Ginkgo is a genus of unusual non-flowering plants. The scientific name is used as the English name; the order to which it belongs, first appeared in the Permian, 270 million years ago derived from "seed ferns" of the order Peltaspermales, now only contains this single genus and species. The rate of evolution within the genus has been slow, all its species had become extinct by the end of the Pliocene; the relationships between ginkgos and other groups of plants are not resolved. The ginkgo is a living fossil, with fossils similar to modern ginkgo from the Permian, dating back 270 million years; the most plausible ancestral group for the order Ginkgoales is the Pteridospermatophyta known as the "seed ferns" the order Peltaspermales. The closest living relatives of the clade are the cycads, which share with the extant G. biloba the characteristic of motile sperm. Fossils attributable to the genus Ginkgo first appeared in the Early Jurassic, the genus diversified and spread throughout Laurasia during the middle Jurassic and Early Cretaceous.
It declined in diversity as the Cretaceous progressed with the extinction of species such as Ginkgo huolinhensis, by the Palaeocene, only a few Ginkgo species, Ginkgo cranei and Ginkgo adiantoides, remained in the Northern Hemisphere, while a markedly different form persisted in the Southern Hemisphere. At the end of the Pliocene, Ginkgo fossils disappeared from the fossil record everywhere except in a small area of central China, where the modern species survived, it is doubtful whether the Northern Hemisphere fossil species of Ginkgo can be reliably distinguished. Given the slow pace of evolution and morphological similarity between members of the genus, there may have been only one or two species existing in the Northern Hemisphere through the entirety of the Cenozoic: present-day G. biloba and G. gardneri from the Palaeocene of Scotland. At least morphologically, G. gardneri and the Southern Hemisphere species are the only known post-Jurassic taxa that can be unequivocally recognised. The remainder may have been subspecies.
The implications would be that G. biloba had occurred over an wide range, had remarkable genetic flexibility and, though evolving genetically, never showed much speciation. While it may seem improbable that a species may exist as a contiguous entity for many millions of years, many of the ginkgo's life-history parameters fit; these are: extreme longevity. Modern-day G. biloba grows best in well-watered and drained environments, the similar fossil Ginkgo favoured similar environments. Ginkgo therefore presents an "ecological paradox" because, while it possesses some favourable traits for living in disturbed environments, many of its other life-history traits are the opposite of those exhibited by modern plants that thrive in disturbed settings. Given the slow rate of evolution of the genus, it is possible that Ginkgo represents a pre-angiosperm strategy for survival in disturbed streamside environments. Ginkgo evolved in an era before flowering plants, when ferns and cycadeoids dominated disturbed streamside environments, forming a low, shrubby canopy.
The large seeds of Ginkgo and its habit of "bolting"—growing to a height of 10 metres before elongating its side branches—may be adaptations to such an environment. Diversity in the genus Ginkgo dropped through the Cretaceous at the same time the flowering plants were on the rise, which supports the notion that flowering plants, with their better adaptations to disturbance, displaced Ginkgo and its associates over time. Ginkgo has been used for classifying plants with leaves that have more than four veins per segment, while Baiera for those with less than four veins per segment. Sphenobaiera has been used to classify plants with broadly wedge-shaped leaves that lacks distinct leaf stems. Trichopitys is distinguished by having multiple-forked leaves with cylindrical, thread-like ultimate divisions; as of February 2013, molecular phylogenetic studies have produced at least six different placements of Ginkgo relative to cycads, conifers and angiosperms. The two most common are that Ginkgo is a sister to a clade composed of conifers and gnetophytes or that Ginkgo and cycads form a clade within the gymnosperms.
A 2013 study examined the reasons for the discrepant results, concluded that the best support was for the monophyly of Ginkgo and cycads
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group and are variously used in swimming, attachment and sensation. Ciliates are an important group of protists, common anywhere there is water — in lakes, oceans and soils. About 3,500 species have been described, the potential number of extant species is estimated at 30,000. Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliate species range in size from as little as 10 µm to as much as 4 mm in length, include some of the most morphologically complex protozoans. In most systems of taxonomy, "Ciliophora" is ranked as a phylum, under either the kingdom Protista or Protozoa. In some systems of classification, ciliated protozoa are placed within the class "Ciliata,".
