Embryophyte
The Embryophyta, or land plants, are the most familiar group of green plants that form vegetation on earth. Embryophyta is a clade within the Phragmoplastophyta, a larger clade that includes several green algae groups, within this large clade the embryophytes are sister to the Zygnematophyceae/Mesotaeniaceae and consist of the bryophytes plus the polysporangiophytes. Living embryophytes therefore include hornworts, mosses, lycophytes and flowering plants; the Embryophyta are informally called land plants because they live in terrestrial habitats, while the related green algae are aquatic. All are complex multicellular eukaryotes with specialized reproductive organs; the name derives from their innovative characteristic of nurturing the young embryo sporophyte during the early stages of its multicellular development within the tissues of the parent gametophyte. With few exceptions, embryophytes obtain their energy by photosynthesis, by using the energy of sunlight to synthesize their food from carbon dioxide and water.
The evolutionary origins of the embryophytes are discussed further below, but they are believed to have evolved from within a group of complex green algae during the Paleozoic era from terrestrial unicellular charophytes, similar to extant Klebsormidiophyceae. Embryophytes are adapted for life on land, although some are secondarily aquatic. Accordingly, they are called land plants or terrestrial plants. On a microscopic level, the cells of embryophytes are broadly similar to those of green algae, but differ in that in cell division the daughter nuclei are separated by a phragmoplast, they are eukaryotic, with a cell wall composed of cellulose and plastids surrounded by two membranes. The latter include chloroplasts, which conduct photosynthesis and store food in the form of starch, are characteristically pigmented with chlorophylls a and b giving them a bright green color. Embryophyte cells generally have an enlarged central vacuole enclosed by a vacuolar membrane or tonoplast, which maintains cell turgor and keeps the plant rigid.
In common with all groups of multicellular algae they have a life cycle which involves'alternation of generations'. A multicellular generation with a single set of chromosomes – the haploid gametophyte – produces sperm and eggs which fuse and grow into a multicellular generation with twice the number of chromosomes – the diploid sporophyte; the mature sporophyte produces haploid spores which grow into a gametophyte, thus completing the cycle. Embryophytes have two features related to their reproductive cycles which distinguish them from all other plant lineages. Firstly, their gametophytes produce sperm and eggs in multicellular structures, fertilization of the ovum takes place within the archegonium rather than in the external environment. Secondly, most the initial stage of development of the fertilized egg into a diploid multicellular sporophyte, take place within the archegonium where it is both protected and provided with nutrition; this second feature is the origin of the term'embryophyte' – the fertilized egg develops into a protected embryo, rather than dispersing as a single cell.
In the bryophytes the sporophyte remains dependent on the gametophyte, while in all other embryophytes the sporophyte generation is dominant and capable of independent existence. Embryophytes differ from algae by having metamers. Metamers are repeated units of development, in which each unit derives from a single cell, but the resulting product tissue or part is the same for each cell; the whole organism is thus constructed from repeating parts or metamers. Accordingly, these plants are sometimes termed'metaphytes' and classified as the group Metaphyta. In all land plants a disc-like structure called a phragmoplast forms where the cell will divide, a trait only found in the land plants in the streptophyte lineage, some species within their relatives Coleochaetales and Zygnematales, as well as within subaerial species of the algae order Trentepohliales, appears to be essential in the adaptation towards a terrestrial life style. All green algae and land plants are now known to form a single evolutionary lineage or clade, one name for, Viridiplantae.
According to several molecular clock estimates the Viridiplantae split 1,200 million years ago to 725 million years ago into two clades: chlorophytes and streptophytes. The chlorophytes are more diverse and were marine, although some groups have since spread into fresh water; the streptophyte algae are less diverse and adapted to fresh water early in their evolutionary history. They have not spread into marine environments; some time during the Ordovician period one or more streptophytes invaded the land and began the evolution of the embryophyte land plants. Present day embryophytes form. Becker and Marin speculate that land plants evolved from streptophytes rather than any other group of algae because streptophytes were adapted to living in fresh water; this prepared them to tolerate a range of environmental conditions found on land. Fresh water living made.
