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46-471: The Medullosales is an extinct order of pteridospermous seed plants characterised by large ovules with circular cross-section and a vascularised nucellus , complex pollen-organs, stems and rachides with a dissected stele , and frond -like leaves. Their nearest still-living relatives are the cycads . Most medullosales were small to medium-sized trees. The largest specimens were probably of genus Alethopteris , whose fronds could be 7 metres long and

92-724: A polyphyletic grouping of extinct seed-producing plants . The earliest fossil evidence for plants of this type are the lyginopterids of late Devonian age. They flourished particularly during the Carboniferous and Permian periods. Pteridosperms declined during the Mesozoic Era and had mostly disappeared by the end of the Cretaceous Period, though Komlopteris seem to have survived into Eocene times, based on fossil finds in Tasmania . With regard to

138-687: A little longer, with evidence having been found there in the Asselian Stage. There is little evidence to suggest that the Medullosales ranged into the southern latitudes of Gondwana. In the northern hemisphere, there are good late Mississippian records in temperate latitudes of Kazakhstan but evidence from the higher northern latitudes in Siberia (Angara) is more equivocal. Pteridosperms Pteridospermatopsida Pteridospermatophyta , also called " pteridosperms " or " seed ferns " are

184-417: A mature embryo as the resources within the seed are limited. In flowering plants, one sperm nucleus fuses with the egg cell to produce a zygote, the other fuses with the two polar nuclei of the central cell to give rise to the polyploid (typically triploid) endosperm . This double fertilization is unique to flowering plants, although in some other groups the second sperm cell does fuse with another cell in

230-608: A monotypic family ( Codonospermaceae , Colpospermaceae , Polylophospermaceae , and Stephanospermaceae , respectively). The oldest evidence of the Medullosales is of late Mississippian age. The group became particularly diverse and abundant during Moscovian and Kasimovian times when it dominated many habitats in the tropical wetland of Euramerica, especially on clastic substrates. The group became extinct in Euramerica in earliest Permian (Autunian) times. They survived in China for

276-432: A purely curatorial perspective the term pteridosperms is a useful shorthand for describing the fern-like fronds that were probably produced by seed plants, which are commonly found in many Palaeozoic and Mesozoic fossil floras. The concept of pteridosperms goes back to the late 19th century when palaeobotanists came to realise that many Carboniferous fossils resembling fern fronds had anatomical features more reminiscent of

322-461: A resin-like substance. The cortex surrounding the stele also had resin-ducts, and towards the outside of the stem there were radially aligned bands of sclerotic tissue. Numerous species used to be recognised for what appeared to be anatomically different Medullosa stems, but many of these differences have been shown to represent changes that took place with the growth of the stem. Moreover, it has been shown that very similar types of stem could bear

368-493: A variety of cup-, bell- and cigar-shaped configurations, assigned to various fossil genera including Dolerotheca , Whittleseya , Aulacotheca and Potoniea . Unlike with the ovules, there is good anatomical evidence that they were borne on the fronds, attached to the rachis. The pollen that they produce is strictly known as pre-pollen, as it germinated proximally and was thus intermediate in structure between pteridophytic spores and gymnospermous true-pollen. The pollen organs of

414-545: A variety of different types of frond. There is a distinctive type of stem found in Late Pennsylvanian preserved floras, however, that is referred to as Medullosa endocentrica and has consistently slender stems and (uniquely for medullosaleans) axillary branching, and has been interpreted as a climbing plant. Another distinctive type of stem in which the vascular segments are of two different sizes in transverse section (fossil genus Sutcliffia ) has been linked with

460-442: Is in downward position and chalazal end in on the upper position hence, in amphitropous the anatropous arrangement is tilted 90 degrees and in orthotropous it is completely inverted) . The ovule appears to be a megasporangium with integuments surrounding it. Ovules are initially composed of diploid maternal tissue, which includes a megasporocyte (a cell that will undergo meiosis to produce megaspores). Megaspores remain inside

506-492: Is now thought that they in fact consist of a series of more or less overtopped dichotomies. Only one group of fronds, known as parispermacean fronds (fossil genera Paripteris and Linopteris ), lacked this major dichotomy although they were still thought to have been constructed from a series of overtopped dichotomies. The characters used to differentiate the fossil genera have used in descending order of significance to group families based mainly on vegetative characters, notably

