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   1153747篇
  免费   126160篇
  国内免费   814篇
  2018年   10861篇
  2017年   10266篇
  2016年   14790篇
  2015年   19940篇
  2014年   23477篇
  2013年   33795篇
  2012年   37647篇
  2011年   38187篇
  2010年   25930篇
  2009年   23859篇
  2008年   33669篇
  2007年   34674篇
  2006年   32697篇
  2005年   31563篇
  2004年   31214篇
  2003年   29929篇
  2002年   29033篇
  2001年   53679篇
  2000年   53862篇
  1999年   42734篇
  1998年   15011篇
  1997年   15719篇
  1996年   14654篇
  1995年   13684篇
  1994年   13274篇
  1993年   13151篇
  1992年   34959篇
  1991年   33889篇
  1990年   32957篇
  1989年   32021篇
  1988年   29725篇
  1987年   27923篇
  1986年   25963篇
  1985年   25744篇
  1984年   21445篇
  1983年   17963篇
  1982年   13686篇
  1981年   12280篇
  1980年   11548篇
  1979年   19703篇
  1978年   15331篇
  1977年   13942篇
  1976年   12668篇
  1975年   14050篇
  1974年   15099篇
  1973年   14846篇
  1972年   13428篇
  1971年   12232篇
  1970年   10524篇
  1969年   10094篇
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
991.
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   
992.
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   
993.
994.
A peroxidase-anti-peroxidase (PAP) method using polyclonal anti-PR antibodies was used to localize progesterone receptor (PR) electron microscopically in the chick oviduct. The immunoreaction precipitate indicating PR was localized inside the nuclei of epithelial, glandular and stromal cells. In the estrogen withdrawn oviduct cytoplasmic immunoreaction precipitate was not seen. Inside the nucleus unoccupied PR was localized mainly like the heterochromatin. As visualized by the PAP technique, the localization of PR was not systematically changed after progesterone administration. In conclusion, we suggest that progesterone receptor in the chick oviduct is an intranuclear protein.  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