Group Of Gymnosperms Research Articles

The complete plastid genome sequence of Welwitschia mirabilis: an unusually compact plastome with accelerated divergence rates

BackgroundWelwitschia mirabilis is the only extant member of the family Welwitschiaceae, one of three lineages of gnetophytes, an enigmatic group of gymnosperms variously allied with flowering plants or conifers. Limited sequence data and rapid divergence rates have precluded consensus on the evolutionary placement of gnetophytes based on molecular characters. Here we report on the first complete gnetophyte chloroplast genome sequence, from Welwitschia mirabilis, as well as analyses on divergence rates of protein-coding genes, comparisons of gene content and order, and phylogenetic implications.ResultsThe chloroplast genome of Welwitschia mirabilis [GenBank: EU342371] is comprised of 119,726 base pairs and exhibits large and small single copy regions and two copies of the large inverted repeat (IR). Only 101 unique gene species are encoded. The Welwitschia plastome is the most compact photosynthetic land plant plastome sequenced to date; 66% of the sequence codes for product. The genome also exhibits a slightly expanded IR, a minimum of 9 inversions that modify gene order, and 19 genes that are lost or present as pseudogenes. Phylogenetic analyses, including one representative of each extant seed plant lineage and based on 57 concatenated protein-coding sequences, place Welwitschia at the base of all seed plants (distance, maximum parsimony) or as the sister to Pinus (the only conifer representative) in a monophyletic gymnosperm clade (maximum likelihood, bayesian). Relative rate tests on these gene sequences show the Welwitschia sequences to be evolving at faster rates than other seed plants. For these genes individually, a comparison of average pairwise distances indicates that relative divergence in Welwitschia ranges from amounts about equal to other seed plants to amounts almost three times greater than the average for non-gnetophyte seed plants.ConclusionAlthough the basic organization of the Welwitschia plastome is typical, its compactness, gene content and high nucleotide divergence rates are atypical. The current lack of additional conifer plastome sequences precludes any discrimination between the gnetifer and gnepine hypotheses of seed plant relationships. However, both phylogenetic analyses and shared genome features identified here are consistent with either of the hypotheses that link gnetophytes with conifers, but are inconsistent with the anthophyte hypothesis.

Ammonium assimilation and amino acid metabolism in conifers

Conifers are the most important group of gymnosperms, which include tree species of great ecological and economic importance that dominate large ecosystems and play an essential role in global carbon fixation. Nitrogen (N) economy has a special importance in these woody plants that are able to cope with seasonal periods of growth and development over a large number of years. As N availability in the forest soil is extremely low, efficient mechanisms are required for the assimilation, storage, mobilization, and recycling of inorganic and organic forms of N. The cyclic interconversion of arginine and the amides glutamine and asparagine plays a central role in the N metabolism of conifers and the regulation of these pathways is of major relevance to the N economy of the plant. In this paper, details of recent progress in our understanding of the metabolism of arginine and the other major amino acids glutamine, glutamate, aspartate, and asparagine in pine, a conifer model tree, are presented and discussed.

Aromatized arborane/fernane hydrocarbons as biomarkers for cordaites

Previous palaeobotanical and palynological studies on coals from Euramerican Pennsylvanian ( identical with Late Carboniferous) coal basins indicate a major change in coal-swamp floras, especially at the Westphalian-Stephanian ( approximately Kasimovian-Gzhelian, according to Geological Time Scale 2004) boundary. A flora dominated by arborescent lycophytes was replaced by a vegetation dominated by marattialean tree ferns in various Euramerican coal basins. Earlier combined palynological and organic geochemical studies on Westphalian/Stephanian coals and shales from the Saar-Nahe Basin (Germany) revealed that the distribution of aromatized arborane/fernane hydrocarbons in solvent extracts reflects the increasing importance of seed plants, especially cordaites (extinct group of gymnosperms), conifers and pteridosperms. However, the biological source of the precursor molecules could not be specified. To clarify if the arborane/fernane derivatives MATH, MAPH, DAPH 1, and DAPH 2 in Westphalian/Stephanian coals can be assigned to one of the three potential source plant groups, we analyzed coals, sediments and fossil plant remains from different Euramerican locations with respect to their biomarker composition and stable carbon isotopic composition. Thereby, stable carbon isotopic ratios showed only insignificant variations between Westphalian and Stephanian samples and proved to be an unsuitable tool to describe floral changes during the Westphalian/Stephanian of the Saar-Nahe Basin. In contrast, we were able to show for the first time that MATH, MAPH, DAPH 1 and DAPH 2 are prominent constituents only in extracts of cordaitean macrofossils and can therefore be regarded as biomarkers for this group of gymnosperms.

