The plant Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, division Gnetophyta or gnetophytes consists of three genera of woody plants A woody plant is a plant that uses wood as its structural tissue. They are typically perennial plants that have their stems and larger roots reinforced with wood produced adjacent to the vascular tissues: typically the main stem and larger branches and roots are covered by a layer of thickened bark. Woody plants are usually either trees, shrubs, grouped in the gymnosperms The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilized state). Their naked condition stands in contrast to the. The living Gnetophyta are in the genera Gnetum Gnetum is a genus of about 30-35 species of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms they possess vessel elements in the xylem. Some species have been proposed to have been the first plants to be insect pollinated as they occur in (family Gnetaceae), Welwitschia Welwitschia is a monotypic genus of gymnosperm plant, composed solely of the very distinct Welwitschia mirabilis. It is the only genus of the family Welwitschiaceae, in the order Welwitschiales, in the division Gnetophyta. The plant is named after the Austrian botanist Friedrich Welwitsch who discovered it in 1859 and is considered a living fossil (family Welwitschiaceae), and Ephedra Ephedra is a genus of gymnosperm shrubs, the only genus in the family Ephedraceae and order Ephedrales. These plants occur in dry climates over a wide area mainly in the northern hemisphere, across southern Europe, north Africa, southwest and central Asia, southwestern North America, and, in the southern hemisphere, in South America south to (family Ephedraceae). The gnetophytes differ from other gymnosperms (conifers The conifers, division Pinophyta, also known as division Coniferophyta or Coniferae, are one of 13 or 14 division level taxa within the Kingdom Plantae. Pinophytes are gymnosperms. They are cone-bearing seed plants with vascular tissue; all extant conifers are woody plants, the great majority being trees with just a few being shrubs. Typical, cycads Cycads are a group of seed plants characterized by a large crown of compound leaves and a stout trunk. They are evergreen, gymnospermous, dioecious plants having large pinnately compound leaves. They are frequently confused with and mistaken for palms or ferns, but are only distantly related to both, and instead belong to the division Cycadophyta, and ginkgos Ginkgo is a genus of highly unusual non-flowering plants with one extant species, G. biloba, which is regarded as a living fossil) in having vessel elements A vessel element is one of the cell types found in xylem, the water conducting tissue of plants. Vessel elements are typically found in the angiosperms but absent from most gymnosperms such as the conifers as in the flowering plants The flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies, or derived characteristics. In some classifications, all three genera are placed in a single order (Gnetales) but in others distributed among three orders, each containing a single family and genus. Most morphological and molecular studies confirm that Gnetum and Welwitschia diverged from each other more recently than from Ephedra.^[1][2][3][4]

Ecology and Morphology

The three genera of the gnetophytes are highly specialized to their respective environments, making it difficult to identify homologous characters.^[5] The three extant Extant is a term commonly used in biology to refer to taxa that are still in existence (living). The term extant contrasts with extinct. For example, Brandt's Cormorant is an extant species, while the Spectacled Cormorant is an extinct species. Likewise, of the group of molluscs known as the cephalopods, there are approximately 600 extant species genera of gnetophytes, a “bizarre and enigmatic” trio^[2], are likely aberrant members of the group, which was diverse and dominant in the Tertiary The Tertiary is a term for a geologic period 65 million to 2.588 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Tertiary extinction event, at start of the Cenozoic era, spanning to beginning of the most.^[5] Some synapomorphies of the gnetophytes include enveloping bracts In botany, a bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis, or cone scale. Bracts are often different from foliage leaves, for example being smaller, larger, or of a different color or texture around the ovules Ovule literally means "small egg." In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integuments forming its outer layer, the nucellus , and the megaspore-derived female gametophyte (or megagametophyte) in its center. The megagametophyte (also called and microsporangia, and a micropylar projection of the outer membrane of the ovule that produces a pollination Pollination is the process by which pollen is transferred in plants, thereby enabling fertilization and sexual reproduction. Pollen grains, which contain the male gametes to where the female gamete(s) are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself. The receptive part of the carpel is called a droplet.^[6]

