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   89890篇
  免费   21023篇
  国内免费   3674篇
  2023年   785篇
  2022年   1038篇
  2021年   2915篇
  2020年   4197篇
  2019年   6034篇
  2018年   5973篇
  2017年   5753篇
  2016年   6461篇
  2015年   7810篇
  2014年   8267篇
  2013年   9096篇
  2012年   7886篇
  2011年   7085篇
  2010年   6303篇
  2009年   4897篇
  2008年   4281篇
  2007年   3496篇
  2006年   3022篇
  2005年   2757篇
  2004年   2300篇
  2003年   2044篇
  2002年   1740篇
  2001年   1480篇
  2000年   1215篇
  1999年   1080篇
  1998年   646篇
  1997年   631篇
  1996年   583篇
  1995年   520篇
  1994年   495篇
  1993年   388篇
  1992年   521篇
  1991年   443篇
  1990年   380篇
  1989年   286篇
  1988年   273篇
  1987年   218篇
  1986年   156篇
  1985年   205篇
  1984年   122篇
  1983年   104篇
  1982年   74篇
  1981年   54篇
  1980年   44篇
  1979年   65篇
  1978年   66篇
  1977年   48篇
  1976年   49篇
  1975年   40篇
  1973年   48篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
3,4‐Methylenedioxymethamphetamine (MDMA, ecstasy) use may have long‐term neurotoxic effects. In this study, positron emission tomography with the tracer alpha‐[11C]methyl‐l ‐tryptophan (11C‐AMT) was used to compare human brain serotonin (5‐HT) synthesis capacity in 17 currently drug‐free MDMA polydrug users with that in 18 healthy matched controls. Gender differences and associations between regional 11C‐AMT trapping and characteristics of MDMA use were also examined. MDMA polydrug users exhibited lower normalized 11C‐AMT trapping in pre‐frontal, orbitofrontal, and parietal regions, relative to controls. These differences were more widespread in males than in females. Increased normalized 11C‐AMT trapping in MDMA users was also observed, mainly in the brainstem and in frontal and temporal areas. Normalized 11C‐AMT trapping in the brainstem and pre‐frontal regions correlated positively and negatively, respectively, with greater lifetime accumulated MDMA use, longer durations of MDMA use, and shorter time elapsed since the last MDMA use. Although the possibility of pre‐existing 5‐HT alterations pre‐disposing people to use MDMA cannot be ruled out, regionally decreased 5‐HT synthesis capacity in the forebrain could be interpreted as neurotoxicity of MDMA on distal (frontal) brain regions. On the other hand, increased 5‐HT synthesis capacity in the raphe and adjacent areas could be due to compensatory mechanisms.

  相似文献   

4.
5.
6.
Polyploidy is a fundamental mechanism in evolution, but is hard to detect in taxa with agmatoploidy or aneuploidy. We tested whether a combination of chromosome counting, microsatellite analyses and flow cytometric measurements represents a suitable approach for the detection of basic chromosome numbers and ploidy in Kobresia (Cyperaceae). Chromosome counting resulted in 2n = 64 for Kobresia pygmaea and K. cercostachys, 2n = 58 and 64 for K. myosuroides, and 2n = 72 for K. simpliciuscula. We characterized eight microsatellite loci for K. pygmaea, which gave a maximum of four alleles per individual. Cross‐species amplification was tested in 26 congeneric species and, on average, six of eight loci amplified successfully. Using flow cytometry, we confirmed tetraploidy in K. pygmaea. Basic chromosome numbers and ploidy were inferred from chromosome counts and the maximum number of alleles per locus. We consider the basic numbers as x = 16 and 18, with irregularities derived from agmatoploidy and aneuploidy. Across all Kobresia taxa, ploidy ranged from diploid up to heptaploid. The combination of chromosome counts and microsatellite analyses is an ideal method for the determination of basic chromosome numbers and for inferring ploidy, and flow cytometry is a suitable tool for the identification of deviating cytotypes. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 22–35.  相似文献   
7.
The causes of recurrent spontaneous abortion (RSA) and fetal malformations are multifactorial and unclear in most cases. Environmental, maternal, and genetic factors have been shown to contribute to these defects. Whole-exome sequencing (WES) is widely used to detect genetic variations associated with human diseases and has recently been successfully applied to unveil genetic causes of unexplained recurrent spontaneous abortion (URSA) and fetal malformations. Here, we review the current discovery and diagnosis strategies to identify the underlying pathogenic mutations of URSA and fetal malformations using WES technology and propose to further develop WES, both to advance our understanding of these diseases and to eventually lead to targeted therapies for reproductive disorders.  相似文献   
8.
It is a long-standing question as to which genes define the characteristic facial features among different ethnic groups. In this study, we use Uyghurs, an ancient admixed population to query the genetic bases why Europeans and Han Chinese look different. Facial traits were analyzed based on high-dense 3D facial images; numerous biometric spaces were examined for divergent facial features between European and Han Chinese, ranging from inter-landmark distances to dense shape geometrics. Genome-wide association studies(GWAS) were conducted on a discovery panel of Uyghurs. Six significant loci were identified, four of which, rs1868752, rs118078182, rs60159418 at or near UBASH3B, COL23A1, PCDH7 and rs17868256 were replicated in independent cohorts of Uyghurs or Southern Han Chinese. A prospective model was also developed to predict 3D faces based on top GWAS signals and tested in hypothetic forensic scenarios.  相似文献   
9.
孢粉学是解决植物分类中疑难类群物种微形态分化的重要方法,随着分子系统学的发展,结合这两门学科的优势可以更加有效地解决疑难类群的分类学问题。鳞盖蕨属(Microlepia)是一个分类困难的疑难类群,采用孢粉学与分子系统学一一对应的方法,以及居群取样方式,选取280份样本,联合4个叶绿体片段(rbcL、trnL-F、psbA-trnH和rps4),采用最大似然法和贝叶斯法构建该属的系统发生关系,在此基础上对凭证标本中100份材料的孢子进行观察和分析。综合分子系统学和孢粉学的研究结果,得出结论:(1)在形态学研究中广泛被接受的15个物种得到了单系支持,并厘清了分类困难的复合群;(2)发现边缘鳞盖蕨(M. marginata)可能存在隐性种;(3)建议恢复过去归并处理为异名的瑶山鳞盖蕨(M. yaoshanica)、罗浮鳞盖蕨(M. lofoushanensis)、四川鳞盖蕨(M. szechuanica)以及滇西鳞盖蕨(M. subspeluncae);(4)提出鳞盖蕨属可能存在杂交现象;(5)提出鳞盖蕨属完整的属下分类建议。  相似文献   
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