further preparation. Pyrite sections are kept flat during polymerization of the plastic by means of a binder clip press. The technique for limonite permineralizations is characterized by the use of plastic not only for re,embedding individual sections but also for adhesion of sections to glass slides. Following this, sections are ground thin and demineralized with hydrochloric acid. We strongly recommend use of the thin-section technique for all studies utilizing limonite permineralizations because this method permits a much greater resolution of detail than is possible with other methods.

Yunia dichotoma, a lower devonian plant from Yunnan, China

Abstract This paper describes a new genus and species, Yunia dichotoma , collected from the Posonchong Formation of Siegenian age in the Wenshan district of Yunnan, China. The spiny axes are characterized by cruciate dichotomy. Associated, but not in organic connection, with the vegetative axes, are numerous elongate-elliptical or ovoid sporangia. One small branching system consisting of a single dichotomy bears the basal part of a sporangium, suggesting the possibility that the sporangia were borne terminally in pairs. Sections of permineralized segments of the axis reveal a columnar protostele, circular to elliptical in transverse section containing one or two roughly circular to transversely elongate (often lunate) regions of large parenchyma cells sparsely interspersed peripherally with tracheids of small diameter. We interpret these regions of parenchyma and tracheids to be protoxylem strands and development of primary xylem to be centrarch. During branching of an axis, the initially single protoxylem region in a branch stele divides precociously prior to the separation of the daughter axes. Thus, the stele may contain two protoxylem strands through considerable lengths of daughter axes between levels of dichotomy. Yunia is tentatively assigned to the Trimerophytales.

Structure of the Earliest Leaves: Adaptations to High Concentrations of Atmospheric CO2

New fossils of the early Devonian plant Eophyllophyton bellum provide morphological and anatomical information that contributes to an understanding of the origin and early evolution of the megaphyll. These earliest leaves are characterized by leaf divisions, apparently arranged in several planes, that possess branching venation and a mesophyll of longitudinally elongate cells lacking differentiation into palisade and spongy layers. These features support the hypothesis that the megaphyll was derived from an axial branching system. The small size and deeply incised margin of these leaves indicate an adaptation to an environment of heat stress and high concentration of atmospheric CO2. Whereas the available evidence indicates that the megaphyll evolved during the early Devonian, a period of harsh environmental conditions, the evolution and radiation of larger megaphylls was apparently correlated with a massive drop in atmospheric CO2 during the late Devonian and early Carboniferous.

Sieve cells in Phloem of a middle devonian progymnosperm.

Phloem tissue from a Middle Devonian member of the Aneurophytales (Progymnospermopsida) is described. This may be the oldest firm evidence of conducting elements of the phloem, extending our knowledge of this tissue back some 35 million years. The discovery indicates a close phylogenetic relation between progymnosperms and gymnosperms and provides a basis for investigating patterns of specialization in the phloem of these groups of plants.

The primary vascular system of Callixylon

Beck, C.B., 1979. The primary vascular system of Callixylon. Rev. Palaeobot. Palynol., 28: 103--115. The primary vascular system of Callixylon (main stem axes of Archaeopteris) is composed of a discontinuous cylinder of longitudinal bundles from some of which traces diverge radially in a regular, helical pattern. Tangential divisions and fusions of primary xylem strands occur in approximately equal frequency and, thus, the number of bundles in the system remains constant in the specimen studied. Following divergence, traces increase in size and become ribbed distally. The available evidence suggests that all of the traces observed in this study are branch traces. Branch bases, embedded in secondary xylem, have the anatomical characteristics of penultimate axes of lateral branch systems of Archaeopteris. It is suggested that some regions of main axes of Archaeopteris might have borne lateral branch systems only, lacking entirely simple leaves. Lateral branch systems, therefore, might have been more similar to compound leaves than previously supposed.