Gnetum Gnetum is a genus of about 30-35 species of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms they possess vessel elements in the xylem. Some species have been proposed to have been the first plants to be insect pollinated as they occur in species are mostly woody Wood is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression. In the strict sense wood is produced as secondary xylem in the stems of trees (and other woody plants). In a living tree it transfers water and nutrients to the leaves and other growing tissues, and has a support function, climbers The term vine may refer to a climbing or trailing plant. The word, derived from Latin vīnea, in the original sense referred to the grapevines . The modern extended sense is mostly restricted to North American English, which uses "grapevine" to refer to the grape-bearing Vitis species. British English tends to use "climber" to in tropical forests. However, the most well-known member of this group, Gnetum gnemon Gnetum gnemon is a species of Gnetum native to southeast Asia and the western Pacific Ocean islands, from Assam south and east through Indonesia and Malaysia to the Philippines and Fiji. Common names include melinjo or belinjo , bago (Malay language, Tagalog language), peesae (Thai language), dae (Kwara'ae language) and bét, rau bép, rau danh or, is a tree. Its seeds are used to produce a crispy krupuk Krupuk, kerupuk, or kroepoek in Indonesia; keropok in Malaysia; kropek in the Philippines; bánh phồng tôm in Vietnam; prawn crackers in British English, shrimp chips or shrimp crackers in American English; is a popular snack in parts of East and Southeast Asia. Krupuk are deep fried crackers made from prawns and starch snack known as emping or krupuk belinjo.

Welwitschia Welwitschia is a monotypic genus of gymnosperm plant, composed solely of the very distinct Welwitschia mirabilis. It is the only genus of the family Welwitschiaceae, in the order Welwitschiales, in the division Gnetophyta. The plant is named after the Austrian botanist Friedrich Welwitsch who discovered it in 1859 and is considered a living fossil comprises only one species, Welwitschia mirabilis. It grows only in the deserts of Namibia Namibia, officially the Republic of Namibia , is a country in Southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana and Zimbabwe to the east, and South Africa to the south and east. It gained independence from South Africa on 21 March 1990 following the Namibian War of and Angola Angola, officially the Republic of Angola , is a country in south-central Africa bordered by Namibia on the south, Democratic Republic of the Congo on the north, and Zambia on the east; its west coast is on the Atlantic Ocean. The exclave province of Cabinda has a border with the Republic of the Congo and the Democratic Republic of the Congo. This strange species has only two large strap-like leaves that grow continuously from the base throughout the plant's life.

Plants of the genus Ephedra Ephedra is a genus of gymnosperm shrubs, the only genus in the family Ephedraceae and order Ephedrales. These plants occur in dry climates over a wide area mainly in the northern hemisphere, across southern Europe, north Africa, southwest and central Asia, southwestern North America, and, in the southern hemisphere, in South America south to are known as jointfirs because they have long slender branches which bear tiny scale-like leaves at their nodes. Ephedra has been traditionally used as a stimulant Stimulants, also sometimes called psychostimulants, are psychoactive drugs which induce temporary improvements in either mental or physical function or both. Examples of these kinds of effects may include enhanced alertness, wakefulness, and locomotion, among others. Due to their effects typically having an "up" quality to them,, but is a controlled substance A controlled substance is generally a drug or chemical whose manufacture, possession, and use are regulated by a government. This may include illegal drugs and prescription medications today in many jurisdictions because of the risk of harmful or even fatal overdosing The term drug overdose describes the ingestion or application of a drug or other substance in quantities greater than are recommended or generally practiced. An overdose is widely considered harmful and dangerous as it can result in death.