Marchantiophyta
The Marchantiophyta are a division of non-vascular land plants referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information, it is estimated. Some of the more familiar species grow as a flattened leafless thallus, but most species are leafy with a form much like a flattened moss. Leafy species can be distinguished from the similar mosses on the basis of a number of features, including their single-celled rhizoids. Leafy liverworts differ from most mosses in that their leaves never have a costa and may bear marginal cilia. Other differences are not universal for all mosses and liverworts, but the occurrence of leaves arranged in three ranks, the presence of deep lobes or segmented leaves, or a lack of differentiated stem and leaves all point to the plant being a liverwort. Liverworts are small from 2–20 mm wide with individual plants less than 10 cm long, are therefore overlooked.
However, certain species may cover large patches of ground, trees or any other reasonably firm substrate on which they occur. They are distributed globally in every available habitat, most in humid locations although there are desert and Arctic species as well; some species can be a weed in gardens. Most liverworts are small, measuring from 2–20 millimetres wide with individual plants less than 10 centimetres long, so they are overlooked; the most familiar liverworts consist of a prostrate, ribbon-like or branching structure called a thallus. However, most liverworts produce flattened stems with overlapping scales or leaves in two or more ranks, the middle rank is conspicuously different from the outer ranks. Liverworts can most reliably be distinguished from the similar mosses by their single-celled rhizoids. Other differences are not universal for all mosses and all liverworts. Unlike any other embryophytes, most liverworts contain unique membrane-bound oil bodies containing isoprenoids in at least some of their cells, lipid droplets in the cytoplasm of all other plants being unenclosed.
The overall physical similarity of some mosses and leafy liverworts means that confirmation of the identification of some groups can be performed with certainty only with the aid of microscopy or an experienced bryologist. Liverworts have a gametophyte-dominant life cycle, with the sporophyte dependent on the gametophyte. Cells in a typical liverwort plant each contain only a single set of genetic information, so the plant's cells are haploid for the majority of its life cycle; this contrasts with the pattern exhibited by nearly all animals and by most other plants. In the more familiar seed plants, the haploid generation is represented only by the tiny pollen and the ovule, while the diploid generation is the familiar tree or other plant. Another unusual feature of the liverwort life cycle is that sporophytes are short-lived, withering away not long after releasing spores. In other bryophytes, the sporophyte is persistent and disperses spores over an extended period; the life of a liverwort starts from the germination of a haploid spore to produce a protonema, either a mass of thread-like filaments or else a flattened thallus.
The protonema is a transitory stage in the life of a liverwort, from which will grow the mature gametophore plant that produces the sex organs. The male organs produce the sperm cells. Clusters of antheridia are enclosed by a protective layer of cells called the perigonium; as in other land plants, the female organs are known as archegonia and are protected by the thin surrounding perichaetum. Each archegonium has a slender hollow tube, the "neck", down which the sperm swim to reach the egg cell. Liverwort species may be either monoicous. In dioicous liverworts and male sex organs are borne on different and separate gametophyte plants. In monoicous liverworts, the two kinds of reproductive structures are borne on different branches of the same plant. In either case, the sperm must move from the antheridia where they are produced to the archegonium where the eggs are held; the sperm of liverworts is biflagellate, i.e. they have two tail-like flagellae that enable them to swim short distances, provided that at least a thin film of water is present.