Hettangian (Early Jurassic) plant fossils from Puale Bay (Peninsular terrane, Alaska)

Middle Hettangian (Early Jurassic) plant macrofossils from the Kamishak Formation at Puale Bay, Alaska occur mainly as leaves and leafy shoots found together with ammonites that allow precise biostratigraphic age assignment. This new locality is the first in the Jurassic of Alaska where the plant material shows preserved cuticle. Four species of three genera are identified representing three different gymnosperm groups: Otozamites mimetes and O. tenuatus (Bennettitales), Brachyphyllum crucis (Coniferales), and Sagenopteris pualensis Barbacka n. sp. (Pteridospermophyta) which is described as new. Their macro- and micromorphology show xeromorphic characters that suggest either an arid environment or a habitat with soil of increased salinity. The latter inference is preferred because it is consistent with conditions expected on an oceanic island, the mode of preservation, and accompanying marine fossils. Adaptation to coastal habitats, possibly occupied by a halophytic, mangrove-type vegetation, is implied. The new species is thus far only known from Alaska whereas the three other taxa from Puale Bay also occur at a few localities of the Middle Jurassic in the UK but they are not known from any other areas. Paleoclimatic and paleogeographic implications of the Puale Bay flora help constrain models for the tectonic evolution of the Peninsular terrane. The paleobotanic evidence favors a low-latitude setting for the Early Jurassic Talkeetna volcanic island arc to which the Kamishak Formation belongs. Plant dispersal requires proximity of the Peninsular terrane to cratonic North America at least by Middle Jurassic time.

Extinction and recovery patterns of the vegetation across the Cretaceous–Palaeogene boundary — a tool for unravelling the causes of the end-Permian mass-extinction

High-resolution palynofloral signatures through the Cretaceous–Palaeogene boundary succession show several features in common with the Permian–Triassic transition but there are also important differences. Southern Hemisphere Cretaceous–Palaeogene successions, to date studied at high resolution only in New Zealand, reveal a diverse palynoflora abruptly replaced by fungi-dominated assemblages that are in turn succeeded by low diversity suites dominated by fern spores, then gymnosperm- and angiosperm-dominated palynofloras of equivalent diversity to those of the Late Cretaceous. This palynofloral signature is interpreted to represent instantaneous (days to months) destruction of diverse forest communities associated with the Chicxulub impact event. The pattern of palynofloral change suggests wholesale collapse of vascular plant communities and short-term proliferation of saprotrophs followed by relatively rapid successional recovery of pteridophyte and seed–plant communities. The Permian–Triassic transition records global devastation of gymnosperm-dominated forests in a short zone synchronous with one or more peaks of the fungal/algal palynomorph Reduviasporonites. This zone is typically succeeded by assemblages rich in lycophyte spores and/or acritarchs. Higher in the succession, these assemblages give way to diverse palynofloras dominated by new groups of gymnosperms. Although different plant families were involved in the mass-extinctions, the general pattern of extinction and recovery is consistent between both events. The major difference is the longer duration for each phase of the Triassic recovery vegetation compared to that of the Paleocene. The protracted extinction-recovery succession at the Permian–Triassic boundary is incompatible with an instantaneous causal mechanism such as an impact of a celestial body but is consistent with hypotheses invoking extended environmental perturbations through flood-basalt volcanism and release of methane from continental shelf sediments.