Knowledge of fossils of the gnetophytes has increased greatly since the 1980s.^[1] There are fossils from the Permian The Permian[note 1] is a geologic period and system characterized among land vertebrates by the diversification of the early amniotes into the ancestral groups of the mammals, turtles, lepidosaurs and archosaurs. The Permian period follows the Carboniferous and extends from 299.0 ± 0.8 to 251.0 ± 0.4 Ma . It is the last period of the Paleozoic,^[7] the Triassic The Triassic is a geologic period and system that extends from about 251 to 199 Ma . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic period has recently been more, and the Jurassic The Jurassic is a geologic period and system that extends from about 199.6± 0.6 Ma to 145.5± 4 Ma, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the "Age of Reptiles". The start of the period is marked by the major Triassic– which may belong to the gnetophytes, but this is uncertain.^[8] The fossil record is richer starting in the early Cretaceous The Cretaceous , Latin language for "chalky", usually abbreviated K for its German translation Kreide (chalk), is a geologic period and system from circa 145.5 ± 4 to 65.5 ± 0.3 million years ago (Ma). In the geologic timescale, the Cretaceous follows on the Jurassic period and is followed by the Paleogene period of the Cenozoic era, with fossils of plants as well as seeds A seed ( /ˈsiːd/ ) is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant. The formation of the seed completes the process of reproduction in seed and pollen Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants. When pollen lands which can be clearly assigned to the gnetophytes.^[8]

Classification

The evolutionary relationships among the seed plants The spermatophytes (also known as phanerogams) comprise those plants that produce seeds. They are a subset of the embryophytes or land plants. The living spermatophytes form five groups: are highly unresolved, and the gnetophytes have played an important role in the formation of phylogenetic In biology, phylogenetics is the study of evolutionary relatedness among various groups of organisms , which is discovered through molecular sequencing data and morphological data matrices. The term phylogenetics is of Greek origin from the terms phyle/phylon (φυλή/φῦλον), meaning "tribe, race," and genetikos (γενετικός hypotheses. Molecular phylogenies of extant gymnosperms have conflicted with morphological characters with regard to whether the gymnosperms as a whole (including gnetophytes) comprise a monophyletic group A clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[note 1] or a paraphyletic In phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all the descendants of that ancestor one that gave rise to angiosperms. At issue is whether the Gnetophyta are the sister group Cladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry. It can be distinguished from other taxonomic systems, such as phenetics, by its focus on evolutionary relationships; while other systems usually use morphological similarities to group similar species into genera, families and other of angiosperms The flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies, or derived characteristics, or whether they are sister to, or nested within, other extant gymnosperms. Numerous fossil gymnosperm clades once existed that are morphologically at least as distinctive as the four living gymnosperm The gymnosperms are a group of seed-bearing plants that includes conifers, cycads, Ginkgo and Gnetales. The term "gymnosperm" comes from the Greek word gymnospermos , meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilized state). Their naked condition stands in contrast to the groups, such as Bennettitales Bennettitales is an extinct order of seed plants that first appeared in the Triassic period and became extinct toward the end of the Cretaceous. Some were characterized by thick trunks and pinnately compound leaves that bore a superficial resemblance to those of cycads, differing primarily in stomatal arrangement, Caytonia and the glossopterids. When these gymnosperm fossils are considered, the question of gnetophyte relationships to other seed plants becomes even more complicated. Several hypotheses, illustrated below, have been presented to explain seed plant evolution.

Anthophyte hypothesis

From the early twentieth century, the anthophyte hypothesis was the prevailing explanation for seed plant The spermatophytes (also known as phanerogams) comprise those plants that produce seeds. They are a subset of the embryophytes or land plants. The living spermatophytes form five groups: evolution, based on shared morphological In biology morphology is the form, structure and configuration of an organism. This includes aspects of the outward appearance [citation needed] as well as the form and structure of the internal parts like bones and organs. This is in contrast to physiology, which deals primarily with function characters between the gnetophytes and angiosperms The flowering plants or angiosperms are the most diverse group of land plants. The flowering plants and the gymnosperms are the only extant groups of seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies, or derived characteristics. In this hypothesis, the gnetophytes, along with the extinct order Bennettitales Bennettitales is an extinct order of seed plants that first appeared in the Triassic period and became extinct toward the end of the Cretaceous. Some were characterized by thick trunks and pinnately compound leaves that bore a superficial resemblance to those of cycads, differing primarily in stomatal arrangement, are sister to the angiosperms, forming the “anthophytes”.^[6] Some morphological characters that were suggested to unite the anthophytes include vessels in wood, net-veined leaves (in Gnetum Gnetum is a genus of about 30-35 species of gymnosperms, the sole genus in the family Gnetaceae and order Gnetales. They are tropical evergreen trees, shrubs and lianas. Unlike other gymnosperms they possess vessel elements in the xylem. Some species have been proposed to have been the first plants to be insect pollinated as they occur in only), lignin Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose, chemistry, the layering of cells in the apical meristem A meristem is the tissue in all plants consisting of undifferentiated cells and found in zones of the plant where growth can take place, pollen Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants. When pollen lands and megaspore features (including thin megaspore wall), short cambial initials, and lignin syringal groups.^{[6][9][10][11]} However, most genetic studies have rejected the anthophyte hypothesis.^{[2][12][13][14][15][16][17][18][19][20]} Several of these studies have suggested that the gnetophytes and angiosperms have independently derived characters, including flower-like reproductive structures and tracheid vessel elements, that appear shared but are actually the result of parallel evolution.^[2][13][6]

