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A pulsed ruby laser (wavelength, 694.3 nanometers) was used to measure the dependence on light intensity of light-induced electron paramagnetic resonance (ERR) signal 1 for short flashes of uniform duration (400 microseconds). Approximately 10(18) photons per square centimeter per flash from the unattenuated beam were available to the sample of subchloroplast "system 1" particles from spinach. The experimental dependence of the EPR signal height plotted as a function of the total number of incident photons per flash was exponential. From measurement of the slope at a very low relative photon flux and the saturated EPR signal amplitude, the value for the cross section or "effective size" of the light-induced paramagnetic unit, sigma(EPR), was found to be 300 x 10(-17) square centimeter. This result is compared with a measured optical absorption cross section, sigma(694nm), of 2.5 x 10(-17) square centimeter, for the identical sample at the laser wavelength. The hundredfold difference in size supports the thesis that the paramagnetic state is a property of an aggregate of chlorophyll molecules of the same general size as the photosynthetic unit. 相似文献
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A microbially mediated carbon-nitrogen cycle involving a newly isolated facultatively methylotrophic pseudomonad is described. The new isolate utilizes formamide as its sole carbon, nitrogen, and energy source. Other organisms involved in the proposed cycle are cyanogenic plants; phytopathogenic fungi, which convert cyanogenic glycosides to formamide; and nitrifying microorganisms. This cycle may be quantitatively important in view of the large variety of cyanogenic plants known to exist. 相似文献
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During Drosophila metamorphosis, most larval cells die. Pupal and adult tissues form from imaginal cells, tissue-specific progenitors allocated in embryogenesis that remain quiescent during embryonic and larval life. Clonal analysis and fate mapping of single, identified cells show that tracheal system remodeling at metamorphosis involves a classical imaginal cell population and a population of differentiated, functional larval tracheal cells that reenter the cell cycle and regain developmental potency. In late larvae, both populations are activated and proliferate, spread over and replace old branches, and diversify into various stalk and coiled tracheolar cells under control of fibroblast growth factor signaling. Thus, Drosophila pupal/adult tissue progenitors can arise both by early allocation of multipotent cells and late return of differentiated cells to a multipotent state, even within a single tissue. 相似文献
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S Backhaus SV Pereverzev A Loshak JC Davis RE Packard 《Science (New York, N.Y.)》1997,278(5342):1435-1438
Direct measurements of the current-phase relation, I versus Deltaphi, for a weak link coupling two reservoirs of B-phase superfluid helium-3 (3He-B) were made over a wide range of temperatures. The weak link consists of a square array of 100-nanometer-diameter apertures. For temperatures T such that T/Tc >/= 0.6 (where Tc is the superfluid transition temperature), I approximately sin(Deltaphi). At lower temperatures, I(Deltaphi) approaches a straight line. Several remarkable phenomena heretofore inaccessible to superconducting Josephson junctions, including direct observation of quantum oscillations and continuous knowledge of Deltaphi, were also observed. 相似文献
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Baines KH Simon-Miller AA Orton GS Weaver HA Lunsford A Momary TW Spencer J Cheng AF Reuter DC Jennings DE Gladstone GR Moore J Stern SA Young LA Throop H Yanamandra-Fisher P Fisher BM Hora J Ressler ME 《Science (New York, N.Y.)》2007,318(5848):226-229
Although lightning has been seen on other planets, including Jupiter, polar lightning has been known only on Earth. Optical observations from the New Horizons spacecraft have identified lightning at high latitudes above Jupiter up to 80 degrees N and 74 degrees S. Lightning rates and optical powers were similar at each pole, and the mean optical flux is comparable to that at nonpolar latitudes, which is consistent with the notion that internal heat is the main driver of convection. Both near-infrared and ground-based 5-micrometer thermal imagery reveal that cloud cover has thinned substantially since the 2000 Cassini flyby, particularly in the turbulent wake of the Great Red Spot and in the southern half of the equatorial region, demonstrating that vertical dynamical processes are time-varying on seasonal scales at mid- and low latitudes on Jupiter. 相似文献
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Enumeration of Rhizobium in soil is usually accomplished by the most probable number (MPN) plant infection method. The accuracy of MPN counts as compared to plate counts was determined for five strains of cowpea rhizobia. Host plants included cowpea (Vigna unguiculata (L.) Walp.), siratro (Macroptilium atropurpureum (DC.) Urb.) and peanut (Arachis hypogaea L.). Plastic growth pouches were used primarily for cowpea and siratro while plastic cups containing vermiculite were used for peanut. The number of rhizobia determined by the MPN method using cowpea, siratro and peanut underestimated the population from 10- to 100-fold. A control experiment using soybeans (Glycine max (L.) Merrill) indicated that the MPN method was accurate for R. japonicum. Experimentors using the MPN method should be aware of its accuracy for particular Rhizobiutn-legume combinations. 相似文献