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   1133902篇
  免费   114541篇
  国内免费   436篇
  2018年   11767篇
  2017年   11010篇
  2016年   15480篇
  2015年   20187篇
  2014年   23537篇
  2013年   34065篇
  2012年   37819篇
  2011年   38263篇
  2010年   26147篇
  2009年   23756篇
  2008年   34022篇
  2007年   35158篇
  2006年   33104篇
  2005年   31401篇
  2004年   31256篇
  2003年   30022篇
  2002年   29140篇
  2001年   45384篇
  2000年   45036篇
  1999年   36357篇
  1998年   13906篇
  1997年   13993篇
  1996年   13343篇
  1995年   12436篇
  1994年   12037篇
  1993年   12069篇
  1992年   30273篇
  1991年   29771篇
  1990年   29116篇
  1989年   28689篇
  1988年   26609篇
  1987年   25402篇
  1986年   23794篇
  1985年   24099篇
  1984年   19976篇
  1983年   17391篇
  1982年   13466篇
  1981年   12119篇
  1980年   11363篇
  1979年   19045篇
  1978年   14997篇
  1977年   13735篇
  1976年   13124篇
  1975年   14561篇
  1974年   15745篇
  1973年   15478篇
  1972年   14272篇
  1971年   12885篇
  1970年   11310篇
  1969年   11202篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
5.
6.
7.
The alga Analipus japonicus (Harv.) Wynne displays a distinct seasonal pattern in its development in Peter the Great Bay (Sea of Japan). In winter and spring, it occurs only in the form of basal crusts, and vertical axes develop in the summer–autumn period. It reproduces mostly asexually from July to November. Algae with unilocular sporangia occur very seldom, only in June and July.  相似文献   
8.
9.
10.
Aim The Chilean endemic Dioscorea biloba (Dioscoreaceae) is a dioecious geophyte that shows a remarkable 600 km north–south disjunction in the peripheral arid area of the Atacama Desert. Its restricted present‐day distribution and probable Neogene origin indicate that its populations have a history linked to that of the Atacama Desert, making this an ideal model species with which to investigate the biogeography of the region. Location Chile, Atacama Desert and peripheral arid area. Methods Two hundred and seventy‐five individuals from nine populations were genotyped for seven nuclear microsatellite loci, and plastid trnL–F and trnT–L sequences were obtained for a representative subset of these. Analyses included the estimation of genetic diversity and population structure through clustering, Bayesian and analysis of molecular variance analyses, and statistical parsimony networks of chloroplast haplotypes. Isolation by distance was tested against alternative dispersal hypotheses. Results Microsatellite markers revealed moderate to high levels of genetic diversity within populations, with those from the southern Limarí Valley showing the highest values and northern populations showing less exclusive alleles. Bayesian analysis of microsatellite data identified three genetic groups that corresponded to geographical ranges. Chloroplast phylogeography revealed no haplotypes shared between northern and southern ranges, and little haplotype sharing between the two neighbouring southern valleys. Dispersal models suggested the presence of extinct hypothetical populations between the southern and northern ranges. Main conclusions Our results are consistent with prolonged isolation of the northern and southern groups, mediated by the life‐history traits of the species. Significant isolation was revealed at both large and moderate distances as gene flow was not evident even between neighbouring valleys. Bayesian analyses of microsatellite and chloroplast haplotype diversity identified the southern area of Limarí as the probable area of origin of the species. Our data do not support recent dispersal of D. biloba from the southern range into Antofagasta, but indicate the fragmentation of an earlier wider range, concomitant with the Pliocene–Pleistocene climatic oscillations, with subsequent extinctions of the Atacama Desert populations and the divergence of the peripheral ones as a consequence of genetic drift.  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