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   83771篇
  免费   7215篇
  国内免费   38篇
  2022年   419篇
  2021年   1119篇
  2020年   720篇
  2019年   872篇
  2018年   1142篇
  2017年   1071篇
  2016年   1795篇
  2015年   3245篇
  2014年   3425篇
  2013年   4510篇
  2012年   5668篇
  2011年   5665篇
  2010年   3731篇
  2009年   3365篇
  2008年   4758篇
  2007年   4853篇
  2006年   4573篇
  2005年   4606篇
  2004年   4505篇
  2003年   4150篇
  2002年   4108篇
  2001年   1150篇
  2000年   955篇
  1999年   1124篇
  1998年   1213篇
  1997年   841篇
  1996年   802篇
  1995年   776篇
  1994年   757篇
  1993年   789篇
  1992年   821篇
  1991年   740篇
  1990年   655篇
  1989年   687篇
  1988年   622篇
  1987年   596篇
  1986年   562篇
  1985年   628篇
  1984年   667篇
  1983年   558篇
  1982年   650篇
  1981年   589篇
  1980年   505篇
  1979年   406篇
  1978年   420篇
  1977年   440篇
  1976年   348篇
  1975年   326篇
  1974年   376篇
  1973年   360篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
1.
The influence of fruit ingestion by the bat, Sturnira lilium, on germination of the seeds of the tree Solanum riparium was studied in a secondary rain forest in northwestern Argentina. Bat frequencies in disturbed areas were analyzed by mist net captures. Germination rates were determined for seeds collected from trees and bat feces. S. lilium was the most abundant fruit bat in the study area. Fruit digestion and the passage of seeds through the intestine did not significantly affect germination in S. riparium. In this case the fruit bats, therefore, probably provide only seed dispersal.  相似文献   
2.
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   
3.
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   
4.
5.
6.
When cotyledonary tissue of G., barbadense cotton are treated with the mutagen ethyl methanesulfonate and then germinated, an enhanced, unscheduled DNA synthesis response is observed, along with a concomitant increase in the thymidine triphosphate precursor pool size. The implications of these results are discussed in this paper.  相似文献   
7.
In this work, incorporation of plasmid DNA, pre-complexed with PEI, into polyelectrolyte multilayers has been studied to further develop platforms for local gene delivery. Polyplex embedding in synthetic and naturally degradable architectures was efficient for transfection of human hepato-cellular carcinoma cells.  相似文献   
8.
9.
10.
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