Seed ferns and the origin of angiosperms

Doyle, J.A. (Section of Evolution and Ecology, University of California, Davis, California 95616, USA). Seed ferns and the origin of angiosperms. J. Torrey Bot. Soc. 133: 169–209. 2006.—If molecular analyses are correct in indicating that Gnetales are related to conifers and no other living gymnosperm group is directly related to angiosperms, studies on the origin of angiosperms must focus on fossil taxa, including “seed ferns.” Some authors have homologized the angiosperm carpel with the cupule of seed ferns, but because angiosperm ovules have two integuments rather than one, cupules are more likely to be homologous with the outer integument. Cupules of the earliest seed ferns may be derived from fertile appendages of “progymnosperms,” but those of later taxa appear to be modified leaves or leaflets, with ovules borne on the abaxial surface in some (peltasperms, corystosperms), the adaxial surface in others (glossopterids, Caytonia). Positional relationships and developmental genetic data suggest that the bitegmic ovule is comparable to a cupule with adaxial ovules. Analysis of a critically revised morphological data set for seed plants indicates that trees in which Gnetales are nested in conifers, as in molecular analyses, are almost as parsimonious as those in which Gnetales are linked with angiosperms, suggesting that the molecular arrangement should be accepted. When living taxa are constrained into the molecular topology, angiosperms are linked with glossopterids, Pentoxylon, Bennettitales, and Caytonia, supporting the homology of the cupule and the bitegmic ovule. Origin of the carpel poses more problems; it could correspond to the leaf portion of the glossopterid leaf-cupule complex, but its homologies in Caytonia are more obscure. New data on currently unknown characters of glossopterids, “Mesozoic seed ferns,” and Bennettitales are needed to test these hypotheses.

Evolution of Genome Size in Conifers

AbstractConifers are the most widely distributed group of gymnosperms in the world. They have large genome size (1C-value) compared with most animal and plant species. The genome size ranges from ~6,500 Mb to ~37,000 Mb in conifers. How and why conifers have evolved such large genomes is not understood. The conifer genome contains ~75% highly repetitive DNA. Most of the repetitive DNA is composed of non-coding DNA, including ubiquitous transposable elements. Conifers have relatively larger rDNA repeat units, larger gene families generated by gene duplications, larger nuclear volume, and perhaps larger genes, as compared to angiosperm plants. These genomic components may partially account for the large genome size, as well as variation in genome size, in conifers. One of the major mechanisms for genome size expansion and evolution of species is polyploidy, which is widespread in angiosperms, but it is rare in conifers. There are only a few natural polyploids in one family of conifers, Cupressaceae. Other conifers, including well-studied pines, are nearly all diploids. Whether ancient polyploidy has played a role in the evolution of genome size in conifers still remains an open question. The mechanisms that account for the variation and evolution of genome size in conifers are addressed in this review.

Spring and summer hydraulic conductivity of 14 woody species of the sub-boreal forest in British Columbia

Four hydraulic properties of the xylem, the Huber value (HV), hydraulic conductivity (Kh), and specific (Ks) and leaf-specific hydraulic conductivity (K1) were studied during the spring before leaf-out and during the summer after full leaf-out for 14 co-occurring tree and shrub species in the sub-boreal mixedwood forests of central British Columbia. The 14 studied species were divided into deciduous and evergreen angiosperm and gymnosperm groups, including one deciduous gymnosperm species, the tamarack, Larix laricina (Du Roi) K. Koch. I tested whether species with different life forms and classification divisions have different hydraulic properties. HVs were statistically similar for all angiosperms but significantly different among gymnosperms. Long-lived, late-successional species had higher HVs than pioneer and early-successional species. Ks and K1 values for all species except one, subalpine fir, Abies lasiocarpa var. lasiocarpa (Hook.) Nutt., were significantly higher in the spring than in the summer. Conductivity was higher in angiosperms than in gymnosperms in both the spring and the summer, and was highest in deciduous species in both seasons. The results indicate that requirements for mechanical strength may account for the high HVs in conifers, and that the high HVs compensate for low Ks and K1 values. The high hydraulic conductivities observed in the spring coincide with high water availability and high water demand by trees with emerging leaves. The subsequent reduction in Ks and K1 values could be caused by drought-induced embolism and cavitation in early summer.

A boreal early cradle of Angiosperms? Angiosperm-like pollen from the Middle Triassic of the Barents Sea (Norway)

Abstract. The origin of flowering plants is still a matter of dispute. Several lines of evidence suggest that their origin may go back to the Triassic. This paper reports on pollen grains with angiosperm-like morphologies from marine Middle Triassic sediments of the Boreal Realm (Norwegian Arctic, Barents Sea area). The morphology of these pollen grains is comparable to forms recorded from the Early Cretaceous, which are generally attributed to angiosperms. The new finds of angiosperm-like pollen are the earliest in the fossil record so far and show an astonishing high diversity. In contrast to other early records, they come from high palaeolatitudes with an inferred warm-temperate climate. The new finds suggest the presence of the first angiosperms during the Middle Triassic (242–227 Ma) or, alternatively, provide evidence for an as-yet unknown group of gymnosperms, possibly an extinct sister group of the flowering plants.

