A Tetrahedral Megaspore Arrangement in a Seed Fern Ovule of Pennsylvanian Age

Schabilion, J. T.; Brotzman, N. C.

doi:10.1002/j.1537-2197.1979.tb06278.x

Cited by 6 publications

(2 citation statements)

References 15 publications

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“…According to Pettitt (1969), all Pennsylvanian ovules except Pachytesta vera produced linear tetrads as adjudged by the absence of triradiate sutures and abortive megaspores. To Schabilion and Brotzman (1979), in agreement with Taylor (1965), evidence for a tetrahedral arrangement in P. vera is inconclusive, but they reported a tetrahedral tetrad for the Pennsylvanian pteridosperm, Conostoma anglogermanicum. Kremp, however, commented only on variation in the relationship of nutrition to aperture form in energy-storage spores.…”

Section: Origin Of the Ovule: A New Conceptsupporting

confidence: 70%

The origin of the ovule

Herr

1995

American J of Botany

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Of several theories for the origin of the ovule advanced in this century and based largely on fossil evidence, the telomic concept is widely considered the most plausible. Its principal tenet is the evolution of the integument through fusion of sterile branches or telomes around a terminal megasporangium. The only point of agreement in these theories is that the entire nucellus is a megasporangium that retains a single megaspore and the endosporic female gametophyte. Their differences center on the origin of the integument. A new concept offered here on evidence from ovules of both Paleozoic and extant seed plants significantly alters the telomic theory. It proposes that the nucellus is a sporangiophore of stem origin that bears a terminal megasporangium; that at least some of the fused integumentary telomes were fertile; and that among all features cited to characterize ovules, the unique nature of the retained megaspore alone defines the seed habit. Changes in the seed plant megaspore that extended the period of nutrient absorption over the whole course of female gametophyte development, along with complex physiological changes in the nucellus, were probably achieved along a single phylogenetic line beginning in a Late Devonian population of progymnosperms. For such a combination of events to have occurred more than once is highly unlikely, and, therefore, a monophyletic origin for seed plants is proposed. Several primitive features in ovule structure, some not evidenced since the Lower Carboniferous Period, occur in a mutant form of Arabidopsis thaliana isolated from genetically transformed plants. Their recurrence provides additional support for the proposed concept of ovule origin and also suggests that the genetic mechanisms for expression of primitive features in advanced taxa could be initiated in each case by mutation of a single homeotic gene.

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Section: Origin Of the Ovule: A New Conceptsupporting

confidence: 70%

The origin of the ovule

Herr

1995

American J of Botany

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“…The megaspore membrane was thick (Oliver & Scott, 1904) and may be up to 5 Am in some lyginopterid pteridosperms (Taylor & Millay, 1981). It is unknown whether the megaspore of L. lomaxi had a trilete mark, but this feature does occur in other lyginopterid megaspores (Pettitt, 1969;Schabilion & Brotzman, 1979), and in Stamnostoma huttonense there is a cluster of three aborted spores at the apex of the megaspore (Long, 1975). This suggests that lyginopterid megaspores were derived from a tetrahedral rather than linear tetrad.…”

Section: Medullosansmentioning

confidence: 89%

Phylogenetic Analysis of Seed Plants and the Origin of Angiosperms

Crane¹

1985

Annals of the Missouri Botanical Garden

547

389

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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.. Missouri Botanical Garden Press is collaborating with JSTOR to digitize, preserve and extend access to Annals of the Missouri Botanical Garden. ABSTRACT Principles of phylogenetic analysis (cladistics) are introduced with an examination of relationships among extant genera of Gnetales. The Gnetales can be supported as a monophyletic group, with Gnetum and Welwitschia more closely related to each other than either is to Ephedra. Characters of the progymnosperm Archaeopteris and 19 extinct and extant seed plant taxa are then reviewed as a basis for a cladistic analysis of their interrelationships. The seed plant taxa included are: Bennettitales are also presented. Results suggest that seed plants are a monophyletic group, and Lyginopteris is resolved as the sister taxon to all other seed plants considered. The cordaites, Cordaixylon and Mesoxylon, along with Lebachia, extant conifers, and Ginkgo constitute a monophyletic group. Pentoxylon is the sister taxon to Bennettitales, and the Gnetales are the sister group to angiosperms. Together the Gnetales plus the angiosperms form the sister group to the Bennettitales plus Pentoxylon. If the outer integument of bennettitalean and angiosperm ovules is interpreted as homologous with the "cupule" of Caytonia and corystosperms, then the corystosperms are resolved as the sister group to the Bennettitales plus Pentoxylon plus Gnetales plus angiosperm clade. Under this interpretation all the seed plant taxa considered except Lyginopteris, cycads, and medullosans are part of a single clade in which flattened seeds and saccate pollen are primitive. The principal difficulties with the cladistic analysis concern necessary inferences on unknown characters in certain plants, and the current absence of a large base of comparative data. Results of the analysis suggest that the seed ferns as currently circumscribed are not a meaningful group for phylogenetic purposes and permit an evaluation of the possible phylogenetic position of Eremopteris, Nystroemia, Spermopteris, Phasmatocycas, Vojnovskiales, Leptostrobus, and several other enigmatic groups of plant fossils. Comparison of the phylogenetic analysis with previous theories of angiosperm origin shows that it reconciles the ideas of Arber and Parkin, that Bennettitales and Gnetales are closely related to flowering plants, with more recent hypotheses that invoke Caytonia and corystosperms as highly relevant to the angiosperm problem. The results suggest that increased understanding of Triassic Bennettitales, Gnetales, and corystosperms will be of maximum interest in further elucidating the phylogenetic relationships of flowering plants.' This paper is dedicated to the late Dr. P. D. W. Barnard, who...

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