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552-401: Is part of the inner structure of the ovule, forming a layer of diploid ( sporophytic ) cells immediately inside the integuments. It is structurally and functionally equivalent to the megasporangium . In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoes sporogenesis via meiosis . In the megasporocyte of Arabidopsis thaliana , meiosis depends on

598-433: Is the chalaza where the nucellus is joined to the integuments. Nutrients from the plant travel through the phloem of the vascular system to the funiculus and outer integument and from there apoplastically and symplastically through the chalaza to the nucellus inside the ovule. In chalazogamous plants, the pollen tubes enter the ovule through the chalaza instead of the micropyle opening. The nucellus (plural: nucelli)

644-546: The Petriellales , Corystospermales and Caytoniales . Their discovery attracted considerable attention at the time, as the pteridosperms were the first extinct group of vascular plants to be identified solely from the fossil record. In the 19th century the Carboniferous Period was often referred to as the "Age of Ferns" but these discoveries during the first decade of the 20th century made it clear that

690-436: The embryo sac ) is much smaller and typically consists of only seven cells and eight nuclei. This type of megagametophyte develops from the megaspore through three rounds of mitotic divisions. The cell closest to the micropyle opening of the integuments differentiates into the egg cell, with two synergid cells by its side that are involved in the production of signals that guide the pollen tube. Three antipodal cells form on

736-534: The "Age of Pteridosperms" was perhaps a better description. During the 20th century the concept of pteridosperms was expanded to include various Mesozoic groups of seed plants with fern-like fronds, such as the Corystospermaceae . Some palaeobotanists also included seed plant groups with entire leaves such as the Glossopteridales and Gigantopteridales , which was stretching the concept. In

782-402: The ancestry of flowering plants (angiosperms). A 2009 study concluded that " phylogenetic analysis techniques have surpassed the hard data needed to formulate meaningful phylogenetic hypotheses" regarding the relationships of "seed ferns" to living plant groups. Ovule In flowering plants , the ovule is located inside the portion of the flower called the gynoecium . The ovary of

828-544: The architecture of the frond: (1) the overall architecture of the frond, (2) the epidermis and cuticles, (3) how the pinnules are attached to the rachis, and (4) the veining pattern of the pinnules. Also commonly known as Potonieaceae, and less commonly Rachivestitaceae or Hexapterospermaceae. "Trigonocarpalean" ovules are generally attributed to medullosans. Examples are listed below. Codonospermum , Colpospermum, Polylophospermum , and Stephanospermum are so anatomically distinctive that some authors classify them each within

874-539: The context of modern phylogenetic models, the groups often referred to as pteridosperms appear to be liberally spread across a range of clades, and many palaeobotanists today would regard pteridosperms as little more than a paraphyletic 'grade-group' with no common lineage. One of the few characters that may unify the group is that the ovules were borne in a cupule , a group of enclosing branches, but this has not been confirmed for all "pteridosperm" groups. It has been speculated that some seed fern groups may be close to

920-500: The critical discovery that some of these fronds (genus Lyginopteris ) were associated with seeds (genus Lagenostoma ) that had identical and very distinctive glandular hairs, and concluded that both fronds and seeds belonged to the same plant. Soon, additional evidence came to light suggesting that seeds were also attached to the Carboniferous fern-like fronds Dicksonites , Neuropteris and Aneimites . Initially it

966-425: The developing megasporophyte, may be described as either tenuinucellate or crassinucellate. The former has either no cells or a single cell layer between the megasporophyte and the epidermal cells, while the latter has multiple cell layers between. Embryos may be described by a number of terms including Linear (embryos have axile placentation and are longer than broad), or rudimentary (embryos are basal in which

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1012-477: The early extinct seed ferns , ovules were borne on the surface of leaves. In the most recent of these taxa, a cupule (a modified branch or group of branches) surrounded the ovule (e.g. Caytonia or Glossopteris ). Ovule orientation may be anatropous , such that when inverted the micropyle faces the placenta (this is the most common ovule orientation in flowering plants), amphitropous , campylotropous , or orthotropous ( anatropous are common and micropyle

1058-414: The embryo within the seed through a mechanism of asexual reproduction called nucellar embryony . The haploid megaspore inside the nucellus gives rise to the female gametophyte , called the megagametophyte . In gymnosperms, the megagametophyte consists of around 2000 nuclei and forms archegonia , which produce egg cells for fertilization. In flowering plants, the megagametophyte (also referred to as