In the taxonomic scheme proposed by the International Society of Protistologists, which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata. Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid micronucleus, a large, polyploid macronucleus; the latter is generated from the micronucleus by amplification of the heavy editing. The micronucleus does not express its genes; the macronucleus provides the nuclear RNA for vegetative growth. Division of the macronucleus occurs by amitosis, the segregation of the chromosomes occurs by a process whose mechanism is unknown; this process is not perfect, after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters are exchanged and fuse to form new micronuclei and macronuclei.
Food vacuoles are formed through phagocytosis and follow a particular path through the cell as their contents are digested and broken down by lysosomes so the substances the vacuole contains are small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches. Most ciliates have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. In some genera, such as Paramecium, these have a distinctive star shape, with each point being a collecting tube. Cilia are arranged in rows called kineties. In some forms there are body polykinetids, for instance, among the spirotrichs where they form bristles called cirri. More body cilia are arranged in mono- and dikinetids, which include one and two kinetosomes, each of which may support a cilium; these are arranged into rows called kineties. The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, include various fibrils and microtubules involved in coordinating the cilia.
The infraciliature is one of the main components of the cell cortex. Others are the alveoli, small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are generally present; the presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates and dinoflagellates. These superficially dissimilar groups make up the alveolates. Most ciliates are heterotrophs, feeding on smaller organisms, such as bacteria and algae, detritus swept into the oral groove by modified oral cilia; this includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids, groups of many cilia together with associated structures. The food is moved by the cilia through the mouth pore into the gullet. Feeding techniques vary however; some ciliates are mouthless and feed by absorption, while others are predatory and feed on other protozoa and in particular on other ciliates.
Some ciliates parasitize animals, although only one species, Balantidium coli, is known to cause disease in humans. Ciliates reproduce asexually, by various kinds of fission. During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis; the cell divides in two, each new cell obtains a copy of the micronucleus and the macronucleus. The cell is divided transversally, with the anterior half of the ciliate forming one new organism, the posterior half forming another. However, other types of fission occur in some ciliate groups; these include budding.
In cell biology, the cytoplasm is all of the material within a cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm; the main components of the cytoplasm are cytosol – a gel-like substance, the organelles – the cell's internal sub-structures, various cytoplasmic inclusions. The cytoplasm is about 80% water and colorless; the submicroscopic ground cell substance, or cytoplasmatic matrix which remains after exclusion the cell organelles and particles is groundplasm. It is the hyaloplasm of light microscopy, high complex, polyphasic system in which all of resolvable cytoplasmic elements of are suspended, including the larger organelles such as the ribosomes, the plant plastids, lipid droplets, vacuoles. Most cellular activities take place within the cytoplasm, such as many metabolic pathways including glycolysis, processes such as cell division; the concentrated inner area is called the endoplasm and the outer layer is called the cell cortex or the ectoplasm.
Movement of calcium ions in and out of the cytoplasm is a signaling activity for metabolic processes. In plants, movement of the cytoplasm around vacuoles is known as cytoplasmic streaming; the term was introduced by Rudolf von Kölliker in 1863 as a synonym for protoplasm, but it has come to mean the cell substance and organelles outside the nucleus. There has been certain disagreement on the definition of cytoplasm, as some authors prefer to exclude from it some organelles the vacuoles and sometimes the plastids; the physical properties of the cytoplasm have been contested in recent years. It remains uncertain how the varied components of the cytoplasm interact to allow movement of particles and organelles while maintaining the cell’s structure; the flow of cytoplasmic components plays an important role in many cellular functions which are dependent on the permeability of the cytoplasm. An example of such function is cell signalling, a process, dependent on the manner in which signaling molecules are allowed to diffuse across the cell.
