首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   898篇
  免费   51篇
  国内免费   26篇
林业   42篇
农学   45篇
基础科学   20篇
  124篇
综合类   245篇
农作物   50篇
水产渔业   53篇
畜牧兽医   332篇
园艺   34篇
植物保护   30篇
  2024年   2篇
  2023年   6篇
  2022年   20篇
  2021年   35篇
  2020年   35篇
  2019年   39篇
  2018年   36篇
  2017年   28篇
  2016年   25篇
  2015年   37篇
  2014年   48篇
  2013年   39篇
  2012年   61篇
  2011年   86篇
  2010年   47篇
  2009年   49篇
  2008年   31篇
  2007年   45篇
  2006年   41篇
  2005年   27篇
  2004年   25篇
  2003年   28篇
  2002年   22篇
  2001年   24篇
  2000年   16篇
  1999年   14篇
  1998年   4篇
  1997年   11篇
  1996年   4篇
  1995年   4篇
  1994年   5篇
  1993年   8篇
  1991年   5篇
  1990年   4篇
  1989年   8篇
  1988年   4篇
  1987年   4篇
  1986年   6篇
  1985年   5篇
  1984年   2篇
  1983年   3篇
  1982年   2篇
  1978年   3篇
  1975年   3篇
  1974年   2篇
  1973年   2篇
  1972年   4篇
  1970年   2篇
  1968年   5篇
  1956年   2篇
排序方式: 共有975条查询结果,搜索用时 31 毫秒
71.
Single-domain microcrystals of LaC(2) encapsulated within nanoscale polyhedral carbon particles have been synthesized in a carbon arc. Typical particle sizes are on the order of 20 to 40 nanometers. The stoichiometry and phase of the La-containing crystals have been assigned from characteristic lattice spacings observed by high-resolution transmission electron microscopy and energy dispersive spectroscopy (EDS). EDS spectra show that La and C are the only elements present. Characteristic interatomic distances of 3.39 and 2.78 angstroms identify the compound inside the nanoparticle cavities as alpha-LaC(2), the phase of LaC(2) that is stable at room temperature. Bulk alpha-LaC(2) is metallic and hydrolytic. Observation of crystals of pure encapsulated alpha-LaC(2) that were exposed to air for several days before analysis indicates that the LaC(2) is protected from degradation bythe carbon polyhedral shells of the nanoparticles. A high percentage of the carbon nanoparticles have encapsulated LaC(2) single crystals. These carbon-coated metal crystals form a new class of materials that can be protected in their pure or carbide forms and may have interesting and useful properties.  相似文献   
72.
Kim E  Chan MH 《Science (New York, N.Y.)》2004,305(5692):1941-1944
We report on the observation of nonclassical rotational inertia in solid helium-4 confined to an annular channel in a sample cell under torsional motion, demonstrating superfluid behavior. The effect shows up as a drop in the resonant oscillation period as the sample cell is cooled below 230 millikelvin. Measurement of 17 solid samples allows us to map out the boundary of this superfluid-like solid or supersolid phase from the melting line up to 66 bars. This experiment indicates that superfluid behavior is found in all three phases of matter.  相似文献   
73.
RNA silencing genes control de novo DNA methylation   总被引:1,自引:0,他引:1  
  相似文献   
74.
Spatially-variable application of inputs to agricultural fields is dependent upon accuracy of the input variables. The effects of boundary determination, interpolation method, and GPS location errors were studied for determining a variable rate nitrogen application map based upon yield maps of a 3.6 ha Florida citrus block. A general linear model for the mean absolute error approximated the error effects.  相似文献   
75.
A conserved domain in the extracellular region of the 60- and 80-kilodalton tumor necrosis factor receptors (TNFRs) was identified that mediates specific ligand-independent assembly of receptor trimers. This pre-ligand-binding assembly domain (PLAD) is physically distinct from the domain that forms the major contacts with ligand, but is necessary and sufficient for the assembly of TNFR complexes that bind TNF-alpha and mediate signaling. Other members of the TNFR superfamily, including TRAIL receptor 1 and CD40, show similar homotypic association. Thus, TNFRs and related receptors appear to function as preformed complexes rather than as individual receptor subunits that oligomerize after ligand binding.  相似文献   
76.
