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91.
Brimhall GH Chadwick OA Lewis CJ Compston W Williams IS Danti KJ Dietrich WE Power ME Hendricks D Bratt J 《Science (New York, N.Y.)》1992,255(5045):695-702
Soils are differentiated vertically by coupled chemical, mechanical, and biological transport processes. Soil properties vary with depth, depending on the subsurface stresses, the extent of mixing, and the balance between mass removal in solution or suspension and mass accumulation near the surface. Channels left by decayed roots and burrowing animals allow organic and inorganic detritus and precipitates to move through the soil from above. Accumulation occurs at depths where small pores restrict further passage. Consecutive phases of translocation and root growth stir the soil; these processes constitute an invasive dilatational process that leads to positive cumulative strains. In contrast, below the depth of root penetration and mass additions, mineral dissolution by descending organic acids leads to internal collapse under overburden load. This softened and condensed precursor horizon is transformed into soil by biological activity, which stirs and expands the evolving residuum by invasion by roots and macropore networks that allows mixing of materials from above. 相似文献
92.
93.
Lewis FT 《Science (New York, N.Y.)》1924,60(1554):333-334
94.
95.
Although relative molar size has been considered a major taxonomic criterion, separating the Australopithecines and some erectus fossils from sapiens man, the Me > Ml, or " fossil" size sequence is found in 33 percent of Ohio whites and Pima Indians, yet is not necessarily the major sequence in the level and below ( 9; 10). Stanley M. Garn. 相似文献
96.
The vertical flux of nitrate across the thermocline in the upper ocean imposes a rigorous constraint on the rate of export of organic carbon from the surface layer of the sea. This export is the primary means by which the oceans can serve as a sink for atmospheric carbon dioxide. For the oligotrophic open ocean regions, which make up more than 75% of the world's ocean, the rate of export is currently uncertain by an order of magnitude. For most of the year, the vertical flux of nitrate is that due to vertical turbulent transport of deep water rich in nitrate into the relatively impoverished surface layer. Direct measurements of rates of turbulent kinetic energy dissipation, coupled with highly resolved vertical profiles of nitrate and density in the oligotrophic eastern Atlantic showed that the rate of transport, averaged over 2 weeks, was 0.14 (0.002 to 0.89, 95% confidence interval) millimole of nitrate per square meter per day and was statistically no different from the integrated rate of nitrate uptake as measured by incorporation of (15)N-labeled nitrate. The stoichiometrically equivalent loss of carbon from the upper ocean, which is the relevant quantity for the carbon dioxide and climate question, is then fixed at 0.90 (0.01 to 5.70) millimole of carbon per square meter per day. These rates are much lower than recent estimates based on in situ changes in oxygen over annual scales; they are consistent with a biologically unproductive oligotrophic ocean. 相似文献
97.
Organ CL Schweitzer MH Zheng W Freimark LM Cantley LC Asara JM 《Science (New York, N.Y.)》2008,320(5875):499
We report a molecular phylogeny for a nonavian dinosaur, extending our knowledge of trait evolution within nonavian dinosaurs into the macromolecular level of biological organization. Fragments of collagen alpha1(I) and alpha2(I) proteins extracted from fossil bones of Tyrannosaurus rex and Mammut americanum (mastodon) were analyzed with a variety of phylogenetic methods. Despite missing sequence data, the mastodon groups with elephant and the T. rex groups with birds, consistent with predictions based on genetic and morphological data for mastodon and on morphological data for T. rex. Our findings suggest that molecular data from long-extinct organisms may have the potential for resolving relationships at critical areas in the vertebrate evolutionary tree that have, so far, been phylogenetically intractable. 相似文献
98.
Lewis NG Yamamoto E Wooten JB Just G Ohashi H Towers GH 《Science (New York, N.Y.)》1987,237(4820):1344-1346
Lignins and suberins are complex plant cell-wall macromolecules that are composed mainly of phenylpropanoid residues derived from L-phenylalanine. Lignins and suberins are considered to be covalently linked to carbohydrates and to lipids, respectively. The bonding of these important structural materials within cell walls has never been established. By feeding specifically labeled [(13)C] ferulic acid over extended durations to seedlings of Triticum aestivum L. and by using solid-state carbon-13 nuclear magnetic resonance techniques, the major resonances due to specific carbons in the propanoid side chains of these cell-wall polymers have been identified in situ. The signals were found to differ significantly from those of synthetic lignins, which have usually been considered to be good approximations of natural lignin structure. 相似文献
99.
100.
Conserved sequence and structural elements in the HIV-1 principal neutralizing determinant 总被引:103,自引:0,他引:103
G J LaRosa J P Davide K Weinhold J A Waterbury A T Profy J A Lewis A J Langlois G R Dreesman R N Boswell P Shadduck 《Science (New York, N.Y.)》1990,249(4971):932-935
The principal neutralizing determinant (PND) of human immunodeficiency virus HIV-1 is part of a disulfide bridged loop in the third variable region of the external envelope protein, gp120. Analysis of the amino acid sequences of this domain from 245 different HIV-1 isolates revealed that the PND is less variable than thought originally. Conservation to better than 80 percent of the amino acids in 9 out of 14 positions in the central portion of the PND and the occurrence of particular oligopeptide sequences in a majority of the isolates suggest that there are constraints on PND variability. One constraining influence may be the structural motif (beta strand--type II beta turn--beta strand--alpha helix) predicted for the consensus PND sequence by a neural network approach. Isolates with a PND similar to the commonly investigated human T cell lymphoma virus IIIB (HTLV-IIIB) and LAV-1 (BRU) strains were rare, and only 14 percent of sera from 86 randomly selected HIV-1 seropositive donors contained antibodies that recognized the PND of these virus isolates. In contrast, over 65 percent of these sera reacted with peptides containing more common PND sequences. These results suggest that HIV vaccine immunogens chosen because of their similarity to the consensus PND sequence and structure are likely to induce antibodies that neutralize a majority of HIV-1 isolates. 相似文献