While small signaling molecules like calcium ions are able to diffuse with ease, larger molecules and subcellular structures require aid in moving through the cytoplasm. The irregular dynamics of such particles have given rise to various theories on the nature of the cytoplasm. There has long been evidence, it is thought that the component molecules and structures of the cytoplasm behave at times like a disordered colloidal solution and at other times like an integrated network, forming a solid mass. This theory thus proposes that the cytoplasm exists in distinct fluid and solid phases depending on the level of interaction between cytoplasmic components, which may explain the differential dynamics of different particles observed moving through the cytoplasm, it has been proposed that the cytoplasm behaves like a glass-forming liquid approaching the glass transition. In this theory, the greater the concentration of cytoplasmic components, the less the cytoplasm behaves like a liquid and the more it behaves as a solid glass, freezing larger cytoplasmic components in place.
A cell's ability to vitrify in the absence of metabolic activity, as in dormant periods, may be beneficial as a defence strategy. A solid glass cytoplasm would freeze subcellular structures in place, preventing damage, while allowing the transmission of small proteins and metabolites, helping to kickstart growth upon the cell's revival from dormancy. There has been research examining the motion of cytoplasmic particles independent of the nature of the cytoplasm. In such an alternative approach, the aggregate random forces within the cell caused by motor proteins explain the non-Brownian motion of cytoplasmic constituents; the three major elements of the cytoplasm are the cytosol and inclusions. The cytosol is the portion of the cytoplasm not contained within membrane-bound organelles. Cytosol makes up about 70% of the cell volume and is a complex mixture of cytoskeleton filaments, dissolved molecules, water; the cytosol's filaments include the protein filaments such as actin filaments and microtubules that make up the cytoskeleton, as well as soluble proteins and small structures such as ribosomes and the mysterious vault complexes.
The inner and more fluid portion of the cytoplasm is referred to as endoplasm. Due to this network of fibres and high concentrations of dissolved macromolecules, such as proteins, an effect called macromolecular crowding occurs and the cytosol does not act as an ideal solution; this crowding effect alters. Organelles, are membrane-bound structures inside the cell that have specific functions; some major organelles that are suspended in the cytosol are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, in plant cells, chloroplasts. The inclusions are small particles of insoluble substances suspended in the cytosol. A huge range of inclusions exist in different cell types, range from crystals of calcium oxalate or silicon dioxide in plants, to granules of energy-storage materials such as starch, glycogen, or polyhydroxybutyrate. A widespread example are lipid droplets, which are spherical droplets composed of lipids and proteins that are used in both prokaryotes and eukaryotes as a way of storing lipids such as fatty acids and sterols.
Lipid droplets make up much of the volume of adipocytes, which are specialized lipid-st
Drosophila melanogaster is a species of fly in the family Drosophilidae. The species is known as the common fruit fly or vinegar fly. Starting with Charles W. Woodworth's proposal of the use of this species as a model organism, D. melanogaster continues to be used for biological research in genetics, microbial pathogenesis, life history evolution. As of 2017, eight Nobel prizes had been awarded for research using Drosophila. D. Melanogaster is used in research because it can be reared in the laboratory, has only four pairs of chromosomes and lays many eggs, its geographic range includes all continents, including islands. D. melanogaster is a common pest in homes and other places where food is served. Flies belonging to the family Tephritidae are called "fruit flies"; this can cause confusion in the Mediterranean and South Africa, where the Mediterranean fruit fly Ceratitis capitata is an economic pest. Wildtype fruit flies are yellow-brown, with brick-red eyes and transverse black rings across the abdomen.
