全文获取类型
收费全文 | 268篇 |
免费 | 0篇 |
国内免费 | 1篇 |
专业分类
系统科学 | 38篇 |
丛书文集 | 2篇 |
教育与普及 | 1篇 |
现状及发展 | 33篇 |
研究方法 | 34篇 |
综合类 | 158篇 |
自然研究 | 3篇 |
出版年
2017年 | 1篇 |
2016年 | 6篇 |
2015年 | 2篇 |
2014年 | 3篇 |
2013年 | 1篇 |
2012年 | 7篇 |
2011年 | 16篇 |
2010年 | 6篇 |
2009年 | 6篇 |
2008年 | 12篇 |
2007年 | 10篇 |
2006年 | 12篇 |
2005年 | 6篇 |
2004年 | 11篇 |
2003年 | 10篇 |
2002年 | 13篇 |
2001年 | 16篇 |
2000年 | 10篇 |
1999年 | 4篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 7篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1984年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 2篇 |
1971年 | 4篇 |
1970年 | 9篇 |
1969年 | 4篇 |
1968年 | 3篇 |
1967年 | 4篇 |
1966年 | 5篇 |
1965年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有269条查询结果,搜索用时 15 毫秒
261.
Product of per locus of Drosophila shares homology with proteoglycans 总被引:23,自引:0,他引:23
262.
263.
264.
The abundances of highly siderophile (iron-loving) elements (HSEs) in the Earth's mantle provide important constraints on models of the Earth's early evolution. It has long been assumed that the relative abundances of HSEs should reflect the composition of chondritic meteorites--which are thought to represent the primordial material from which the Earth was formed. But the non-chondritic abundance ratios recently found in several types of rock derived from the Earth's mantle have been difficult to reconcile with standard models of the Earth's accretion, and have been interpreted as having arisen from the addition to the primitive mantle of either non-chondritic extraterrestrial material or differentiated material from the Earth's core. Here we report in situ laser-ablation analyses of sulphides in mantle-derived rocks which show that these sulphides do not have chondritic HSE patterns, but that different generations of sulphide within single samples show extreme variability in the relative abundances of HSEs. Sulphides enclosed in silicate phases have high osmium and iridium abundances but low Pd/Ir ratios, whereas pentlandite-dominated interstitial sulphides show low osmium and iridium abundances and high Pd/Ir ratios. We interpret the silicate-enclosed sulphides as the residues of melting processes and interstitial sulphides as the crystallization products of sulphide-bearing (metasomatic) fluids. We suggest that non-chondritic HSE patterns directly reflect processes occurring in the upper mantle--that is, melting and sulphide addition via metasomatism--and are not evidence for the addition of core material or of 'exotic' meteoritic components. 相似文献
265.
采用定理和定性结合的方法,分析陕西省房地产价格波动特点及影响因素.实证结果显示,陕西省新建住宅、新建商品住宅及二手住宅价格均呈平稳中温和上涨的整体运行态势.房地产价格波动的主要影响因素为政府的宏观调控政策、保障房供应、土地供给加大等.根据分析结果提出稳定房地产价格、促进陕西房地产市场健康发展的政策建议. 相似文献
266.
Stream denitrification across biomes and its response to anthropogenic nitrate loading 总被引:11,自引:0,他引:11
Mulholland PJ Helton AM Poole GC Hall RO Hamilton SK Peterson BJ Tank JL Ashkenas LR Cooper LW Dahm CN Dodds WK Findlay SE Gregory SV Grimm NB Johnson SL McDowell WH Meyer JL Valett HM Webster JR Arango CP Beaulieu JJ Bernot MJ Burgin AJ Crenshaw CL Johnson LT Niederlehner BR O'Brien JM Potter JD Sheibley RW Sobota DJ Thomas SM 《Nature》2008,452(7184):202-205
Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20-25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks. 相似文献
267.
d'Adda di Fagagna F Reaper PM Clay-Farrace L Fiegler H Carr P Von Zglinicki T Saretzki G Carter NP Jackson SP 《Nature》2003,426(6963):194-198
Most human somatic cells can undergo only a limited number of population doublings in vitro. This exhaustion of proliferative potential, called senescence, can be triggered when telomeres--the ends of linear chromosomes-cannot fulfil their normal protective functions. Here we show that senescent human fibroblasts display molecular markers characteristic of cells bearing DNA double-strand breaks. These markers include nuclear foci of phosphorylated histone H2AX and their co-localization with DNA repair and DNA damage checkpoint factors such as 53BP1, MDC1 and NBS1. We also show that senescent cells contain activated forms of the DNA damage checkpoint kinases CHK1 and CHK2. Furthermore, by chromatin immunoprecipitation and whole-genome scanning approaches, we show that the chromosome ends of senescent cells directly contribute to the DNA damage response, and that uncapped telomeres directly associate with many, but not all, DNA damage response proteins. Finally, we show that inactivation of DNA damage checkpoint kinases in senescent cells can restore cell-cycle progression into S phase. Thus, we propose that telomere-initiated senescence reflects a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres. 相似文献
268.
Nikita Minhas Meilang Xue Christopher J. Jackson 《Cellular and molecular life sciences : CMLS》2017,74(10):1895-1906
Activated protein C (APC) is a natural anticoagulant with strong anti-inflammatory, anti-apoptotic, and barrier stabilizing properties. These cytoprotective properties of APC are thought to be exerted through its pathway involving the binding of APC to endothelial protein C receptor and cleavage of protease-activated receptors. In this study, we found that APC enhanced endothelial barrier integrity via a novel pathway, by binding directly to and activating Tie2, a transmembrane endothelial tyrosine kinase receptor. Binding assays demonstrated that APC competed with the only known ligands of Tie2, the angiopoietins (Angs). APC bound directly to Tie2 (Kd ~3 nM), with markedly stronger binding affinity than Ang2. After binding, APC rapidly activated Tie2 to enhance endothelial barrier function as shown by Evan’s blue dye transfer across confluent cell monolayers and in vivo studies. Blocking Tie2 restricted endothelial barrier integrity. This study highlights a novel mechanism by which APC binds directly to Tie2 to enhance endothelial barrier integrity, which helps to explain APC’s protective effects in vascular leakage-related pathologies. 相似文献
269.
The composition of four radiocarbon-dated, late Pleistocene woodrat middens is reported and analyzed. A date of 11,850 ±550 BP records the first reported macrofossil occurrence in this region of late Pleistocene Pinus monophylla-Juniperus osteosperma woodlands. A 7,800 ±350 BP date documents the most recent J. osteosperma woodlands in this presently coniferless desert area, while a date of 12,100 ±400 BP is the oldest record of juniper woodlands among the four middens. Other juniper and creosote bush desert flora radiocarbon dates along with six pollen profiles were obtained. The research suggests that as recently as 7,800 BP this part of the Mohave Desert was subject to a cooler, moister climate than at present, and that the aboriginal food resources of pinyon seeds and juniper berries were probably available to early prehistoric man in this area. 相似文献