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   78877篇
  免费   6316篇
  国内免费   1243篇
  2023年   395篇
  2022年   828篇
  2021年   1767篇
  2020年   1089篇
  2019年   1439篇
  2018年   1628篇
  2017年   1328篇
  2016年   2183篇
  2015年   3479篇
  2014年   3844篇
  2013年   4752篇
  2012年   5888篇
  2011年   5733篇
  2010年   3712篇
  2009年   3365篇
  2008年   4561篇
  2007年   4557篇
  2006年   4176篇
  2005年   4091篇
  2004年   3875篇
  2003年   3657篇
  2002年   3547篇
  2001年   1053篇
  2000年   830篇
  1999年   1017篇
  1998年   1029篇
  1997年   755篇
  1996年   756篇
  1995年   708篇
  1994年   656篇
  1993年   665篇
  1992年   645篇
  1991年   545篇
  1990年   457篇
  1989年   457篇
  1988年   444篇
  1987年   381篇
  1986年   378篇
  1985年   446篇
  1984年   452篇
  1983年   391篇
  1982年   468篇
  1981年   420篇
  1980年   370篇
  1979年   247篇
  1978年   299篇
  1977年   289篇
  1976年   233篇
  1974年   241篇
  1973年   222篇
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
1.
The phylogeny of the fungus gnat family Mycetophilidae (Diptera) is reconstructed with a focus on the species‐rich and taxonomically difficult subfamilies Gnoristinae and Mycomyinae. The multigene phylogenetic analyses are based on five nuclear (18S, 28S, CAD, MCS, ITS2) and four mitochondrial (12S, 16S, COI, CytB) gene markers. The analyses strongly support the monophyly of Mycetophilidae and the subfamilies Manotinae, Sciophilinae, Leiinae, and Mycomyinae, although Gnoristinae is paraphyletic with respect to Mycetophilinae. All the genera and groups of genera included are supported as monophyletic, except for Acomoptera Vockeroth, Boletina Staeger, Dziedzickia Johannsen, Ectrepesthoneura Enderlein, and Neoempheria Osten Sacken. Ancestral character state reconstructions were applied to two morphological features present in Gnoristinae and Mycomyinae (i.e. presence of setae on wing membrane and wing vein R4) in order to assess their evolution. The wing vein R4 appears as an unstable character, spread throughout different clades. A dated phylogeny of the family Mycetophilidae showed that most of the subfamilies of Mycetophilidae originated and diversified during the Cretaceous. The youngest subfamilies, originated in the Paleogene, appear to be Mycomyinae and Mycetophilinae.  相似文献   
2.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982).  相似文献   
3.
4.
5.
6.
7.
8.
9.

Background  

The function and viability of cultured, transplanted, or encapsulated pancreatic islets is often limited by hypoxia because these islets have lost their vasculature during the isolation process and have to rely on gradient-driven passive diffusion, which cannot provide adequate oxygen transport. Pancreatic islets (islets of Langerhans) are particularly susceptible due to their relatively large size, large metabolic demand, and increased sensitivity to hypoxia. Here, finite element method (FEM) based multiphysics models are explored to describe oxygen transport and cell viability in avascular islets both in static and in moving culture media.  相似文献   
10.
Using hot alkaline solution, the elastic laminae were extracted from aortas and observed with scanning electron microscopy. Vascular structures were found in the elastin layers of the tunica media in descending thoracic aortas of sheep, dogs, and pigs, and these tube-like structures were filled with elastomer which was injected through the heart of the animal in vivo. Sub-intimal microvessels were also found to be filled with the elastomer and it is concluded that vasa vasorum can exist close to the internal elastic lamina in these animals.  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