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   82674篇
  免费   6864篇
  国内免费   23篇
  2023年   329篇
  2021年   905篇
  2020年   653篇
  2019年   759篇
  2018年   1499篇
  2017年   1464篇
  2016年   1939篇
  2015年   2493篇
  2014年   2802篇
  2013年   3982篇
  2012年   5967篇
  2011年   6410篇
  2010年   3618篇
  2009年   2846篇
  2008年   5255篇
  2007年   5375篇
  2006年   5022篇
  2005年   4578篇
  2004年   4449篇
  2003年   4154篇
  2002年   4093篇
  2001年   2023篇
  2000年   2165篇
  1999年   1417篇
  1998年   941篇
  1997年   697篇
  1996年   700篇
  1995年   640篇
  1994年   645篇
  1993年   616篇
  1992年   677篇
  1991年   575篇
  1990年   518篇
  1989年   481篇
  1988年   463篇
  1987年   433篇
  1986年   368篇
  1985年   504篇
  1984年   582篇
  1983年   482篇
  1982年   471篇
  1981年   442篇
  1980年   374篇
  1979年   362篇
  1978年   361篇
  1977年   348篇
  1976年   303篇
  1975年   349篇
  1974年   275篇
  1973年   276篇
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
1.
2.
3.
Elevated levels of intracellular calcium are a major cause of myocardial dysfunction. To find possible mediators of the deregulated calcium we searched for EF-hand calcium-binding proteins of the S100 family. By PCR technology we identified three members of the S100 protein family (S100 alpha, CACY, and CAPL) in the human heart. We cloned the corresponding cDNAs and examined their expression levels in various human tissues by Northern blot analysis. All three proteins are expressed at high levels in the human heart. Whereas CACY and CAPL mRNAs are expressed ubiquitously, S100 alpha mRNA is restricted to heart, skeletal muscle, and brain. Interestingly, the expression pattern of S100 alpha, CACY, and CAPL in human tissues differs significantly from that in rodent tissues.  相似文献   
4.
5.
6.
7.
8.
9.
White‐sand forests are patchily distributed ecosystems covering just 5% of Amazonia that host many specialist species of birds not found elsewhere, and these forests are threatened due to their small size and human exploitation of sand for construction projects. As a result, many species of birds that are white‐sand specialists are at risk of extinction, and immediate conservation action is paramount for their survival. Our objective was to evaluate current survey methods and determine the relative effect of the size of patches of these forests on the presence or absence of white‐sand specialists. Using point counts and autonomous recorders, we surveyed avian assemblages occupying patches of white‐sand forest in the Peruvian Amazon in April 2018. Overall, we detected 126 species, including 21 white‐sand forest specialists. We detected significantly more species of birds per survey point with autonomous recorders than point counts. We also found a negative relationship between avian species richness and distance from the edge of patches of white‐sand forest, but a significant, positive relationship when only counting white‐sand specialists. Although we detected more species with autonomous recorders, point counts were more effective for detecting canopy‐dwelling passerines. Therefore, we recommend that investigators conducting surveys for rare and patchily distributed species in the tropics use a mixed‐method approach that incorporates both autonomous recorders and visual observation. Finally, our results suggest that conserving large, continuous patches of white‐sand forest may increase the likelihood of survival of species of birds that are white‐sand specialists.  相似文献   
10.
In a section of the Liassic southwest of the village of Préjano (Prov. Logroño) Sinemurian Dasyclad algae were found for the first time in the “Bankkalk-Series” of the “Carniolas-Formation” which is not dated up to that time. Six species,Dissocladella lucasi (Cros & Lemoine),Dissocladella iberica nov. sp.,Dissocladella ebroensis nov. sp.,Sestrosphaera liasina Pia,Gyroporella retica (Zanin) andMacroporella nov. sp. aff. sturi Bystricky are described.  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