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   40517篇
  免费   17050篇
  国内免费   12篇
  2022年   191篇
  2021年   599篇
  2020年   2286篇
  2019年   3840篇
  2018年   3999篇
  2017年   4248篇
  2016年   4379篇
  2015年   4553篇
  2014年   4203篇
  2013年   4785篇
  2012年   2628篇
  2011年   2365篇
  2010年   3628篇
  2009年   2285篇
  2008年   1414篇
  2007年   1046篇
  2006年   1016篇
  2005年   1064篇
  2004年   987篇
  2003年   956篇
  2002年   1015篇
  2001年   477篇
  2000年   374篇
  1999年   372篇
  1998年   244篇
  1997年   191篇
  1996年   178篇
  1995年   164篇
  1994年   155篇
  1993年   182篇
  1992年   177篇
  1991年   175篇
  1990年   220篇
  1989年   194篇
  1988年   180篇
  1987年   176篇
  1986年   149篇
  1985年   152篇
  1984年   157篇
  1983年   163篇
  1982年   165篇
  1981年   137篇
  1980年   119篇
  1979年   104篇
  1978年   135篇
  1977年   116篇
  1976年   98篇
  1975年   86篇
  1974年   91篇
  1973年   76篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
141.
142.
A series of new chiral thiosemicarbazones derived from homochiral amines in both enantiomeric forms were synthesized and evaluated for their in vitro antiproliferative activity against A549 (human alveolar adenocarcinoma), MCF‐7 (human breast adenocarcinoma), HeLa (human cervical adenocarcinoma), and HGC‐27 (human stomach carcinoma) cell lines. Some of compounds showed inhibitory activities on the growth of cancer cell lines. Especially, compound 17b exhibited the most potent activity (IC50 4.6 μM) against HGC‐27 as compared with the reference compound, sindaxel (IC50 10.3 μM), and could be used as a lead compound to search new chiral thiosemicarbazone derivatives as antiproliferative agents. Chirality 27:177–188, 2015. © 2014 Wiley Periodicals, Inc.  相似文献   
143.
144.
Although Ficus (Moraceae) is a keystone plant genus in the tropics, providing resources to many frugivorous vertebrates, its population genetic structure, which is an important determinant of its long‐term survival, has rarely been investigated. We examined the population genetic structure of two dioecious fig species (Ficus hispida and Ficus exasperata) in the Indian Western Ghats using co‐dominant nuclear microsatellite markers. We found high levels of microsatellite genetic diversity in both species. The regression slopes between genetic relationship coefficients (fij) and spatial distances were significantly negative in both species indicating that, on average, individuals in close spatial proximity were more likely to be related than individuals further apart. Mean parent–offspring distance (σ) calculated using these slopes was about 200 m in both species. This should be contrasted with the very long pollen dispersal distances documented for monoecious Ficus species. Nevertheless, overall population genetic diversity remained large suggesting immigrant gene flow. Further studies will be required to analyze broader scale patterns.  相似文献   
145.
146.
Space use including territoriality and spatial arrangement within a population can reveal important information on the nature, dynamics, and evolutionary maintenance of alternative strategies in color polymorphic species. Despite the prevalence of color polymorphic species as model systems in evolutionary biology, the interaction between space use and genetic structuring of morphs within populations has rarely been examined. Here, we assess the spatial and genetic structure of male throat color morphs within a population of the tawny dragon lizard, Ctenophorus decresii. Male color morphs do not differ in morphology but differ in aggressive and antipredator behaviors as well as androgen levels. Despite these behavioral and endocrine differences, we find that color morphs do not differ in territory size, with their spatial arrangement being essentially random with respect to each other. There were no differences in genetic diversity or relatedness between morphs; however, there was significant, albeit weak, genetic differentiation between morphs, which was unrelated to geographic distance between individuals. Our results indicate potential weak barriers to gene flow between some morphs, potentially due to nonrandom pre‐ or postcopulatory mate choice or postzygotic genetic incompatibilities. However, space use, spatial structure, and nonrandom mating do not appear to be primary mechanisms maintaining color polymorphism in this system, highlighting the complexity and variation in alternative strategies associated with color polymorphism.  相似文献   
147.
148.
149.
150.
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