Gnetifer hypothesis

In the gnetifer hypothesis, the gnetophytes are sister to the conifers, and the gymnosperms are a monophyletic group, sister to the angiosperms. The gnetifer hypothesis first formally emerged in the mid-twentieth century, when vessel elements in the gnetophytes were interpreted as being derived from tracheids with circular bordered pits, as in conifers.^[6] It did not gain strong support, however, until the emergence of molecular data in the late 1990s.^{[12][18][21][22]} Although the morphological evidence still largely supports the anthophyte hypothesis, there are some morphological commonalities between the gnetophytes and conifers that lend support to the gnetifer hypothesis. These shared traits include: tracheids with scalariform pits with tori interspersed with annular thickenings, absence of scalariform pitting in primary xylem, scale-like and strap-shaped leaves of Ephedra and Welwitschia; and reduced sporophylls.^[9][17][23]

Gnepine hypothesis

The gnepine hypothesis is a modification of the gnetifer hypothesis, and suggests that the gnetophytes belong within the conifers as a sister group to the Pinaceae.^[6] According to this hypothesis, the conifers as currently defined are not a monophyletic group, in contrast with molecular findings that support its monophyly.^[21]. All existing evidence for this hypothesis comes from molecular studies within the last decade.^{[2][3][13][20][17][15][18][23]} However, the morphological evidence remains difficult to reconcile with the gnepine hypothesis. If the gnetophytes are nested within conifers, they must have lost several shared derived characters of the conifers (or these characters must have evolved in parallel in the other conifer lineages): narrowly triangular leaves (gnetophytes have diverse leaf shapes), resin canals, a tiered proembryo, and flat woody ovuliferous cone scales.^[12][15].

Gnetophyte-sister hypothesis

Some partitions of the genetic data suggest that the gnetophytes are sister to all of the other extant seed plant groups.^{[6][4][20][17][21]} However, there is no morphological evidence nor evidence in the fossil record to support the gnetophyte-sister hypotheses.^[23]