1104-511: The enduring utility of this division , many palaeobotanists still use the pteridosperm grouping in an informal sense to refer to the seed plants that are not angiosperms , coniferoids ( conifers or cordaites ), ginkgophytes or cycadophytes (cycads or bennettites ). This is particularly useful for extinct seed plant groups whose systematic relationships remain speculative, as they can be classified as pteridosperms with no valid implications being made as to their systematic affinities. Also, from

1150-465: The exact homologies of these tissues, and it has been argued that the vascularised nucellus was in fact the nucellus and integument that have become fused together, and that the 'integument' was homologous to a cupule that contained only one ovule. Most medullosalean ovules preserved as casts or adpressions show three longitudinal ribs and are assigned to the fossil genus Trigonocarpus . When such ovules are preserved as petrifactions, they are assigned to

1196-441: The expression of genes that facilitate DNA repair and homologous recombination . In gymnosperms, three of the four haploid spores produced in meiosis typically degenerate, leaving one surviving megaspore inside the nucellus. Among angiosperms, however, a wide range of variation exists in what happens next. The number (and position) of surviving megaspores, the total number of cell divisions, whether nuclear fusions occur, and

1242-477: The figure) there is no separation of the megaspores following meiosis, then the nuclei fuse to form a triploid nucleus and a haploid nucleus. The subsequent arrangement of cells is similar to the Polygonum pattern, but the ploidy of the nuclei is different. After fertilization, the nucellus may develop into the perisperm that feeds the embryo. In some plants, the diploid tissue of the nucellus can give rise to

1288-556: The final number, position and ploidy of the cells or nuclei all vary. A common pattern of embryo sac development (the Polygonum type maturation pattern) includes a single functional megaspore followed by three rounds of mitosis. In some cases, however, two megaspores survive (for example, in Allium and Endymion ). In some cases all four megaspores survive, for example in the Fritillaria type of development (illustrated by Lilium in

1334-451: The fossil genera Pachytesta or Stephanospermum , depending mainly on differences in the apical form of the ovule. Another group of medullosalean seeds, usually associated with parispermacean fronds (see later), have six longitudinal ribs and are referred to as Hexagonocarpus when found as adpressions or casts, and Hexapterospermum when found as petrifactions. The pollen producing organs consisted of clusters of elongate sacs formed into

1380-491: The gynoecium produces one or more ovules and ultimately becomes the fruit wall. Ovules are attached to the placenta in the ovary through a stalk-like structure known as a funiculus (plural, funiculi). Different patterns of ovule attachment, or placentation , can be found among plant species, these include: In gymnosperms such as conifers, ovules are borne on the surface of an ovuliferous (ovule-bearing) scale, usually within an ovulate cone (also called megastrobilus ). In

1426-400: The inner integument (which is integral to the formation of ovules from megasporangia) has been proposed to be by enclosure of a megasporangium by sterile branches (telomes). Elkinsia , a preovulate taxon, has a lobed structure fused to the lower third of the megasporangium, with the lobes extending upwards in a ring around the megasporangium. This might, through fusion between lobes and between

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1472-465: The kind produced by the Caytoniales or Glossopteridales may have evolved into the outer integument of angiosperms. The integuments develop into the seed coat when the ovule matures after fertilization. The integuments do not enclose the nucellus completely but retain an opening at the apex referred to as the micropyle . The micropyle opening allows the pollen (a male gametophyte ) to enter

1518-480: The latter being the largest known ovules produced by any non-angiosperm seed-plant. It was traditionally believed that the ovules were borne directly on the fronds, replacing one of the pinnules on the ultimate pinnae. However, there is a strong possibility that this reconstruction was based on the chance finds of ovules having been preserved just lying on a piece of pinna rather than in organic attachment to it. A number of cases are now coming to light that suggest that

1564-464: The length of the stem, superficially resembling the polysteles seen in tree ferns. However, detailed study of these vascular strands has shown that they merge and split along the length of the stem and in fact represents a single dissected stele. As the stems increased in size, the vascular segments also expanded by adding secondary wood. The vascular segments of the stele are embedded in ground tissue that contain canals or ducts thought to have contained

1610-418: The megagametophyte to produce a second embryo. The plant stores nutrients such as starch , proteins , and oils in the endosperm as a food source for the developing embryo and seedling, serving a similar function to the yolk of animal eggs. The endosperm is also called the albumen of the seed. the zygote then develops into a megasporophyte, which in turn produces one or more megasporangia. The ovule, with