New Insights into Patterns of Nuclear Genome Size Evolution in Plants

In land plants (Embryophyta), comprising bryophytes, pteridophytes, gymnosperms and angiosperms, DNA Cvalue data has increased significantly in recent years. Estimates are available for over 3,800 species, ranging over 2000- fold (1C = 0.06 - 127.4 pg). Evaluating the phylogenetic component of this variation is essential to understand its evolutionary significance. Consequently, C-value data were superimposed onto well-supported phylogenies. This showed: (1) Most angiosperms (52%) had small C-values (defined as 1C £ 3.5 pg) within five times the mode (0.6 pg). Very large C-values (1C ³ 35.0 pg) occurred in only two distantly related groups (monocots and Santalales) suggesting that ancestral angiosperms had small genomes, and that very large genomes represented a derived condition that arose independently at least twice. (2) In contrast, extant gymnosperms (sister to angiosperms) typically had larger genomes whose modal Cvalue (15.8 pg) was over 20 times greater than angiosperms. Large C-values (³ 14.0 pg) were even typical in extant cycads (mean 1C = 14.7 pg), regarded as the basal group of gymnosperms. Thus within extant seed plants (gymnosperms and angiosperms), possession of a small genome seems unique to angiosperms. (3) C-value data in pteridophytes and bryophytes are still too sparse to permit analysis of evolutionary trends. However, low C-values (1C = 0.06 - 2.1 pg) in lycophytes, a pteridophyte group sister to all other extant vascular plants, suggest that first vascular plants had small genomes. Further, the narrow range and small size of C-values in bryophytes and Selaginella and Lycopodium (0.06 - 2.1 pg) seem likely to be under tight nucleotypic control. Nucleotypic correlations between cell size and nuclear DNA Cvalues in fossil plants may reveal major trends of plant genome size evolution over geological time. Knowledge of genome size, its origin and consequences will further unify our understanding of the evolutionary processes, structural components and DNA sequences responsible for p lant genome diversity.

The Cyanobacterial: Cycad Symbiosis

THE CYANOBACTERJAL- CYCAD SYMBIOSIS P. Lindblad and J.-L. Costa P Lindblad (corresponding author, e-mailpeterlindblad@ ebc uu se) and J -L Costa,Department of Physiological Botany, Evolutionary Biology Centre, Uppsala University, Villavagen 6, SE-752 36 Uppsala, Sweden INTRODUCTION Cycads are an ancient group of gymnosperms occurring naturally in tropical and subtropical re gions. They have a unique capacity to form symbi otic associations with heterocystous cyanobacteria (Nostoc) within apogeotropical coralloid roots This is the only example of a symbiosis between a gymnosperm and a nitrogen-fixing organism In this paper we present an overview of the location, establishment, charactenstics and specificity of the symbioses, aswell as information on the transfer of metabolites between the two partners. Cycads first appeared in the Pennsylvanian pernod and have existed for approximately 300 million years. Nearly 100 million years ago, cycads had a wide geographical distribution, extending from Alaska and Siberia to the Antarctic Today they are found on every continent except Europe and Antarctica but are restricted to small popula tions in the tropics and subtropics of both henm spheres. Moreover, many are facing possible extinction in nature The approximately 160 spe cies of cycads are distributed in three familles Cycadaceae, with a single genus, Cycas; Stangerl aceae, with two genera, Stangeria and Bowenia, and Zamiaceae, with eight genera, Ceratozamia, Chigua, Dioon, Encephalartos, Lepidozamia, Macrozamia, Microcycas and Zamia Cycads are mostly terrestrial and arborescent, except for at least one species (Zamia pseudoparasitica Yates), which is truly epi phytic, and can grow in a number of different habitats. Some species may be found in forests (both rainforest and seasonally dry forest), and others grow in open stands in grasslands Struc turally, cycads have a stem, normally columnar and woody, with the exception of Zamia pygmaea Sims ,which has a subterranean stem The aerial stem, which may be branched, is covered by persistent leaf bases In general, the vegetative shoots of cycads produce two types of leaves: scale leaves (called cataphylls) and foliage leaves The leaves are born terminally and are mainly pinnate, with the exception of the genus Bowenia, which has bipinnate leaves. Reproductively, cycads are dioecious, producing either pollen-bearing or ovule-beanng modified leaves called sporophylls The ovules, like those of true gymnosperms, are naked Cycads produce three types of root (1) a taproot equivalent to the primary root system found inmost plants (restricted to the genus Cycas, it is adventitious, arises from the lower side of trunk offsets and grows downwards close to the trunk); (ni) lateral roots, and (1ii) highly speciallsed lateral roots usually termed 'corallold roots' WHERE ARE THE SYMBIOTIC CYANOBACTERIA LOCATED? Corallold roots have been known to exist for a long time and have been recorded in all genera and all cycad species examined The ability to form this type of root is encoded by genes in the cycad, and the roots are formed before being invaded by symbiotic cyanobacterna Instead of themore com mon downward growth pattern, these roots show a marked negative geotropism and grow laterally and upwards toward the surface of the soil In seedlings of Macrozamia, the process of corallold root development begins with the initiation of paplllose roots called 'pre-corallold'. The first 'pre corallold' roots are adventitious and emerge from the hypocotyl immediately under the cotyledonary petioles Their subsequent development involves phases of maturation, cyanobactenal invasion, corallold formation, senescence and regeneration. When they are infected, the cyanobacteria are present in a specific cortical layer inside the root (the cyanobacterial zone) The presence of cyanobacteria inside the root induces irreversible modifications to the growth and development of the root The growth in length of these roots is much retarded compared with normal roots of the same age, but growth in diameter increases notice ably. Furthermore, the cells in the cyanobactenal zone undergo marked differentiation and elongate radially to connect the two adjacent cortical layers It has been suggested that these elongated cells are speciallsed cells responsible for the transfer of metabolhtes between the partners (Lindblad et al 1991; Pate et at 1988; Costa et a[ 1999, Adams 2000). ESTABLISHMENT OF THE SYMBIOSIS The process of infection is still unclear Invasion may occur at any stage of development of the root, but...

