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   116274篇
  免费   4568篇
  国内免费   88篇
  2021年   924篇
  2020年   695篇
  2019年   887篇
  2018年   1936篇
  2017年   2071篇
  2016年   4057篇
  2015年   7560篇
  2014年   7358篇
  2013年   7692篇
  2012年   7297篇
  2011年   4519篇
  2010年   3698篇
  2009年   3408篇
  2008年   2344篇
  2007年   2287篇
  2006年   2233篇
  2005年   7911篇
  2004年   6650篇
  2003年   4766篇
  2002年   2275篇
  2001年   1650篇
  2000年   857篇
  1999年   1921篇
  1998年   693篇
  1992年   2132篇
  1991年   2186篇
  1990年   2240篇
  1989年   2121篇
  1988年   2065篇
  1987年   1909篇
  1986年   1730篇
  1985年   1742篇
  1984年   1209篇
  1983年   910篇
  1982年   557篇
  1981年   499篇
  1979年   1125篇
  1978年   812篇
  1977年   653篇
  1976年   670篇
  1975年   919篇
  1974年   1069篇
  1973年   1047篇
  1972年   978篇
  1971年   955篇
  1970年   855篇
  1969年   866篇
  1968年   764篇
  1967年   770篇
  1966年   599篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
We present a time‐calibrated phylogeny of the charismatic green lacewings (Neuroptera: Chrysopidae). Previous phylogenetic studies on the family using DNA sequences have suffered from sparse taxon sampling and/or limited amounts of data. Here we combine all available previously published DNA sequence data and add to it new DNA sequences generated for this study. We analysed these data in a supermatrix using Bayesian and maximum likelihood methods and provide a phylogenetic hypothesis for the family that recovers strong support for the monophyly of all subfamilies and resolves relationships among a large proportion of chrysopine genera. Chrysopinae tribes Leucochrysini and Belonopterygini were recovered as monophyletic sister clades, while the species‐rich tribe Chrysopini was rendered paraphyletic by Ankylopterygini. Relationships among the subfamilies were resolved, although with relatively low statistical support, and the topology varied based on the method of analysis. Greatest support was found for Apochrysinae as sister to Nothochrysinae and Chrysopinae, which is in contrast to traditional concepts that place Nothochrysinae as sister to the rest of the family. Divergence estimates suggest that the stem groups to the various subfamilies diverged during the Triassic‐Jurassic, and that stem groups of the chrysopine tribes diverged during the Cretaceous.  相似文献   
63.
Following treatment of hen erythrocyte nuclei with dimethyl 3,3'-dithiobispropionimidate, dimers between histones H1a, H1b, and H5 were extracted with 5% perchloric acid. They resolved electrophoretically into four sub-bands and these were identified by non-reducing/reducing gel electrophoresis. The H5-H5 homodimer species was purified by gel electrophoresis and was treated sequentially with BrCN and dithiothreitol. The pattern of resulting fragments indicates that cross-links were mainly formed between the COOH-terminal portions and at a significantly lower frequency between the COOH-terminal and the NH2-terminal portions.  相似文献   
64.
Enzyme activity capable of converting the glycine-extended substance P precursor, substance P-Gly12, into substance P was purified from human cerebrospinal fluid. The conversion reaction was monitored by radioimmunoassay measurement of substance P formation. The chemical identity of the product was verified by reversed-phase HPLC. The enzyme reaction was stimulated by Cu(II) ion and ascorbic acid and inhibited by the presence of diethyldithiocarbamate. By HPLC molecular sieving, the major enzyme activity appeared as a protein of 26,000 molecular weight.  相似文献   
65.
The ultrasonic calls produced by three day old mice when separated from the nest mother and siblings increase in number when naloxone is injected.  相似文献   
66.
67.
In has been shown that cortisol immobilized on polyvinylpyrrolidone (PVP-GC) affects cyclic AMP production stimulated by adenosine and isoproterenol in rat thymocytes. This effect of PVP-GC is specific for cortisol: antiglucocorticoid progesterone (at a concentration of 10(-5) M) inhibited completely the action of PVP-GC on the intracellular cAMP level. It is suggested that cortisol effect on cAMP production is one of the mechanisms of glucocorticoid hormone action in target cells.  相似文献   
68.
We sampled macroinvertebrates at 75 locations in the Mondego river catchment, Central Portugal, and developed a predictive model for water quality assessment of this basin, based on the Reference Condition Approach. Sampling was done from June to September 2001. Fifty-five sites were identified as “Reference sites” and 20 sites were used as “Test sites” to test the model. At each site we also measured 40 habitat variables to characterize water physics and chemistry, habitat type, land use, stream hydrology and geographic location. Macroinvertebrates were generally identified to species or genus level; a total of 207 taxa were found. By Unweighted Pair Group Method with Arithmetic mean (UPGMA) clustering and analysis of species contribution to similarities percentage (SIMPER), two groups of reference sites were established. Using Discriminant Analysis (stepwise forward), four variables correctly predicted 78% of the reference sites to the appropriate group: stream order, pool quality, substrate quality and current velocity. Test sites’ environmental quality was established from their relative distance to reference sites, in MDS ordination space, using a series of bands (BEAST methodology). The model performed well at upstream sites, but at downstream sites it was compromised by the lack of reference sites. As with the English RIVPACS predictive model, the Mondego model should be continually improved with the addition of new reference sites. The adaptation of the Mondego model methodology to the Water Framework Directive is possible and would consist mainly of the integration of the WFD typology and increasing the number of ellipses that define quality bands. Handling editor: K. Martens  相似文献   
69.
70.
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