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   474963篇
  免费   52130篇
  国内免费   209篇
  2018年   4310篇
  2017年   4264篇
  2016年   5932篇
  2015年   8320篇
  2014年   9655篇
  2013年   14136篇
  2012年   16093篇
  2011年   16348篇
  2010年   10948篇
  2009年   10106篇
  2008年   14815篇
  2007年   15043篇
  2006年   14427篇
  2005年   13870篇
  2004年   13871篇
  2003年   13189篇
  2002年   12972篇
  2001年   19731篇
  2000年   19692篇
  1999年   16324篇
  1998年   6494篇
  1997年   6462篇
  1996年   6245篇
  1995年   5707篇
  1994年   5820篇
  1993年   5628篇
  1992年   13493篇
  1991年   12929篇
  1990年   12837篇
  1989年   12831篇
  1988年   11647篇
  1987年   11101篇
  1986年   10268篇
  1985年   10450篇
  1984年   8711篇
  1983年   7581篇
  1982年   6011篇
  1981年   5427篇
  1980年   5242篇
  1979年   8169篇
  1978年   6493篇
  1977年   5818篇
  1976年   5650篇
  1975年   5931篇
  1974年   6278篇
  1973年   6162篇
  1972年   5445篇
  1971年   4833篇
  1970年   4319篇
  1969年   4000篇
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
811.
Published results of the distribution of anthocyanins in the Sterculiaceae have been re-interpreted on the basis of the phylogenetic status of the compounds present. A flavonoid score system was less useful than a cladistic interpretation based on Hennigian arguments.  相似文献   
812.
813.
Abstract Porcine Escherichia coli strains isolated from cases fo postweaning diarrhea or edema disease were analysed for the presence of fedA , the major subunit gene of F107 fimbriae. The E. coli isolates were known to contain colonisation factor '8813', or to express F107, 2134P or other fimbriae, different from F4, F5, F6, and F41. PCR with fedA -specific primers, restriction enzyme digestion of the PCR product, and nucleotide sequence analysis demonstrated that 2134P pili, colonisation factor '8813' and fimbriae identified on Australian strains of the O141 serotype belong to one family of F107 fimbrial antigens.  相似文献   
814.
Regional Evaluation of Carbon Budget of Forests (RECBF), was used to study the dynamics of carbon balance in Russian forests in 1988–2015. The carbon sink (excess of absorption over losses) to forests was minimal in 1988. Since the first half of the 1990s, its increase has started. This increase was associated with the reduction of logging volume in connection with socioeconomic reforms. Since 2008, the carbon sink was gradually reduced due to increasing losses in logging operations, forest fires, and decreased carbon absorption.  相似文献   
815.
Vaginal microbicides potentially empower women to protect themselves from HIV and other sexually transmitted infections (STIs), especially when culture, religion, or social status may prevent them from negotiating condom use. The open literature contains minimal information on factors that drive user acceptability of women’s health products or vaginal drug delivery systems. By understanding what women find to be most important with regard to sensory properties and product functionality, developers can iteratively formulate a more desirable product. Conjoint analysis is a technique widely used in market research to determine what combination of elements influence a consumer’s willingness to try or use a product. We applied conjoint analysis here to better understand what sexually-active woman want in a microbicide, toward our goal of formulating a product that is highly acceptable to women. Both sensory and non-sensory attributes were tested, including shape, color, wait time, partner awareness, messiness/leakage, duration of protection, and functionality. Heterosexually active women between 18 and 35 years of age in the United States (n = 302) completed an anonymous online conjoint survey using IdeaMap software. Attributes (product elements) were systematically presented in various combinations; women rated these combinations of a 9-point willingness-to-try scale. By coupling systematic combinations and regression modeling, we can estimate the unique appeal of each element. In this population, a multifunctional product (i.e., broad spectrum STI protection, coupled with conception) is far more desirable than a microbicide targeted solely for HIV protection; we also found partner awareness and leakage are potentially strong barriers to use.  相似文献   
816.
817.
818.
Are all plant populations metapopulations?   总被引:2,自引:1,他引:1  
  相似文献   
819.
The behaviour of Na+/K+ ATPase during cell growth has been studied. Human cultured fibroblasts were used in the presence or absence of EGF. Sample and control cultures were stopped by gathering and washing the cells with tris buffer. Homogenates were tested for Na+/K+ ATPase activity by the method of incubating and for the -SH groups content (Ellman). Na+/K+ ATPase activity that slightly increases in the controls is strongly reduced by the addition of EGF. The behaviour shows evidence for a double mechanism of action: I) involvement of the cAMP system 2) decrease of the -SH group availability.  相似文献   
820.
Proteoglycans (PG) have been purified by classical methods from human articular cartilage in order to set up a radioimmunoassay. Conditions of labelling, purification of labelled PG, and optimal conditions of buffer, temperature, duration of incubations and dilution of antiserum are described. Separation of free and bound PG is performed by immunoprecipitation. It is demonstrated that human articular PG can be assayed quantitatively by RIA procedure, with the sensitivity of +/- 2 femto-moles (+/- 5 ng) per tube.  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