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   608篇
  免费   0篇
  国内免费   2篇
系统科学   7篇
教育与普及   2篇
理论与方法论   4篇
现状及发展   149篇
研究方法   73篇
综合类   361篇
自然研究   14篇
  2017年   3篇
  2016年   3篇
  2013年   7篇
  2012年   25篇
  2011年   44篇
  2010年   4篇
  2009年   4篇
  2008年   19篇
  2007年   16篇
  2006年   20篇
  2005年   26篇
  2004年   22篇
  2003年   15篇
  2002年   13篇
  2001年   25篇
  2000年   25篇
  1999年   14篇
  1998年   3篇
  1997年   2篇
  1994年   3篇
  1992年   6篇
  1991年   8篇
  1990年   8篇
  1989年   13篇
  1988年   19篇
  1987年   15篇
  1986年   10篇
  1985年   14篇
  1984年   12篇
  1983年   9篇
  1982年   7篇
  1981年   4篇
  1980年   5篇
  1979年   17篇
  1978年   13篇
  1977年   12篇
  1976年   9篇
  1975年   12篇
  1974年   15篇
  1973年   14篇
  1972年   4篇
  1971年   8篇
  1970年   13篇
  1969年   12篇
  1968年   11篇
  1967年   12篇
  1966年   15篇
  1965年   12篇
  1957年   1篇
  1956年   1篇
排序方式: 共有610条查询结果,搜索用时 15 毫秒
41.
We have used hybridization analysis to detect ancient DNA in wheat seeds collected from three archaeological sites in Europe and the Middle East. One of these samples, carbonizedT. spelta dated to the first millennium BC, has yielded PCR products after amplification with primers directed at the leader regions of the HMW (high molecular weight) glutenin alleles. Sequences obtained from these products suggest that the DNA present in the Danebury seeds is chemically damaged, as expected for ancient DNA, and also indicate that it should be possible to study the genetic variability of archaeological wheat by ancient DNA analysis. Finally, we describe a PCR-based system that enables tetraploid and hexaploid wheats to be distinguished.  相似文献   
42.
J T Webb  G W Brown 《Experientia》1980,36(8):903-904
Specific activity of glutamine synthetase in Squalus acanthias (spiny dogfish) central nervous system regions was highest in the cerebellum and lowest in the spinal cord. The levels of activity may relate to the excitability of each region by regulating the glutamate pool.  相似文献   
43.
44.
Defenders of value-free science appeal to cognitive attitudes as part of a wedge strategy, to mark a distinction between science proper and the uses of science for decision-making, policy, etc. Distinctions between attitudes like belief and acceptance have played an important role in defending the value-free ideal. In this paper, I will explore John Dewey's pragmatist philosophy of science as an alternative to the philosophical framework the wedge strategy rests on. Dewey does draw significant and useful distinctions between different sorts of cognitive attitudes taken by inquirers, but none can be used to support the wedge strategy.  相似文献   
45.
We investigated genetic variation within and between 3 populations of Gaura neomexicana ssp. coloradensis , a federally listed threatened species, using inter-simple sequence repeats. The data matrix included 24 individuals scored for 88 bands, with 3.4% missing data. Cluster analysis showed that members of the 3 populations are very similar and are intermixed in the phenogram. Principal coordinate analysis indicated that members of the Crow Creek population are different from the Diamond Creek and ""unnamed drainage"" populations. This could be the result of historic differences, current changes in vegetation at the Crow Creek site, or an artifact of limited sampling. In any case, for management purposes it is best to preserve the dwindling population at Crow Creek as well as the thriving populations at the other 2 sites.  相似文献   
46.
We used the computer program RAMAS to explore the sensitivity of an extinction-risk model for the Gila trout ( Oncorhynchus gilae ) to management of wildfires and number of populations of the species. The Gila trout is an endangered salmonid presently restricted to very few headwaters of the Gila and San Francisco river tributaries in southwestern New Mexico. Life history data for 10 extant populations were used to examine sensitivity of the species viability to changes in a variety of factors including population size, fecundity, life stage structure, number of populations, severity and probability of forest fires, and a regulated fishery. The probability and severity of forest fires and number of populations had the greatest effect on viability. Results indicate that successful conservation of Gila trout requires establishment of additional populations and reduction of the severity of forest fires through a program incorporating more frequent, but less severe, fires.  相似文献   
47.
48.
49.
Summary Chicken spinal cord adenosine triphosphatases (both Na+, K+ stimulated and ouabain insensitive) were inhibited by tri-o-tolyl phosphate (TOTP, a neurotoxic organophosphate which is not a cholinesterase inhibitor) and mevinphos (a non-neurotoxic compound but inhibitor of cholinesterases). The inhibition was concentration and time dependent, with an initial rapid drop in activity followed by a gradual exponential decline.  相似文献   
50.
Acknowledgment of priority  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