A meristem is a tissue in plants that consists of undifferentiated cells capable of cell division. Meristems are responsible for growth. Differentiated plant cells cannot divide or produce cells of a different type. Meristematic cells are undifferentiated or incompletely differentiated, are totipotent and capable of continued cell division. Division of meristematic cells provides new cells for expansion and differentiation of tissues and the initiation of new organs, providing the basic structure of the plant body; the cells are small, with no or small vacuoles and protoplasm fills the cell completely. The plastids, are present in rudimentary form. Meristematic cells are packed together without intercellular spaces; the cell wall is a thin primary cell wall. The term meristem was first used in 1858 by Carl Wilhelm von Nägeli in his book Beiträge zur Wissenschaftlichen Botanik, it is derived from the Greek word merizein, meaning to divide, in recognition of its inherent function. There are three types of meristematic tissues: apical and lateral.
At the meristem summit, there is a small group of dividing cells, called the central zone. Cells of this zone are essential for meristem maintenance; the proliferation and growth rates at the meristem summit differ from those at the periphery. Apical meristems are the undifferentiated meristems in a plant; these differentiate into three kinds of primary meristems. The primary meristems in turn produce the two secondary meristem types; these secondary meristems are known as lateral meristems because they are involved in lateral growth. There are two types of apical meristem tissue: shoot apical meristem, which gives rise to organs like the leaves and flowers, root apical meristem, which provides the meristematic cells for future root growth. SAM and RAM cells divide and are considered indeterminate, in that they do not possess any defined end status. In that sense, the meristematic cells are compared to the stem cells in animals, which have an analogous behavior and function; the number of layers varies according to plant type.
In general the outermost layer is called the tunica. In monocots, the tunica determine the physical characteristics of the leaf margin. In dicots, layer two of the corpus determine the characteristics of the edge of the leaf; the corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems. Apical meristems are found in two locations: the stem; some Arctic plants have an apical meristem in the lower/middle parts of the plant. It is thought. Shoot apical meristems are the source such as leaves and flowers. Cells at the shoot apical meristem summit serve as stem cells to the surrounding peripheral region, where they proliferate and are incorporated into differentiating leaf or flower primordia; the shoot apical meristem is the site of most of the embryogenesis in flowering plants. Primordia of leaves, petals and ovaries are initiated here at the rate of one every time interval, called a plastochron, it is. One of these indications might be the loss of apical dominance and the release of otherwise dormant cells to develop as auxiliary shoot meristems, in some species in axils of primordia as close as two or three away from the apical dome.
The shoot apical meristem consists of 4 distinct cell groups: Stem cells The immediate daughter cells of the stem cells A subjacent organizing center Founder cells for organ initiation in surrounding regionsThe four distinct zones mentioned above are maintained by a complex signalling pathway. In Arabidopsis thaliana, 3 interacting CLAVATA genes are required to regulate the size of the stem cell reservoir in the shoot apical meristem by controlling the rate of cell division. CLV1 and CLV2 are predicted to form a receptor complex to. CLV3 shares some homology with the ESR proteins of maize, with a short 14 amino acid region being conserved between the proteins. Proteins that contain these conserved regions have been grouped into the CLE family of proteins. CLV1 has been shown to interact with several cytoplasmic proteins that are most involved in downstream signalling. For example, the CLV complex has been found to be associated with Rho/Rac small GTPase-related proteins; these proteins may act as an intermediate between the CLV complex and a mitogen-activated protein kinase, involved in signalling cascades.
KAPP is a kinase-associated protein phosphatase, shown to interact with CLV1. KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it. Another important gene in plant meristem maintenance is WUSCHEL, a target of CLV signaling in addition to positively regulating CLV, thus forming a feedback loop. WUS is expressed in the cells below the stem cells of the meristem and its presence prevents the differentiation of the stem cells. CLV1 acts to promote cellular differentiation by repressing WUS activity outside of the central zone containing the stem cells; the function of WUS in the shoot apical meristem is linked to the phytohormone cytokinin. Cytokinin activates histidine kin
Neohelos is an extinct diprotodontid marsupial, that lived from the early to middle-Miocene. There are four species assigned to this genus, Neohelos tirarensis, the type species, N. stirtoni, N. solus and N. davidridei. N. davidridei is the most derived species of the genus, its premolar morphology shows that it is structurally and ancestor of the genus Kolopsis. All four species are from Riversleigh of Australia. Neohelos is known from many specimens, assigned to all the species. N. tirarensis includes a partial skull, maxillas and dentarys. A revision of Neohelos found a set of features in all the species that are absent in all other diprotodontoids, they are listed below: four−cusped P3 with a tall, subcentral parametacone, a distinct anterior parastyle, a moderately developed protocone and a small to moderate hypocone. Neohelos includes many species. Among them, N. davidridei is the most derived. N. davidridei shows many features that are found in Kolopsis, is thought to be the ancestor of it and its species.