References

1. ^ ^{a b} Peter R. Crane, Patrick Herendeen and Else Marie Friis (2004). "Fossils and plant phylogeny". American Journal of Botany 91: 1683–1699. doi:10.3732/ajb.91.10.1683. http://www.amjbot.org/cgi/content/full/91/10/1683.
2. ^ ^{a b c d e} Bowe, L.M.; Coat, G.; and dePamphilis, C.W. (2000). "Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales’ closest relatives are conifers". Proceedings of the National Academy of Science 97: 4092-4097. doi:10.1073/pnas.97.8.4092. http://www.pnas.org/content/97/8/4092.full.
3. ^ ^{a b} Gugerli, F.; Sperisen, C.; Buchler, U.; Brunner, L.; Brodbeck, S.; Palmer, J.D.; and Qiu, Y.L. (2001). "The evolutionary split of Pinaceae from other conifers: evidence from an intron loss and a multigene phylogeny". Molecular Phylogeny and Evolution 21: 167-175. doi:10.1006/mpev.2001.1004. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WNH-456JS33-H&_user=18704&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1138931837&_rerunOrigin=google&_acct=C000002018&_version=1&_urlVersion=0&_userid=18704&md5=0a3650492a24edd77c6acd1ffef1ae87.
4. ^ ^{a b} Rai, H.S.; Reeves, P.A.; Peakall, R.; Olmstead, R.G.; and Graham, S.W. (2008). "Inference of higher-order conifer relationships from a multi-locus plastid data set". Botany 86: 658-669. doi:10.1139/B08-062. http://columbia.library.ingentaconnect.com/content/nrc/bot/2008/00000086/00000007/art00004.
5. ^ ^{a b} Arber, E.A.N. and Parkin, J. (1908). "Studies on the evolution of the angiosperms: the relationship of the angiosperms to the Gnetales". Annals of Botany 22: 489-515. http://aob.oxfordjournals.org/content/volos-22/issue3/index.dtl.
6. ^ ^{a b c d e f g} Judd, W.S.; Campbell, C.S.; Kellogg, E.A.; Stevens, P.F.; and Donoghue, M.J. (2008) Plant Systematics: A Phylogenetics Approach. 3rd ed. Sunderland, Massachusetts, USA: Sinauer Associates, Inc.
7. ^ Zi-Qiang Wang (2004). "A New Permian Gnetalean Cone as Fossil Evidence for Supporting Current Molecular Phylogeny". Annals of Botany 94 (2): 281–288. doi:10.1093/aob/mch138. PMID 15229124. http://aob.oxfordjournals.org/cgi/content/full/94/2/281.
8. ^ ^{a b} Catarina Rydin, Kaj Raunsgaard Pedersen, Peter R. Crane and Else Marie Friis (2006). "Former Diversity of Ephedra (Gnetales): Evidence from Early Cretaceous Seeds from Portugal and North America". Annals of Botany 98 (1): 123–140. doi:10.1093/aob/mcl078. PMID 16675607. http://aob.oxfordjournals.org/cgi/content/full/98/1/123.
9. ^ ^{a b} Donaghue, M.J. and Doyle, J.A. (2000). "Seed plant phylogeny: demise of the anthophyte hypothesis?". Current Biology 10: R106-R109. doi:10.1016/S0960-9822(00)00304-3. http://linkinghub.elsevier.com/retrieve/pii/S0960982200003043.
10. ^ Loconte, H. and Stevenson, D.W. (1990). "Cladistics of the Spermatophyta". Brittonia 42: 197-211. doi:10.2307/2807216. http://www.springerlink.com/content/93518n8n2237k054/.
11. ^ Nixon, K.C.; Crepet, W.L.; Stevenson, D.; and Friis, E.M. (1994). "A reevaluation of seed plant phylogeny". Annals of the Missouri Botanical Garden 81: 494-533. doi:110.2307/2399901. http://www.jstor.org/stable/2399901.
12. ^ ^{a b c} Chaw, S.M.; Aharkikh, A.; Sung, H.M.; Lau, T.C.; and Li, W.H. (1997). "Molecular phylogeny of extant gymnosperms and seed plant evolution: analysis of nuclear 18S rRNA sequences". Molecular Biology and Evolution 14: 56-68. http://mbe.oxfordjournals.org/cgi/content/abstract/14/1/56.
13. ^ ^{a b c} Chaw, S.M.; Parkinson, C.L.; Cheng, Y.; Vincent, T.M.; Palmer, J.D. (2000). "Seed plant phylogeny inferred from all three plant genomes: monophyly of extant gymnosperms and the origin of Gnetales from conifers". Proceedings of the National Academy of Science USA 97: 4086-4091. doi:10.1073/pnas.97.8.4086. http://www.pnas.org/content/97/8/4086.abstract.