Systematic relationships of podocarpales, cephalotaxales, and taxales based on comparative seed anatomy and biochemistry data

The morphology of female reproductive organs and anatomy, ultrastructure, and ultrasculpture of seed integuments were studied in more than 150 species of Podocarpales, Cephalotaxales, and Taxales. The main trends of morphological-evolutionary transformations of female fructifications were established in three studied groups of gymnosperms. The taxonomic weight of structural features of the female reproductive organs of Podocarpales, Cephalotaxales, and Taxales was determined. Seeds of 20 species of these orders were investigated using amino acid and Western blot analysis. The index of remoteness from the least specialized hypothetical taxon and degree of difference between taxa were calculated from data on amino acid composition of seeds and embryos. The differences in the degree of similarity between Podocarpales and other groups of gymnosperms were demonstrated. An original concept was proposed for phylogenetic relations between Podocarpales, Cephalotaxales, and Taxales.

Palynomorphs of Permian Gondwana coal from borehole GDH-38, Barapukuria Coal Basin, Bangladesh

Thirty-two core samples of Permian Gondwana coal from three coal beds of borehole GDH-38, Barapukuria Coal Basin, Dinajpur, the north-northwestern part of Bangladesh, have been collected for palynological analysis. All samples except one yielded palynomorphs and some samples contain well-preserved and abundant palynomorphs of the gymnospermal and cryptogamic groups that are considered to be useful for future correlation studies. The lower coal bed (331.6–372.5 m) can easily be differentiated from the upper two coal beds by the presence of Alisporites, Cordaitina, Corisaccites, Hamiapollenites, Leuckisporites, Nuskoisporites, Tumoripollenites, Vestgisporites and Vittatina. It is difficult to palynologically differentiate the middle (198.1–208 m) and upper (162.3–172.9 m) coal beds as they contain a very limited number of specimens by which they can be identified. The middle bed is distinguished by the presence of Microbaculispora and Weylandites and the upper bed by the presence of a single taxon Acanthotriletes. Some of the vesiculate or saccate taxa extracted from these coal beds are typical of those occurring in Permian strata of Gondwana in India, South Africa, South America, Russia, Australia and Antarctica. They are thought to be derived from Glossopteris flora, which is characterised by an abundance of Pteridospermic plants of the gymnosperm group.

Seed plant phylogeny inferred from all three plant genomes: monophyly of extant gymnosperms and origin of Gnetales from conifers.

