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   76449篇
  免费   5058篇
  国内免费   10篇
  81517篇
  2023年   532篇
  2022年   660篇
  2021年   1347篇
  2020年   1097篇
  2019年   1274篇
  2018年   2298篇
  2017年   2124篇
  2016年   2796篇
  2015年   3661篇
  2014年   3808篇
  2013年   5081篇
  2012年   5892篇
  2011年   5363篇
  2010年   3478篇
  2009年   2826篇
  2008年   4115篇
  2007年   3865篇
  2006年   3709篇
  2005年   3163篇
  2004年   3082篇
  2003年   2761篇
  2002年   2550篇
  2001年   1704篇
  2000年   1640篇
  1999年   1314篇
  1998年   655篇
  1997年   493篇
  1996年   479篇
  1995年   457篇
  1994年   350篇
  1993年   335篇
  1992年   664篇
  1991年   567篇
  1990年   513篇
  1989年   512篇
  1988年   477篇
  1987年   445篇
  1986年   405篇
  1985年   396篇
  1984年   404篇
  1983年   276篇
  1982年   270篇
  1981年   208篇
  1980年   208篇
  1979年   252篇
  1978年   243篇
  1975年   220篇
  1974年   245篇
  1973年   245篇
  1972年   211篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Primates have long been used as indicator species for assessing overall ecosystem health. However, area‐wide census methods are time consuming, costly, and not always feasible under many field conditions. Therefore, it is important to establish whether monitoring a subset of a population accurately reflects demographic changes occurring in the population at large. Over the past 35 years, we have conducted 15 area‐wide censuses in Sector Santa Rosa, Costa Rica. These efforts have revealed important trends in population growth patterns of capuchin monkeys following the protection and subsequent regeneration of native forests. During this same period, we have also intensively studied a subset of the capuchin groups. Comparing these two datasets, we investigate whether the population structures of the closely monitored groups are reliable indicators of area‐wide demographic patterns. We compare the overall group size and the individual age/sex class compositions of study groups and nonstudy groups (i.e., those contacted during area‐wide censuses only). Our study groups contained more individuals overall with a larger proportion of infants, and there were indications that the proportion of adult and subadult males was lower. These differences can be ascribed either to sampling errors or real differences attributable to human presence and/or better habitat quality for the study groups. No other sex/age classes differed, and major demographic changes were simultaneously evident in both study and nonstudy groups. This study suggests that the Santa Rosa capuchin population is similarly impacted by large‐scale ecological patterns observable within our study groups.  相似文献   
3.
4.
Jan Scheirs  Luc De Bruyn 《Oikos》2002,97(1):139-144
The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account.  相似文献   
5.
A formalin-fixed paraffin-embedded (FFPE) sample usually yields highly degraded DNA, which limits the use of techniques requiring high-quality DNA, such as Infinium Methylation microarrays. To overcome this restriction, we have applied an FFPE restoration procedure consisting of DNA repair and ligation processes in a set of paired fresh-frozen (FF) and FFPE samples. We validated the FFPE results in comparison with matched FF samples, enabling us to use FFPE samples on the Infinium HumanMethylation450 Methylation array.  相似文献   
6.
7.
8.
Administration of purified pertussis toxin to rats induced persistent tachycardia, (observed in conscious rats but not after pithing); as little as 0.05 microgram/100 g produced a significant effect. Pertussis toxin-treatment did not affected the pressor response produced in the pithed rats by the alpha 2-adrenergic agonist methoxamine but markedly diminished the pressor effect of the alpha 2-adrenergic agonists clonidine and azepexole. A role of adenylate cyclase inhibition in the action of postsynaptic vascular alpha 2-adrenergic receptors is suggested.  相似文献   
9.
10.
Clostridium thermocellum encodes a cellulosomal, modular, and thermostable serine protease inhibitor (serpin), PinA. PinA stability but not inhibitory activity is affected by the Fn(III) and Doc(I) domains, and PinA is a broad inhibitor of subtilisin-like proteases and may play a key role in protecting the cellulosome from protease attack.  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