1656-534: The modern-day seed plants, the cycads . In 1899 the German palaeobotanist Henry Potonié coined the term " Cycadofilices " ("cycad-ferns") for such fossils, suggesting that they were a group of non-seed plants intermediate between the ferns and cycads. Shortly afterwards, the British palaeobotanists Frank Oliver and Dukinfield Henry Scott (with the assistance of Oliver's student at the time, Marie Stopes ) made

1702-462: The next sporophyte generation. In flowering plants, a second sperm nucleus fuses with other nuclei in the megagametophyte forming a typically polyploid (often triploid) endosperm tissue, which serves as nourishment for the young sporophyte. An integument is a protective layer of cells surrounding the ovule. Gymnosperms typically have one integument (unitegmic) while angiosperms typically have two integuments (bitegmic). The evolutionary origin of

1748-411: The opposite (chalazal) end of the ovule and later degenerate. The large central cell of the embryo sac contains two polar nuclei . The pollen tube releases two sperm nuclei into the ovule. In gymnosperms, fertilization occurs within the archegonia produced by the female gametophyte. While it is possible that several egg cells are present and fertilized, typically only one zygote will develop into

1794-440: The ovule and divide by mitosis to produce the haploid female gametophyte or megagametophyte, which also remains inside the ovule. The remnants of the megasporangium tissue (the nucellus) surround the megagametophyte. Megagametophytes produce archegonia (lost in some groups such as flowering plants), which produce egg cells. After fertilization, the ovule contains a diploid zygote and then, after cell division begins, an embryo of

1840-403: The ovule for fertilization. In gymnosperms (e.g., conifers), the pollen is drawn into the ovule on a drop of fluid that exudes out of the micropyle, the so-called pollination drop mechanism. Subsequently, the micropyle closes. In angiosperms, only a pollen tube enters the micropyle. During germination , the seedling 's radicle emerges through the micropyle. Located opposite from the micropyle

1886-425: The parispermacean fronds. Fragments of the fronds are the most frequently found fossils of the Medullosales, and they have been widely used for biostratigraphy and biogeographical studies. Most are characterised by a major fork of the main rachis in the lower (proximal) part of the frond. Each branch produced by the fork has an essentially pinnate appearance, superficially resembling the fronds of many ferns, but it

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1932-540: The parispermacean species (fossil genus Potoniea ) produced spherical pre-pollen with a trilete mark. Most other medullosaleans produced large ovoid pre-pollen with a monolete mark, and assigned to the genus Schopfipollenites . Most medullosaleans had unbranched, upright stems that produced a crown of fronds at the top. The most widespread are referred to the fossil genus Medullosa when found as petrifactions with anatomy preserved. When viewed in transverse section they appear to have several vascular segments passing along

1978-425: The seeds were borne in clusters on relatively slender, branching axes, and that these trusses of ovules would have been produced from the top of the trunk among the crown of fronds. The seed megaspore was surrounded by two layers of tissue: a vascularised nucellus and a usually three-layered integument ; the nucellus and integument were completely free except at the base of the ovule. There has been some debate as to

2024-441: The structure and the megasporangium, have produced an integument. The origin of the second or outer integument has been an area of active contention for some time. The cupules of some extinct taxa have been suggested as the origin of the outer integument. A few angiosperms produce vascular tissue in the outer integument, the orientation of which suggests that the outer surface is morphologically abaxial. This suggests that cupules of

2070-557: The trees were perhaps up to 10 metres tall. Especially in Moscovian times, many medullosales were rather smaller, with fronds only about 2 metres long, and apparently growing in dense, mutually supporting stands. During Kasimovian and Gzhelian times there were also non-arboreal forms with smaller fronds (e.g. Odontopteris ) that were probably scrambling or possibly climbing plants. Ovules in different medullosalean species could vary from maybe 1 cm to over 10 cm long -

2116-626: Was still thought that they were " transitional fossils " intermediate between the ferns and cycads, and especially in the English-speaking world they were referred to as "seed ferns" or "pteridosperms". Today, despite being regarded by most palaeobotanists as only distantly related to ferns, these spurious names have nonetheless established themselves. Nowadays, four orders of Palaeozoic seed plants tend to be referred to as pteridosperms: Lyginopteridales , Medullosales , Callistophytales and Peltaspermales , with "Mesozoic seed ferns" including

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