Phylogenetic relationships among the five groups of extant seed plants are presently quite unclear. For example, morphological studies consistently identify the Gnetales as the extant sister group to angiosperms (the so-called "anthophyte" hypothesis), whereas a number of molecular studies recover gymnosperm monophyly, and few agree with the morphology-based placement of Gnetales. To better resolve these and other unsettled issues, we have generated a new molecular data set of mitochondrial small subunit rRNA sequences, and have analyzed these data together with comparable data sets for the nuclear small subunit rRNA gene and the chloroplast rbcL gene. All nuclear analyses strongly ally Gnetales with a monophyletic conifers, whereas all mitochondrial analyses and those chloroplast analyses that take into account saturation of third-codon position transitions actually place Gnetales within conifers, as the sister group to the Pinaceae. Combined analyses of all three genes strongly support this latter relationship, which to our knowledge has never been suggested before. The combined analyses also strongly support monophyly of extant gymnosperms, with cycads identified as the basal-most group of gymnosperms, Ginkgo as the next basal, and all conifers except for Pinaceae as sister to the Gnetales + Pinaceae clade. According to these findings, the Gnetales may be viewed as extremely divergent conifers, and the many morphological similarities between angiosperms and Gnetales (e.g., double fertilization and flower-like reproductive structures) arose independently.

First evidence of Pentoxylales in Antarctica

Plant remains from the Byers Peninsula, South Shetlands Islands, are described. These are leaves referred toTaeniopterissp. and female fructifications referred toCarnoconites llambiasiiCésari sp. nov. They are considered to belong to the Pentoxylales, originally described from India, Australia and New Zealand. This is the first record of this group of gymnosperms from Antarctica. The occurrence contributes to further differentiation between floras of this part of Gondwana and Laurasia.

Non‐angiosperm phytochromes and the evolution of vascular plants

The phytochromes, a class of plant light‐sensing pigments, are a gene family with a long, complex evolutionary history. Angiosperms each have five or more phytochromes (designated A to E in Arabidopsis) with distinct functions as light receptors and only moderate sequence identities for different types within a species. The long‐term challenge taken up here is to trace the origin and function of the various motifs within the angiosperm phytochromes through gymnosperm phytochromes (types N, O and P) and lower plant phytochromes, sometimes reaching even to bacterial progenitor molecules. Particularly intriguing are the findings of homology of a C‐terminal region of phytochromes with bacterial transmitter modules and of a large N‐terminal region with a protein encoded by a gene from the cyanobacterum Synechocystis. Phylogenetic analysis helps to answer general questions such as the times of divergence of mono‐ and dicotyledons, of groups of gymnosperms or of ferns. Phytochrome sequences suggest (1) that mono‐ and dicotyledons became separated 150‐200 million years earlier than indicated by the fossil record and (2) that Ginkgo and Cycas have been separated unexpectedly late from the lineage giving rise to the Pinidae. (3) The status of Psilotum as a close relative of the primeval vascular plants is not supported. Phytochrome gene sequences additionally reveal that (4) moss and fern phytochromes have erratically acquired C‐termini which, though kinase‐like, are different from the common ones and that (5) introns have been lost, gained or shifted in position from algae to angiosperms. Phytochromes promise to be a rich source of phylogenetic information into the future as more sequences and functional data emerge, not least from studies of lower plants.

The Pentoxyleae: an overview

The fossil plants of the Group Pentoxyleae collected from the Rajmahal Hills suggest an assemblage of small trees with multimorphic branches, polystelic vasculature and diploxylic bundles in leaves. Microsporophylls are radial, branched and have balloon-shaped microsporangia with monocolpate spores. Ovules originate directly from cone axis. Its relationship has been discussed with different groups of gymnosperms (Medullosales, Cycadales, Bennettitales, Ginkgoales, Coniferales and Gnetales) and angiosperms.

