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   555938篇
  免费   51893篇
  国内免费   1549篇
  609380篇
  2018年   17265篇
  2017年   16184篇
  2016年   14078篇
  2015年   8936篇
  2014年   9894篇
  2013年   13273篇
  2012年   20303篇
  2011年   30426篇
  2010年   24123篇
  2009年   19610篇
  2008年   25254篇
  2007年   27827篇
  2006年   13736篇
  2005年   14201篇
  2004年   14133篇
  2003年   13535篇
  2002年   12885篇
  2001年   19244篇
  2000年   19137篇
  1999年   15114篇
  1998年   5637篇
  1997年   5674篇
  1996年   5389篇
  1995年   5161篇
  1994年   4862篇
  1993年   4888篇
  1992年   12506篇
  1991年   12511篇
  1990年   12209篇
  1989年   11700篇
  1988年   10859篇
  1987年   10241篇
  1986年   9754篇
  1985年   9522篇
  1984年   7904篇
  1983年   6856篇
  1982年   5159篇
  1981年   4637篇
  1980年   4320篇
  1979年   7355篇
  1978年   5975篇
  1977年   5307篇
  1976年   4962篇
  1975年   5786篇
  1974年   6309篇
  1973年   6123篇
  1972年   6084篇
  1971年   5660篇
  1970年   4283篇
  1969年   4213篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
It has been found by gel-penetrating chromatography and quantitative 13C NMR spectroscopy that mechanochemical processing of arabinogalactan isolated from the wood of the Siberian larch changes the molecular mass distribution, monosaccharide composition, and the degree of branching of its macromolecules. This is due to the partial destruction of polysaccharide macromolecules and subsequent recombination of resulting fragments. The intensity of these processes depends on the conditions of mechanochemical processing. Based on the data of IR and 13C NMR spectra, the conclusion has been drawn that no functionalization of arabinogalactan macromolecules occurs under these conditions. The toxicopharmacological properties of mechanically processed arabinogalactan have been studied. Its acute toxicity is >5000 mg/kg.  相似文献   
62.
63.
64.
With DNA sequencing now getting cheaper more quickly than data storage or computation, the time may have come for genome informatics to migrate to the cloud.  相似文献   
65.
66.
Puerto Rican populations of two species of sea anemones (Bunodosoma cavernata and B. granulifera) which had previously been considered one were assayed electrophoretically for enzymes encoded by 12 loci. The two species shared no common allozymes at 6 of the 12 loci. Genetic distance and identity values based on these allozymes were computed for the Puerto Rican populations and for B. cavernata from Florida and B. granulifera from Panama. The Puerto Rican populations of both species had much higher genetic identities for their geographically distant conspecifics than for each other. These results indicate that the two species are reproductively isolated and should be considered as separate valid species. Average heterozygosities are presented which are the first published for coelenterate species.  相似文献   
67.
Rice fields are an alternative habitat for waterbirds and provide food and shelter for many avian species, but there is a lack of information about how the use of rice fields translates into population level effects. The aim of this study was to test the relationship between the use of rice fields by European waterbirds and trends in their populations. We tested this relationship during the autumn migration season and during the breeding season. Based on counts conducted over the last 23 years in natural marshes and areas of rice fields in Doñana (SW Spain), an index of rice field use was constructed for 76 bird species, which was then compared to these species’ European population trends obtained from the literature. A positive relationship was found between waterbird population trends and the use of rice fields during autumn migration season. Our study suggests that changes in the Common Agriculture Policy in Europe leading to reductions in areas of rice cultivation may have important effects on waterbirds. The restoration of former marsh areas and the maintenance of rice cultivation would seem to be more environmentally friendly approaches than the use of these areas to grow alternative crops or solar farms.  相似文献   
68.
Previous studies have shown that the concentration of red blood cell (RBC) magnesium is significantly lower in subjects carrying an HLA-BW 35 antigen (p less than 0.001) than in non-carriers. As this finding might be related to modifications of the RBC membrane sialoglycoconjugates, RBC sialic acid was comparatively determined in BW 35+ and BW 35- subjects. Pyruvate-kinase activity mean RBC volume, and reticulocyte count have also been determined in order to estimate whether some significant variations in the level of these age markers could be detected between the HLA BW 35+ and BW 35- subjects. A significant negative correlation between sialic acid and RBC magnesium concentrations was observed for the whole population tested (n 57, p less than 0.005), 61% of the BW 35+ and only 25% of the BW 35- individuals having sialic acid values above, and magnesium values below the overall mean (p less than 0.01). The variance of mean RBC volume was also larger for the BW 35+ group. Other determinations did not show any significant variations, suggesting that the results are not related to RBC age.  相似文献   
69.
70.
EEG activity was recorded in rats submitted to osmotic opening of the BBB by intracarotid mannitol infusion.This procedure produced an immediate short-lasting depression of the EEG and a tardive paroxysmal EEG activity. Both these phenomena were more relevant on the ipsilateral hemisphere. In some instances a tonico-clonic seizure was recorded.Pre-treatment with diazepam abolished the occurrence of the tardive EEG and behavioral modifications.In accord with previous findings, focal seizure activity is likely to be responsible for the metabolic abnormalities associated with osmotic opening of the BBB. This preparation therefore produces in the brain unphysiological states in respect to local metabolism and electrical function.  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