Their journey may be assisted by the splashing of raindrops. In 2008, Japanese researchers discovered that some liverworts are able to fire sperm-containing water up to 15 cm in the air, enabling them to fertilize female plants growing more than a metre from the nearest male; when sperm reach the archegonia, fertilisation occurs, leading to the production of a diploid sporophyte. After fertilisation, the immature sporophyte within the archegonium develops three distinct regions: a foot, which both anchors the sporophyte in place and receives nutrients from its "mother" plant, a spherical or ellipsoidal capsule, inside which the spores will be produced for dispersing to new locations, a seta which lies between the other two
Fertilisation
Fertilisation or fertilization known as generative fertilisation, pollination, fecundation and impregnation, is the fusion of gametes to initiate the development of a new individual organism or offspring. This cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. In Antiquity, Aristotle conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging in a mode called by him as epigenetic. In 1784, Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. In 1827, von Baer observed a therian mammalian egg for the first time. Oscar Hertwig, in Germany, described the fusion of ova from sea urchin; the evolution of fertilisation is related to the origin of meiosis, as both are part of sexual reproduction, originated in eukaryotes.
There are two conflicting theories on how the couple meiosis–fertilisation arose. One is; the other is. The gametes that participate in fertilisation of plants are the pollen, the egg cell. Various families of plants have differing methods. In Bryophyte land plants, fertilisation takes place within the archegonium. In flowering plants a second fertilisation event involves another sperm cell and the central cell, a second female gamete. In flowering plants there are two sperm from each pollen grain. In seed plants, after pollination, a pollen grain germinates, a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle; the sperm are transferred from the pollen through the pollen tube to the ovule. Pollen tube growth Unlike animal sperm, motile, plant sperm is immotile and relies on the pollen tube to carry it to the ovule where the sperm is released; the pollen tube penetrates the stigma and elongates through the extracellular matrix of the style before reaching the ovary.
Near the receptacle, it breaks through the ovule through the micropyle and the pollen tube "bursts" into the embryo sac, releasing sperm. The growth of the pollen tube has been believed to depend on chemical cues from the pistil, however these mechanisms were poorly understood until 1995. Work done on tobacco plants revealed a family of glycoproteins called TTS proteins that enhanced growth of pollen tubes. Pollen tubes in a sugar free pollen germination medium and a medium with purified TTS proteins both grew. However, in the TTS medium, the tubes grew at a rate 3x that of the sugar-free medium. TTS proteins were placed on various locations of semi in vevo pollinated pistils, pollen tubes were observed to extend toward the proteins. Transgenic plants lacking the ability to produce TTS proteins exhibited slower pollen tube growth and reduced fertility. Rupture of pollen tube The rupture of the pollen tube to release sperm in Arabidopsis has been shown to depend on a signal from the female gametophyte.
Specific proteins called FER protein kinases present in the ovule control the production of reactive derivatives of oxygen called reactive oxygen species. ROS levels have been shown via GFP to be at their highest during floral stages when the ovule is the most receptive to pollen tubes, lowest during times of development and following fertilization. High amounts of ROS activate Calcium ion channels in the pollen tube, causing these channels to take up Calcium ions in large amounts; this increased uptake of calcium causes the pollen tube to rupture, release its sperm into the ovule. Pistil feeding assays in which plants were fed diphenyl iodonium chloride suppressed ROS concentrations in Arabidopsis, which in turn prevented pollen tube rupture. Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called "non-vascular plants"; some bryophytes do have specialised tissues for the transport of water. A fern is a member of a group of 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers.
They differ from mosses by being vascular. They leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided; the gymnosperms are a group of seed producing plants that includes conifers, Cycads and Gnetales. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος, meaning "naked seeds", after the unenclosed condition of their seeds, their naked condition stands in contrast to the seeds and ovules of flowering plants, which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves modified to form cones, or at the end of short stalks as in Ginkgo. After being fertilised, the ovary starts to develop into the fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule; the growth of the pollen tube is controlled by the vegetative cytoplasm. Hydrolytic enzymes are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style
Heterospory
Heterospory is the production of spores of two different sizes and sexes by the sporophytes of land plants. The smaller of these, the microspore, is male and the larger megaspore is female. Heterospory evolved during the Devonian period from isospory independently in several plant groups: the clubmosses, the arborescent horsetails, progymnosperms; this occurred as part of the process of evolution of the timing of sex differentiation. Heterospory developed due to natural selection pressures that encouraged an increase in propagule size; this may first have led to an increase in spore size and resulted in the species producing larger megaspores as well as smaller microspores. Heterospory evolved from homospory many times, but the species in which it first appeared are now extinct. Heterosporic plants that produce seeds are their most widespread descendants. Seed plants constitute the largest subsection of heterosporic plants. Microspores are haploid spores that in endosporic species contain the male gametophyte, carried to the megaspores by wind, water currents or animal vectors.
