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A subclass of the plant class Bryopsida (mosses). The Archidiidae consists of a single genus, <i>Archidium</i>, with 26 species, occurring in ephemeral habitats, especially in wet, grassy places.
Industry:Science
A subclass of the true mosses (Bryopsida) largely limited to the South Pacific. The subclass consists of a single genus, <i>Dawsonia</i>, of nine species. The gametophytes are remarkably similar to those of the Polytrichidae, but the origin and structure of the peristome are very different.
Industry:Science
A subdiscipline in the field of microbiology concerned with the study of bacteria, fungi, and viruses that grow in or are transmitted by foods. While bacteria are frequently associated with food spoilage and food poisoning, some species preserve foods through fermentation or produce food ingredients. Food microbiology is a broad field that can include not only microbiology but also sanitation, epidemiology, biochemistry, engineering, statistics, and mathematical modeling.
Industry:Science
A subdiscipline of condensed-matter physics that focuses on the properties of solids in a size range intermediate between bulk matter and individual atoms or molecules. The size scale of interest is determined by the appearance of novel physical phenomena absent in bulk solids and has no rigid definition; however, the systems studied are normally in the range of 100 nanometers (10<sup>−7</sup> meter, the size of a typical virus) to 1000 nm (the size of a typical bacterium).
Other branches of science, such as chemistry and molecular biology, also deal with objects in this size range, but mesoscopic physics has dealt primarily with artificial structures of metal or semiconducting material which have been fabricated by the techniques employed for producing microelectronic circuits. Thus it has a close connection to the fields of nanofabrication and nanotechnology. The boundaries of this field are not sharp; nonetheless, its emergence as a distinct area of investigation was stimulated by the discovery of three categories of new phenomena in such systems: interference effects, quantum size effects, and charging effects.
Industry:Science
A subdiscipline of paleobiology that investigates the processes of preservation and their influence on information in the fossil record. Coined by J. A. Efremov in 1940, the term “taphonomy” involves all processes that affected the organism during its life, its transferral from the living world (biosphere) to the geological realm (lithosphere), and all physical and chemical interactions from the time of burial until collection. Besides the conspicuous characteristics of the preserved organism that can be seen easily—either morphological (external) and/or anatomical (internal) features—there are often less prominent details that record the fossil's history. Taphonomists are forensic scientists. By analyzing preserved details, paleontologists can understand an organism's mode of death or disarticulation; the biological processes that may have modified the remains before burial, including their use by hominids; the response of the organism or one of its parts to transport by animals, sediment, water, or wind; its residency time in a depositional setting before final entombment; and the alterations of tissues or skeletal parts within a wide range of chemical settings. The processes of fossilization appear to be environmentally site-specific, resulting in a mosaic of preservational traits in terrestrial and marine environments. Few fossil assemblages are exactly identical with regard to formative processes, but general patterns exist. An understanding of taphonomic assemblage features within its environmental context allows for a more accurate interpretation of the fossil record.
Industry:Science
A subdiscipline of stratigraphy and geochemistry that involves correlation and dating of marine sediments and sedimentary rocks through the use of trace-element concentrations, molecular fossils, and certain isotopic ratios that can be measured on components of the rocks. The isotopes used in chemostratigraphy can be divided into three classes: radiogenic (strontium, neodymium, osmium), radioactive (radiocarbon, uranium, thorium, lead), and stable (oxygen, carbon, sulfur). Trace-element concentrations (that is, metals such as nickel, copper, molybdenum, and vanadium) and certain organic molecules (called biological markers or biomarkers) are also employed in chemostratigraphy.
