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   1071495篇
  免费   110991篇
  国内免费   431篇
  1182917篇
  2018年   10046篇
  2016年   13391篇
  2015年   17374篇
  2014年   20617篇
  2013年   30194篇
  2012年   33416篇
  2011年   34383篇
  2010年   23498篇
  2009年   21757篇
  2008年   30780篇
  2007年   32149篇
  2006年   30126篇
  2005年   28914篇
  2004年   28809篇
  2003年   27814篇
  2002年   27086篇
  2001年   44360篇
  2000年   44020篇
  1999年   35538篇
  1998年   13452篇
  1997年   13641篇
  1996年   12971篇
  1995年   12079篇
  1994年   11703篇
  1993年   11778篇
  1992年   29727篇
  1991年   29224篇
  1990年   28604篇
  1989年   28136篇
  1988年   26172篇
  1987年   24934篇
  1986年   23331篇
  1985年   23603篇
  1984年   19527篇
  1983年   16987篇
  1982年   13116篇
  1981年   11800篇
  1980年   11055篇
  1979年   18562篇
  1978年   14629篇
  1977年   13385篇
  1976年   12784篇
  1975年   14124篇
  1974年   15290篇
  1973年   15045篇
  1972年   13869篇
  1971年   12507篇
  1970年   10904篇
  1969年   10755篇
  1968年   9621篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
994.
Five species of sand eels (Ammodytidae) are regularly found in the north-east Atlantic. Some of these species (i.e. Ammodytes tobianus L. and A. marinus Raitt) are difficult to identify by morphological criteria. The aims of the study reported were (a) to reveal unambiguous genetic traits for species classification and (b) to study possible population structure of the more common species in Norwegian waters. In total more than 900 specimens were analysed for inter- and intraspecific allozyme variation. Reference samples were obtained from Scotland and Denmark. By combining the patterns at different loci, all five species could be unambiguously defined. In Norwegian waters A. marinus was found together with minor numbers of Hyperoplus lanceolatus (Le Sauvage). As expected, Gymnammodytes semisquamatus (Jourdain) and H. immaculatus (Corbin) were not found, but not finding A. tobianus in these waters was unexpected. Distribution of alleles at three polymorphic loci did not indicate any structure of A. marinus in Norwegian waters.  相似文献   
995.
996.
Species groups formed in light coniferous forests prevailing in southeastern Russia are considered using the example of butterflies. Problems concerning the formation of species composition and its originality in some insects groups are discussed. A classification analysis of species checklists for twelve local faunas of Transbaikalia and the southern Far East of Russia is performed. It is shown that the faunas of areas occupied by light coniferous forests have a common background.  相似文献   
997.
Fast, efficient and selective deprotection of the tert-butoxycarbonyl (Boc) group of various amino acids and peptides was achieved by using hydrogen chloride (4 m) in anhydrous dioxane solution for 30 min at room temperature. In the cases studied in our laboratory, this protocol provided superior selectivity to deprotect Nalpha-Boc groups in the presence of tert-butyl esters and tert-butyl ethers, including thio-tert-butyl ethers, but not phenolic tert-butyl ethers.  相似文献   
998.
A high-performance liquid chromatographic method is described for the determination of opipramol in human plasma. Opipramol was extracted into tert.-butylmethyl ether, separated on a cyanopropyl silica column and detected at 254 nm. Imipramine was used as internal standard. The limit of quantitation was 250 pg/ml using 1.5 ml plasma. Precision was better than 9%, inaccuracy less than 8%. The assay is more sensitive than previously published methods, and it has been applied to the analysis of plasma samples from a pharmacokinetic study.  相似文献   
999.
1000.
The cost of mutualism in a fly-fungus interaction   总被引:2,自引:0,他引:2  
The movement ability of individuals has become increasingly important to a variety of ecological questions. In this study, I investigate how plant structure and changes in body size through development affect the movement ability of a predaceous stinkbug, Podisus maculiventris, on three species of goldenrod (Solidago spp.) representing a wide range of surface complexities. I adapt existing techniques for quantifying movement in two dimensions to the study of movement on natural plant structures in three dimensions. These experiments indicate that plant structure and insect size are significant factors affecting the movement ability of P. maculiventris. Changes in movement ability due to factors of ontogeny and different habitat structures suggest that the scale of an individual’s ambit or ecological sphere of influence may vary within its lifespan. Considering the influence of ontogeny and habitat structure on movement ability may be useful to investigations of population dynamics, foraging behavior, and pest management. Received: 14 July 1999 / Accepted: 23 March 2000  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