Cloning through seeds has potential revolutionary applications in agriculture, because it would allow vigorous hybrids to be propagated indefinitely. However, asexual seed formation or apomixis, avoiding meiosis and fertilization, is not found in the major food crops. To develop de novo synthesis of apomixis, we crossed Arabidopsis MiMe and dyad mutants that produce diploid clonal gametes to a strain whose chromosomes are engineered to be eliminated after fertilization. Up to 34% of the progeny were clones of their parent, demonstrating the conversion of clonal female or male gametes into seeds. We also show that first-generation cloned plants can be cloned again. Clonal reproduction through seeds can therefore be achieved in a sexual plant by manipulating two to four conserved genes.  相似文献   
77.
78.
With data from 33 nations, we illustrate the differences between cultures that are tight (have many strong norms and a low tolerance of deviant behavior) versus loose (have weak social norms and a high tolerance of deviant behavior). Tightness-looseness is part of a complex, loosely integrated multilevel system that comprises distal ecological and historical threats (e.g., high population density, resource scarcity, a history of territorial conflict, and disease and environmental threats), broad versus narrow socialization in societal institutions (e.g., autocracy, media regulations), the strength of everyday recurring situations, and micro-level psychological affordances (e.g., prevention self-guides, high regulatory strength, need for structure). This research advances knowledge that can foster cross-cultural understanding in a world of increasing global interdependence and has implications for modeling cultural change.  相似文献   
79.
Apple polyphenols (AP) are an excellent source of dietary antioxidants. The present study investigated the effect of AP on the lifespan of fruit flies and their interaction with gene expressions of superoxide dismutase (SOD), catalase (CAT), methuselah (MTH), Rpn11, and cytochrome c oxidase (CcO) subunits III and VIb. Results showed the mean lifespan was significantly extended by 10% in fruit flies fed the AP diet. This was accompanied by up-regulation of genes SOD1, SOD2, and CAT and down-regulation of MTH in the aged fruit flies. Paraquat and H(2)O(2) challenge tests demonstrated that AP prolonged the survival time only for Oregon R wild type flies but not for SOD(n108) or Cat(n1) mutants, in which either SOD or CAT was knocked out. Chronic paraquat exposure could shorten the maximum lifespan from 68 to 31 days and reduce the climbing ability by 60%, whereas AP could partially reverse the paraquat-induced mortality and decline in climbing ability. AP could up-regulate Rpn11 at day 30, whereas it appeared to have no significant effect on gene expression of ubiquitinated protein, CcO subunits III and VIb. These AP-induced changes were unlikely associated with caloric restriction as the gustatory assay found no difference in average body weight and stomach redness index between the control and AP fruit flies. It was therefore concluded that the antiaging activity of AP was, at least in part, mediated by its interaction with genes SOD, CAT, MTH, and Rpn11.  相似文献   
80.
Sargassum hemiphyllum , a kind of brown seaweed generally found along coastlines in East Asia, has long served as a traditional Chinese medicine. S. hemiphyllum has shown an anti-inflammatory effect; however, its mechanism has not been elucidated clearly. This study explored S. hemiphyllum for its biomedical effects. S. hemiphyllum sulfated polysaccharide extract (SHSP) was first prepared; the mouse macrophage cell line (RAW 264.7) activated by lipopolysaccharide (LPS) was used as a model system. The secretion profiles of pro-inflammatory cytokines, including IL-1β, IL-6, TNF-α, and NO, were found significantly to be reduced in 1-5 mg/mL dose ranges of SHSP treatments. RT-PCR analysis suggested SHSP inhibits the LPS-induced mRNA expressions of IL-β, iNOS, and COX-2 in a dose-dependent manner. At protein levels, Western blot analysis demonstrated a similar result for NF-κB (p65) in cytosol/nuclear. Taken together, the anti-inflammatory properties of SHSP may be attributed to the down-regulation of NF-κB in nucleus.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号