14. ^ Goremykin, V.; Bobrova, V.; Pahnke, J.; Troitsky, A.; Antonov, A.; and Martin, W. (1996). "Noncoding sequences from the slowly evolving chloroplast inverted repeat in addition to rbcL data do not support gnetalean affinities of angiosperms". Molecular Biology and Evolution 13: 383-396. http://mbe.oxfordjournals.org/cgi/content/abstract/13/2/383.
15. ^ ^{a b c} Hajibabaei, M.; Xia, J.; and Drouin, G. (2006). "Seed plant phylogeny: Gnetophytes are derived conifers and a sister group to Pinaceae". Molecular Phylogenetics and Evolution 40: 208-217. doi:10.1016/j.ympev.2006.03.006. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WNH-4JRVDYS-B&_user=18704&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1138965526&_rerunOrigin=google&_acct=C000002018&_version=1&_urlVersion=0&_userid=18704&md5=699cd25dedc2e55d5b819401b1972968.
16. ^ Hansen, A.; Hansmann, S.; Samigullin, T.; Antonov, A.; and Martin, W. (1999). "Gnetum and the angiosperms: molecular evidence that their shared morphological characters are convergent rather than homologous". Molecular Biology and Evolution 16: 1006-1009. http://mbe.oxfordjournals.org/cgi/content/citation/16/7/1006.
17. ^ ^{a b c d} Magallon, S. and Sanderson, M.J. (2002). "Relationships among seed plants inferred from highly conserved genes: sorting conflicting phylogenetic signals among ancient lineages". American Journal of Botany 89: 1991-2006. doi:10.3732/ajb.89.12.1991. http://www.jstor.org/stable/4122754.
18. ^ ^{a b c} Qiu, Y.L.; Lee, J.; Bernasconi-Quadroni, F.; Soltis, D.E.,; Soltis, P.S.; Zanis, M.; Zimmer, E.A.; Chen, Z.; Savalainen, V.; and Chase, M.W. (1999). "The earliest angiosperms: evidence from mitochondrial, plastid and nuclear genomes". Nature 402: 404-407. http://www.nature.com/nature/journal/v402/n6760/full/402404a0.html.
19. ^ Samigullin, T.K.; Martin, W.F.; Troitsky, A.V.; and Antonov, A.S. (1999). "Molecular data from the chloroplast rpoC1 gene suggest a deep and distinct dichotomy of contemporary spermatophytes into two monophyla: gymnosperms (including Gnetalaes) and angiosperms". Journal of Molecular Evolution 49: 310-315. doi:10.1007/PL00006553. http://www.springerlink.com/content/77e79hf2e7vv9u6e/.
20. ^ ^{a b c} Sanderson, M.J.; Wojciechowski, M.F.; Hu, J.M.; Sher Khan, T.; and Brady, S.G. (2000). "Error, bias, and long-branch attraction in data for two chloroplast photosystem genes in seed plants". Molecular Biology and Evolution 17: 782-797. http://mbe.oxfordjournals.org/cgi/content/abstract/17/5/782.
21. ^ ^{a b c} Rydin, C.; Kallersjo, M.; and Friist, E.M. (2002). "Seed plant relationships and the systematic position of Gnetales based on nuclear and chloroplast DNA: conflicting data, rooting problems, and the monophyly of conifers". International Journal of Plant Sciences 163: 197-214. doi:10.1086/338321. http://www.jstor.org/stable/3080238.
22. ^ Braukmann, T.W.A.; Kuzmina, M.; and Stefanovic, S. (2009). "Loss of all plastid nhd genes in Gnetales and conifers: extent and evolutionary significance for the seed plant phylogeny". Current Genetics 55: 323-337. http://www.nature.com/nrg/journal/v2/n3/full/nrg0301_186a.html.
23. ^ ^{a b c} Burleigh, J.G. and Mathews, S. (2007). "Phylogenetic signal in nucleotide data from seed plants: implications for resolving the seed plant tree of life". International Journal of Plant Science 168: 125-135. http://www.amjbot.org/cgi/content/abstract/91/10/1599.

Other Sources:

• Gifford, Ernest M., Adriance S. Foster. 1989. Morphology and Evolution of Vascular Plants. Third edition. WH Freeman and Company, New York.
• Hilton, Jason, and Richard M. Bateman. 2006. Pteridosperms are the backbone of seed-plant phylogeny. Journal of the Torrey Botanical Society 133: 119-168 (abstract)

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