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   497830篇
  免费   55729篇
  国内免费   763篇
  2018年   4889篇
  2017年   4679篇
  2016年   6892篇
  2015年   10313篇
  2014年   11751篇
  2013年   15882篇
  2012年   19012篇
  2011年   19438篇
  2010年   12902篇
  2009年   11631篇
  2008年   16926篇
  2007年   17427篇
  2006年   16380篇
  2005年   15770篇
  2004年   15818篇
  2003年   14782篇
  2002年   14430篇
  2001年   19558篇
  2000年   19317篇
  1999年   15721篇
  1998年   6376篇
  1997年   6178篇
  1996年   5891篇
  1995年   5702篇
  1994年   5369篇
  1993年   5379篇
  1992年   12970篇
  1991年   12871篇
  1990年   12557篇
  1989年   12024篇
  1988年   11210篇
  1987年   10513篇
  1986年   10050篇
  1985年   9862篇
  1984年   8286篇
  1983年   7161篇
  1982年   5566篇
  1981年   5003篇
  1980年   4635篇
  1979年   7571篇
  1978年   6233篇
  1977年   5550篇
  1976年   5182篇
  1975年   5973篇
  1974年   6516篇
  1973年   6314篇
  1972年   5570篇
  1971年   5193篇
  1970年   4398篇
  1969年   4307篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.

Background  

Recent studies point to a great diversity of non-ribosomal peptide synthesis systems with major roles in amino acid and co-factor biosynthesis, secondary metabolism, and post-translational modifications of proteins by peptide tags. The least studied of these systems are those utilizing tRNAs or aminoacyl-tRNA synthetases (AAtRS) in non-ribosomal peptide ligation.  相似文献   
92.
Antibodies to calcitonin, parathyroid hormone and cortisol are detected in acute stage of infection-caused bronchial asthma. The appearance of antibodies is paralleled by marked hypercalcaemia. The antibodies may bind excessive hormones in the blood, preventing further hormonal imbalance. Ten-day treatment with glucocorticoids decreased the amount of antibodies possibly due to normalization of hormonal secretion and restoration of their balance. As a result, calcium blood levels returned to normal.  相似文献   
93.
94.
95.
96.
97.
  1. Fishing is a strong selective force and is supposed to select for earlier maturation at smaller body size. However, the extent to which fishing‐induced evolution is shaping ecosystems remains debated. This is in part because it is challenging to disentangle fishing from other selective forces (e.g., size‐structured predation and cannibalism) in complex ecosystems undergoing rapid change.
  2. Changes in maturation size from fishing and predation have previously been explored with multi‐species physiologically structured models but assumed separation of ecological and evolutionary timescales. To assess the eco‐evolutionary impact of fishing and predation at the same timescale, we developed a stochastic physiologically size‐structured food‐web model, where new phenotypes are introduced randomly through time enabling dynamic simulation of species'' relative maturation sizes under different types of selection pressures.
  3. Using the model, we carried out a fully factorial in silico experiment to assess how maturation size would change in the absence and presence of both fishing and predation (including cannibalism). We carried out ten replicate stochastic simulations exposed to all combinations of fishing and predation in a model community of nine interacting fish species ranging in their maximum sizes from 10 g to 100 kg. We visualized and statistically analyzed the results using linear models.
  4. The effects of fishing on maturation size depended on whether or not predation was enabled and differed substantially across species. Fishing consistently reduced the maturation sizes of two largest species whether or not predation was enabled and this decrease was seen even at low fishing intensities (F = 0.2 per year). In contrast, the maturation sizes of the three smallest species evolved to become smaller through time but this happened regardless of the levels of predation or fishing. For the four medium‐size species, the effect of fishing was highly variable with more species showing significant and larger fishing effects in the presence of predation.
  5. Ultimately our results suggest that the interactive effects of predation and fishing can have marked effects on species'' maturation sizes, but that, at least for the largest species, predation does not counterbalance the evolutionary effect of fishing. Our model also produced relative maturation sizes that are broadly consistent with empirical estimates for many fish species.
  相似文献   
98.
A random sample of schoolchildren, 119 boys and 153 girls, was tested in the fall of 1983. The data presented here are anthropometric data (height, weight, fat % and vital capacity) and oxygen uptake directly measured on a bicycle ergometer. The mean height and weight for boys were 179.1 cm and 67.7 kg, and those for girls were 168.0 cm and 59.6 kg. The mean fat content was 9.1% for boys and 19.1% for girls, and their mean vital capacities were 4.91 and 3.61 respectively. The boys had a high maximal oxygen uptake (51.7 ml X kg-1 X min-1) showing no reduction over the age span studied. The girls' maximal oxygen uptake was lower (overall mean 40.0 ml X kg-1 X min-1) with a small reduction from 16 to 19 years of age. When comparing maximal oxygen uptake per kg lean body mass in the two sexes, the boys had 18.4% higher values than the girls, indicating that girls of this age have the lower fitness level. The results of maximal aerobic power measurement in the boys compare well with findings from other investigations using direct measurements, indicating that the fitness of teenage boys is kept at a high level. Comparable data from various countries for girls show different pictures, but it appears that in general they have a low fitness level.  相似文献   
99.
100.
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