Microspores are nearly all nonflagellated, are therefore not capable of active movement. The morphology of the microspore consists of an outer double walled structures surrounding the dense cytoplasm and central nucleus. Megaspores contain the female gametophytes in heterosporic plant species, they develop archegonia that produce egg cells that are fertilized by sperm of the male gametophyte originating from the microspore. This results in the formation of a fertilized diploid zygote, that develops into the sporophyte embryo. While heterosporous plants produce fewer megaspores, they are larger than their male counterparts. In exosporic species, the smaller spores germinate into free-living male gametophytes and the larger spores germinate into free-living female gametophytes. In endosporic species, the gametophytes of both sexes are highly reduced and contained within the spore wall; the microspores of both exosporic and endosporic species are free-sporing, distributed by wind, water or animal vectors, but in endosporic species the megaspores and the megagametophyte contained within are retained and nurtured by the sporophyte phase.
Endosporic species are thus dioecious, a condition that promotes outcrossing. Some exosporic species produce micro- and megaspores in the same sporangium, a condition known as homoangy, while in others the micro- and megaspores are produced in separate sporangia; these may both be borne on the same monoecious sporophyte or on different sporophytes in dioicous species. Heterospory was a key event in the evolution of surviving plants; the retention of megaspores and the dispersal of microspores allow for both dispersal and establishment reproductive strategies. This adaptive ability of heterospory increases reproductive success as any type of environment favors having these two strategies. Heterospory stops self-fertilization from occurring in a gametophyte, but does not stop two gametophytes that originated from the same sporophyte from mating; this specific type of self-fertilization is termed as sporophytic selfing, it occurs most among angiosperms. While heterospory stops extreme inbreeding from occurring, it does not prevent inbreeding altogether as sporophytic selfing can still occur.
A complete model for the origin of heterospory, known as the Haig-Westoby model, establishes a connection between minimum spore size and successful reproduction of bisexual gametophytes. For the female function, as minimum spore size increases so does the chance for successful reproduction. For the male function, reproductive success does not change as the minimum spore size increases
Carboniferous
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
Fern
A fern is a member of a group of vascular plants that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e. having specialized tissues that conduct water and nutrients and in having life cycles in which the sporophyte is the dominant phase. Like other vascular plants, ferns have complex leaves called megaphylls, that are more complex than the microphylls of clubmosses. Most ferns are leptosporangiate ferns, sometimes referred to as true ferns, they produce coiled fiddleheads that expand into fronds. The group includes about 10,560 known extant species. Ferns are defined here in the broad sense, being all of the Polypodiopsida, comprising both the leptosporangiate and eusporangiate ferns, the latter itself comprising ferns other than those denominated true ferns, including horsetails or scouring rushes, whisk ferns, marattioid ferns, ophioglossoid ferns. Ferns first appear in the fossil record about 360 million years ago in the late Devonian period, but many of the current families and species did not appear until 145 million years ago in the early Cretaceous, after flowering plants came to dominate many environments.