Together and Kolopsis make up Zygomaturinae along with Zygomaturus and other genera. Neohelos lived in the early to Middle Miocene of Australia, it is one of few diprotodont genera that existed in Bullock Creek in both Riversleigh and Northern Territory, in the Wipajiri Formation. The Book of Life: An Illustrated History of the Evolution of Life on Earth by Jean-Paul Tibbles, Peter Andrews, John Barber, Michael Benton Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution by John A. Long, Michael Archer, Timothy Flannery, Suzanne Hand Number 6 Murray P et al. 2000 Morphology and evolution of the marsupial genus Neohelos Stirton
Macroscope is a science fiction novel by British-American writer Piers Anthony. It was nominated for the Hugo Award for Best Novel in 1970. Macroscope was first published in 1969 and in some respects reflects the idealistic values of that time; the plot involves, among other things, an extension of the Peckham Experiment, mathematicians John Conway and Michael Paterson's game of sprouts, the poetry of Sidney Lanier, the history of Phoenicia, commentary on the value of a dedicated teacher of a subject contrasted with a practicing engineer of that subject attempting to teach it, all in a kaleidoscopic combination. The book fills a unique place in Anthony's work as one that has garnered good reviews from hard-core science fiction fans as well as his usual audience of fantasy fans; the central plot device is the "macroscope", a large crystal that can be used to focus a newly discovered type of particle, the "macron". Macrons are not subject to many of the effects that interfere with light, as a result the macroscope can focus on any location in space-time with exceptional clarity, producing what is a telescope of infinite resolution in the space-time continuum.
The macroscope has been built into a solar-orbiting space station where scientists visit to book time on the device. Using it, they are able to explore space like never before. Among their many discoveries are two intelligent alien races. Using the macroscope, observers were able to look into one race's historical records, finding numerous parallels with human life on Earth; the race is now in societal decline, the implications are worrying. The macroscope's clear view across space makes it an ideal communications system for intelligent races, who broadcast signals by generating macrons, a technique not yet understood on Earth. However, over-riding all of these signals is another of enormous power, one of such strength that it must have been constructed by a Type II civilization; this signal repeats itself, starting with instructions on basic math and progressing to ever-more complex information. Viewers with high enough intelligence, an IQ of 150, reach a point where the information causes them to go insane or die.
Those without the intelligence to understand the advanced portions of the signal are unaffected. The signal appears to be a deliberate attempt to "jam" macroscopic communications, blocking those with the ability to understand the other signals from being able to see them, they refer to the blocking signal as the "destroyer". The protagonist of the story, Ivo Archer, is taken to the macroscope station on the invitation of a childhood friend, Brad Carpenter. Ivo has an unexplained link to a mysterious hyper-intelligent character, Schön, Brad believes Schön may be able to break through the destroyer signal. Ivo is not so convinced, is reluctant to ask for Schön's help; this reluctance is cause for some misunderstanding with Brad's girlfriend, Afra Summerfield, with whom Ivo falls hopelessly in love. While inadvertently viewing the destroyer signal with Brad and a Senator visiting the project, only Ivo survives the experience. Afra concludes Ivo is not intelligent enough to be affected, finds him somewhat beneath her as a result.
The Senator's death sparks a series of events that lead to Ivo and two other station members and Beatryx, stealing the macroscope. Afra, still in love with Brad and hoping to find a cure, brings his body along, now in a vegetative state, they detach the macroscope from the station and fly off with it while United Nations ships give chase. With time on their hands, Ivo turns to the macroscope and finds a way to avoid being overwhelmed by the destroyer signal; this reveals a number of broadcasts from farther out in space. Ivo demonstrates this technique to the others, allowing Harold and Afra to view the signals being hidden. Harold, a talented engineer, uses the information from one of the signals to build a device reducing their bodies to a liquid state, allowing them to accelerate at 10 g and escape the pursuing ships, they travel following a cryptic message left by Schön. Here they set up camp on Triton. Afra attempts to cure Brad by shocking him while he is being reconstituted from the liquid state, but the attempt fails and Brad is killed.
Over time, with further help from Ivo, the group watches a benign form of the entire broadcast, which they come to call "traveller". The liquefaction technique is only the first of many shown in the traveller signal, which describes an entire suite of technologies that allow interstellar travel. Using the instructions in the signal, they convert Neptune into an interstellar spaceship. Schön makes his first appearance during construction, revealing himself to be an alternate personality within Ivo's brain. Schön is ostensibly the body's "owner", having created the Ivo personality to avoid being the subject of experimentation. Schön has frightening intelligence, but having given over his body to Ivo at the age of five, is still a child and without morals. In an attempt to take control of the body, Schön traps Ivo in a historical drama running in his own brain. Over time, Ivo sees parallels between the characters in the drama and the group in the ship, escapes the illusion and re-asserts control.
The group decides to hunt down the source of the destroyer signal in an effort to turn it off. Travelling 15,000 light years from Earth, they find it is being broadcast from an abandoned space station. Exploring the inner portions of the station, a large museum, they are individually drawn into a series of visions that reveal different aspects of the nature of the destroyer and its history; the traveller signal had arrived late in the history of the galaxy, in the midst of flourishing communications