Wood, Bark, and Stem Anatomy of Gnetales: A Summary

Data from a series of eight papers, representing a survey of Gnetales at the species level, are summarized in order to develop concepts on the relationship between the anatomical data and ecology, phylogeny, and systematics. Most of the characters and character states newly reported have phylogenetic and ecological correlations for Gnetales as a whole rather than systematic significance within the genera. Wood features are sensitively related to habit, although strategies in lianoid species of Ephedra differ from those in lianoid Gnetum species. Data also bear close relationships to organography. Vessel details are given special attention because workers have questioned whether vessels originated in Gnetales independently of those in angiosperms, and thus whether or not vessel origin in the two groups is a synapomorphy. The evidence cited here favors origin of vessels in Gnetales independent of vessel origins in angiosperms Hitherto unappreciated contrasts between vessels of Gnetales and those of angiosperms are detailed: the torusmargo structure of pits in Gnetales (both Ephedra and Gnetum), and the aspiration ability of gnetalean pits represent modes of conduction and promotion of conductive safety different from those in angiosperms, and the shape difference (circular vs scalariform), cited in data matrices of cladograms, is secondary to the different functional syndromes. The intercalation of circular bordered pits into helical secondary wall thickenings of primary xylem tracheary elements of Ephedra and Gnetum (a feature found also in conifers and Ginkgo but not in angiosperms) is given new functional interpretations. Ephedra and Gnetum contain both tracheids and fiber-tracheids that co-occur in a mode different from that in angiosperms which contain both cell types together. Ray structure like that of Gnetales occurs widely within seed plants and likely offers little conclusive information about relationships. Wood data on Gnetales are compatible with the hypothesis that a vesselless group of gymnosperms, such as Pentoxylales or Bennettitales, is the closest sister group of angiosperms. Formation of successive cambia in Gnetum differs from that in Welwitschia: the latter has phellem but no other bark. Because Welwitschia has only phellem, there are more numerous bark features in common between Ephedra and Gnetum than one might have expected. The three genera appear monophyletic on the basis of wood and bark. Infrageneric anatomical features of systematic importance are few, although the arboreal Gnetum gnemon differs from the lianoid Gnetum species in significant ways

Pollen morphology and ultrastructure of the Bennettitales: In situ pollen ofCycadeoidea

The micromorphology and ultrastructure of in situ pollen from Cycadeoidea dacotensis are described from permineralized specimens collected from the Lower Cretaceous of North America. Pollen grains are ovoid and relatively small, averaging 25 μm in length and 12 μm in width. Grains are monosulcate with the exine typically invaginated in apertural regions. Exine ornamentation ranges from punctate to psilate. The exine averages 0.73 μm in thickness and is composed of a light‐staining sexine and a dark‐staining nexine. The sexine consists of a thin, homogeneous tectum, typically with a well‐defined inner boundary, and a thicker granular infratectum. The infratectal granules are relatively uniform in size, however, variation occurs in the arrangement of granules. In some grains, the sexine appears homogeneous because there is little lacunal space between the individual granules. The granular infratectum is in direct contact with the underlying nexine. The nexine is uniform in thickness in both apertural and nonapertural regions, and it lacks lamellae throughout. Pollen morphology and ultrastructure are compared with those of the bennettitalean genus Leguminanthus and the dispersed genus Monosulcites. In addition, the fine structure of Cycadeoidea pollen is compared to that of the gymnosperm groups with which the Bennettitales are regarded to be most closely related, including Gnetales, Pentoxylales, and Eucommiidites‐type pollen‐producing plants.

Pollen Morphology and Ultrastructure of the Bennettitales: In Situ Pollen of Cycadeoidea

The micromorphology and ultrastructure of in situ pollen from Cycadeoidea dacotensis are described from permineralized specimens collected from the Lower Cretaceous of North America. Pollen grains are ovoid and relatively small, averaging 25 μm in length and 12 μm in width. Grains are monosulcate with the exine typically invaginated in apertural regions. Exine ornamentation ranges from punctate to psilate. The exine averages 0.73 μm in thickness and is composed of a light-staining sexine and a dark-staining nexine. The sexine consists of a thin, homogeneous tectum, typically with a well-defined inner boundary, and a thicker granular infratectum. The infratectal granules are relatively uniform in size, however, variation occurs in the arrangement of granules. In some grains, the sexine appears homogeneous because there is little lacunal space between the individual granules. The granular infratectum is in direct contact with the underlying nexine. The nexine is uniform in thickness in both apertural and nonapertural regions, and it lacks lamellae throughout. Pollen morphology and ultrastructure are compared with those of the bennettitalean genus Leguminanthus and the dispersed genus Monosulcites. In addition, the fine structure of Cycadeoidea pollen is compared to that of the gymnosperm groups with which the Bennettitales are regarded to be most closely related, including Gnetales, Pentoxylales, and Eucommiidites-type pollen-producing plants.