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   426013篇
  免费   48044篇
  国内免费   162篇
  2018年   3916篇
  2016年   5298篇
  2015年   6928篇
  2014年   8187篇
  2013年   11282篇
  2012年   12848篇
  2011年   13310篇
  2010年   9116篇
  2009年   8461篇
  2008年   12165篇
  2007年   12672篇
  2006年   11883篇
  2005年   11358篇
  2004年   11408篇
  2003年   10680篇
  2002年   10500篇
  2001年   17465篇
  2000年   17481篇
  1999年   13955篇
  1998年   5038篇
  1997年   5293篇
  1996年   4924篇
  1995年   4632篇
  1994年   4504篇
  1993年   4553篇
  1992年   11654篇
  1991年   11583篇
  1990年   11333篇
  1989年   10958篇
  1988年   10528篇
  1987年   10110篇
  1986年   9391篇
  1985年   9266篇
  1984年   7786篇
  1983年   6738篇
  1982年   5187篇
  1981年   4633篇
  1980年   4493篇
  1979年   7442篇
  1978年   5893篇
  1977年   5423篇
  1976年   5225篇
  1975年   5626篇
  1974年   6318篇
  1973年   6192篇
  1972年   5789篇
  1971年   5246篇
  1970年   4658篇
  1969年   4596篇
  1968年   4431篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Censuses of buffalo and wildebeest in the Serengeti-Mara region have been carried out by various workers at intervals since 1958. The methods of these censuses, which normally employ a total count, are described and the errors inherent in such techniques are analysed in detail. After the appropriate corrections had been made the results showed that both populations have increased considerably over the years 1961–1971.  相似文献   
42.
43.
44.
Zooplankton data collected during September 1995 in the NorthWest Atlantic at 4139'N, 4958'W (the location of the siteof the ‘Titanic’ wreck) were analysed. The regioninvestigated was characterized by a very sharp frontal zonebetween the Gulf Stream and the main stream of the LabradorCurrent. The total plankton biomass in the water column wasvery high. The macroplankton biomass values below the 600 mlayer were significantly higher as compared with the similarvalues measured before in other productive boreal regions ofthe Atlantic and Pacific oceans. A lot of dead mesoplanktonanimals occurred in the deep layers. The reason was that thecold-water mesoplankton advected by the Labrador Current diedoff intensively within the deep layers of the frontal zone andwere used as a food resource by the macroplankton carnivoresand scavengers that were very abundant there.  相似文献   
45.
Reconstruction of bony structures of the face is always a problem as big as the defect and the function that must be replaced. Everything from simple grafts to sophisticated distant bone flaps has been used. Based on the studies of Cutting et al., Psillakis et al., and Casanova et al., we have developed the full-thickness galeoparietal bone flap, initially for mandibular reconstruction, but of great use for all maxillofacial reconstructions. From July of 1987 to December of 1988, 14 patients have been operated on. The experience with this flap is shown in four patients as follows: primary reconstruction of a mandible as a result of ameloblastoma, secondary reconstruction of a mandible with associated old fractures and malalignment of segments, bilateral malar reconstruction in a patient with Treacher Collins syndrome, and severe sequelae of an already treated Romberg case. Small variations could be made to best accommodate the technique used to the defect we were treating. Some technical details, the experience, the results, and possible sequelae or complications are also discussed.  相似文献   
46.
47.
48.
49.
50.
An agar-degrading marine bacterium identified as a Microscilla species was isolated from coastal California marine sediment. This organism harbored a single 101-kb circular DNA plasmid designated pSD15. The complete nucleotide sequence of pSD15 was obtained, and sequence analysis indicated a number of genes putatively encoding a variety of enzymes involved in polysaccharide utilization. The most striking feature was the occurrence of five putative agarase genes. Loss of the plasmid, which occurred at a surprisingly high frequency, was associated with loss of agarase activity, supporting the sequence analysis results.  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