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   1207387篇
  免费   127072篇
  国内免费   861篇
  2018年   11567篇
  2017年   11117篇
  2016年   16784篇
  2015年   24156篇
  2014年   27338篇
  2013年   37471篇
  2012年   40918篇
  2011年   39558篇
  2010年   27297篇
  2009年   25250篇
  2008年   33971篇
  2007年   34936篇
  2006年   32997篇
  2005年   36769篇
  2004年   35372篇
  2003年   32642篇
  2002年   29776篇
  2001年   54049篇
  2000年   53957篇
  1999年   43199篇
  1998年   15177篇
  1997年   15769篇
  1996年   14693篇
  1995年   13752篇
  1994年   13334篇
  1993年   13198篇
  1992年   35672篇
  1991年   34705篇
  1990年   33865篇
  1989年   32922篇
  1988年   30601篇
  1987年   28735篇
  1986年   26692篇
  1985年   26528篇
  1984年   21974篇
  1983年   18372篇
  1982年   13895篇
  1981年   12498篇
  1980年   11728篇
  1979年   20252篇
  1978年   15717篇
  1977年   14275篇
  1976年   13045篇
  1975年   14490篇
  1974年   15681篇
  1973年   15451篇
  1972年   13946篇
  1971年   12768篇
  1970年   11004篇
  1969年   10615篇
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
991.
Dr A C Srivastava has written to us to describe a case that raises the suggestion that people infected with the human immuno-deficiency virus (HIV) should carry identity cards. We asked two physicians, a general practitioner working with patients with the acquired immune deficiency syndrome (AIDS), and a general practitioner with a special interest in medical ethics to respond to the broad issues raised by Dr Srivastava''s letter.  相似文献   
992.
We sampled macroinvertebrates at 75 locations in the Mondego river catchment, Central Portugal, and developed a predictive model for water quality assessment of this basin, based on the Reference Condition Approach. Sampling was done from June to September 2001. Fifty-five sites were identified as “Reference sites” and 20 sites were used as “Test sites” to test the model. At each site we also measured 40 habitat variables to characterize water physics and chemistry, habitat type, land use, stream hydrology and geographic location. Macroinvertebrates were generally identified to species or genus level; a total of 207 taxa were found. By Unweighted Pair Group Method with Arithmetic mean (UPGMA) clustering and analysis of species contribution to similarities percentage (SIMPER), two groups of reference sites were established. Using Discriminant Analysis (stepwise forward), four variables correctly predicted 78% of the reference sites to the appropriate group: stream order, pool quality, substrate quality and current velocity. Test sites’ environmental quality was established from their relative distance to reference sites, in MDS ordination space, using a series of bands (BEAST methodology). The model performed well at upstream sites, but at downstream sites it was compromised by the lack of reference sites. As with the English RIVPACS predictive model, the Mondego model should be continually improved with the addition of new reference sites. The adaptation of the Mondego model methodology to the Water Framework Directive is possible and would consist mainly of the integration of the WFD typology and increasing the number of ellipses that define quality bands. Handling editor: K. Martens  相似文献   
993.
A new cestode species, Diphyllobothrium klebanovskii sp. n., is described on the basis of experimental examination and morphological study of 51 specimens expelled from men in lower part of the Amur river and 18 specimens expelled from golden hamsters. The larval stage of this cestode is plerocercoid of type F, which is localized in muscles of Oncorhynchus keta and O. gorbuscha. D. klebanovskii sp. n. is a marine species, it differs from D. latum and other species of the genus Diphyllobothrium.  相似文献   
994.
995.
996.
997.
998.
999.
1000.
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