They exhibit sexual dimorphism. Males are distinguished from females based on colour differences, with a distinct black patch at the abdomen, less noticeable in emerged flies, the sexcombs. Furthermore, males have a cluster of spiky hairs surrounding the reproducing parts used to attach to the female during mating. Extensive images are found at FlyBase. Under optimal growth conditions at 25 °C, the D. melanogaster lifespan is about 50 days from egg to death. The developmental period for D. melanogaster varies with temperature, as with many ectothermic species. The shortest development time, 7 days, is achieved at 28 °C. Development times increase at higher temperatures due to heat stress. Under ideal conditions, the development time at 25 °C is 8.5 days, at 18 °C it takes 19 days and at 12 °C it takes over 50 days. Under crowded conditions, development time increases. Females lay some 400 eggs, about five at a time, into rotting fruit or other suitable material such as decaying mushrooms and sap fluxes.
The eggs, which are about 0.5 mm long, hatch after 12–15 hours. The resulting larvae grow for about 4 days while molting twice, at about 48 h after hatching. During this time, they feed on the microorganisms that decompose the fruit, as well as on the sugar of the fruit itself; the mother puts feces on the egg sacs to establish the same microbial composition in the larvae's guts that has worked positively for herself. The larvae encapsulate in the puparium and undergo a 4-day-long metamorphosis, after which the adults eclose; the female fruit fly prefers a shorter duration. Males, prefer it to last longer. Males perform a sequence of five behavioral patterns to court females. First, males orient themselves while playing a courtship song by horizontally extending and vibrating their wings. Soon after, the male positions himself at the rear of the female's abdomen in a low posture to tap and lick the female genitalia; the male curls his abdomen and attempts copulation. Females can reject males by moving away and extruding their ovipositor.
Copulation lasts around 15–20 minutes, during which males transfer a few hundred long sperm cells in seminal fluid to the female. Females store the sperm in two mushroom-shaped spermathecae. A last male precedence is believed to exist; this precedence was found to occur through both incapacitation. The displacement is attributed to sperm handling by the female fly as multiple matings are conducted and is most significant during the first 1–2 days after copulation. Displacement from the seminal receptacle is more significant than displacement from the spermathecae. Incapacitation of first male sperm by second male sperm becomes significant 2–7 days after copulation; the seminal fluid of the second male is believed to be responsible for this incapacitation mechanism which takes effect before fertilization occurs. The delay in effectiveness of the incapacitation mechanism is believed to be a protective mechanism that prevents a male fly from incapacitating his own sperm should he mate with the same female fly repetitively.
Sensory neurons in the uterus of female D. melanogaster respond to a male protein, sex peptide, found in sperm. This protein makes the female reluctant to copulate for about 10 days after insemination; the signal pathway leading to this change in behavior has been determined. The signal is sent to a brain region, a homolog of the hypothalamus and the hypothalamus controls sexual behavior and desire. Gonadotropic hormones in Drosophila maintain homeostasis and govern reproductive output via a cyclic interrelationship, not unlike the mammalian estrous cycle. Sex Peptide perturbs this homeostasis and shifts the endocrine state of the female by inciting juvenile hormone synthesis in the corpus allatum. D. Melanogaster is used for life extension studies, such as to identify genes purported to increase lifespan when mutated. Females become receptive to courting males about 8–12 hours after emergence. Specific neuron groups in females have been found to affect copulation behavior a
In cell biology, the centrosome is an organelle that serves as the main microtubule organizing center of the animal cell, as well as a regulator of cell-cycle progression. The centrosome is thought to have evolved only in the metazoan lineage of eukaryotic cells. Fungi and plants lack centrosomes and therefore use structures other than MTOCs to organize their microtubules. Although the centrosome has a key role in efficient mitosis in animal cells, it is not essential in certain fly and flatworm species. Centrosomes are composed of two centrioles arranged at right-angles to each other, surrounded by an amorphous mass of protein termed the pericentriolar material; the PCM contains proteins responsible for microtubule nucleation and anchoring including γ-tubulin and ninein. In general, each centriole of the centrosome is based on a nine triplet microtubule assembled in a cartwheel structure, contains centrin and tektin. In many cell types the centrosome is replaced by a cilium during cellular differentiation.