The fern Osmunda claytoniana is a paramount example of evolutionary stasis. Ferns are not of major economic importance, but some are used for food, medicine, as biofertilizer, as ornamental plants and for remediating contaminated soil, they have been the subject of research for their ability to remove some chemical pollutants from the atmosphere. Some fern species, such as bracken and water fern are significant weeds world wide; some fern genera, such as Azolla can fix nitrogen and make a significant input to the nitrogen nutrition of rice paddies. They play certain roles in mythology and art. Like the sporophytes of seed plants, those of ferns consist of stems and roots. Stems: Fern stems are referred to as rhizomes though they grow underground only in some of the species. Epiphytic species and many of the terrestrial ones have above-ground creeping stolons, many groups have above-ground erect semi-woody trunks; these can reach up to 20 meters tall in a few species. Leaf: The green, photosynthetic part of the plant is technically a megaphyll and in ferns, it is referred to as a frond.
New leaves expand by the unrolling of a tight spiral called a crozier or fiddlehead fern. This uncurling of the leaf is termed circinate vernation. Leaves are divided into a sporophyll. A trophophyll frond is a vegetative leaf analogous to the typical green leaves of seed plants that does not produce spores, instead only producing sugars by photosynthesis. A sporophyll frond is a fertile leaf that produces spores borne in sporangia that are clustered to form sori. In most ferns, fertile leaves are morphologically similar to the sterile ones, they photosynthesize in the same way. In some groups, the fertile leaves are much narrower than the sterile leaves, may have no green tissue at all; the anatomy of fern leaves can either be simple or divided. In tree ferns, the main stalk that connects the leaf to the stem has multiple leaflets; the leafy structures that grow from the stipe are known as pinnae and are again divided into smaller pinnules. Roots: The underground non-photosynthetic structures that take up water and nutrients from soil.
They are always fibrous and structurally are similar to the roots of seed plants. Like all other vascular plants, the diploid sporophyte is the dominant phase or generation in the life cycle; the gametophytes of ferns, are different from those of seed plants. They are free-living and resemble liverworts, whereas those of seed plants develop within the spore wall and are dependent on the parent sporophyte for their nutrition. A fern gametophyte consists of: Prothallus: A green, photosynthetic structure, one cell thick heart or kidney shaped, 3–10 mm long and 2–8 mm broad; the prothallus produces gametes by means of: Antheridia: Small spherical structures that produce flagellate sperm. Archegonia: A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck. Rhizoids: root-like structures that consist of single elongated cells, that absorb water and mineral salts over the whole structure. Rhizoids anchor the prothallus to the soil. Ferns first appear in the fossil record in the early Carboniferous period.
By the Triassic, the first evidence of ferns related to several modern families appeared. The great fern radiation occurred in the late Cretaceous, when many modern families of ferns first appeared. Ferns were traditionally classified in the class Filices, in a Division of the Plant Kingdom named Pteridophyta or Filicophyta. Pteridophyta is no longer recognised as a valid taxon; the ferns are referred to as Polypodiophyta or, when treated as a subdivision of Tracheophyta, although this name sometimes only refers to leptosporangiate ferns. Traditionally, all of the spore producing vascular plants were informally denominated the pteridophytes, rendering the term synonymous with ferns and fern allies; this can be confusing because members of the division Pteridophyta were denominated pteridophytes. Traditionally, three discrete groups have be
Gymnosperm
The gymnosperms known as Acrogymnospermae, are a group of seed-producing plants that includes conifers, cycads and gnetophytes. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος, meaning "naked seeds"; the name is based on the unenclosed condition of their seeds. The non-encased condition of their seeds stands in contrast to the seeds and ovules of flowering plants, which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves, which are modified to form cones, or solitary as in Yew, Ginkgo; the gymnosperms and angiosperms together compose the spermatophytes or seed plants. The gymnosperms are divided into six phyla. Organisms that belong to the Cycadophyta, Ginkgophyta and Pinophyta phyla are still in existence while those in the Pteridospermales and Cordaitales phyla are now extinct. By far the largest group of living gymnosperms are the conifers, followed by cycads and Ginkgo biloba. Roots in some genera have fungal association with roots in the form of mycorrhiza, while in some others small specialised roots called coralloid roots are associated with nitrogen-fixing cyanobacteria.
