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   1464328篇
  免费   133388篇
  国内免费   1526篇
  2021年   18213篇
  2019年   16388篇
  2018年   18366篇
  2017年   17157篇
  2016年   28723篇
  2015年   43098篇
  2014年   51220篇
  2013年   77523篇
  2012年   40607篇
  2011年   29875篇
  2010年   45019篇
  2009年   45941篇
  2008年   28020篇
  2007年   26312篇
  2006年   30882篇
  2005年   31715篇
  2004年   31091篇
  2003年   28599篇
  2002年   26754篇
  2001年   39564篇
  2000年   36876篇
  1999年   35408篇
  1998年   26168篇
  1997年   26021篇
  1996年   25490篇
  1995年   23557篇
  1994年   23436篇
  1993年   22500篇
  1992年   31017篇
  1991年   29388篇
  1990年   27895篇
  1989年   28704篇
  1988年   26283篇
  1987年   25107篇
  1986年   23599篇
  1985年   25531篇
  1984年   24705篇
  1983年   21768篇
  1982年   21692篇
  1981年   20723篇
  1980年   19409篇
  1979年   21275篇
  1978年   19281篇
  1977年   18667篇
  1976年   17635篇
  1975年   17424篇
  1974年   18192篇
  1973年   18537篇
  1972年   15868篇
  1971年   14669篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
51.
52.
53.
54.
Monographic data rely on specimens deposited in herbaria and museums, which have been thoroughly revised by experts. However, monographic data have been rarely used to map species richness at large scale, mainly because of the difficulties caused by spatially heterogeneous sampling effort. In this paper we estimate patterns of species richness and narrow endemism, based on monographic data of 4,055 Neotropical angiosperm species. We propose a geometric interpolation method to derive species ranges at a 1° grid resolution. To this we apply an inverse distance-weighted summation scheme to derive maps of species richness and endemism. In the latter we also adjust for heterogeneous sampling effort. Finally, we test the robustness of the interpolated species ranges and derived species richness by applying the same method but using a leave-one-out-cross-validation (LOOCV). The derived map shows four distinct regions of elevated species richness: (1) Central America, (2) the Northern Andes, (3) Amazonia and (4) the Brazilian Atlantic coast (‘Mata Atlantica’). The region with the highest estimated species richness is Amazonia, with Central America following closely behind. Centers of narrow endemism are located over the entire Neotropics, several of them coinciding with regions of elevated species richness. Sampling effort has a minor influence on the interpolation of overall species richness, but it substantially influences the estimation of regions of narrow endemism. Thus, in order to improve maps of narrow endemism and resulting conservation efforts, more collection and identification activity is required.  相似文献   
55.
56.
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   
57.
58.
1. Examination of the cerebrospinal fluid (CSF) of head-injured patients reveals that the concentration of intraventricular xanthine is elevated and that of uridine is decreased relative to those of adult lumbar CSF. 2. No correlations were observed between CSF lactate and CSF hypoxanthine, xanthine, or uridine, suggesting that changes in purine metabolites and the pyrimidine nucleoside do not index similar cellular events as does lactic acid production. 3. Ventricular CSF from hydrocephalic infants had uridine and hypoxanthine concentrations not significantly different from those of normal adult lumbar CSF, but xanthine was significantly elevated. 4. Since uridine has anticonvulsant properties and is a crucial substrate for cerebral metabolism, it may be useful to evaluate this pyrimidine for use in the management of patients with head injury.  相似文献   
59.
60.
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