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   4096篇
  免费   392篇
  2022年   24篇
  2021年   57篇
  2020年   45篇
  2019年   55篇
  2018年   61篇
  2017年   61篇
  2016年   77篇
  2015年   161篇
  2014年   155篇
  2013年   212篇
  2012年   236篇
  2011年   262篇
  2010年   165篇
  2009年   152篇
  2008年   189篇
  2007年   220篇
  2006年   199篇
  2005年   202篇
  2004年   242篇
  2003年   206篇
  2002年   160篇
  2001年   73篇
  2000年   80篇
  1999年   62篇
  1998年   72篇
  1997年   40篇
  1996年   35篇
  1995年   27篇
  1994年   40篇
  1993年   35篇
  1992年   35篇
  1991年   40篇
  1990年   29篇
  1989年   33篇
  1988年   30篇
  1987年   45篇
  1986年   48篇
  1985年   41篇
  1984年   35篇
  1983年   48篇
  1982年   32篇
  1981年   32篇
  1980年   27篇
  1979年   32篇
  1978年   39篇
  1977年   25篇
  1976年   23篇
  1975年   19篇
  1974年   27篇
  1971年   24篇
排序方式: 共有4488条查询结果,搜索用时 15 毫秒
11.
Hurricane risk characteristics are examined across the U. S. Gulf of Mexico coastline using a hexagonal tessellation. Using an extreme value model, parameters are collected representing the rate or λ (frequency), the scale or σ (range), and the shape or ξ (intensity) of the extreme wind distribution. These latent parameters and the 30-year return level are visualized across the grid. The greatest 30-year return levels are located toward the center of the Gulf of Mexico, and for inland locations, along the borders of Louisiana, Mississippi, and Alabama. Using a geographically weighted regression model, the relationship of these parameters to sea surface temperature (SST) is found to assess sensitivity to change. It is shown that as SSTs increase near the coast, the frequency of hurricanes in these grids decrease significantly. This reinforces the importance of SST in areas of likely tropical cyclogenesis in determining the number of hurricanes near the coast, along with SSTs along the lifespan of the storm, rather than simply local SST. The range of hurricane wind speeds experienced near Florida is shown to increase with increasing SSTs (insignificant), suggesting that increased temperatures may allow hurricanes to maintain their strength as they pass over the Florida peninsula. The modifiable areal unit problem is assessed using multiple grid sizes. Moran’s I and the local statistic G are calculated to examine spatial autocorrelation in the parameters. This research opens up future questions regarding rapid intensification and decay close to the coast and the relationship to changing SSTs.  相似文献   
12.
13.
Nucleotide sequencing of the second Synechocystis 6803 psbG gene, psbG2 shows the predicted polypeptide to be 219 amino acids long. It is less similar to chloroplast psbG genes than is the Synechocystis psbG1 copy. Alignment with seven other psbG protein sequences, including that from the Paramecium mitochondrial genome reveals a central highly conserved region common to each. This is discussed as evidence supporting the proposal that the psbG polypeptide is a NAD(P)H dehydrogenase (complex I) subunit in cyanobacteria, chloroplasts and mitochondria.  相似文献   
14.
15.
Apolipoprotein E has key functions in lipoprotein metabolism, and polymorphisms in the apolipoprotein E gene are associated with distinct lipoprotein patterns. The possibility of gene-nutrient interactions for apolipoprotein E has been addressed in many studies. Although results have generally been mixed, the indications for such an interaction have been more common in studies employing a metabolic challenge. Studies directly designed to examine apolipoprotein E gene-nutrient interactions are needed.  相似文献   
16.
17.
Using an enzyme-linked immunosorbent assay (ELISA) based technique, Schistocephalus solidus infection was shown to considerably reduce levels of spiggin in the kidney of male three-spined sticklebacks Gasterosteus aculeatus from an oligotrophic upland lake. These results suggested a graduated effect of the infection on the reproductive physiology of male three-spined sticklebacks.  相似文献   
18.
19.
Penicillium citrinum cultures have been germinated on an H2O-based medium, resuspended on a D2O-based medium and treated with [l,2-13C2] acetate. The resulting citrinin (1) has been analysed by2H and13C nuclear magnetic resonance spectroscopy and information about the metabolism of hydrogen in citrinin biosynthesis has been deduced.  相似文献   
20.
Summary Mice selectively bred for either high or low levels of thermoregulatory nest building were cold-acclimated (5°C) for 3 weeks without nesting material; then body weight and food intake were measured. The mice selected for low nest building (Lows) of both sexes showed lower feed efficiencies than the high nest-building mice (Highs), although their body weights were not significantly different (Table 1). This adds to a large body of evidence which suggests that nest building and feed efficiency were influenced by a common mechanism (Lacy et al. 1978; Sulzbach and Lynch 1984; Lunch et al. 1981; Lynch and Roberts 1984).Brown adipose tissue mitochondrial GDP binding and cytochrome c oxidase activity were measured in the above mice. In females, the Lows had 100% higher levels of total GDP binding than the Highs, while no difference between the lines was seen in males (Fig. 2). Thus in the High females, lower energy expenditure through brown fat thermogenesis may account for their greater feed efficiency. In males, the genetic differences in feed efficiency must be due to differences in either thermogenesis in tissues other than brown fat, or mechanisms which reduce heat loss.Abbreviations Highs Mice from lines selectively bred for high levels of nest-building;Lows mice from the low nest-building selected lines  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