However, once the cell starts to divide, the cilium is replaced again by the centrosome. The centrosome was discovered by Edouard Van Beneden in 1883, described and named in 1888 by Theodor Boveri. Centrosomes are associated with the nuclear membrane during the prophase stage of the cell cycle. In mitosis the nuclear membrane breaks down and the centrosome nucleated microtubules can interact with the chromosomes to build the mitotic spindle; the mother centriole, the older of the two in the centriole pair has a central role in making cilia and flagella. The centrosome is copied only once per cell cycle so that each daughter cell inherits one centrosome, containing two structures called centrioles; the centrosome replicates during the S phase of the cell cycle. During the prophase in the process of cell division called mitosis, the centrosomes migrate to opposite poles of the cell; the mitotic spindle forms between the two centrosomes. Upon division, each daughter cell receives one centrosome. Aberrant numbers of centrosomes in a cell have been associated with cancer.
Doubling of a centrosome is similar to DNA replication in two respects: the semiconservative nature of the process and the action of CDK2 as a regulator of the process. But the processes are different in that centrosome doubling does not occur by template reading and assembly; the mother centriole just aids in the accumulation of materials required for the assembly of the daughter centriole. Centrioles however, are not required for the progression of mitosis; when the centrioles are irradiated by a laser, mitosis proceeds with a morphologically normal spindle. Moreover, development of the fruit fly Drosophila is normal when centrioles are absent due to a mutation in a gene required for their duplication. In the absence of the centrioles, the microtubules of the spindle are focused by motors allowing the formation of a bipolar spindle. Many cells can undergo interphase without centrioles. Unlike centrioles, centrosomes are required for survival of the organism. Cells without centrosomes lack radial arrays of astral microtubules.
They are defective in spindle positioning and in the ability to establish a central localization site in cytokinesis. The function of centrosome in this context is hypothesized to ensure the fidelity of cell division because it increases the efficacy; some cell types arrest in the following cell cycle. This is not a universal phenomenon; when the nematode C. elegans egg is fertilized the sperm delivers a pair of centrioles. These centrioles will form the centrosomes which will direct the first cell division of the zygote and this will determine its polarity. It's not yet clear whether the role of the centrosome in polarity determination is microtubule dependent or independent. Theodor Boveri, in 1914, described centrosome aberrations in cancer cells; this initial observation was subsequently extended to many types of human tumors. Centrosome alterations in cancer can be divided in two subgroups, structural or numeric aberrations, yet both can be found in a tumor, they appear due to uncontrolled expression of centrosome components, or due to post-translational modifications which are not adequate for those components.
These modifications may produce variations in centrosome size. In addition, because centrosomal proteins have the tendency to form aggregates centrosome-related bodies are observed in ectopic places. Both enlarged centrosomes and CRBs are similar to the centrosomal structures observed in tumors. More, these structures can be induced in culture cells by overexpression of specific centrosomal proteins, such as CNap-1 or Nlp; these structures may look similar, yet detailed studies reveal that they may present different properties, depending on their proteic composition. For instance, their capacity to incorporate γ-TuRC complexes can be variable, so their capacity to nucleate microtubules, therefore affecting in different way the shape and motility of implicated tumor cells; the presence of an inadequate number of centrosomes is often linked to the appearance of genome instability and the loss of tissue differentiation. However, the method to count the centrosome number is not precise, because it is assessed using fluorescence microscopy, whose optical resolution is not high enough to resolve centrioles that are close to each other.
It is clear that the presence of an excess of centrosomes is a common event in human tu
Enzymes are macromolecular biological catalysts. Enzymes accelerate chemical reactions; the molecules upon which enzymes may act are called substrates and the enzyme converts the substrates into different molecules known as products. All metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon enzymes to catalyze individual steps; the study of enzymes is called enzymology and a new field of pseudoenzyme analysis has grown up, recognising that during evolution, some enzymes have lost the ability to carry out biological catalysis, reflected in their amino acid sequences and unusual'pseudocatalytic' properties. Enzymes are known to catalyze more than 5,000 biochemical reaction types. Most enzymes are proteins; the latter are called ribozymes. Enzymes' specificity comes from their unique three-dimensional structures. Like all catalysts, enzymes increase the reaction rate by lowering its activation energy; some enzymes can make their conversion of substrate to product occur many millions of times faster.
