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   167756篇
  免费   6791篇
  国内免费   919篇
  175466篇
  2022年   487篇
  2021年   942篇
  2020年   567篇
  2019年   721篇
  2018年   12558篇
  2017年   11345篇
  2016年   8758篇
  2015年   2961篇
  2014年   2996篇
  2013年   4023篇
  2012年   8716篇
  2011年   17268篇
  2010年   14841篇
  2009年   10877篇
  2008年   13694篇
  2007年   15323篇
  2006年   4362篇
  2005年   4568篇
  2004年   5107篇
  2003年   4753篇
  2002年   4533篇
  2001年   1116篇
  2000年   762篇
  1999年   912篇
  1998年   1194篇
  1997年   875篇
  1996年   794篇
  1995年   760篇
  1994年   744篇
  1993年   715篇
  1992年   644篇
  1991年   639篇
  1990年   588篇
  1989年   622篇
  1988年   545篇
  1987年   523篇
  1986年   460篇
  1985年   593篇
  1984年   753篇
  1983年   656篇
  1982年   763篇
  1981年   796篇
  1980年   722篇
  1979年   499篇
  1978年   557篇
  1977年   527篇
  1976年   526篇
  1974年   499篇
  1972年   518篇
  1971年   538篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
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   
3.
4.
5.
6.
7.
8.
The photosynthetic reaction center complex from the green sulfur bacteriumChlorobium vibrioforme has been isolated under anaerobic conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals polypeptides with apparent molecular masses of 80, 40, 30, 18, 15, and 9 kDa. The 80- and 18-kDa polypeptides are identified as the reaction center polypeptide and the secondary donor cytochromec 551 encoded by thepscA andpscC genes, respectively. N-terminal amino acid sequences identify the 40-kDa polypeptide as the bacteriochlorophylla-protein of the baseplate (the Fenna-Matthews-Olson protein) and the 30-kDa polypeptide as the putative 2[4Fe-4S] protein encoded bypscB. Electron paramagnetic resonance (EPR) analysis shows the presence of an iron-sulfur cluster which is irreversibly photoreduced at 9K. Photoaccumulation at higher temperature shows the presence of an additional photoreduced cluster. The EPR spectra of the two iron-sulfur clusters resemble those of FA and FB of Photosystem I, but also show significantly differentg-values, lineshapes, and temperature and power dependencies. We suggest that the two centers are designated Center I (with calculatedg-values of 2.085, 1.898, 1.841), and Center II (with calculatedg-values of 2.083, 1.941, 1.878). The data suggest that Centers I and II are bound to thepscB polypeptide.  相似文献   
9.
John L. Graner 《CMAJ》1985,133(9):855-857,880
In 1849 Thomas Addison described the clinical entity now known as pernicious anemia. In 1855 he reported several cases of adrenal insufficiency, or Addison''s disease. Considering the importance of these works, there remains a great deal of confusion about them. Contrary to what many historians have written, a review of Addison''s original publications demonstrates a firm appreciation of the distinction between pernicious anemia and adrenal insufficiency, based particularly on the discoloration of the skin in these conditions. Three major sources of possible confusion for historians who are attempting to understand Addison''s views include Addison''s early attempts to link pernicious anemia with disease of the supra-renal capsules, Addison''s redefinition of pernicious anemia in his monograph on adrenal disease, and several confusing statements made by Wilks and Daldy in the first reprint of Addison''s monograph.  相似文献   
10.
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