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   1178811篇
  免费   121845篇
  国内免费   604篇
  2018年   14178篇
  2017年   13615篇
  2016年   17018篇
  2015年   19434篇
  2014年   22976篇
  2013年   33194篇
  2012年   38176篇
  2011年   41690篇
  2010年   28997篇
  2009年   27109篇
  2008年   36806篇
  2007年   39071篇
  2006年   32743篇
  2005年   31977篇
  2004年   31531篇
  2003年   30550篇
  2002年   29686篇
  2001年   51672篇
  2000年   51290篇
  1999年   40798篇
  1998年   14569篇
  1997年   15083篇
  1996年   14226篇
  1995年   13170篇
  1994年   12686篇
  1993年   12745篇
  1992年   33306篇
  1991年   32341篇
  1990年   31409篇
  1989年   30712篇
  1988年   28155篇
  1987年   26832篇
  1986年   24975篇
  1985年   24951篇
  1984年   20585篇
  1983年   17725篇
  1982年   13410篇
  1981年   12234篇
  1980年   11417篇
  1979年   19289篇
  1978年   15131篇
  1977年   13707篇
  1976年   12900篇
  1975年   14319篇
  1974年   15439篇
  1973年   15159篇
  1972年   14225篇
  1971年   12845篇
  1970年   10937篇
  1969年   10586篇
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
991.
992.
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   
993.
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   
994.
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   
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