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   71597篇
  免费   5725篇
  国内免费   35篇
  2022年   466篇
  2021年   957篇
  2020年   614篇
  2019年   745篇
  2018年   964篇
  2017年   907篇
  2016年   1546篇
  2015年   2722篇
  2014年   2828篇
  2013年   3841篇
  2012年   4832篇
  2011年   4761篇
  2010年   3161篇
  2009年   2780篇
  2008年   4032篇
  2007年   4083篇
  2006年   3889篇
  2005年   3935篇
  2004年   3836篇
  2003年   3652篇
  2002年   3603篇
  2001年   836篇
  2000年   620篇
  1999年   855篇
  1998年   1020篇
  1997年   736篇
  1996年   737篇
  1995年   733篇
  1994年   692篇
  1993年   743篇
  1992年   642篇
  1991年   586篇
  1990年   500篇
  1989年   504篇
  1988年   521篇
  1987年   443篇
  1986年   425篇
  1985年   502篇
  1984年   605篇
  1983年   509篇
  1982年   587篇
  1981年   550篇
  1980年   483篇
  1979年   345篇
  1978年   417篇
  1977年   378篇
  1976年   315篇
  1975年   309篇
  1974年   314篇
  1973年   288篇
排序方式: 共有10000条查询结果,搜索用时 15 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.
  1. Birds colliding with turbine rotor blades is a well‐known negative consequence of wind‐power plants. However, there has been far less attention to the risk of birds colliding with the turbine towers, and how to mitigate this risk.
  2. Based on data from the Smøla wind‐power plant in Central Norway, it seems highly likely that willow ptarmigan (the only gallinaceous species found on the island) is prone to collide with turbine towers. By employing a BACI‐approach, we tested if painting the lower parts of turbine towers black would reduce the collision risk.
  3. Overall, there was a 48% reduction in the number of recorded ptarmigan carcasses per search at painted turbines relative to neighboring control (unpainted) ones, with significant variation both within and between years.
  4. Using contrast painting to the turbine towers resulted in significantly reduced number of ptarmigan carcasses found, emphasizing the effectiveness of such a relatively simple mitigation measure.
  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