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   112086篇
  免费   1762篇
  国内免费   886篇
  114734篇
  2023年   84篇
  2022年   86篇
  2021年   202篇
  2020年   156篇
  2019年   164篇
  2018年   12102篇
  2017年   10920篇
  2016年   7915篇
  2015年   1460篇
  2014年   1106篇
  2013年   1478篇
  2012年   5649篇
  2011年   14145篇
  2010年   12720篇
  2009年   8826篇
  2008年   10893篇
  2007年   12449篇
  2006年   1389篇
  2005年   1559篇
  2004年   1910篇
  2003年   1917篇
  2002年   1610篇
  2001年   660篇
  2000年   578篇
  1999年   384篇
  1998年   159篇
  1997年   160篇
  1996年   118篇
  1995年   120篇
  1994年   112篇
  1993年   111篇
  1992年   229篇
  1991年   254篇
  1990年   200篇
  1989年   158篇
  1988年   182篇
  1987年   149篇
  1986年   132篇
  1985年   116篇
  1984年   116篇
  1983年   103篇
  1982年   83篇
  1981年   105篇
  1979年   108篇
  1978年   99篇
  1976年   91篇
  1975年   82篇
  1974年   89篇
  1972年   293篇
  1971年   350篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
We propose a technique for separating the climatic signal which is contained in two tree-ring parameters widely used in dendroclimatology. The method is based on the removal of the relationship between tree-ring width (TRW) and maximum latewood density (MXD) observed for narrow tree rings from high latitudes. The new technique is tested on data from three larch stands located along the northern timberline in Eurasia. Correlations were calculated between the temperatures of pentads (five consecutive days), TRW chronologies and MXD chronologies calculated according to the standard and proposed methods. The analysis confirms the great importance of summer temperature for tree radial growth and tree-ring formation. TRW is positively correlated with the temperature of four to eight pentads (depending on the region) at the beginning of the growth season, but MXD as obtained by the standard technique is correlated with temperature over a much longer period. For maximum density series from which the relationship between MXD and TRW has been removed (MXD′), there is a clear correlation with temperatures in the second part of the growing season. These results are consistent with the known dynamics of tree-ring growth in high latitudes and mechanisms of tree-ring formation.  相似文献   
42.
43.
44.
Systems biology is an approach to explain the behaviour of a system in relation to its individual components. Synthetic biology uses key hierarchical and modular concepts of systems biology to engineer novel biological systems. In my opinion the next step in biology is to use molecule-to-phenotype data using these approaches and integrate them in the form a periodic table. A periodic table in biology would provide chassis to classify, systematize and compare diversity of component properties vis-a-vis system behaviour. Using periodic table it could be possible to compute higher-level interactions from component properties. This paper examines the concept of building a bio-periodic table using protein fold as the fundamental unit.  相似文献   
45.
Ferruh Aşçi 《Biologia》2009,64(6):1146-1149
In this study, the structural characteristics, unique features, various organ measurements of males and females of the water mite Atractides (Atractides) turcicus sp. n. from Turkey are described. In addition, the study compares their characteristics with related species.  相似文献   
46.
47.
Biological screening for hereditary thrombophilia must be performed with constant concern for quality of the results and the interpretation. Different guidelines are available common to most laboratory tests, common to hemostasis tests, thrombophilia screening or specific for each test. These different steps are discussed in this paper with a special focus on the diagnosis of antithrombin, protein C and protein S deficiencies.  相似文献   
48.
A general in vitro cloning system was established for four Helleborus species: H. argutifolius, H. foetidus, H. niger and H. orientalis. The plant material was introduced in vitro from axillary buds. A Murashige and Skoog (MS)—based medium (Murashige and Skoog 1962) was used supplemented with 2% (w/v) sucrose, 2-isopentenyladenine (2-iP) and 6-benzylaminopurine (BA). Multiplication rates depended on the genotype and varied from 1.3 for H. foetidus till 3.8 for H. niger. The first results showed that the rooting phase could be done ex vitro. Rooting was induced by a drench for one week in a solution of indole-3-butyric acid (IBA -3 mg l−1) and 1-naphthaleneacetic acid (NAA-1 mg l−1) at 5°C.  相似文献   
49.
50.
Acidic lipase finds its commercial values in medical applications and bioremediation of food wastes. In this work, approaches for rapid screening of lipase-producing bacteria were developed and the feasibility assessment of the screening methods was performed. From food waste samples, the proposed screening procedures allowed isolation of sixteen pure bacterial strains expressing higher lipase activity at acidic pH (pH 6.0) than at alkaline pH (pH 9.0). To enhance the accuracy of lipase activity determination under acidic conditions, a novel assay procedure was also developed by deactivating lipase activity by microwave treatment prior to back titration. This additional step could minimize interferences arising from residual lipase activity during conventional direct back-titration methods in measuring lipase activity at acidic pH. Using the four strategies proposed in this work, the best acidic-lipase-producing isolate was obtained by strategy C (SSC) and was identified as Aeromonas sp. C14, displaying an optimal lipase activity of 0.7 U/ml at an acidic pH of 6.0.  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