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   719115篇
  免费   66247篇
  国内免费   1150篇
  2021年   5489篇
  2018年   18206篇
  2017年   16739篇
  2016年   16848篇
  2015年   13917篇
  2014年   15481篇
  2013年   21905篇
  2012年   27848篇
  2011年   35130篇
  2010年   26484篇
  2009年   21314篇
  2008年   28872篇
  2007年   30468篇
  2006年   19753篇
  2005年   18596篇
  2004年   18847篇
  2003年   17898篇
  2002年   16933篇
  2001年   30916篇
  2000年   30648篇
  1999年   23798篇
  1998年   7673篇
  1997年   8012篇
  1996年   7448篇
  1995年   6875篇
  1994年   6545篇
  1993年   6496篇
  1992年   18155篇
  1991年   17289篇
  1990年   16673篇
  1989年   16223篇
  1988年   14783篇
  1987年   13809篇
  1986年   12854篇
  1985年   12672篇
  1984年   10420篇
  1983年   8724篇
  1982年   6528篇
  1981年   5863篇
  1980年   5547篇
  1979年   9640篇
  1978年   7362篇
  1977年   6804篇
  1976年   6122篇
  1975年   6859篇
  1974年   7442篇
  1973年   7205篇
  1972年   6805篇
  1971年   6211篇
  1970年   5271篇
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
51.
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   
52.
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   
53.
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   
54.
Formation of rings from Drosophila DNA fragments   总被引:1,自引:0,他引:1  
  相似文献   
55.
56.
Transcriptive expression during sea urchin embryogenesis   总被引:2,自引:0,他引:2  
  相似文献   
57.
58.
59.
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and structural-functional organization of the studied organs.  相似文献   
60.
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