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   573260篇
  免费   47565篇
  国内免费   1043篇
  2018年   17334篇
  2017年   16084篇
  2016年   14111篇
  2015年   8598篇
  2014年   9315篇
  2013年   13370篇
  2012年   19186篇
  2011年   29081篇
  2010年   23261篇
  2009年   18874篇
  2008年   23854篇
  2007年   26576篇
  2006年   12722篇
  2005年   13497篇
  2004年   13516篇
  2003年   13175篇
  2002年   12325篇
  2001年   18729篇
  2000年   18691篇
  1999年   15015篇
  1998年   5447篇
  1997年   5748篇
  1996年   5490篇
  1995年   5086篇
  1994年   4998篇
  1993年   4997篇
  1992年   12968篇
  1991年   12760篇
  1990年   12673篇
  1989年   12432篇
  1988年   11759篇
  1987年   11040篇
  1986年   10291篇
  1985年   10693篇
  1984年   8900篇
  1983年   7636篇
  1982年   5890篇
  1981年   5280篇
  1980年   4953篇
  1979年   8409篇
  1978年   6597篇
  1977年   6174篇
  1976年   5849篇
  1975年   6357篇
  1974年   6937篇
  1973年   6802篇
  1972年   6901篇
  1971年   6268篇
  1970年   4897篇
  1969年   4921篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
It is known that skin releases volatile organic compounds to the environment, and also that its emission pattern changes with aging of the skin. It could be considered, that these compounds are intermediaries in cell metabolism, since many intermediaries of metabolic pathways have a volatile potential. In this work, a simple and non-destructive method consisting of SPME sampling and GC/MS analysis was developed to identify volatile organic emanations from cell cultures. This technique, applied to skin cells culture, indicates that the cells or cell metabolism produce several skin emissions. Chemometric analysis was performed in order to explore the relationship between a volatile profile and the senescence of cell cultures. Volatile profiles were different for cell cultures in different degrees of senescence, indicating that volatile compound patterns could be used to provide information about the age of skin cells.  相似文献   
32.
A promising producer of bioactive compounds isolated from a Brazilian tropical soil was tested for its range of antimicrobial activities. Strain 606, classified as Streptomyces sp., could not be identified up to species level, suggesting a possible new taxon. The supernatant and 10 extracts and fractions, obtained by extraction and chromatographic techniques, presented antimicrobial activity using antibiograms. The methanolic fraction was highly active against pathogenic bacteria, phytopathogenic fungi and the human pathogenic yeast Candida albicans. It also possessed high antiviral activity inhibiting the propagation of an acyclovir-resistant herpes simplex virus type 1 strain on HEp-2 cells at non-cytotoxic concentration. The strong cytotoxic effect suggests an antitumour action. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
33.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses, which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity infanticide add to the growing database of the phenomenon in wild chimpanzees.  相似文献   
34.
The literature relating to chemical, biochemical and biological aspects of the steroidal glycoalkaloid, α-tomatine, is reviewed. The alkaloid, which can be used as a starting compound for the synthesis of steroidal hormones, is toxic to a wide range of living organisms. The significance of tomatine to plants which elaborate it is discussed and some possible uses of the compound are mentioned.  相似文献   
35.
36.
37.
38.
39.
40.
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