Industry:Science
A subject which embraces all applications of radioactive isotopes to chemistry. It is not precisely defined and is closely linked to nuclear chemistry. The widespread use of isotopes in chemistry is based on two fundamental properties exhibited by all radioactive substances. The first property is that the disintegration rate of an isotopic sample is directly proportional to the number of radioactive atoms in the sample. Thus, measurement of its disintegration rate (with a Geiger counter, for example) serves to analyze a radioactive compound. With nearly all chemical elements (the most notable exceptions being nitrogen and oxygen, which have no suitable radioactive isotopes), an isotope may be incorporated in a chemical compound, and thereafter, masses of this compound as small as 10<sup>−6</sup> to 10<sup>−10</sup> g may be measured with a high precision.
Because experimental chemistry depends largely upon analysis, isotopes may be employed in most chemical problems, especially those requiring high analytical sensitivity. The second fundamental property is that the disintegration rate is completely unaffected by the chemical form of the isotope, and conversely, the property of radioactivity does not affect the chemical properties of the isotope. By substituting or labeling a particular atom within a molecule, isotopes can be used to trace the fate of that atom during a chemical reaction. In contrast to physical migration tracer studies, the compounds arising in a reaction must first be isolated in separated pure forms before radioactive assays can be performed.
Industry:Science
A submillimeter extraterrestrial particle that has survived entry into the atmosphere without melting. Meteoroids are natural interplanetary objects that orbit the Sun, and they range in size from small dust grains to objects that are miles (kilometers) in diameter. Particles below 0.04 in. (1 mm) in diameter are considered micrometeoroids, and the micrometeoroids that enter the atmosphere without melting are called micrometeorites. Meteoroids of all sizes enter the atmosphere with velocities in excess of the Earth's escape velocity of 7.0 mi/s (11.2 km/s), and all but the smallest ones are heated sufficiently by air friction to produce at least partial melting.
Micrometeorites survive entry without severe heating because they are small and they totally decelerate from cosmic velocity at high altitudes near 55 mi (90 km).
In the thin air at such altitudes the power generated by frictional heating is low enough to be radiated away without a particle reaching its melting point, typically about 2400°F (1300°C) for common meteoritic samples. Larger objects penetrate deeper into the atmosphere before slowing down, and are melted and partially vaporized by friction with the comparatively dense air. Most of the mass of extraterrestrial matter that annually collides with the Earth is in the micrometeoroid size range, a total of about 10<sup>4</sup> tons (10<sup>7</sup> kg), but only a small fraction survives as micrometeorites. Usually only the particles smaller than 0.1 mm survive as true unmelted micrometeorites, although the survival of an individual micrometeorite depends on entry velocity, angle of entry, melting point, and density as well as size.
The flux of micrometeorites falling onto the Earth's surface is approximately 1 per square meter per day (0.1 per square foot per day) for particles with diameters of at least 10 micrometers and approximately 1 per square meter per year (0.1 per square foot per year) for particles with diameters of at least 100 μm.
Industry:Science
A suborder of amphipod crustaceans. Most hyperiids can be recognized by the large eyes which cover nearly the entire surface of the head. The first maxillae and especially the maxillipeds are greatly reduced in comparison to the suborder Gammaridea. In the prehensile pereiopods, the claw is formed by the fifth and sixth segments, the carpus and propodus, rather than by the sixth and seventh segments or the propodus and dactyl, as in the Gammaridea. The second and third somites of the urosome are always fused, a condition rarely found in the Gammaridea.
Industry:Science
A suborder of the Acarina, class Arachnida, comprising the ticks. Ticks differ from mites, their nearest relatives, in their larger size and in having a pair of breathing pores, or spiracles, behind the third or fourth pair of legs. They have a gnathosoma (or so-called head or capitulum), which consists of a base (basis capituli), a pair of palps, and a rigid, elongated, ventrally toothed hypostome which anchors the parasite to its host. They also have a pair of protrusible cutting organs, or chelicerae, which permit the insertion of the hypostome. The stages in the life cycle are egg, larva, nymph, and adult. The larvae have three pairs of legs; nymphs and adults, four. The 600 or so known species are all bloodsucking, external parasites of vertebrates including amphibians, reptiles, birds, and mammals.
Industry:Science
