全文获取类型
收费全文 | 369篇 |
免费 | 15篇 |
专业分类
林业 | 21篇 |
农学 | 6篇 |
基础科学 | 2篇 |
65篇 | |
综合类 | 85篇 |
农作物 | 15篇 |
水产渔业 | 36篇 |
畜牧兽医 | 117篇 |
园艺 | 9篇 |
植物保护 | 28篇 |
出版年
2023年 | 4篇 |
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 8篇 |
2018年 | 8篇 |
2017年 | 10篇 |
2016年 | 12篇 |
2015年 | 8篇 |
2014年 | 9篇 |
2013年 | 15篇 |
2012年 | 18篇 |
2011年 | 26篇 |
2010年 | 22篇 |
2009年 | 6篇 |
2008年 | 26篇 |
2007年 | 14篇 |
2006年 | 16篇 |
2005年 | 19篇 |
2004年 | 33篇 |
2003年 | 21篇 |
2002年 | 28篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1995年 | 3篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1972年 | 1篇 |
1965年 | 1篇 |
1963年 | 1篇 |
1961年 | 1篇 |
1959年 | 2篇 |
1958年 | 1篇 |
1953年 | 1篇 |
排序方式: 共有384条查询结果,搜索用时 984 毫秒
371.
372.
Berrettini W Bierut L Crowley TJ Cubells JF Frascella J Gelernter J Hewitt JK Kreek MJ Lachman H Leppert M Li MD Madden P Miner C Pollock JD Pomerleau O Rice JP Rutter JL Shurtleff D Swan GE Tischfield JA Tsuang M Uhl GR Vanyukov M Volkow ND Wanke K 《Science (New York, N.Y.)》2004,304(5676):1445-7; author reply 1445-7
373.
We performed high-resolution computer simulations of impacts into homogeneous and layered martian terrain analogs to try to account for the ages and characteristics of the martian meteorite collection found on Earth. We found that craters as small as approximately 3 kilometers can eject approximately 10(7) decimeter-sized fragments from Mars, which is enough to expect those fragments to appear in the terrestrial collection. This minimum crater diameter is at least four times smaller than previous estimates and depends on the physical composition of the target material. Terrain covered by a weak layer such as an impact-generated regolith requires larger, therefore rarer, impacts to eject meteorites. Because older terrain is more likely to be mantled with such material, we estimate that the martian meteorites will be biased toward younger ages, which is consistent with the meteorite collection. 相似文献
374.
Continuous plasma outflows from the edge of a solar active region as a possible source of solar wind
Sakao T Kano R Narukage N Kotoku J Bando T Deluca EE Lundquist LL Tsuneta S Harra LK Katsukawa Y Kubo M Hara H Matsuzaki K Shimojo M Bookbinder JA Golub L Korreck KE Su Y Shibasaki K Shimizu T Nakatani I 《Science (New York, N.Y.)》2007,318(5856):1585-1588
The Sun continuously expels a huge amount of ionized material into interplanetary space as the solar wind. Despite its influence on the heliospheric environment, the origin of the solar wind has yet to be well identified. In this paper, we report Hinode X-ray Telescope observations of a solar active region. At the edge of the active region, located adjacent to a coronal hole, a pattern of continuous outflow of soft-x-ray-emitting plasmas was identified emanating along apparently open magnetic field lines and into the upper corona. Estimates of temperature and density for the outflowing plasmas suggest a mass loss rate that amounts to approximately 1/4 of the total mass loss rate of the solar wind. These outflows may be indicative of one of the solar wind sources at the Sun. 相似文献
375.
D'Hondt S Jørgensen BB Miller DJ Batzke A Blake R Cragg BA Cypionka H Dickens GR Ferdelman T Hinrichs KU Holm NG Mitterer R Spivack A Wang G Bekins B Engelen B Ford K Gettemy G Rutherford SD Sass H Skilbeck CG Aiello IW Guèrin G House CH Inagaki F Meister P Naehr T Niitsuma S Parkes RJ Schippers A Smith DC Teske A Wiegel J Padilla CN Acosta JL 《Science (New York, N.Y.)》2004,306(5705):2216-2221
Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere. 相似文献
376.
Siegel JS 《Science (New York, N.Y.)》2004,304(5675):1256-1258
377.
The immunological synapse is a specialized cell-cell junction that is defined by large-scale spatial patterns of receptors and signaling molecules yet remains largely enigmatic in terms of formation and function. We used supported bilayer membranes and nanometer-scale structures fabricated onto the underlying substrate to impose geometric constraints on immunological synapse formation. Analysis of the resulting alternatively patterned synapses revealed a causal relation between the radial position of T cell receptors (TCRs) and signaling activity, with prolonged signaling from TCR microclusters that had been mechanically trapped in the peripheral regions of the synapse. These results are consistent with a model of the synapse in which spatial translocation of TCRs represents a direct mechanism of signal regulation. 相似文献
378.
379.
Diaz J Ingall E Benitez-Nelson C Paterson D de Jonge MD McNulty I Brandes JA 《Science (New York, N.Y.)》2008,320(5876):652-655
The in situ or authigenic formation of calcium phosphate minerals in marine sediments is a major sink for the vital nutrient phosphorus. However, because typical sediment chemistry is not kinetically conducive to the precipitation of these minerals, the mechanism behind their formation has remained a fundamental mystery. Here, we present evidence from high-sensitivity x-ray and electrodialysis techniques to describe a mechanism by which abundant diatom-derived polyphosphates play a critical role in the formation of calcium phosphate minerals in marine sediments. This mechanism can explain the puzzlingly dispersed distribution of calcium phosphate minerals observed in marine sediments worldwide. 相似文献
380.
Most enterotoxigenic Escherichia coli (ETEC) isolated from neonatal cattle with diarrhea (enteric colibacillosis) exhibit the colonization factor antigen, K99. The K99 pili are necessary for the bacteria to bind to a receptor, N-glycolylneuraminic acid-GM3 on the host cells in the small intestine where the bacteria multiply and secrete toxins that cause the diarrhea. When the attachment of the ETEC to host cell is inhibited, the bacteria do not accumulate sufficiently in the gut to cause disease. Since purified K99 pili block K99+ ETEC from binding to host epithelia, three recombinant K99 proteins of different sizes were developed and produced to demonstrate inhibition with in vitro competitive binding assays. The full-length recombinant protein, rK99-476 inhibited the binding of ETEC with an activity similar to that of the native purified K99, whereas the truncated recombinant K99 protein had no inhibitory activity. Thus this binding activity of rK99-476, which is specific and effective in blocking the receptors on the host cells, may be able to competitively inhibit K99+ ETEC infections in cattle. 相似文献