The current formal classification of the living gymnosperms is the "Acrogymnospermae", which form a monophyletic group within the spermatophytes. The wider "Gymnospermae" group is thought to be paraphyletic; the fossil record of gymnosperms includes many distinctive taxa that do not belong to the four modern groups, including seed-bearing trees that have a somewhat fern-like vegetative morphology. When fossil gymnosperms such as these and the Bennettitales and Caytonia are considered, it is clear that angiosperms are nested within a larger gymnospermae clade, although which group of gymnosperms is their closest relative remains unclear; the extant gymnosperms include 12 main families and 83 genera which contain more than 1000 known species. Subclass Cycadidae Order Cycadales Family Cycadaceae: Cycas Family Zamiaceae: Dioon, Macrozamia, Encephalartos, Ceratozamia, Zamia. Subclass Ginkgoidae Order Ginkgoales Family Ginkgoaceae: GinkgoSubclass Gnetidae Order Welwitschiales Family Welwitschiaceae: Welwitschia Order Gnetales Family Gnetaceae: Gnetum Order Ephedrales Family Ephedraceae: EphedraSubclass Pinidae Order Pinales Family Pinaceae: Cedrus, Cathaya, Pseudotsuga, Pseudolarix, Nothotsuga, Abies Order Araucariales Family Araucariaceae: Araucaria, Agathis Family Podocarpaceae: Phyllocladus, Prumnopitys, Halocarpus, Lagarostrobos, Saxegothaea, Pherosphaera, Dacrycarpus, Falcatifolium, Nageia, Podocarpus Order Cupressales Family Sciadopityaceae: Sciadopitys Family Cupressaceae: Cunninghamia, Athrotaxis, Sequoia, Cryptomeria, Taxodium, Austrocedrus, Pilgerodendron, Diselma, Callitris, Thuja, Chamaecyparis, Cupressus, Xanthocyparis, Tetraclinis, Microbiota Family Taxaceae: Austrotaxus, Taxus, Amentotaxus, Torreya There are over 1000 living species of gymnosperm.
It is accepted that the gymnosperms originated in the late Carboniferous period, replacing the lycopsid rainforests of the tropical region. This appears to have been the result of a whole genome duplication event around 319 million years ago. Early characteristics of seed plants were evident in fossil progymnosperms of the late Devonian period around 383 million years ago, it has been suggested that during the mid-Mesozoic era, pollination of some extinct groups of gymnosperms was by extinct species of scorpionflies that had specialized proboscis for feeding on pollination drops. The scorpionflies engaged in pollination mutualisms with gymnosperms, long before the similar and independent coevolution of nectar-feeding insects on angiosperms. Evidence has been found that mid-Mesozoic gymnosperms were pollinated by Kalligrammatid lacewings, a now-extinct genus with members which resembled the modern butterflies that arose far later. Conifers are by far the most abundant extant group of gymnosperms with six to eight families, with a total of 65-70 genera and 600-630 species.
Conifers most are evergreens. The leaves of many conifers are long and needle-like, other species, including most Cupressaceae and some Podocarpaceae, have flat, triangular scale-like leaves. Agathis in Araucariaceae and Nageia in Podocarpaceae have flat strap-shaped leaves. Cycads are the next most abundant group of gymnosperms, with two or three families, 11 genera, 338 species. A majority of cycads are native to tropical climates and are most abundantly found in regions near the equator; the other extant groups are one species of Ginkgo. Gymnosperms have major economic uses. Pine, fir and cedar are all examples of conifers that are used for lumber, paper production, resin; some other common uses for gymnosperms are soap, nail polish, food and perfumes. Gymnosperms, like all vascular plants, have a sporophyte-dominant life cycle, which means they spend most of their life cycle with diploid cells, while