An extreme example is orotidine 5'-phosphate decarboxylase, which allows a reaction that would otherwise take millions of years to occur in milliseconds. Chemically, enzymes are like any catalyst and are not consumed in chemical reactions, nor do they alter the equilibrium of a reaction. Enzymes differ from most other catalysts by being much more specific. Enzyme activity can be affected by other molecules: inhibitors are molecules that decrease enzyme activity, activators are molecules that increase activity. Many therapeutic drugs and poisons are enzyme inhibitors. An enzyme's activity decreases markedly outside its optimal temperature and pH, many enzymes are denatured when exposed to excessive heat, losing their structure and catalytic properties; some enzymes are used commercially, in the synthesis of antibiotics. Some household products use enzymes to speed up chemical reactions: enzymes in biological washing powders break down protein, starch or fat stains on clothes, enzymes in meat tenderizer break down proteins into smaller molecules, making the meat easier to chew.
By the late 17th and early 18th centuries, the digestion of meat by stomach secretions and the conversion of starch to sugars by plant extracts and saliva were known but the mechanisms by which these occurred had not been identified. French chemist Anselme Payen was the first to discover an enzyme, diastase, in 1833. A few decades when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that this fermentation was caused by a vital force contained within the yeast cells called "ferments", which were thought to function only within living organisms, he wrote that "alcoholic fermentation is an act correlated with the life and organization of the yeast cells, not with the death or putrefaction of the cells."In 1877, German physiologist Wilhelm Kühne first used the term enzyme, which comes from Greek ἔνζυμον, "leavened" or "in yeast", to describe this process. The word enzyme was used to refer to nonliving substances such as pepsin, the word ferment was used to refer to chemical activity produced by living organisms.
Eduard Buchner submitted his first paper on the study of yeast extracts in 1897. In a series of experiments at the University of Berlin, he found that sugar was fermented by yeast extracts when there were no living yeast cells in the mixture, he named the enzyme that brought about the fermentation of sucrose "zymase". In 1907, he received the Nobel Prize in Chemistry for "his discovery of cell-free fermentation". Following Buchner's example, enzymes are named according to the reaction they carry out: the suffix -ase is combined with the name of the substrate or to the type of reaction; the biochemical identity of enzymes was still unknown in the early 1900s. Many scientists observed that enzymatic activity was associated with proteins, but others argued that proteins were carriers for the true enzymes and that proteins per se were incapable of catalysis. In 1926, James B. Sumner crystallized it; the conclusion that pure proteins can be enzymes was definitively demonstrated by John Howard Northrop and Wendell Meredith Stanley, who worked on the digestive enzymes pepsin and chymotrypsin.
These three scientists were awarded the 1946 Nobel Prize in Chemistry. The discovery that enzymes could be crystallized allowed their structures to be solved by x-ray crystallography; this was first done for lysozyme, an enzyme found in tears and egg whites that digests the coating of some bacteria. This high-resolution structure of lysozyme marked the beginning of the field of structural biology and the effort to understand how enzymes work at an atomic level of detail. An enzyme's name is derived from its substrate or the chemical reaction it catalyzes, with the word ending in -ase. Examples are alcohol dehydrogenase and DNA polymerase. Different enzymes that catalyze the same chemical reaction are called isozymes; the International Union of Biochemistry and Molecular Biology have developed a nomenclature for enzymes, the EC numbers. The first number broadly classifies the enzyme based on its mechanism; the top-level classification is: EC 1, Oxidoreductases: catalyze oxidation/reducti
Bryophytes are an informal group consisting of three divisions of non-vascular land plants: the liverworts and mosses. They are characteristically limited in size and prefer moist habitats although they can survive in drier environments; the bryophytes consist of about 20,000 plant species. Bryophytes produce enclosed reproductive structures, they reproduce via spores. Bryophytes are considered to be a paraphyletic group and not a monophyletic group, although some studies have produced contrary results. Regardless of their status, the name is convenient and remains in use as an informal collective term; the term "bryophyte" comes from Greek βρύον, bryon "tree-moss, oyster-green" and φυτόν, phyton "plant". The defining features of bryophytes are: Their life cycles are dominated by the gametophyte stage Their sporophytes are unbranched They do not have a true vascular tissue containing lignin Bryophytes exist in a wide variety of habitats, they can be found growing in a range of temperatures and moisture.
Bryophytes can grow where vascularized plants cannot because they do not depend on roots for an uptake of nutrients from soil. Bryophytes can survive on bare soil. Like all land plants, bryophytes have life cycles with alternation of generations. In each cycle, a haploid gametophyte, each of whose cells contains a fixed number of unpaired chromosomes, alternates with a diploid sporophyte, whose cell contain two sets of paired chromosomes. Gametophytes produce haploid sperm and eggs which fuse to form diploid zygotes that grow into sporophytes. Sporophytes produce haploid spores by meiosis. Bryophytes are gametophyte dominant, meaning that the more prominent, longer-lived plant is the haploid gametophyte; the diploid sporophytes appear only and remain attached to and nutritionally dependent on the gametophyte. In bryophytes, the sporophytes produce a single sporangium. Liverworts and hornworts spend most of their lives as gametophytes. Gametangia and antheridia, are produced on the gametophytes, sometimes at the tips of shoots, in the axils of leaves or hidden under thalli.
Some bryophytes, such as the liverwort Marchantia, create elaborate structures to bear the gametangia that are called gametangiophores. Sperm are flagellated and must swim from the antheridia that produce them to archegonia which may be on a different plant. Arthropods can assist in transfer of sperm. Fertilized eggs become zygotes. Mature sporophytes remain attached to the gametophyte, they consist of a stalk called a single sporangium or capsule. Inside the sporangium, haploid spores are produced by meiosis; these are dispersed, most by wind, if they land in a suitable environment can develop into a new gametophyte. Thus bryophytes disperse by a combination of swimming sperm and spores, in a manner similar to lycophytes and other cryptogams; the arrangement of antheridia and archegonia on an individual bryophyte plant is constant within a species, although in some species it may depend on environmental conditions. The main division is between species in which the antheridia and archegonia occur on the same plant and those in which they occur on different plants.
The term monoicous may be used where antheridia and archegonia occur on the same gametophyte and the term dioicous where they occur on different gametophytes. In seed plants, "monoecious" is used where flowers with anthers and flowers with ovules occur on the same sporophyte and "dioecious" where they occur on different sporophytes; these terms may be used instead of "monoicous" and "dioicous" to describe bryophyte gametophytes. "Monoecious" and "monoicous" are both derived from the Greek for "one house", "dioecious" and "dioicous" from the Greek for two houses. The use of the "oicy" terminology is said to have the advantage of emphasizing the difference between the gametophyte sexuality of bryophytes and the sporophyte sexuality of seed plants. Monoicous plants are hermaphroditic, meaning that the same plant has both sexes; the exact arrangement of the antheridia and archegonia in monoicous plants varies. They may be borne on different shoots, on the same shoot but not together in a common structure, or together in a common "inflorescence".
Dioicous plants are unisexual. All four patterns occur in species of the moss genus Bryum. Traditionally, all living land plants without vascular tissues were classified in a single taxonomic group a division. More phylogenetic research has questioned whether the bryophytes form a monophyletic group and thus whether they should form a single taxon. Although a 2005 study supported the traditional view that the bryophytes form a monophyletic group, by 2010 a broad consensus had emerged among systematists that bryophytes as a whole are not a natural group, although each of the three extant groups is monophyletic; the three bryophyte clades are the Marchantiophyta and Anthocerotophyta. The vascular plants or tracheophytes form a fourth, unranked clade of land plants called the "Polysporangiophyta". In this analysis, hornworts are sister